A julolidine-fused anthracene derivative: synthesis,photophysical properties,and oxidative dimerization

We describe the synthesis and characterization of a julolidine-fused anthracene derivative J-A, which exhibits a maximum absorption of 450 nm and a maximum emission of 518 nm. The fluorescent quantum yield was determined to be 0.55 in toluene. J-A dimerizes in solution via oxidative coupling. Structure of the dimer was characterized using single crystal X-ray diffraction.

A julolidine fused anthracene derivative with unique photophysical and redox properties was presented.

Julolidine¹ is a popular structural subunit in various fluorescent dyes (Chart 1).² The restricted motion and strong electron-donating capability of the fused julolidine moiety are quite effective for improving the photophysical properties. For instance, julolidine-fused fluorophores normally display desirable photophysical characteristics, such as high quantum yield, red-shifted absorption and emission, and good photostability. Recently, julolidine derivatives have been widely exploited in various applications such as sensing,³ imaging,⁴ and nonlinear optical materials.⁵ Several julolidine dyes have been used in dye-sensitized solar cells due to their large π-conjugated system and the promising electron donating property.⁶Open in a separate windowChart 1The structure of julolidine, J-A and DAA.In this paper, we report a julolidine-fused anthracene derivative J-A, which exhibits attractive photophysical properties not observed in DAA, a dimethyl-amino substituted analogue. Both the absorption and emission of J-A show a dramatic red-shift (ca. 74 and 131 nm, respectively), compared with the unmodified anthracene (Fig. 2). The fluorescence quantum yield of J-A was determined to be 0.55 in toluene, while the emission of DAA was completely quenched. The observed spectral properties were rationalized by DFT calculations. In addition, we found that J-A was stable in the solid state, but reactive in solution. J-A dimer was formed through oxidative coupling at the para-position of the N-atom in a dichloromethane solution under air atmosphere. The structure of the dimerized product was characterized using single crystal X-ray diffraction, which unambiguously reveals the structural feature of the julolidine-fused anthracene compound. Preparation of J-A is shown in Scheme 1. Detailed synthesis and characterizations are provided in the ESI.^†Open in a separate windowScheme 1Synthetic route of J-A.Open in a separate windowFig. 2(A) Absorption spectra of J-A, AN, and DAA (1 × 10⁻⁴ mol L⁻¹ in dichloromethane); (B) emission spectra of J-A, AN, and DAA (1 × 10⁻⁵ mol L⁻¹ in dichloromethane). Excitation wavelength: 350 nm. 1 cm cuvette was used in both of the experiments. Inset: visualized fluorescence in solution was shown. ¹H-NMR signals of J-A shift to the high-field significantly, compared with DAA (Fig. 1), which indicates that the fused structure of J-A facilitates electron delocalization from the nitrogen atom to the anthracene moiety, and thus resulting in a stronger shielding effect. In the case of DAA, however, electron delocalization from the dimethyl amino group to the anthracene core is essentially inhibited due to steric hindrance, which will explain the fact that it displays a spectral feature similar to that of the unmodified anthracene.Open in a separate windowFig. 1Comparison of the ¹H-NMR spectrum between J-A and DAA in CDCl₃. Partial resonance signals in aromatic region are shown.Fusing with julolidine will exert significant effects on the photophysical properties of anthracene. The absorption and fluorescence spectra of J-A, DAA, and anthracene are shown in Fig. 2. The maximum absorption of J-A is 450 nm, which displays a red-shift of about 70 nm compared with the unmodified anthracene. J-A emits green light (^maxλ_em = 518 nm, Φ = 0.55), while anthracene emits blue light (^maxλ_em = 380 nm, Φ = 0.22). In contrast, the absorption of DAA essentially overlaps with that of anthracene, with only a minor red shift of ca. 10 nm, but its fluorescence is quenched significantly (Fig. 2). This spectral feature indicates that the dimethyl amino group is electronically separated from the anthracene moiety in the ground state, a result in good accordance with the ¹H-NMR data shown above. The quenched fluorescence of DAA may result from the photo-induced electron transfer⁷ from the lone pair of the nitrogen atom to the anthracene moiety in the excited state.The observed photophysical properties of J-A were reproduced by DFT calculations. The HOMO and LUMO orbitals are evenly distributed over the anthracene moiety and the N-atom in the julolidine, indicating the existence of a conjugated structure. The HOMO–LUMO transition (f = 0.10) corresponds to the absorption band at 450 nm. The sharper absorption at 390 nm can be assigned to the HOMO to LUMO + 1 transition. In contrast, the HOMO and LUMO orbitals of DAA resemble those of anthracene, because the dimethyl-amino group is orthogonal to the conjugated π-system (Fig. 3).Open in a separate windowFig. 3Molecular orbitals of J-A and DAA calculated at the B3LYP/6-31G(d) level of theory (iso value = 0.02). Orbital energies were given in parentheses. Excitation energies were computed by TD-DFT at the same level. Values in parentheses represent the oscillator strengths (f).J-A is stable in the solid state, but reactive in solution. The cyclic voltammetry (CV) diagram of J-A shows an irreversible oxidation potential at 0.007 V (vs. Fc/Fc⁺), indicating that J-A is easy to be oxidized (Fig. S3^†). Single crystals suitable for X-ray diffraction study were obtained by slow evaporation of a dichloromethane solution of J-A under air atmosphere. To our surprise, instead of J-A, X-ray data discloses the formation of a dimeric product (Scheme 2) of J-A. We hypothesized that the dimeric compound 5 formed via oxidative coupling reaction, a mechanism well-documented for the dimerization of the dimethylaniline compounds.⁸ The ¹H-NMR spectrum of 5 is distinct from that of J-A. All protons of the anthracene part (b′–d′) appear as a group of multiplet resonance signals (6.93–7.04 ppm) (Scheme 2). In addition, mass spectrometric analysis indicates that two hydrogen atoms were removed after the dimerization of J-A. Compound 5 exhibits a maximum absorption at 460 nm and a very weak emission (^maxλ_em = 530 nm, Φ = 0.03, Fig. S1^†). Two quasi-reversible oxidation waves were identified in the CV diagram of 5 at −0.010 V and 0.135 V (vs. Fc/Fc⁺), respectively (Fig. S5^†).Open in a separate windowScheme 2Oxidative dimerization of J-A. Inset: partial ¹H-NMR of 5 is shown.X-ray structure of 5 is shown in Fig. 4. The two connected anthracene planes are found to be orthogonal to each other with a dihedral angle of 90.17°, as a result of steric repulsion. Specifically, the bond length of N–C3 is 1.389 Å, which is similar to those of the other reported julolidine compounds (1.359–1.393 Å), while significantly shorter than that of the dimethyl-amino anthracene (1.433 Å).⁹ This result testifies the presence of electron delocalization between the fused julolidine nitrogen and the anthracene π-plane, which is in good agreement with the DFT calculations (Fig. 3). However, the two anthracene π-planes connected by the single bond (C4–C5, 1.489 Å) might not exhibit electron delocalization because of the orthogonal conformation. The fused julolidine ring-i and -ii are symmetric to each other, and both of them adopt an “envelope” conformation. The fused julolidine is nonplanar (bond angle, C3–N–C2, 115.73°, C3–N–C1, 115.92°, C1–N–C2, 113.81°). 5 is closely packed in the crystal (Fig. 4B), and no intercalated solvent molecules were observed. The closest distance between two adjacent anthracene planes is 3.922 Å, indicating a weak π–π stacking. Detailed crystal data are summarized in Table S5.^†Open in a separate windowFig. 4(A) Single crystal X-ray structure of 5. (B) View along a-axis.In summary, we report the synthesis and characterizations of a julolidine-fused anthracene derivative J-A, which demonstrates significantly red-shifted absorption (^maxλ_ab = 450 nm) and emission (^maxλ_em = 518 nm, Φ = 0.55), compared with the unmodified anthracene. The photophysical properties of J-A also contrast dramatically with a dimethyl-amino analogue DAA, which were rationalized by DFT calculations. In addition, J-A could be transformed into 5, a dimeric product, whose single crystal X-ray structure unambiguously confirmed the structural feature of the julolidine-fused anthracene.     

A novel fluorescent probe for imaging the process of HOCl oxidation and Cys/Hcy reduction in living cells

A new on-off-on fluorescent probe, CMOS, based on coumarin was developed to detect the process of hypochlorous acid (HOCl) oxidative stress and cysteine/homocysteine (Cys/Hcy) reduction. The probe exhibited a fast response, good sensitivity and selectivity. Moreover, it was applied for monitoring the redox process in living cells.

A new on–off–on fluorescent probe, CMOS, was designed and applied to detect the process of HOCl oxidation and Cys/Hcy reduction.

Reactive oxygen species (ROS) are indispensable products and are closely connected to various physiological processes and diseases.¹ For instance, endogenous hypochlorous acid (HOCl) as one of the most important ROS, which is mainly produced from the reaction of hydrogen peroxide with chloride catalyzed by myeloperoxidase (MPO), is a potent weapon against invading pathogens of the immune system.^2,3 However, excess production of HOCl may also give rise to oxidative damage via oxidizing or chlorinating the biomolecules.⁴ The imbalance of cellular homostasis will cause a serious pathogenic mechanism in numerous diseases, including neurodegenerative disorders,⁵ renal diseases,⁶ cardiovascular disease,⁷ and even cancer.⁸ Fortunately, cells possess an elaborate antioxidant defense system to cope with the oxidative stress.⁹ Therefore, it is necessary and urgent to study the redox process between ROS and antioxidants biosystems.Fluorescence imaging has been regarded as a powerful visual methodology for researching various biological components as its advantages of high sensitivity, good selectivity, little invasiveness and real-time detection.^10,11 To date, amounts of small molecular fluorescent probes have been reported for detection and visualization of HOCl in vivo and in vitro.^12–22,29 The designed strategies of HOCl sensitive probes are based on various HOCl-reactive functional groups, such as p-methoxyphenol,¹³p-alkoxyaniline,¹⁴ dibenzoyl-hydrazine,¹⁵ selenide,¹⁶ thioether,¹⁷ oxime,¹⁸ hydrazide,¹⁹ hydrazone.²⁰ But, many of these probes display a delayed response time and low sensitivity. And, only few fluorescent probes can be applied for investigating the changes of intracellular redox status.²¹ Besides, it''s worth noting that most of the redox fluorescent probes rely on the organoselenium compounds.²² Even though these probes are well applied for detection of cellular redox changes, excessive organic selenium is harmful to organisms and the synthesis of organoselenium compounds is high requirement and costly. Additionally, almost all the reports have only investigated the reduction effects of glutathione (GSH) as an antioxidant in the redox events. While, there are the other two important biothiols, cysteine (Cys) and homocysteine (Hcy), which not only present vital antioxidants, but also are tightly related to a wide variety of pathological effects in biosystem, such as slowed growth, liver damage, skin lesions,²³ cardiovascular,²⁴ and Alzheimer''s diseases.²⁵ However, the fluorescent probes for specially studying internal redox changes between HOCl and Cys/Hcy are rarely reported. In this respect, a novel redox-responsive fluorescent probe, CMOS, was designed and synthesized in this work, and we hope that it can be a potential tool for studying their biological relevance in living cells.Based on literature research, the aldehyde group has excellent selectivity in identification of Cys/Hcy, and the thiol atom in methionine can be easily oxidized to sulfoxide and sulfone by HOCl.^26,27 Considering these two points, we utilized 2-mercaptoethanol to protect the 3-aldehyde of 7-diethylamino-coumarin as the recognition part of HOCl, meaning that two kinds of potential recognition moieties are merged into one site. Fluorescent probe CMOS can be easily synthesized by the acetal reaction in one step (Scheme S1^†). A control molecule CMOS-2 was also prepared by 3-acetyl-7-diethylaminocoumarin (CMAC) similarly. The structure of all these compounds have been convinced by ¹H NMR, ¹³C NMR, and HR-MS (see ESI^†).As shown in Scheme 1a, we estimated that both CMOS and CMOS-2 can be rapidly oxidized in the appearance of HOCl. The oxidation product CMCHO of CMOS, which has the aldehyde moiety, can further react with Cys/Hcy to obtain the final product CMCys and CMHcy, respectively. In contrast, the oxidation product CMAC of CMOS-2 cannot combine with Cys/Hcy or other biothiols anymore (Scheme 1b).Open in a separate windowScheme 1Proposed reaction mechanism of CMOS and CMOS-2 to HOCl and Cys/Hcy.In order to confirm our design concept, the basic photo-physical characteristics of CMOS, CMCHO, CMOS-2 and CMAC were tested (Table S1, Fig. S1^†). Under the excitation wavelength 405 nm, CMOS and CMOS-2 exhibited strong fluorescence centred at 480 nm in PBS buffer solution, while the fluorescence of CMCHO and CMAC was weak around this band. The emission properties of CMOS and CMCHO were also investigated at the excitation wavelength 448 nm under the same experimental conditions as well (Fig. S2^†). After careful consideration, we chose 405 nm as the excitation wavelength in the follow-up experiments in vitro and in vivo.Next, the sensitivity of CMOS and CMOS-2 to HOCl and Cys/Hcy were investigated. As we expected, both the CMOS and CMOS-2 exhibited good response to HOCl. The fluorescence intensity of CMOS and CMOS-2 decreased gradually with addition of NaOCl (Fig. 1a, S3a^†), indicating that the fluorescence was switched off obviously in the presence of HOCl. The variation of intensity displayed good linearity with concentration of HOCl in the range of 0–20 μM (R² = 0.993, Fig. S4^†), and the detection limit of CMOS to HOCl was calculated to be 21 nM (S/N = 3). Subsequently, when Cys/Hcy was added to the final solution in Fig. 1a, the fluorescence intensity increased gradually within 180 min (Fig. 1b, S5^†). However, the fluorescence cannot be recovered by addition thiols to the CMOS-2 solution with excess HOCl (Fig. S3b^†). These results indicate that the probe CMOS can response to HOCl and Cys/Hcy in a fluorescence on-off-on manner, and can be used for monitoring the redox process with high sensitivity.Open in a separate windowFig. 1(a) Fluorescence responses of CMOS (2 μM) to different concentrations of NaOCl (0–200 μM). (b) Fluorescence responses of the CMOS solution (2 μM) with HOCl (200 μM) to Cys/Hcy (5 mM). (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex = 405 nm).To further identify the recognizing mechanism of probe CMOS, high performance liquid chromatography (HPLC) and mass spectral (MS) analysis were used to detect the redox process. Initially, probe CMOS displayed a single peak with a retention time at 3.7 min (Fig. 2a, S6^†) while reference compound CMCHO produced a single peak with a retention time at 2.5 min (Fig. 2b, S7^†). Upon the addition of HOCl to the solution of CMOS, the peak at 3.7 min weakened while 2.5 min and 2.2 min appeared (Fig. 2c). According to corresponding mass spectra, the new main peak at 2.5 min is related to compound CMCHO (Fig. S8^†). The other new peak of 2.2 min corresponds to the compound C3, which can be predicted as an intermediate in the oxidation process (Fig. S8^†).²⁸ The addition of Cys to the solution of CMCHO also caused a new peak with a retention time at 2.1 min, which has been confirmed to be the thioacetal product CMCys (Fig. S9^†). The possible sensing mechanism is depicted in Fig. S10.^†Open in a separate windowFig. 2The reversed-phase HPLC with absorption (400 nm) detection. (a) 10 μM CMOS. (b) 10 μM CMCHO. (c) 10 μM CMOS in the presence of 50 μM HOCl for 30 s. (d) 10 μM CMCHO in the presence of 1 mM Cys for 30 min. (Eluent: CH₃CN containing 0.5% CH₃COOH; 100% CH₃CN (0–7 min), 0.5 ml min⁻¹, 25 °C; injection volume, 5.0 μL).To study the selectivity of CMOS towards HOCl, we performed fluorescence response to different reactive oxygen species (ROS), reactive nitrogen species (RNS) and reactive sulfur species (RSS). As shown in Fig. 3a, CMOS exhibited significant change of fluorescence intensity only in the presence of HOCl, while other ROS and RNS, such as singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), hydroxyl radical (HO·), superoxide anion (O₂⁻), nitric oxide (NO), tert-butylhydroperoxide (t-BuOOH) and tert-butoxy radical (t-BuOO·) had no obvious fluorescence emission changes. Additionally, RSS which are abundant in biological samples, showed no influence in this process under the identical condition. The detection of reducing process was also investigated. As displayed in Fig. 3b, only cysteine and homocysteine induced excellent fluorescence recovery towards other reducing materials, such as RSS and various amino acids. Furthermore, the selectivity of CMOS-2 was also studied in the same condition. As expected, CMOS-2 could selectively detect HOCl, and not alter fluorescence intensity under various kinds of biothiols (Fig. S11^†). Therefore, our design strategy for the on–off–on probe is confirmed by results obtained above, with which CMOS can be utilized for detecting the redox process between HOCl and Cys/Hcy with high selectivity.Open in a separate windowFig. 3(a) Fluorescence response of CMOS (2 μM) to different ROS, RNS and RSS (200 μM). Bars represent emission intensity ratios before (F₀) and after (F₁) addition of each analytes. (a) HOCl; (b) KO₂; (c) H₂O₂; (d) ¹O₂; (e) HO·; (f) t-BuOOH; (g) t-BuOO·; (h) NO₂⁻; (i) NO₃⁻; (j) NO; (k) GSH; (l) Cys; (m) Hcy; (n) Na₂S; (o) Na₂S₂O₃; (p) Na₂S₂O₈; (q) NaSCN; (r) DTT; (s) Na₂SO₃. (b) Fluorescence response of the solution added HOCl in (a) to different RSS and amino acids. Bars represent emission intensity ratios before (F₂) and after (F₃) addition of each analytes (5 mM). (a) Cys; (b) Hcy; (c) Na₂S; (d) Na₂S₂O₃; (e) Na₂S₂O₈; (f) NaSCN; (g) DTT; (h) Na₂SO₃; (i) Ala; (j) Glu; (k) Gly; (l) His; (m) Ile; (n) Leu; (o) Met; (p) Phe; (q) Pro; (r) Ser; (s) Trp; (t) Vc; (u) GSH. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex/λ_em = 405/480 nm).Subsequently, the influence of pH on probe CMOS was measured. The fluorescence intensity of CMOS and CMCHO perform no significant variances in wide pH ranges (pH = 4–11, Fig. S12a^†). Fluorescence intensity changes could be observed immediately when HOCl was added into the solution of probe CMOS, especially in alkaline condition (Fig. 4a). Considering the pK_a of HOCl is 7.6,²⁹CMOS is responsive to both HOCl and OCl⁻. Alkaline condition was also benefit for the fluorescence recovery of CMOS from Cys/Hcy (Fig. S12b^†). It is reasonable to consider that thiol atom displays higher nucleophilicity in alkaline condition. From the stop-flow test, the UV-visible absorbance of probe CMOS sharply decreased at the wavelength of 400 nm (Fig. 4b). The response time was within 10 s and the kinetic of the reaction was fitted to a single exponential function (k_obs = 0.67 s⁻¹). The ability of instantaneous response is extremely necessary to intracellular HOCl detection.Open in a separate windowFig. 4(a) Fluorescence responses of CMOS to HOCl under different pH values. Squares represent emission intensity ratios after (F₁) and before (F₀) addition of 200 μM HOCl (λ_ex/λ_em = 405/480 nm). (b) Time-dependent changes in the absorption intensity of CMOS (1 μM) before and after addition of HOCl. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_abs = 400 nm).With these data in hand, we next applied CMOS for fluorescence imaging of the redox changes with HOCl and Cys/Hcy in living cells. After incubation with 5 μM CMOS at 37 °C for 30 min, intense fluorescence was observed of the SKVO-3 cells in the optical window 425–525 nm (Fig. 5a and d), indicating the probe can easily penetrate into cells. Treating the cells with 100 μM NaOCl led to remarkable fluorescence quenching as the probe sensed the HOCl-induced oxidative stress (Fig. 5b and e). After 3 min, the cells were washed with PBS buffer three times, and added 5 mM Cys/Hcy for 1 h, respectively. Then the fluorescence was recovered obviously (Fig. 5c and f). Experimental results clearly declare that the probe CMOS was successfully used to detect the process of HOCl oxidative stress and Cys/Hcy reducing repair in living cells.Open in a separate windowFig. 5Fluorescence imaging of the process of HOCl oxidative stress and thiols repair in CMOS-labeled SKVO-3 cells. Fluorescence images of SKVO-3 cells loaded with 5 μM CMOS at 37 °C for 30 min (a and d). Dye-loaded cells treated with 100 μM NaOCl at 25 °C for 3 min (b and e). Dye-loaded, NaOCl-treated cell incubated with 5 mM Cys (c), 5 mM Hcy (f) for 1 h. Emission intensities were collected in an optical window 425–525 nm, λ_ex = 405 nm, intensity bar: 0–3900.     

Correction: Dipyrrolyl-bis-sulfonamide chromophore based probe for anion recognition

Correction for ‘Dipyrrolyl-bis-sulfonamide chromophore based probe for anion recognition’ by Namdev V. Ghule et al., RSC Adv., 2014, 4, 27112–27115, DOI: 10.1039/C4RA04000G.

The authors regret that an incorrect version of Fig. 1 was included in the original article. The correct version of Fig. 1 is presented below.Open in a separate windowFig. 1Color changes of receptor DPBS in chloroform upon addition of 5 equiv. of F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻ and AcO⁻ (tetrabutylammonium salts).The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Insight into trifluoromethylation – experimental electron density for Togni reagent I

3,3-Dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole represents a popular reagent for trifluoromethylation. The σ hole on the hypervalent iodine atom in this “Togni reagent” is crucial for adduct formation between the reagent and a nucleophilic substrate. The electronic situation may be probed by high resolution X-ray diffraction: the experimental charge density thus derived shows that the short intermolecular contact of 3.0 Å between the iodine and a neighbouring oxygen atom is associated with a local charge depletion on the heavy halogen in the direction of the nucleophile and visible polarization of the O valence shell towards the iodine atom. In agreement with the expectation for λ3-iodanes, the intermolecular O⋯I–C_aryl halogen bond deviates significantly from linearity.

The experimentally observed electron density for the “Togni reagent” explains the interaction of the hypervalent iodine atom with a nucleophile.

A “halogen bond” denotes a short contact between a nucleophile acting as electron density donor and a (mostly heavy) halogen atom as electrophile;^1,2 halogen bonds are a special of σ hole interactions.^3–5 Such interactions do not only play an important role in crystal engineering;^6–11 rather, the concept of a nucleophile approaching the σ hole of a neighbouring atom may also prove helpful for understanding chemical reactivity.The title compound provides an example for such a σ hole based reactivity: 3,3-dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole, 1, (Scheme 1) commonly known as “Togni reagent I”, is used for the electrophilic transfer of a trifluoromethyl group by reductive elimination. The original articles in which the application of 1¹² and other closely related “Togni reagents”¹³ were communicated have been and still are highly cited. Trifluoromethylation is not the only application for hypervalent iodine compounds; they have also been used as alkynylating¹⁴ or azide transfer reagents.^15,16 The syntheses of hypervalent iodanes and their application in organofluorine chemistry have been reviewed,¹⁷ and a special issues of the Journal of Organic Chemistry has been dedicated to Hypervalent Iodine Catalysis and Reagents.¹⁸ Recently, Pietrasiak and Togni have expanded the concept of hypervalent reagents to tellurium.¹⁹Open in a separate windowScheme 1Lewis structure of the Togni reagent, 1.Results from theory link σ hole interactions and chemical properties and indicate that the electron density distribution associated with the hypervalent iodine atom in 1 is essential for the reactivity of the molecule in trifluoromethylation.²⁰ Lüthi and coworkers have studied solvent effects and shown that activation entropy and volume play relevant roles for assigning the correct reaction mechanism to trifluoromethylation via1. These authors have confirmed the dominant role of reductive elimination and hence the relevance of the σ hole interaction for the reactivity of 1 in solution by ab initio molecular dynamics (AIMD) simulations.^21,22 An experimental approach to the electron density may complement theoretical calculations: low temperature X-ray diffraction data of sufficient resolution allow to obtain the experimental charge density and associate it with intra- and inter-molecular interactions.^23–25 Such advanced structure models based on aspherical scattering factors have also been applied in the study of halogen bonds.^26–30 In this contribution, we provide direct experimental information for the electronic situation in Togni reagent I, 1; in particular, we analyze the charge distribution around the hypervalent iodine atom.Excellent single crystals of the title compound were grown by sublimation.^§ The so-called independent atom model (IAM), i.e. the structure model based on conventional spherical scattering factors for neutral atoms, confirms the solid state structure reported by the original authors,¹² albeit with increased accuracy. As depicted in Fig. 1, two molecules of 1 interact via a crystallographic center of inversion. The pair of short intermolecular O⋯I contacts thus generated can be perceived as red areas on the interaction-sensitive Hirshfeld surface.³¹Open in a separate windowFig. 1Two neighbouring molecules of 1, related by a crystallographic center of inversion. The short intermolecular I⋯O contacts show up in red on the Hirshfeld surface³² enclosing the left molecule. (90% probability ellipsoids, H atoms omitted, symmetry operator 1 − x, 1 − y, 1 − z).The high resolution of our diffraction data ^¶ for 1 allowed an atom-centered multipole refinement^33,34 and thus an improved model for the experimental electron density which takes features of chemical bonding and lone pairs into account. Fig. 2 shows the deformation density, i.e. the difference electron density between this advanced multipole model and the IAM in the same orientation as Fig. 1.^|| The orientation of an oxygen lone pair (blue arrow) pointing towards the σ hole of the heavy halogen in the inversion-related molecule and the region of positive charge at this iodine atom (red arrow) are clearly visible. Single-bonded terminal halides are associated with one σ hole opposite to the only σ bond, thus resulting in a linear arrangement about the halogen atom. Different geometries and potentially more than a single σ hole are to be expected for λ3-iodanes such as our target molecule, and as a tendency, the resulting halogen bonds are expected to be weaker than those subtended by single-bonded iodine atoms.³⁵ In agreement with these theoretical considerations, the closest I⋯O contacts in 1 amount to 2.9822(9) Å. This distance is significantly shorter than the sum of the van-der-Waals radii (I, 1.98 Å; O, 1.52 Å (ref. 36)) but cannot compete with the shortest halogen bonds between iodine and oxygen^37,38 or iodine and nitrogen.^39–41Fig. 1 and ​and22 show that the C_aryl–I⋯O contacts are not linear; they subtend an angle C10–I1⋯O1′ of 141.23(3)° at the iodine atom. On the basis of theoretical calculations, Kirshenboim and Kozuch³⁵ have suggested that the split σ holes should be situated in the plane of the three substituents of the hypervalent atom and that halogen and covalent bonds should be coplanar. Fig. 3 shows that the C_aryl–I⋯O interaction in 1 closely matches this expectation, with the next oxygen neighbour O1′ only 0.47 Å out of the least-squares plane through the heavy halogen I1 and its three covalently bonded partners C1, O1 and C10.Open in a separate windowFig. 2Deformation density for the pair of neighbouring molecules in 1; the dashed blue and red arrows indicate regions of opposite charge. (Contour interval 0.10e Å⁻³; blue lines positive, red lines negative, green lines zero contours, symmetry operator 1 − x, 1 − y, 1 − z).Open in a separate windowFig. 3A molecule of 1, shown [platon] along O1⋯C1, and its halogen-bonded neighbour O1ⁱ. Symmetry operator 1 − x, 1 − y, 1 − z.The Laplacian, the scalar derivative of the gradient vector field of the electron density, emphasizes local charge accumulations and depletions and it allows to assess the character of intra- and inter-molecular interactions. A detailed analysis of all bonds in 1 according to Bader''s Atoms In Molecules theory⁴² is provided in the ESI.^‡ We here only mention that the electron density in the bond critical point (bcp) of the short intermolecular I⋯O contact amounts to 0.102(5)e Å⁻³; we are not aware of charge density studies on λ3-iodanes, but both the electron density and its small positive Laplacian match values experimentally observed for halogen bonds involving O and terminal I in the same distance range.⁴³The crystal structure of 1 necessarily implies additional contacts beyond the short halogen bond shown in Fig. 1 and ​and2.2. The shortest among these secondary interactions is depicted in Fig. 4: it involved a non-classical C–H⋯F contact with a H⋯F distance of 2.55 Å.Open in a separate windowFig. 4C–H⋯F contact in 1; additional information has been compiled in the ESI.^‡ Symmetry operators i = 1 − x, 1 − y, 1 − z; ii = 1 − x, 1 − y, −z.The topological analysis of the experimental charge density reveals that this non-classical C–H⋯F hydrogen bond and all other secondary contacts are only associated with very small electron densities in the bcps. Table S8 in the ESI^‡ provides a summary of this analysis and confirms that the I⋯O halogen bond discussed in Fig. 1 and ​and22 represents by far the most relevant intermolecular interaction.The relevance of this halogen bond extends beyond the crystal structure of 1: Insight into the spatial disposition of electrophilic and nucleophilic regions and hence into the expected reactivity of a molecule may be gained from another electron-density derived property, the electrostatic potential (ESP). The ESP for the pair of interacting molecules in 1 is depicted in Fig. 5.Open in a separate windowFig. 5Electrostatic potential for a pair of molecules in 1 mapped on an electron density isosurface (ρ = 0.5e Å⁻³; program MoleCoolQt^44,45). Fig. 5 underlines the complementary electrostatic interactions between the positively charged iodine and the negatively charged oxygen atoms. One can easily imagine to “replace” the inversion-related partner molecule in crystalline 1 by an incoming nucleophile.The ESP tentatively obtained for a single molecule in the structure of 1 did not differ significantly from that derived for the inversion-related pair (Fig. 5), and even the results from theoretical calculations in the gas phase for an isolated molecule²⁰ are in good qualitative agreement with our ESP derived from the crystal structure. In the absence of very short contacts, polarization by neighbouring molecules only has a minor influence on the ESP. The experimentally observed electron density matches the proven reactivity for the title compound, and we consider it rewarding to extend our charge density studies on related hypervalent reagents.     

Proton transfer network with luminescence display controls OFF/ON catalysis that generates a high-speed slider-on-deck

A three-component network for OFF/ON catalysis was built from a protonated nanoswitch and a luminophore. Its activation by addition of silver(i) triggered the proton-catalyzed formation of a biped and the assembly of a fast slider-on-deck (k₂₉₈ = 540 kHz).

Upon addition/removal of silver(i) ions and due to efficient inter-component communication, a supramolecular multicomponent network acts as an OFF/ON proton relay with luminescence display enabling switchable catalysis.

Proton relays in living systems¹ are often coupled to bio-machinery with enzymatic catalysis.² Frequently, such proton transfer systems comprise intricate networks with information exchange^3,4 amongst multiple components. Using manmade molecules,⁵ however, it is still arduous to create networks that approach the complexity⁶ and functions of biosystems.Although activation via proton transfer plays an eminent role in molecular logic,⁷ switching,^8–10 and machine operation,^11–15 proton transfer networks in combination with the regulation of catalysis remain scarce.^16,17 Beyond that, we present the in situ catalytic synthesis of a highly dynamic supramolecular device by a new proton transfer network requiring interference-free information handling,¹⁸ self-sorting^19–21 and communication²² among multiple components.^23–25Recently, our group has developed various chemically actuated networks of communicating components and used them for the ON/OFF regulation of transition metal catalysis.^26–28 Herein, we describe a proton relay system that reversibly toggles between two networked states and is useful for regulation of acid catalysis. It catalyzes an amine deprotection which was utilized to fabricate in situ a high-speed slider-on-deck,^29,30 for the first time based on aniline → zinc-porphyrin³¹ (N_an → ZnPor) interactions.The communicating network consists of nanoswitch 1 (ref. 32) and luminophore 2 (ref. 33) as receptors (Fig. 1). In the self-sorted networked state (NetState-I), the added proton is captured in the cavity of nanoswitch 1 resulting in [H(1)]⁺, while luminophore 2 remains free. NetState-I is catalytically inactive as the encapsulated proton is unable to catalyze the deprotection of the trityl protected slider biped 4. Upon addition of AgBF₄ to NetState-I, the silver(i) coordinates to switch 1 and the proton is released onto luminophore 2 thus leading to formation of [Ag(1)]⁺ and [H(2)]⁺ in NetState-II (see Scheme 1), which may be followed by characteristic ratiometric emission changes. If NetState-II is formed in presence of 3 and 4, the catalytic trityl deprotection of 4 produces the free biped 5 that eventually binds to deck 3 furnishing the slider-on-deck [(3)(5)].Open in a separate windowFig. 1Chemical structures and cartoon representations of the ligands used in the study.Open in a separate windowScheme 1Schematic representation of the reversible proton relay network. Green circle represents catalytic site.The concept of the catalyzed biped formation through a proton transfer network relies on the following considerations: (a) the initial networked state should be catalytically inactive as expected from a self-sorting of the proton inside the nanoswitch leaving the luminophore free. (b) Addition of silver(i) ions should release the proton from the protonated switch thus forming the protonated luminophore which we anticipated to act as catalyst. (c) A di-protected biped should undergo acid catalyzed deprotection by the protonated luminophore and eventually form a slider-on-deck by coordination to a deck. (d) Finally, the product of the catalysis should not intervene in any state of the operation with the main system.As proton receptor we chose the known phenanthroline–terpyridine nanoswitch 1 (ref. 32) in order to tightly capture the proton/silver(i) as HETTAP (HETeroleptic Terpyridine And Phenanthroline) complexes.³⁴ Luminophore 2 was based on the 2,4,6-triarylpyridine scaffold which is known to fluoresce strongly upon protonation due to intramolecular charge transfer (ICT).³⁵To investigate the self-sorting within the network, we mixed nanoswitch 1, luminophore 2 and TFA in 1 : 1 : 1 ratio in CD₂Cl₂ which furnished NetState-I = [H(1)]⁺ + 2. ¹H NMR shifts of the 4/7-H proton signals to 8.43 and 8.69 ppm and of 9-H and e-H proton peaks to 6.60 and 8.48 ppm confirmed formation of [H(1)]⁺. Moreover, proton signals of c′-H and d′-H at 7.07 and 7.92 ppm corroborated presence of the free luminophore 2 (Fig. 2). Addition of one equiv. of AgBF₄ to NetState-I instantaneously generated NetState-II = [Ag(1)]⁺ + [H(2)]⁺. In the ¹H NMR of NetState-II, the 9-H and e-H proton signals shifted to 6.55 and 8.23 ppm, respectively, which substantiated the formation of [Ag(1)]⁺ whereas the shift of proton peaks c′-H and d′-H to 7.12 and 8.00 ppm, respectively, established the protonated luminophore [H(2)]⁺ (Fig. 2). Upon excitation at 300 nm, NetState-I exhibited emissions at λ = 346, 372 and 393 nm which represent an overlap of the emission peaks of the free luminophore 2 at λ = 346 and 372 nm and of [H(1)]⁺ at λ = 376 and 393 nm (Fig. 3c). In NetState-II, the fluorescence spectrum showed a single emission at λ = 461 nm, which is attributed to [H(2)]⁺ (Fig. 3). In contrast, [Ag(1)]⁺ in NetState-II is nonfluorescent as advocated by the full quenching of the emission of [H(1)]⁺ at λ = 376 and 393 nm upon titration with silver(i) ions (ESI, Fig. S18^†). When one equiv. of tetra-n-butylammonium iodide (TBAI) was added to NetState-II, complete restoration of NetState-I was achieved as confirmed from ¹H NMR data. TBAI acted as a scavenger for silver(i) by removing AgI through precipitation thus reversing the translocation sequence. Multiple switching of NetState-I ⇆ NetState-II was readily followed by ¹H NMR (Fig. 2) and luminescence changes. Upon successive addition of AgBF₄ to NetState-I, the emission changed to λ = 461 nm (sky blue emission of [H(2)]⁺) in NetState-II (Fig. 3a and b) in a clean ratiometric manner that allowed monitoring of the amount of liberated protons. Alternating switching between the NetStates was followed by emission spectroscopy over three cycles with a small decline in emission intensity (Fig. 3c and d), which may be due to the progressive accumulation of AgI.Open in a separate windowFig. 2Partial ¹H NMR spectra (500 MHz, CD₂Cl₂, 298 K) showing reliable switching between NetState-I and II over 3 cycles. (a) After mixing of TFA, 1, and 2 in 1 : 1 : 1 ratio; (b) after adding 1.0 equiv. of AgBF₄ furnishing [Ag(1)]⁺ and [H(2)]⁺; (c) after addition of 1.0 equiv. of TBAI; (d) after adding 1.0 equiv. of AgBF₄; (e) after addition of 1.0 equiv. of TBAI, (f) after adding 1.0 equiv. of AgBF₄; (g) after addition of 1.0 equiv. of TBAI. Blue asterisk marked proton signals represent the tetra-butylammonium ion.Open in a separate windowFig. 3(a) Ratiometric emission titration of 2 (2.1 × 10⁻⁵ M) with TFA (5.3 × 10⁻³ M) in CH₂Cl₂. (b) Change of emission upon protonation of 2. (c) Reversibility of the network NetState-I ⇆ II over three cycles upon addition of silver(i) and TBAI (see text, blue arrow assigns direction of switching) monitored by luminescence changes at λ = 461 nm. (d) Multiple cycles monitored by the change of the fluorescence intensity.For probing the catalytic efficiency of each state towards a simple amine deprotection reaction we chose the trityl-protected aniline 6 (Fig. 4) performing two NetState-I ⇆ NetState-II cycles (Fig. 4b). We first prepared NetState-I by mixing 1, 2 and TFA (1 : 1 : 1) in CD₂Cl₂ and then added 6 and TMB (1,3,5-trimethoxybenzene as an internal standard) in a 10 : 10 ratio. After heating for 2 h at 40 °C, the ¹H NMR spectrum revealed no product 7. Clearly, the proton in [H(1)]⁺ is catalytically inactive (ESI, Fig. S10b^†). Addition of one equiv. of AgBF₄ and probing the catalysis again for 2 h at 40 °C revealed formation of product 7 in (30 ± 2)% yield (ESI, Fig. S10c^†). When the catalytic experiment was probed with only [H(2)]⁺ under identical reaction conditions, it provided the same yield (ESI, Fig. S16^†). This suggested that the reversible proton relay network (Scheme 1) functioned reliably also in presence of 6 with complex [H(2)]⁺ as the catalytically active species. Addition of one equiv. of TBAI to the above NetState-II mixture and probing for catalysis, revealed no further product formation. Apparently, addition of TBAI translocated back the proton to nanoswitch 1 thus restoring NetState-I, which resulted in the OFF state for catalysis. Adding one equiv. of AgBF₄ to the mixture and probing again the catalytic activity resulted in formation of further (29 ± 2)% of product 7, which was basically identical to the yield produced in the first cycle of NetState-II (ESI, Fig. S10e^†). Adding TBAI and the consumed amount of substrate showed no further deprotection. In sum, the robust catalytic performance of the proton relay network as reflected in two successive catalytic cycles resulted in no significant decline of the catalytic activity (Fig. 4b).Open in a separate windowFig. 4(a) Representation of the OFF/ON regulation of the trityl deprotection reaction in NetState-I and II. (b) Reversible switching between the network states furnished reproducible amounts of product 7 in NetState-II (two independent runs). Consumed amounts of substrates were added (green asterisk).The OFF–ON switching of the catalytic trityl deprotection of aniline 6 by the proton relay network inspired us to utilize the system to catalytically fabricate a slider-on-deck based on the N_an (= aniline) → ZnPor interaction. Deprotection of the protected biped 4 should afford the bis-aniline biped 5 and enable its attachment to the tris-zinc porphyrin deck 3 thus forming the slider-on-deck [(3)(5)].Addition of one equiv. of biped 5 (for synthesis and characterization, see ESI^†) to deck 3 (ref. 25) in CD₂Cl₂ quantitatively generated the slider-on-deck [(3)(5)], which was unambiguously characterized by ¹H NMR, ¹H DOSY NMR and elemental analysis (ESI, Fig. S5 and S6^†). In the ¹H NMR-spectrum, several diagnostic shifts of biped and deck proton signals attested the binding of biped 5 to the deck. Formation of the slider-on-deck was further confirmed from a single set in the ¹H DOSY in CD₂Cl₂ (ESI, Fig. S9^†).The single set of protons (p-H, r-H, s-H, t-H, u-H, v-H) in the ¹H NMR spectrum for all three-zinc porphyrin (ZnPor) stations of deck 3 unmistakably suggested rapid intrasupramolecular exchange^29a of the aniline biped across all three ZnPor sites requiring fast N_an → ZnPor bond cleavage. At low temperature (−65 °C) the ZnPor p-H proton signals separated into two sets (1 : 2) (ESI, Fig. S7 and S8^†). Kinetic analysis over a wide temperature range provided the exchange frequency (k) with k₂₉₈ = 540 kHz at 298 K (Fig. 5a) and the activation data as ΔH^‡ = 41.5 ± 0.7 kJ mol⁻¹, ΔS^‡ = 2.8 ± 0.7 J mol⁻¹ K⁻¹ and ΔG^‡₂₉₈ = 40.7 kJ mol⁻¹ (Fig. 5).Open in a separate windowFig. 5(a) Experimental and theoretical splitting of p-H proton signal of nanoslider [(3)(5)] in VT ¹H-NMR (600 MHz) furnishing rate data in CD₂Cl₂. (b) Eyring plot providing kinetic parameters.After determining the kinetic parameters of the slider-on-deck [(3)(5)], we investigated its in situ catalyzed formation by the proton relay network. Therefore, we mixed nanoswitch 1, luminophore 2, TFA, trityl-protected biped 4, deck 3 and TMB in 1 : 1 : 1 : 5 : 5 : 5 ratio in CD₂Cl₂ and heated the reaction mixture for 12 h at 40 °C. The ¹H NMR spectrum suggested no formation of any slider-on-deck in NetState-I (ESI, Fig. S13^†). Addition of one equiv. of AgBF₄ to the same mixture (converting NetState-I to NetState-II) and heating it at 40 °C for 12 h revealed full formation of the slider-on-deck as indicated by the disappearance of the 2′-H, 3′-H and 4′-H proton signals of biped 4 at 6.60, 6.31 and 6.51 ppm (ESI, Fig. S11^†), respectively, and the quantitative emergence of 4′-H proton signal of biped 5 bound to deck 3 at 5.84 ppm (ESI, Fig. S12^†). The upfield shift of the deck''s meso t-H protons from 10.36 to 10.29 ppm allowed monitoring of the formation of [(3)(5)], e.g. by the gradually increased chemical shift difference of proton signal t-H (Δδ_t-H) (Fig. 6a; ESI, Fig. S11^†). In sum, the protonated [H(2)]⁺ in NetState-II catalyzed the deprotection of the trityl-protected biped 4 with the effect that the resultant bis-aniline biped 5 would quantitatively bind to deck 3 affording the slider-on-deck [(3)(5)]. Furthermore, the OFF/ON switching of catalysis was probed for shortened time periods of 3 h. As illustrated in Fig. 6b, the process could be readily followed using the proton signal t-H (Δδ_t-H) (ESI, Fig. S14^†) and the growth of proton signal 4′-H at 5.84 ppm of bound biped 5 (ESI, Fig. S15^†). One clearly sees recurring OFF/ON regulation in NetState-I/II.Open in a separate windowFig. 6Plot of the shift difference of proton signal t-H (Δδ_t-H) vs. time for (a) the formation of [(3)(5)] from 3 and 4. Red curve: formation of [(3)(5)] over 12 h catalyzed by NetState-II. Black curve: OFF state of catalysis in NetState-I. (b) OFF/ON catalytic cycles by switching between NetState-I and NetState-II. Addition of Ag⁺, see blue asterisk; addition of TBAI: red asterisk.In conclusion, we have designed a supramolecular multicomponent network³⁶ that acts as an OFF/ON proton relay with a luminescence display³⁷ and is useful for switchable catalysis.³⁸ The network is toggled by chemical input and intercomponent communication^15,16 and as a result is a conceptual complement to photoacids driving networks.^7,8The release and capture of protons is demonstrated by the ON/OFF trityl deprotection of anilines. To demonstrate functioning in a complex setting, the network was utilized to catalyze formation of a high-speed slider-on-deck assembly based on N_an → ZnPor interactions (sliding frequency of k₂₉₈ = 540 kHz). The robust operation of the proton relay furnishing dynamic machinery³⁹ through acid catalysis followed by self-assembly is a valuable step mimicking sophisticated biological proton relays.²     

Mesoporous-silica-coated palladium-nanocubes as recyclable nanocatalyst in C–C-coupling reaction – a green approach

We report the straightforward design of a recyclable palladium-core–silica-shell nanocatalyst showing an excellent balance between sufficient stability and permeability. The overall process – design, catalysis and purification – is characterized by its sustainability and simplicity accompanied by a great recycling potential and ultra high yields in C–C-coupling reactions.

A green approach: in a single-step coating process a mesoporous silica shell was tailored onto palladium-nanocubes. Along with a PEG-matrix this core–shell-nanocatalyst could be recovered after C–C-coupling reactions and reused without any significant decrease in product yield.

In contrast to bulk materials, metal-nanocyrstals (NC) possess unique physical and chemical properties. Both, nano-scale and geometry can dictate their optical, electronic and catalytic behavior.^1–3 Consequently, nanomaterials have been implemented already in various fields like medicine,⁴ sensing,⁵ nano-electronics⁶ and organic synthesis.⁷ Smart strategies for a sustainable usage of limited resources such as precious metals and hydrocarbons are inevitable due to the consistently growing population and economy.^8–10 Generally, catalysis gives rise to novel and energy-saving synthetic routes. However, homogeneous catalysis is not widely used in industrial processes, due to the need of mostly toxic ligands and the costly purification along with a restricted reusability potential.^11–13 Heterogeneous catalysis based on the utilization of metal nanocrystals overcomes most of these limitations. Caused by its high surface-to-volume ratio, catalytic activities are drastically increased in comparison to bulk materials. However, the most common disadvantage in NC-based catalysis lies in the occurrence of aggregates during the reaction, which leads to a decrease of the catalytic active surface. Several studies were already presented in literature to delay that phenomenon using micelle-like- or core–shell-nanostructures as potential nanocatalytic systems.^14–20 However, these surfactants or shells are either potentially harmful for the environment or very step-inefficient to produce. Other approaches used the deposition of small NCs in a mesoporous support which offers a large active surface area.^21–24 In this context, it is crucial to find an adequate balance between permeability for small organic molecules and the overall stability of the nanocatalyst. Overcoming these obstacles can contribute to a sustainable supply of drugs and other organic substances.In the present work palladium-nanocubes (Pd-NCubes) were fabricated in aqueous solution using cetyltrimethyl-ammoniumbromide (CTAB) as surfactant. The procedure is adapted from a previously published study.^25,26 The respective transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern of the as-obtained Pd-NCubes are depicted in Fig. S1.^† The SAED confirms the single-crystallinity of Pd-NCubes bound by {100}-facets. TE micrographs of Pd-NCubes revealed an average edge-length of (18 ± 2) nm (for histogram see Fig. S2^†). The formation of polyhedra and nanorods was found to be less than 1%.To the best of our knowledge, there is no procedure reported that showed the direct fabrication of a mesoporous silica (mSi) shell tailored on Pd-NCs. However, Matsuura et al. demonstrated a single-step coating approach of CTAB-capped gold-nanorods and CdSe/ZnS quantum dots obtaining a mesoporous silica shell.²⁷ The pores were determined to be 4 nm in width with 2 nm thick walls. Since the Pd-NCubes are covered by CTAB, already no surfactant exchange is necessary. Consequently, this procedure could be directly transferred to the as-obtained Pd-NCubes (∼10¹⁵ particles per L) of this study using tetraethyl orthosilicate (TEOS) as silica precursor in an alkaline solution. Here, CTAB serves as organic template for the formation of the mesoporous silica shell.TE micrographs of individual Pd-mSi-nanohybrids are depicted in Fig. 1 showing a spherical silica coating with a thickness of (17 ± 2) nm (for histogram see Fig. S3^†). The porosity is essential to ensure that vacant coordination sites on the palladium-core are present and accessible for catalysis. In contrast to other multistep approaches, pores are formed in situ with no additional etching step necessary.^28,29 This avoids the usage of harmful etching agents such as fluorides or ammonia.³⁰ The silica shell then served as platform for further surface modification using two different PEG-silanes (M_n = 5000 g mol⁻¹ and M_n = 20 000 g mol⁻¹). TE microscopy did not reveal any changes in the structure of the PEG functionalized Pd-mSi-nanohybrids opposed to the unfunctionalized nanocatalyst, since the contrast of polymer is too low (see Fig. S4^†). However, dynamic light scattering (DLS) measurements in diluted aqueous solutions proved an increased hydrodynamic radius with increasing molecular weight of the PEG-chain grafted onto the silica shell (see Fig. 2). These results provide evidence that only individual nanostructures are formed while no larger aggregates are present.Open in a separate windowFig. 1Exemplary TE micrographs of Pd-mSi-nanohybrids.Open in a separate windowFig. 2DLS results along the different stages of the hierarchal fabrication process of the nanocatalyst.The successful functionalization of the Pd-mSi-nanohybrid with PEG provides the dispersibility for the overall nanocatalyst in a PEG matrix. Due to its lack of toxicity and its simple recovery, caused by its melting point at ∼50 °C, PEG is considered as a “green” reaction medium.³¹ It has already been shown that PEG can act as suitable solvent for both homogeneous and heterogeneous catalysis.^32–34 Consequently, further experiments were performed using only the Pd-mSi-nanohybrid functionalized with PEG-silane having an average molecular weight of M_n = 5000 g mol⁻¹ (PEG-5k). For catalytic reactions, Pd-mSi-PEG-5k was dispersed in a PEG matrix (PEG-2000, M_n = 2000 g mol⁻¹) and charged into a Teflon centrifuge tube. Using only one tube for the reaction and the product separation avoids an additional transfer step and prevents any loss of the nanocatalyst between reaction cycles (see recycling Scheme 1). Here, the C–C-coupling between ethyl acrylate and p-iodoanisole served as model Heck-reaction to prove the catalytic activity of the designed catalyst (4.4 mol% overall Pd conc. equal to ca. 0.3 mol% surface-available Pd; conc. is determined by ICP-MS measurements, see Table S1^†). Sodium phosphate was used as base providing the largest product yield when compared to other bases, such as Na₂CO₃, K₂CO₃ and K₃PO₄. Once the catalysis was performed and the PEG-2000 was cooled down, diethyl ether was added to extract the product and separated from the reaction medium via centrifugation.Open in a separate windowScheme 1Recycling process of the Pd-mSi-PEG-5k-nanocatalyst and PEG-2000 as solvent after the Heck-reaction between ethyl acrylate and p-iodoanisole to form ethyl p-methoxycinnamate.To exclude any suspended compounds from the desired product, the mixture was passed through a PTFE-filter. Et₂O was removed in vacuo without the need of column chromatography. Comparing the ¹H-NMR spectra of the as-obtained product with the educts indicate a yield of 98% with only small amounts of PEG-2000 present (identified by the signal at ∼3.6 ppm, < 1 weight-%). The results show that both educts and the base Na₃PO₄ are able to diffuse through the mesoporous silica shell to the palladium core (see Fig. 3). A detailed ¹H-NMR signal assignment of the product is given in Fig. S5.^†Open in a separate windowFig. 3 ¹H-NMR spectra of the educts ethyl acrylate (top) and p-iodoanisole (center) and the product ethyl p-methoxycinnamate (bottom).After recovering the reaction mixture containing Pd-mSi-PEG-5k and PEG-2000, seven further Heck-reaction-cycles were conducted under the same conditions. Results obtained from ¹H-NMR and gravimetry indicate no significant decrease in catalytic activity (see ¹H-NMR spectra in Fig. 4). Along the eight Heck-reactions, product yields were determined between 94% and 99%. Only small traces of p-iodoanisole (identified by the signal at ∼6.75 ppm) were still present while ethyl acrylate could be fully removed in vacuo. The yields obtained after each cycle are displayed in Fig. 5. ICP-MS measurements of the catalysis product were performed to determine the palladium leaching out of the catalytic system. The results are displayed in Table S1^† indicating that leaching is strongly suppressed since the overall Pd-content in the product ranges from 0.3–5.7 ng, only. This corresponds to 0.002–0.044 ppm palladium with respect to the product mass. The data are in good agreement with the high product yields along the eight Heck-reactions. To trace the evolution of the nanocatalyst along the cycles, TE micrographs were taken after the 1^st and the 8^th Heck-cycle (see Fig. 6). It can be seen that the cubical structure of the palladium vanishes during the first reaction (left TEM image). Inside the silica shell spherical palladium particles were formed. An explanation for this rearrangement lies in the suggested Heck-mechanism.³⁵ Here, a Pd²⁺-species forms after the oxidative addition of the p-iodoanisole which can desorb from the Pd-NCube. Once the reductive elimination of the product occurs the Pd⁰-species is generated again that can re-deposit on the palladium-core.²¹ Since the spherical geometry possesses the lowest free surface energy, globules were eventually formed.³ The mesoporous silica shell is not affected significantly by the catalysis and the rearrangement of the palladium. The porosity appears to stay intact, enabling the penetration of further small organic molecules. Control experiments were performed to validate whether these observations are based on either a thermally or a chemically induced rearrangement process of the palladium core. Therefore, only the nanocatalyst was dispersed in PEG-2000 and heated at 110 °C for 24 h without any conducted catalysis reaction. TEM results showed no changes in the structure of neither the palladium core nor the silica shell (see Fig. S6^†). After the 8^th reaction, no core–shell-nanostructures could be detected anymore via TEM. Only small randomly shaped Pd-nanoparticles (≤20 nm) were found indicating a slow leaching of the palladium out of the silica shell. However, no larger aggregates were formed (see right TE micrograph in Fig. 6) which explains the continuously high catalytic activity.Open in a separate windowFig. 4 ¹H-NMR spectra of the product ethyl p-methoxycinnamate after the 1^st Heck-reaction (bottom) up to the 8^th reaction (top).Open in a separate windowFig. 5Conversions of ethyl p-methoxycinnamate obtained via NMR and gravimetry after each respective Heck-reaction (1–8).Open in a separate windowFig. 6TE micrographs of the Pd-mSi-PEG-5k-nanocatalyst taken after the 1^st (left) and the 8^th Heck-reaction (right).     

Ni(ii)-based coordination polymers for efficient electrocatalytic oxygen evolution reaction

The exploration of highly efficient, stable and cheap water oxidation electrocatalysts using earth-abundant elements is still a great challenge. Herein, alkaline-stable cationic Ni(ii) coordination polymers (Ni-CPs) were successfully obtained under hydrothermal conditions, which could stabilize the incorporation of Fe(iii) to form Fe-immobilized Fe@Ni-CPs. The newly developed Ni-based CPs were used for the first time as an effective electrocatalyst for the oxygen evolution reaction in strong alkaline media.

An alkaline-stable cationic Ni(ii) coordination polymer showed remarkable oxygen evolution reaction (OER) catalytic activity due to capturing Fe ions.

The oxygen evolution reaction (OER) plays a vital role in energy storage and conversion applications due to energy issues and the need for sustainable development.^1–4 Because of the sluggish kinetics of the OER, excellent electrocatalysts are required to work in acidic or strong alkaline environments.^5,6 Noble metal-based catalysts like IrO₂ and RuO₂ with high efficiency showed excellent OER catalytic activity.⁷ However, noble metal with high-cost and scarcity are impractical for scale-up applications. Currently, to substitute these precious metal-based materials, transition-metal-based (Fe, Co, Ni and so on) and metal-free (e.g., N, P and S) hybrid materials have been extensively developed.^8–10 For example, metal–organic frameworks (MOFs) such as ZIF-8/67 derived metal–carbon composite materials exhibit promising electrocatalytic performance.^11,12 Unfortunately, the pyrolysis process destroys the framework completely and causes agglomeration of metals, resulting in a decreased number of active sites. Therefore, to explore highly efficient and low-cost OER catalysts that can be directly used in the OER without calcination are desired, including complexes and MOFs.Recently, much of transition bimetallic materials showed excellent electrocatalytic activity.^13–15 However, a handful of examples such as Fe–Co-MOFs or Co–Ni-MOFs have been explored due to instability, poor conductivity and harsh synthesis conditions.^16–20 Notably, the disadvantages of MOFs have limited their usage in the potential OER. Therefore, it is urgent to develop cheap, stable and active OER catalysts to replace the precious metals. However, this is still a great challenge. Coordination polymers (CPs) with a low-dimensional framework similar to MOFs are constructed by metal ion and organic ligands with potential active sites and functional groups, exhibiting wide applications in sensing, photoluminescence and photocatalysis.^21–26 However, rare examples of CPs have been directly explored in the OER. For Fe/Ni-based bimetal electrocatalysts, a novel strategy involves doping Fe(iii) into a functional Ni-based CPs, which could enhance the electrocatalytic OER activities.Herein, we report the hydrothermal synthesis of a Ni-based CPs as a high-performance OER electrocatalyst in strong alkaline solutions (Scheme 1). The blue crystals of [Ni(bp)₃·(H₂O)₂]·(bp)·(ClO₄)₂ (bp = 4,4′-biprydine) were obtained upon the reaction of bp ligands with NiClO₄·6H₂O under hydrothermal systems. Chemical stability tests displayed that Ni-CPs could retain their original framework in water or even a strong alkaline (pH = 14) solution after 12 hours (Fig. S1^†), which is rarely reported for most transition metal CPs. The thermogravimetric analysis (TGA) showed that there is a significant change at about 110 °C due to the weight loss of the partial guest (Fig. S2^†). Interestingly, the cationic Ni-CPs successfully captured the Mohr''s salt (ammonium iron(ii) sulfate) by taking advantage of the post-synthetic strategy. The obtained Fe@Ni-CPs exhibited a high-efficient OER activity under strong alkaline conditions.Open in a separate windowScheme 1Illustration of the synthesis process for Fe doped Ni coordination polymers for the OER.Single-crystal X-ray diffraction analysis revealed that the Ni-CPs crystallized in the C2/c space group (Table S1^†). The obtained Ni coordination polymer was the isostructural compound reported by Talham,²⁷ but their packing modes were distinctly different (Fig. S3–S5^†). It also had a similar coordination environment to the railroad-like double chains synthesized by Yaghi.²⁸ The parallel chains were occupied by 4,4′-bpy, perchlorate and water molecules. In the Ni-CPs, most of the phenyl rings adopted the face-to-face mode. There were evident π⋯π interactions between the adjacent 4,4′-biprydine (Fig. 1a). In addition, there were strong intermolecular hydrogen bonds between 4,4′-bpy and perchlorate (strong Cl–O⋯C amongst adjacent layers) (Fig. 1a). The weak reaction increased the high density of the framework and protected the coordination bonds against external guest attacking. These chains and guests were further packed with a three-dimensional structure along the c-axis (Fig. 1b).Open in a separate windowFig. 1(a) The weak reaction between chains in Ni CPs, showing the Cl–O⋯C (green and black dashed line) and π⋯π interactions (pink dashed line) between the adjacent perchlorate and 4,4′-biprydine, respectively; (b) the packing view of Ni CPs along the c-axis. Cl in green, Ni in pale blue, N in blue, O in red and C in black. H atoms and partial guest molecules were omitted for clarity.The unique cationic Ni-CPs framework has the potential to immobilize some counterpart ions. To demonstrate this, Mohr''s salts were investigated. Most strikingly, the color slowly changed from blue to green in an aqueous solution, given by the optical image (Fig. 2a and b), which not only indicated that Ni-CPs captured Mohr''s salts via ion-exchange, but also suggested an alteration in the valence of Fe ions. It was possible that Fe²⁺ may have been further oxidized to Fe³⁺ under the O₂ and water environment when we prepared the Fe@Ni-CPs (4Fe²⁺ + 2H₂O + O₂ = 4Fe³⁺ + 4OH⁻). The PXRD pattern showed that the frameworks remained unchanged after doping with Fe ions (Fig. S1^†). From the transmission electron microscopy (TEM) images (Fig. 2c), after immobilization, the morphology of the Fe@Ni-CPs was still level and smooth; no ring-like patterns arose corresponding to the selected area for electron diffraction (SAED) (Fig. 2d), indicating that no bulk Fe particles formed during ion-exchange. This was further demonstrated using high resolution TEM (HRTEM) (Fig. 2e), in which there was no lattice fringe of crystallized Fe. The well distribution of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs was demonstrated by elemental mapping (Fig. 2f–l). Energy-dispersive X-ray spectroscopy (EDX) also agreed well with the above mapping data (Fig. S6^†). In addition, the Fe³⁺ uptake was 4.1 wt%, as determined by inductively coupled plasma atomic emission spectroscopy (ICP). These results showed Fe ions to have been successfully immobilized by the Ni-CPs.Open in a separate windowFig. 2(a–b) The optical image of Ni-CPs and Fe@Ni-CPs; (c) TEM images of Fe@ Ni-CPs; (d–e) the corresponding SAED and HRTEM pattern. (f–l) Element mapping of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs.The X-ray photoelectron spectroscopy (XPS) survey spectrum of the Fe@Ni-CPs also showed the presence of C, N, O, Cl, Fe, and Ni elements (Fig. 3a). The Fe 2p high resolution XPS spectrum exhibited peaks at 725 eV and 711 eV (Fig. 3b), further indicating the presence of the Fe³⁺ oxidation state. This could be explained by the transformations of Fe²⁺ to Fe³⁺ during the ion-exchange process. Similarly, in the Ni 2p spectra (Fig. 3c), two main peaks located at 855.8 eV and 873.5 eV can be ascribed to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2, respectively. These peaks are associated with two shakeup satellite peaks, indicating that Ni still remained in a divalent state. The Cl 2p and N 1s region could be corresponded to the ClO₄⁻ and biprydine, respectively (Fig. S7^†). The O 1s spectrum (Fig. 3d) was divided into two peaks at 531.7 eV and 533.2 eV, which could be assigned to the OH group from filled H₂O molecules and partial ClO₄⁻, respectively.Open in a separate windowFig. 3(a) XPS survey spectrum of the Ni-CPs; XPS spectra of the Ni-CPs in the (b) Fe 2p, (c) Ni 2p, and (d) O 1s regions.The above Ni-CPs with Fe doping encouraged us to investigate its electrocatalytic application in oxygen evolution reaction. To study the electrocatalytic activity of Fe@Ni-CPs for the OER, linear sweep voltammetry (LSV) was performed in a strong alkaline solution (pH = 14) for Fe@Ni-CPs@GC (fresh samples coated on glassy carbon electrode with Nafion binder). Fe@Ni-CPs@GC directly acted as working electrodes and showed good OER activity with an onset potential of 1.52 V (Fig. 4a), overpotential of 368 mV at 10 mA cm⁻², and a Tafel slope of 59.3 mV dec⁻¹ (Fig. 4b). These OER performances are close to some reported MOFs catalysts (Table S2^†) and even better than commercial benchmark OER catalysts like RuO₂ working at the same condition (Fig. S8^†). In contrast, the electrocatalytic OER activities of the pristine Ni-CPs@GC without Fe incorporation displayed much worse activity. The onset potential, overpotential (at 10 mA cm⁻²), and the Tafel slope reached 1.62 V, 458 mV, and 96.8 mV dec⁻¹, respectively (Fig. 4a and b). In addition, Fe@Ni-CPs showed a strong durability during the OER process. The chronoamperometric response of Fe@Ni-CPs displayed a slight anodic current attenuation within 12 h due to the peeling of samples during the evolution of a large amount of O₂ gas (Fig. S9^†). Furthermore, LSV of Fe@Ni-CPs showed negligible changes after OER tests for 12 h (Fig. S10^†). These results indicate that the Fe-doped Ni-CPs with more active sites could serve as an excellent candidate for OER in strong alkaline conditions.Open in a separate windowFig. 4(a) OER polarization curves and (b) Tafel plots of various electrocatalysts in a 1 M KOH aqueous solution; (c) linear relationship of the current density at 1.1 V (vs. RHE) vs. scan rates for Fe@Ni-CPs@GC and Ni-CPs@GC; (d) EIS of Fe@Ni-CPs and Ni-CPs electrode.When Fe(iii) was introduced, the resulting Fe@Ni-CPs catalysts greatly improved OER catalytic performance. There were dynamic collisions between Fe³⁺ ions and Ni-CPs, which allowed for more accessible catalytic active sites compared to the Ni-CPs. Particularly, Fe(iii) doping can contribute to the adsorption and reaction of OH⁻ groups in OER process.²⁹ As a result, Fe@Ni-CPs enhanced charge transfer under an apt electronic environment of the mixed Fe⋯Ni systems. In addition, electrochemical impedance spectrum and double-layer capacitance (C_dl) of the Fe@Ni-CP were also studied. The C_dl of Fe@Ni-CPs was confirmed to be 269.7 μF cm⁻² (Fig. 4c and S11^†), which is higher than that of Ni-CPs (C_dl = 174.2 μF cm⁻²) (Fig. 4c and S12^†). The semicircular diameter in EIS of Fe@Ni-PCP was smaller than that of Ni-CPS (Fig. 4d). These results further showed that Fe@Ni-CPs were more effective in enlarging the catalytically active surface area, conductivity and synergistic effects between Fe and Ni in comparison to Ni-CPs coated on electrodes.In conclusion, a new alkaline-stable cationic Ni(ii) coordinated polymers was synthesized under hydrothermal conditions. The Ni CPs could quickly interact with Mohr''s salt. Interestingly, the Ni CPs could act as a unique oxidation matrix to realize the transformation of Fe²⁺ to Fe³⁺ during the ion-exchange process. Furthermore, the resulting Fe@Ni-CPs electrode, for the first time, showed an excellent electrocatalytic activity for OER in strong alkaline media. This study provides a new avenue to explore stable coordinated polymers by incorporating the low-cost and high-activity transition metal, Fe, which will substitute the rare noble metals used in energy-related research.     

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged dimers: acid-responsive molecular rotors

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged complexes were synthesized via oxidative dimerization of a ruthenium(ii) N-confused porphyrin complex using 2,2,6,6-tetramethylpiperidine 1-oxyl. The deformed core planes in the dimers conferred a relatively high ring rotational barrier of ca. 16 kcal mol⁻¹. Rotation of the complexes was controlled by protonating the peripheral nitrogen.

Ring rotation of ruthenium(iv) N-confused porphyrin μ-oxo-dimer was controlled by protonation at the peripheral nitrogen moieties.

The design/creation/operation of molecular rotors, gears, and switches is an interesting topic in machinery nanotechnology, allowing the development of adaptive materials, switchable chiral sensors, stereo-controlled catalysts, etc.¹ The molecular approach to control nanoscale motions (e.g., rotation) is advantageous for realizing well-defined molecular devices or sets of practical supramolecular assembled systems.² As active components of molecular rotors, metal double-decker bisporphyrins and related macrocycles (e.g., phthalocyanines) are representative candidates that afford specific features, such as long-term stability, ease of fabrication, and visual responses that are detectable by microscopy (Fig. 1a).³ Bis-metal μ-oxo (or nitride)-bridged porphyrin complexes are also considered alternatives for modulating complexes'' rotational mode. However, there are few investigations concerning molecular rotation because of the limitation in synthesizing molecular rotamers comprising unsymmetrical core skeletons with diamagnetic and oxophilic metal ions (for which the rotational dynamics can be probed by NMR spectroscopy) (Fig. 1b).⁴Open in a separate windowFig. 1Chemical structures of (a) double-decker metal porphyrin complexes and (b) bis-metal μ-oxo-bridged porphyrin complex as representative motifs of molecular rotors (R¹ = R³ = p-methoxyphenyl, R² = R⁴ = 4-pyridyl). (c) N-confused porphyrin as a key rotor ligand and its (d) bis-iron μ-hydroxo-bridged porphyrin complex. (e) Schematic of the rotational mode of unsymmetrical N-confused porphyrin μ-oxo dimer complex. Sky-blue moiety in the planes is a confused pyrrole ring.To develop new rotors based on metal μ-oxo-bridged porphyrin dimers, we envisioned that a C_s-symmetric N-confused porphyrin (NCP) isomer possessing an inverted pyrrole ring in the macrocyclic scaffold could act as an alternative ring motif for the complexes (Fig. 1c).⁵ The NCPs bind various metal ions at the NNNC core, forming unique organometallic complexes.^5,6 For example, redox-active metal species such as iron facilitate the generation of metal–oxo linkages to form μ-oxo-bridged dimer complexes.⁷ In the case of iron-coordination of NCP, a unique bis-iron μ-hydroxo-bridged NCP dimer was developed, in which the structure was restricted by the coordination of peripheral sodium, as evident from X-ray crystallography (Fig. 1d).⁶We herein report the synthesis of novel ruthenium(iv) NCP μ-oxo-bridged dimers (Ru₂-2 and Ru₂-3) as a porphyrin-based molecular rotor. The ruthenium(ii) NCP complex (Ru-1) with an axial carbonyl and pyridine (=py) exhibits catalytic reactivity for alkene epoxidation (Scheme 1).⁸ The simultaneous formation of isomers can be anticipated owing to the two stereogenic centers in the NCP planes of Ru₂-2 and Ru₂-3, which differ in the relative position of the nitrogen atoms of the confused pyrrole rings in the scaffold. The Ru–μ-oxo dimers showed distinct ring rotation in solution when probed via NMR spectroscopy (Fig. 1e). Moreover, symmetry-dependent restriction of the rotation was achieved through protonation at the peripheral imino-nitrogen moieties in Ru₂-2 and Ru₂-3 using trifluoroacetic acid (TFA).Open in a separate windowScheme 1Synthesis of ruthenium(iv) NCP μ-oxo dimers, Ru₂-2 and Ru₂-3.Treatment of monomer Ru-1 with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) in refluxing o-dichlorobenzene solution under argon atmosphere afforded two products with approximately the same yields (40% each) (Scheme 1). The fast atom bombardment mass spectra of the two products revealed identical parent ion peaks at 1474.2407 (M⁺) and 1474.2400 (M⁺), indicating dimeric ruthenium NCP structures (i.e., Ru₂-2 and Ru₂-3) bridged by an oxygen atom and two axial oxygen atoms (Fig. S1 in ESI^†). A characteristic broad feature tailing up to 900 nm was observed in the absorption spectra of both Ru₂-2 and Ru₂-3 (Fig. S2^†).The structures of Ru₂-2 and Ru₂-3 were unambiguously characterized by single-crystal X-ray diffraction analysis (Fig. 2 and Tables S1 and S2 in ESI^†). In both complexes, two ruthenium atoms coordinated at the NNNC core in the NCPs are bridged by an oxygen atom to form a sandwich-like structure. The characteristic metalaoxirane structure was found at the inner carbons of the confused pyrrole moieties in the scaffolds.^6,9 Such oxidative reactivity is a characteristic of Ru–NCP, which is in sharp contrast to the reactivity of the Ru^II(CO) azuliporphyrin with an NNNC core.¹⁰ Notably, the confused pyrrole rings in Ru₂-2 and Ru₂-3 are positioned opposite to the C17–Ru1–Ru2–C62(C61) torsion angle of 149.9(2)°, which is anticipated to induce molecular dipole moments. The strict structural difference between Ru₂-2 and Ru₂-3 is found in the position of the nitrogen atoms of the confused pyrrole rings; the former product (Ru₂-2) was eluted in the less polar fraction (R_f = 0.125) via silica gel chromatography using toluene/ethyl acetate (v/v = 19/1) and can be regarded as a syn conformer, whereas the polar fraction (R_f = 0.014) obtained under the same chromatographic conditions was eventually determined to be an anti-conformer (Ru₂-3).¹¹ The Ru1/2–O3 lengths for Ru₂-2 and Ru₂-3 are 1.792(3)/1.804(3) Å and 1.796(2)/1.800(2) Å, respectively, which are typical of μ-oxo-bridged ruthenium(iv) porphyrin complexes (e.g., [Ru^IV(TPP)(p-OC₆H₄CH₃)]₂–O: 1.789(11) Å).¹² The bond angles of Ru1–O3–Ru2 for Ru₂-2 and Ru₂-3 are almost linear 175.9(2)° and 176.1(2)°, respectively, similar to those of other reported ruthenium porphyrin μ-oxo dimer complexes (e.g., 177.8(7)°).¹² Due to the internal metal oxirane moieties, the confused pyrrole rings are inclined inward. Therefore, the overall geometries of the NCP planes are nonplanar with estimated mean plane deviation values (defined by 24 atoms of NCP skeleton) of 0.276 Å and 0.294 Å for Ru₂-2 and 0.286 Å and 0.292 Å for Ru₂-3.Open in a separate windowFig. 2Top (left) and side (right) views of crystal structures of (a) Ru₂-2 and (b) Ru₂-3 with thermal ellipsoids set at 50% probability level. Hydrogen atoms and solvent molecules excepting the H-atoms hydrogen-bonded to the peripheral nitrogen atoms were omitted for clarity.As anticipated above, the formal oxidation state of both ruthenium centers is expected to be +4 (d⁴) for the complexes. The X-ray photoelectron spectra of Ru₂-2 and Ru₂-3 revealed higher Ru (3p_3/2) binding energies of 462.2 and 462.1 eV, respectively, than that of the divalent ruthenium (d⁶) complex, Ru-1 (460.9 eV, Fig. S3 in ESI^†).The structures of Ru₂-2 and Ru₂-3 were further characterized by ¹H NMR spectroscopy in 1,1,2,2-tetrachloroethane-d₂ (TCE-d₂) (Fig. 3a and b). At 298 K, complicatedly overlapped signals were observed in the aromatic region (7–10 ppm). To distinguish the signals from the individual groups (β-pyrrole and meso-phenyl) in the complexes, we prepared the deuterated-phenyl derivatives (i.e., Ru₂-2D and Ru₂-3D) as shown in Scheme 1 (see, ESI^†). The ¹H NMR spectra of Ru₂-2D and Ru₂-3D (Fig. S4^†) show two singlets at 9.38 and 8.98 ppm for Ru₂-2 and 9.32 and 9.05 for Ru₂-3. The remaining six β-CHs are expected to have signals around 8 ppm for both complexes. This spectral feature suggests the existence of two possible rotational isomers in solution with an almost 1 : 1 molecular ratio. The 1D exchange nuclear Overhauser effect spectra revealed chemical exchange of the rotamers at 298 K (Fig. S5 and S6^†), which indicated that the NCP rings of Ru₂-2 and Ru₂-3 rotate slowly at 298 K. The rotational mode of the NCP rings was seemingly restricted at 243 K (Fig. S7 and S8^†). With increasing temperature, the signal of the set of α-pyrrole protons gradually disappeared and coalesced into a single peak at 9.15 ppm for Ru₂-2 and 9.27 ppm for Ru₂-3 at 408 K (Fig. 3c and d). Signals of the meso-phenyl protons also appeared with an averaged spectral feature in the region of 7.5–8.0 ppm (Fig. S9 and S10^†). These results suggest that the rotation rate of the NCP rings is faster than the NMR timescale above 353 K. Furthermore, for the ruthenium(iv) tetrakis(phenyl)-substituted porphyrin μ-oxo dimer (e.g., Ru(TPP)₂O) used as the reference complex, the inward and outward CH signals (i.e., o- and m-CHs) of the meso-phenyl rings did not coalesce even at 408 K in the variable temperature (VT) NMR spectra (Fig. S11^†). In this regard, the rotation of the bulky phenyl ring could be restricted by the spatially locked dimeric structure. The rotation mode of the NCP planes could be thus mutually un-synchronized with the rotational motion of the individual meso-phenyl rings.Open in a separate windowFig. 3 ¹H NMR spectra of (a) Ru₂-2 and (b) Ru₂-3 recorded in TCE-d₂ at 298 K. Partial ¹H NMR spectra (495 MHz, black line) of (c) Ru₂-2 and (d) Ru₂-3 in TCE-d₂ as a function of temperature and the line-shape simulated spectra for the compounds (red line).To gain further insight into the rotational barrier of the complexes, a line-shape analysis of the VT ¹H NMR spectra was conducted to estimate the rate constant for interconversion (k) in the specific temperature range. The free Gibbs''s energy (ΔG^‡ = ΔH^‡ − TΔS^‡) of Ru₂-2 and Ru₂-3 was determined to be (16.0 ± 0.8) kcal mol⁻¹ and (16.1 ± 0.4) kcal mol⁻¹, respectively, at 298 K, from the Eyring plots (Fig. S12^†). The rate constant (k < 200 s⁻¹) at 333 K indicates that the energy barrier of the ring rotations of Ru₂-2 and Ru₂-3 is more significant than that of the double-decker metal complexes (k = 760 s⁻¹ at 333 K).^3d Considering the structure of the complexes, the bent confused pyrrole moieties in the planes may play an essential role in the rotational motion beyond the barrier.Upon lowering the temperature, the ratio of the integrated α-CH signals for the rotamer mixture of Ru₂-2 changed to 0.85 (at 9.33 ppm) and 0.15 (at 8.93 ppm) (Fig. S13^†). The ratio of the two-rotamers of Ru₂-3 also changed when the temperature was further lowered (∼223 K), affording a thermodynamically stable conformer (Fig. S14^†). To understand the structure-dependent features of Ru₂-2 and Ru₂-3, we analyzed the thermodynamic energy profiles of the possible rotational isomers (e.g., Ru₂-2a–f and Ru₂-3a–c) by theoretical calculation with the B3LYP method (Tables S3 and S4^†). The most stable isomers (i.e., Ru₂-2c and Ru₂-3c) obtained in the optimization process are highly consistent with the crystal structures. Notably, the torsion angles of C17–Ru1–Ru2–C61(C62) in the structures of Ru₂-2e and Ru₂-3b are −102.8° and 98.5°, respectively, indicating stability (ΔE = +1.19 and +1.45 kcal mol⁻¹, respectively) relative to Ru₂-2c/-3c. Therefore, two energetically close conformers are present in the solution under the experimental conditions.The effect of protonation on the rotation of the rotamers was evaluated.¹³ Upon addition of trifluoroacetic acid (TFA), the absorption spectra of Ru₂-2 and Ru₂-3 gradually changed with isosbestic points (Fig. S15 and S16^†), reflecting protonation at the peripheral imino-nitrogen of the confused pyrrole moieties (Fig. 4a and b).¹⁴ In both complexes, a relatively large amount of TFA (at least 8 equiv) was necessary until spectral saturation (Fig. S15a and S16a). Strikingly, in the case of Ru₂-3, the relatively crowded imino-nitrogen site surrounded by meso-phenyl rings interfered with double protonation, as seen in the acid-spectral titration change after the addition of 8 equiv. of TFA (Fig. 4d, e, S15b and S16b^†). In the ¹H NMR spectrum, the resulting dication species of Ru₂-2 (i.e., Ru₂-2H₂²⁺) was well-resolved, demonstrating the interlocked single isomer at 298 K, as evident from the NMR spectral features (Fig. S17a^†). The VT NMR spectra revealed that ring rotation becomes feasible with increasing temperature due to the dissociation of the TFA molecule from the complex at equilibrium (Fig. S18^†). In sharp contrast, the ¹H NMR spectrum of the protonated species of the anti-symmetrical complex (i.e., Ru₂-3H⁺) exhibited a significantly broadened spectral feature at 298 K in TCE-d₂. The unsymmetrical spectral feature was observed at 243 K (Fig. S17b^†), which may be due to the above steric issue in the conformation of Ru₂-3. Therefore, the predominant formation of mono-protonated species was anticipated in Ru₂-3. Eventually, upon the addition of triethylamine as a base, the original spectral signal of the neutral species was reproduced for both complexes (Fig. S15c and S16c^†).Open in a separate windowFig. 4Symmetry-dependent acid-binding behaviors of Ru₂-2 and Ru₂-3. Schematic of acid-induced rotational control for (a) Ru₂-2 and (b) Ru₂-3. (c)–(e) Side-views of the protonation site.In summary, novel cofacial ruthenium(iv) NCP μ-oxo dimers, Ru₂-2 and Ru₂-3, were synthesized as motifs for molecular rotors. The resulting complexes, Ru₂-2 and Ru₂-3, are syn/anti-stereo-isomers having the nitrogen at different positions of the confused pyrrole rings. The symmetrical difference between the complexes leads to distinct rotational behavior of the Ru–NCP planes through the oxo-axis of the complex, as proven by VT ¹H NMR analysis. Additionally, considering the intrinsic basicity of the peripheral nitrogen sites, the rotational profile can be controlled by protonation. Further study on the cofacial metal–μ-oxo NCP dimers can enable applications in molecular machinery.     

Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

The formation of highly dispersed Pt nanoclusters supported on zeolite-templated carbon (PtNC/ZTC) by a facile electrochemical method as an electrocatalyst for the oxygen reduction reaction (ORR) is reported. The uniform micropores of ZTC serve as nanocages to stabilize the PtNCs with a sharp size distribution of 0.8–1.5 nm. The resultant PtNC/ZTC exhibits excellent catalytic activity for the ORR due to the small size of the Pt clusters and high accessibility of the active sites through the abundant micropores in ZTC.

Electrochemically synthesized highly dispersed Pt nanoclusters (PtNCs) stabilized by the nanocages of zeolite-templated carbon (ZTC) exhibit excellent electrocatalytic performance toward the oxygen reduction reaction.

Platinum (Pt) is currently considered one of the best electrocatalysts for the oxygen reduction reaction (ORR), which occurs at the cathode of a fuel cell and is the key process determining the overall performance.^1–5 However, the high cost and scarcity of Pt limit its wide commercialization in this field. According to the US Department of Energy, the total Pt loading is required to be below 0.125 mg cm⁻², in contrast to a presently used Pt loading of 0.4 mg cm⁻² or more for fuel cell application.⁴ Therefore, reducing the Pt loading without loss or with an improvement of the cathode performance has received significant interest in electrocatalytic research for fuel cell systems.^6–10 In this regard, reducing the size of Pt particles to a nanocluster scale (size < 2 nm) and maximizing the Pt dispersion may offer an efficient way to achieve maximum utilization of the Pt electrocatalyst with appropriate consumption.^4,11–15The size of nanomaterials generally plays a critical role in controlling the physical and chemical properties for catalytic applications.^16–20 With a decrease in the particle size to the nanoscale, quantum size effects are induced, which alter the surface energy of the material due to unsaturated coordination and change in the energy level of the d orbital of metal atoms, leading to spatial localization of the electrons.^17–20 This size-induced effect on the electronic structures at the active sites modifies the capability of binding the reactant molecules in catalytic reactions, thereby altering the activity of the nanocatalyst.²⁰ When the particle contains a few to several dozens of atoms with sizes, ranging from sub-nanometer to 2 nm often termed as nanocluster that bridges nanoparticle and a single atom.²¹ However, the Pt single atom is not an appropriate electrocatalyst for the ORR in a fuel cell system as the fast four-electron (4e⁻) pathway for the reduction of O₂ to H₂O requires at least two neighboring Pt atoms.^22,23 Anderson''s group demonstrated that the ratio between the production of H₂O (product of 4e⁻ process) and H₂O₂ (2e⁻) in the ORR strongly depends on the number of atoms in the Pt cluster. Typically, it requires more than 14 atoms in a Pt cluster to produce H₂O efficiently through the 4e⁻ pathway of the ORR.²⁴ Therefore, Pt nanoclusters having more than a dozen atoms have proven to be highly efficient ORR electrocatalysts for fuel cell systems.^13–15 Upon decreasing the size of the nanoparticles to a nanocluster, the electronic state and structure are known to be changed, leading to an increase of the catalytic activity in the ORR. Therefore, it is highly desirable to synthesize a Pt nanocluster-based material as an ORR electrocatalyst with high catalytic performance. To date, several synthesis strategies, such as wet-chemical, atomic-layer deposition, and photochemical methods, have been applied for the preparation of well-dispersed Pt nanoclusters on different types of support, such as dendrimer, metal oxide, and carbon materials.^{13–15,25–31}An alternative approach to synthesize Pt nanocluster (PtNC) is the encapsulation of the cluster within nanosized pores, for example, by utilizing microporous (diameters less than 2 nm) carbon materials.³² Among the microporous carbons, zeolite-templated carbon (ZTC) has been attractive for supporting Pt clusters due to its ordered microporous structure.^33–37 ZTC is a potentially promising material as catalyst support as it offers the advantages of extremely large surface area and high electrical conductivity of graphene-like carbon frameworks constituting a three-dimensional (3D) interconnected pore structure.³⁶ Moreover, the micropores of ZTC can serve as nanocages for stabilization of the Pt nanoclusters. Coker et al. used Pt²⁺ ion-exchanged zeolite as a carbon template to synthesize Pt nanoparticles in ZTC with size in a range of 1.3 to 2.0 nm.³³ Recently, atomically dispersed Pt ionic species was synthesized via a simple wet-impregnation method on ZTC containing a large amount of sulfur (17 wt%).²³ Itoi et al. synthesized PtNC consisting of 4–5 atoms and a single Pt atom in ZTC using the organoplatinum complex.³⁷ Although these methods produced Pt nanoclusters with narrow size distribution and atomic dispersion, they required multi-step processes and/or high-temperature treatment (>300 °C). High-temperature treatment often induces the sintering of nanoclusters to aggregated clusters. Therefore, it is highly desirable to develop a simple and low-cost method for the preparation of PtNC supported on ZTC (PtNC/ZTC) for use as an efficient ORR electrocatalyst. The electrochemical reduction approach offers an alternate and efficient route for the synthesis of PtNC in the micropores of ZTC. The electrochemical method is one of the popular ways to prepare electrocatalysts because it is a simple single-step procedure and ensures electrical contact between the nanoparticles and the support.^38,39Herein, we report a facile electrochemical method for the formation of PtNC with a narrow size range of 0.8–1.5 nm supported on ZTC. The resultant PtNC/ZTC shows higher electrocatalytic activities towards ORR compared to that of commercial Pt/C. Here, ZTC plays two important roles: (i) it provides nanocages to stabilize the PtNC and (ii) it accelerates the ORR activity by enhancing the accessibility of active sites through its abundant micropores. Fig. 1a shows a schematic representation of the typical electrochemical synthesis of PtNC/ZTC. In the first step, ZTC was impregnated with a Pt-precursor dissolved in a water–ethanol mixture. As ZTC possess ordered micropores (Fig. S1a^†) with high Brunauer–Emmett–Teller (BET) surface area of 3400 m² g⁻¹ (vide infra), the uniform adsorption and anchorage of PtCl₆²⁻ ions into the micropores of ZTC was favored. After impregnating and drying, the resultant ZTC–PtCl₆²⁻ was mixed with water–ethanol and Nafion to make the ink for the preparation of the electrode. Using the prepared electrode, a potential of 0.77 V vs. reversible hydrogen electrode (RHE) (Fig. 1b) was applied followed by potential cycling between 1.12 to −0.02 V vs. RHE until the cyclic voltammogram was stabilized. The Pt content of PtNC/ZTC was determined to be ∼10 wt% (Fig. S2^†) by thermogravimetric analysis (TGA). The obtained PtNC/ZTC was electrochemically characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammogram (Fig. 1c) after potential cycling in fresh KOH electrolyte shows the characteristic Pt peaks corresponding to hydrogen adsorption and desorption. The Nyquist plots (Fig. 1d) demonstrate that PtNC/ZTC has lower electrolyte resistance (42 Ω) than that of ZTC (70 Ω), implying an improvement in the conductivity of ZTC by the presence of PtNC. Due to the increase in the conductivity, PtNC/ZTC could facilitate the electron transfer more effectively than ZTC, enhancing its electrocatalytic activity.Open in a separate windowFig. 1(a) Illustration for the formation of PtNC/ZTC:Pt-precursor was impregnated into ZTC micropores, and then a potential (0.77 V vs. RHE) was exerted on the ZTC–PtCl₆²⁻ composite in a 0.1 M KOH solution to form PtNC/ZTC (b) Chronoamperometric response of ZTC–PtCl₆²⁻ at a constant potential of 0.77 V (vs. RHE) in 0.1 M KOH electrolyte. (c) Cyclic voltammogram of PtNC/ZTC in a fresh 0.1 M KOH at a scan rate of 20 mV s⁻¹. (d) Nyquist plots of ZTC and PtNC/ZTC in 0.1 M KOH. Fig. 2a and b show images from aberration-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF). The HAADF-STEM images exhibit the typical morphology of the final product (PtNC/ZTC) after electrochemical reduction. As shown in Fig. 2a, it is very clear that isolated PtNCs are uniformly dispersed in ZTC. These PtNCs have a homogeneous distribution with a narrow size range (0.8–1.5 nm, Fig. 2b). On further magnification, the STEM image shows a cluster-like structure of Pt (Fig. 2c). The STEM image of selected PtNC (Fig. 2d) reveals that it consists of ∼20 atoms. The number of atom content in PtNC was further determined by matrix-assisted laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry using trans-2-[3-(4-test-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix.^40,41 As shown in Fig. S3,^† MALDI-TOF measurement produces a mass spectra with a predominant peak centered at ∼3700 Da corresponding to the Pt₁₉ cluster. The TEM image (Fig. S4 a and b^†) validates the formation of PtNC with an average size of 0.9 nm. In addition, the energy dispersive X-ray spectrometer (EDS) mapping images clearly shows the uniform dispersion of Pt nanocluster in ZTC (Fig. S4c^†). The X-ray powder diffraction (XRD) pattern (Fig. 2e) of PtNC/ZTC showed three broad peaks associated with small size metallic Pt corresponds to (111), (200), and (311) planes (Fig. 2e, inset), along with peaks of ZTC at 2θ = 7.8° and 14.9° corresponding to the ordered microporous structure. Along with the structural analysis, the porous texture of PtNC/ZTC was examined by Ar adsorption (Fig. 2f). PtNC/ZTC had a high BET surface area of 2360 m² g_ZTC⁻¹, which is 1.4 times lower than that of pristine ZTC (3400 m² g_ZTC⁻¹). The decrease in Ar adsorption capacity after the formation of PtNC in ZTC is interpreted as a result of the filling of ZTC micropores by PtNC. This micropore filling was confirmed in the pore size distributions of the pristine ZTC and the metal-loaded carbon (inset of Fig. 2f). The X-ray photoelectron spectroscopy (XPS) results reveal the signature of Pt in ZTC (Fig. S5^†). The elemental survey (Fig. S5a^†) shows the signature of C 1s, O 1s, F 1s (Nafion), and Pt 4f. The chemical nature of Pt in PtNC/ZTC was inspected by a detailed Pt 4f XPS analysis. The deconvoluted Pt 4f XPS spectra (Fig. S5b^†) reveals the presence of both metallic and ionic Pt species. The peaks observed at 71.0 (4f_7/2) and 74.2 (4f_5/2) eV correspond to metallic Pt whereas the other peaks positioned at 72.6 (4f_7/2) and 76.0 (4f_5/2) are attributed to Pt²⁺ and the peaks at 74.9 (4f_7/2) and 77.8 (4f_5/2) eV are attributed to Pt⁴⁺ originating from the surface oxidation of metallic Pt.⁴²Open in a separate windowFig. 2(a–d) Representative spherical aberration-corrected HAADF-STEM images of PtNC/ZTC at various magnifications. (e) XRD pattern of PtNC/ZTC and (f) Ar adsorption–desorption isotherms of ZTC and PtNC/ZTC. Inset in (e) shows a 30 times magnified high-angle region of XRD of PtNC/ZTC. Inset in (f) shows the pore size distributions of the ZTC and PtNC/ZTC.The formation of narrow sized PtNC by the electrochemical method can be ascribed to the stabilization of PtNC in the ZTC micropores, which serve as cages to impose a spatial limitation on the size of the Pt clusters. For comparison, Pt supported on ZTC was also prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction at high temperature (300 °C). The Pt obtained by this incipient wetness impregnation method shows the formation of Pt nanoparticles on the exterior surface of ZTC (PtNP/ZTC) (Fig. S6^†). The formation of larger Pt nanoparticles is due to the sintering at high temperature, showing that even ZTC micropores could not prevent the aggregation of PtNCs at high temperatures. Fig. 3 shows the electrochemical ORR activity of PtNC/ZTC using linear sweep voltammetry (LSV) technique on a rotating disc electrode (RDE) in a 0.1 M KOH solution saturated with O₂ at a scan rate of 5 mV s⁻¹. The ORR activity of ZTC (without PtNC) was measured for comparison as well. As shown in Fig. 3a, PtNC/ZTC exhibited higher diffusion limiting current density and higher positive onset and half-wave potential compared to ZTC alone, indicating that PtNC is the active center for the ORR. To investigate the effect of the Pt loading amount on the ORR activity, PtNC/ZTC with various Pt loadings, 2–20 wt%, was used for the measurement of LSV at 1600 rpm. With an increase in Pt content, both the onset and half-wave potential shifted towards more positive potential up to 10 wt% loading of Pt (Fig. 3a and S7^†). Upon further increase of loading of Pt on ZTC to 20 wt%, both the onset and half-wave potential of PtNC/ZTC shifted towards less positive potential along with a slight decrease in the diffusion limiting current density (Fig. 3a). The decrease in the ORR activity of PtNC/ZTC at high loading of Pt (20 wt%) was attributed to the decrease in the electrochemically active surface area (Fig. S8^†) and decrease in the specific surface area (Fig. S9^†). The STEM image clearly shows that the aggregated Pt clusters were formed on the exterior surface of ZTC at 20 wt% loading of Pt (Fig. S10c^†), blocking the accessibility of active sites. Therefore, PtNC/ZTC with the optimum loading of 10 wt% of Pt leads to superior ORR activity with a high positive onset potential of 0.99 V, which is similar to commercial Tanaka Pt/C (Pt/C-TKK) (Fig. 3b), and a half-wave potential of 0.87 V, which is ∼10 mV more positive than that of commercial Pt/C-TKK (0.86 V) (Fig. 3b). Compared to the case of PtNC/ZTC, both the onset and half-wave potential of PtNP/ZTC prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction with the same loading of Pt exhibited a less positive value (Fig. S11^†). The poorer activity of PtNP/ZTC is due to the blockage of active sites by larger PtNPs formed on the exterior surface of ZTC (Fig. S6^†).Open in a separate windowFig. 3(a) RDE ORR polarization curves of PtNC/ZTC with different mass loading of Pt. (b) Comparison of PtNC/ZTC (PtNC_10%/ZTC) with commercial Pt/C-TKK at the same loading of 40 μg_Pt cm⁻². (c) RDE ORR polarization curves of PtNC/ZTC at different rotation speeds. Inset in (c) shows the corresponding K–L plots at different potentials. (d) Represents the kinetic current density values of Pt/C-TKK and PtNC/ZTC at the potential of 0.8 V vs. RHE.To investigate the kinetics of the ORR activity of PtNC/ZTC, LSV measurements were performed with RDE at different rotating rates (Fig. 3c), and the kinetics was analyzed using a Koutecký–Levich (K–L) plot (Fig. 3c, inset). From Fig. 3c, it was observed that the current density increases with the increasing speed of rotation of the electrode, which is characteristic of a diffusion-controlled reaction. The corresponding linear K–L plots (Fig. 3c, inset) with a similar slope at different potentials reveal that the number of transferred electrons was ∼4, indicating that O₂ is directly reduced to OH⁻ and the ORR is dominated by the H₂O₂-free 4e⁻ pathway. To estimate the amount of produced peroxide ion, rotating ring-disc electrode (RRDE) measurement was performed and the produce peroxide ion calculated from RRDE curve was < 4% (Fig. S12^†). The kinetic current density (J_k) obtained from K–L plot at the potential of 0.8 V (Fig. 3d) for PtNC/ZTC (J_k = 50 mA cm⁻²) is 2.2 times higher than that of commercial Pt/C-TKK (J_k = 22 mA cm⁻²).As Pt-based electrocatalysts are known to be highly active in an acidic medium, the ORR activity of PtNC/ZTC in O₂-saturated 0.1 M HClO₄ was also evaluated by comparing it with that of commercial Pt/C-TKK with the same loading of Pt on the electrode surface using RDE at a scan rate of 5 mV s⁻¹. The PtNC/ZTC-based electrode exhibited ORR activity with an onset potential of 0.96 V (Fig. 4), which is close to that of Pt/C-TKK (0.98 V), and half-wave potentials of 0.84 V, which is 20 mV more positive than that of Pt/C-TKK (0.82 V). PtNC/ZTC showed a slightly higher diffusion-limiting current density of ∼5.9 mA cm⁻² (0.4–0.7 V) compared with that of the Pt/C-TKK catalyst (∼5.6 mA cm⁻²). The kinetics of the ORR in an acidic medium was further analyzed using RDE at different rotation rates (Fig. S13^†) and it was observed that the current density increases with the increasing speed of rotation of the electrode, as in the case of the alkaline medium. The number of electron involved and the amount of produced H₂O₂ estimated by RRDE measurement were ∼4 and < 5%, respectively (Fig. S14^†). The mass activity of PtNC/ZTC obtained using the mass transport corrected kinetic current at 0.8 V is 0.15 A mg⁻¹, which is 3.2 times higher than that of Pt/C-TKK (0.046 A mg⁻¹).Open in a separate windowFig. 4(a) RDE ORR polarization curves at 1600 rpm and (b) mass activity at 0.8 V of PtNC/ZTC and Pt/C-TKK in 0.1 M HClO₄.Furthermore, the methanol tolerance of PtNC/ZTC was assessed by intentionally adding methanol to the oxygen saturated electrolyte solution (both in alkaline and acidic media). The commercial Pt/C-TKK was used for comparison as well. The peak current densities for methanol oxidation with PtNC/ZTC were ∼2.8 and ∼3 times lower than that of Pt/C-TKK in alkaline (Fig. 5a) and acidic (Fig. 5b) media, respectively. These results indicate that PtNC/ZTC has much higher tolerance towards methanol than Pt/C-TKK does. This higher methanol tolerance of PtNC/ZTC can be attributed to the small size of the Pt cluster, which may not be sufficient to catalyze the oxidation of methanol efficiently, as the oxidation of methanol requires Pt ensemble sites.⁴³Open in a separate windowFig. 5ORR polarization curves of PtNC/ZTC and Pt/C-TKK in the absence (solid line) and presence (dotted line) of 0.1 M of CH₃OH at a rotation rate of 1600 rpm in (a) alkaline and (b) acid media.The durability of PtNC/ZTC was also investigated by the amperometric technique. The test was performed at a constant voltage of the half-wave potential in an O₂-saturated alkaline medium and at 0.7 V in an O₂-saturated acidic medium at a rotation rate of 1600 rpm (Fig. S15a and b^†). The durability of the PtNC/ZTC catalyst in the alkaline medium was higher than that of Pt/C-TKK, exhibiting a 30% decrease compared to a 40% decrease of Pt/C-TKK in 5.5 h of ORR operation (Fig. S15a^†). The higher durability of PtNC/ZTC compared to Pt/C-TKK in the alkaline medium may be due to the stabilization of PtNC by pore entrapment. In the acidic medium, however, PtNC/ZTC exhibited a 54% decrease in the initial current after 5.5 h of operation while a 33% decrease was observed in the case of Pt/C-TKK (Fig. S15b^†). The decrease in ORR activity in the acidic medium may be due to the leaching out of tiny Pt nanoclusters in acid electrolyte from the ZTC micropores. To understand the decrease in the ORR durability with time, STEM measurements of PtNC/ZTC after 5.5 h of ORR operation were performed. In the alkaline medium, the STEM image of post-ORR PtNC/ZTC shows a slight change in the size of PtNC (Fig. S15c^†) while the STEM image of PtNC/ZTC after ORR in the acidic medium exhibited sintering of PtNC into large particles with an average size of 30 nm (Fig. S15d^†), resulting in a decrease of the ORR activity. In the alkaline medium, the decrease in ORR activity with time may be due to the oxidation of the ZTC support in KOH.⁴⁴We attributed the excellent ORR activity of PtNC/ZTC to the interplay between the following: (1) the structure of the Pt cluster possessing a high ratio of surface atoms that benefits the surface reactions,^45–47 (2) the microporous 3D graphene-like structure of the ZTC support that enables easy access of O₂ and electrolyte molecules to the active sites,⁴⁸ and (3) the high conductivity and large accessible surface area of ZTC that facilitates the electron transfer.^49–51     

Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m⁻².

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m^−2.

It is well known that inkjet printing works as precise and versatile patterning method for printed electronics.^1,2 As for its advantages, it is already being exploited widespread for printing electronics.^3,4 Those merits are easily processed from solutions and conveniently used for air-stable electrodes.^5,6Light-emitting electrochemical cells (LECs) have emerged as an active layer,^7,8 arousing tremendous attention over the years.^9–11 Nevertheless, LECs are simpler than organic light-emitting diodes (OLEDs).^12,13 Due to its single layer architecture, low fabrication price and operating voltages, LECs are considered as a promising, next-generation, emissive thin-film technology.^14–16The most efficient device used to date for LECs are biscyclometalated iridium(iii) (Ir) complexes, because they have highly efficient and stable devices spanning the whole visible range.^17–19 However, avoiding the use of Ir is strongly desired because of its high cost and limited supply.^20–22 Till now, thiazole has worked as active component in a LECs system with a long-lived charge-separation molecule, without additional ions in its active layer.^23–26 As for its advantages, it can reduce recombination of charge carriers and facilitate carrier transfer.^27–29 More and more importance has been attached to thiazole compounds, especially bithiazole-based ones.^30–33 Therefore, these have aroused interest in the syntheses of bithiazoles.^34–36Based on our previous research,^37–41 the use of bithiazole ligands already explored by our group for photocatalytic technology has the advantage of an easy transformation of neutral complexes into charged ones by substitution on the N-atom of the thiazole moiety.^42,43 Furthermore, we demonstrate the fabrication of LEC devices by a combination of inkjet printing and spin coating, on A4 paper substrates for low cost, disposable and flexible conductive pattern.^44,45 Their corresponding material characteristics were closely investigated, a detailed report of the material properties of the resulting coatings, and an envisaged proof of concept application are disclosed in this paper. These fully printed devices demonstrate the potential upscaling of the fabrication of optoelectronic devices.Herein we report the design for a range of bithiazole derivatives 1a–1c (Fig. 1, their synthesis is shown in Scheme S1^†), and explore their properties. First of all, the UV-visible diffuse-reflectance (UV-vis DRS) spectra of those specimens at room temperature are displayed in Fig. 2 and the data are collected in Table S1.^† This enhancement was ascribed to the increase of aromatic rings, which has an intense absorption in the visible-light region. And their bandgap energies (E_g) are between 2.97–3.01 eV (Table S1^†), and are estimated from these absorption spectra according to the Kubelka–Munk method. Expanding the conjugated system and electron density with these donor groups, it can lead to a larger bathochromic shift of the absorption maximum.^46,47Open in a separate windowFig. 1Chemical structures of compounds 1a–1c described in this work.Open in a separate windowFig. 2UV-vis DRS spectra of 1a–1c.All of the three compounds are photoluminescent at room temperature, the relevant fluorescence peak maximum ranges from 360 to 398 nm (Fig. 3). They emit blue light when they are being excited. In agreement with Fig. 3, the presence of electron-releasing groups would shift the emission maximum. Among them, the most electron-donating carbazole unit would give the most red-shifted peak at 398 nm for 1c (Table S1^†). The PL quantum yield (ϕ_PL) studies show that both 1a and 1b are as high as 62 and 78%, while another one is 85% (Table S1^†). The data for their fluorescent quantum yields largely depends on interaction effect with molecules 1a–1c in the crystal packing. In this context, π–π intermolecular interactions can inhibit the fluorescence.⁴⁸Open in a separate windowFig. 3Typical PL spectra of 1a–1c.It may indicate that enlarging the conjugation length and electron density with these donor groups plays an important role in increasing the ϕ_PL.⁴⁹ Moreover, those with high ϕ_PL may be suitable for application as efficient light-emitting material in LECs.⁵⁰In addition, time-resolved measurements of the donor lifetime (τ) in 1a–1c were carried out, and their corresponding values are given in Table S1.^† The bithiazole derivatives 1a–1c showed long-lived singlet fluorescence lifetimes (τ_F), ranging between 1.04 and 9.54 ns (Table S1^†). This was on average more than double the fluorescence lifetime of the bithiazole starting material.⁵¹ These three samples possess relatively high decay time, as a result of their inhomogeneity.⁵² Compound 1c exhibited the longest fluorescence lifetime decay τ_F = 9.54 ns. The sensitivity of the emission to the polarity of the solvent is beneficial for an intramolecular charge transfer (ICT)-like emission.⁵³ This difference probably accounts for the high concentration of donor units in the bithiazole backbone, low rates of intersystem crossing to reactive triplet states.^48,54Consequently, the composite 1c was the best performing LEC and chosen for the below test. The EL spectra obtained for both devices were almost identical, with a main peak at 498 nm and other peak at 504 nm wavelength as illustrated in Fig. 4a. It may account for trapping, cavity, and self-absorption effects from within the LEC device 1c multilayer-structure. The spin-coated emission is quantified by the commission Internationale de L’Eclairage (CIE) coordinates of (0.28, 0.42), and a colour rendering index (CRI) of 65 (Table S2^†). While inkjet-printed light emission with CIE coordinates of (0.34, 0.43) and a CRI value of 83 was achieved for device 1c (Table S2^†). It exhibits good colour rendering indices (CRI > 70), this device exhibited a warm-white appearance, and the colour rendering is considered sufficient for indoor lighting applications.Open in a separate windowFig. 4Comparison of device characteristics with spin-cast and inkjet printer emitting layer for 1c: (a) electroluminescent spectra, (b) luminance vs. voltage, (c) current density vs. voltage, (d) efficiency vs. luminance behaviour.The time-dependent brightness and current density under constant biases of 2.9–3.3 V for device 1c are shown in Fig. 4b and c. Interestingly, the device with the inkjet-printed emitting layer (EML) produced a light output of around 3600 cd m⁻², whereas 4600 cd m⁻² was achieved with the spin-coated EML. In other words, the device with the inkjet printed EML obtained about 78.3% of brightness compared with the reference device. The differences between the two devices regarding current efficiency (J) and power efficiency (PE) were rather closed, as listed in the Table S2.^† A luminous efficiency much larger than 10 cd A⁻¹ was achieved for the inkjet-printed EML, which tended to be 73.9%, and is similar to the spin-coated emitting layer.The performance gap between the devices (Fig. 4d) can be attributed to differences in the height and surface roughness of the emitting layer.⁵⁵ Lateral sizing histograms (Fig. S11^†) show that the spin-coated EML possessed a thickness of 35 nm and a very smooth surface. The inferior performance of the inkjet-printed EML is caused by a less homogeneous surface morphology and an overall fatter layer.⁵⁶ The depth of the inkjet-printed EML ranged from 25 nm (pixel centre) to 35 nm (pixel edges). As a consequence, it is an inkjet deposition process and the related evaporation dynamics reduces the light output and efficiency, but also postpones the stability of the device in the system.^10,57We then shifted our attention to the turn-on kinetics, efficiency and long-term stability of the LEC device for 1c; the typical evolution of current density and light emission is presented in Fig. 5a and b, showing a slow turn-on followed by a progressive decay over time.Open in a separate windowFig. 5(a) Current density (closed symbols) and brightness (open symbols) versus time at 3 V for a device with the representative sample 1c; (b) EQE versus time at an applied voltage of 3 V.The build-up of the light output is synchronous with that of the current density. This time-delayed response is one of the striking features of the operation of an electrochemical cell and reflects the mechanism of device operation.²² The champion device in this set exhibited a peak power efficiency (PE) of 17.4 lm W⁻¹ at a luminance of 18 000 cd m⁻². Meanwhile, the current density began to decrease, possibly due to the electrochemical oxidation or electro corrosion that occurred on component 1c through accumulated electroinduced holes, and then it kept up a durative datum.⁵⁸ Apparently it exhibited outstanding long-term operation stability beyond 40 h (Fig. 5b).The time-dependent external quantum efficiency (EQE) of LEC for the representative substrate 1c are shown in Fig. 5b. It also exhibited similar temporal tendency in device efficiency. When a bias was applied on the LECs, the EQE quickly increased since balanced carrier injection was achieved by the formation of the doped layers.⁵⁹ After attaining the crest value, the device current was still rising while the EQE reduced little by little. It implied that both growing the doped layers and weakening of the EQE was resulting from exciton quenching near the bithiazlole core in persistently extended doped layers.⁶⁰ Doping-induced self-absorption was rather adaptable to the temporal roll-off in device efficiency. This configuration shows the best performance (EQE = 12.8%) compared to those devices prepared with the single compounds, which is given in Table S3.^† The reason may be that the steady-state recombination zone in 1c device was close to the central active layer.⁶¹ Besides it contains fluorenes on its side chains, constructing plane structure, benefiting from good electron injection abilities, leading to enhancing the EQE itself.⁶²The fabrication of inkjet-printed bithiazole interdigitated electrode (IDE) is illustrated in Fig. 6. All the prepared varieties of bithiazole-based ink have been found to be highly stable with nil or miniscule precipitation in over 180 days being stored on the shelf (Fig. 6a). After HCl treatment of the paper substrate, the prepared inks have filled within ink cartridges of a low-cost desktop printer Canon iP1188 (Fig. 6b). Fig. 6c shows the photograph of inkjet-printed conductive patterns.Open in a separate windowFig. 6(a) The representative ink based on sample 1c was stable for 180 days; (b) electrode printing using Canon iP1188 printer; (c) scale showing size of printed electrodes; (d) interdigitated electrode (inset shows small size printed pattern).In addition, the most attractive prospect of the bilayer device structure at this stage is the possibility for patterned emission for the creation of a static display. Fig. 7 presents a photograph of a small portion of a larger static display, with a resolution of 170 PPI, which repeatedly exhibits a message in the form of the word “LEC”. As shown in Fig. 7, the pixel array produced are fairly luminous. The EL spectra were collected from each pixel on the substrate, the emission peak wavelength was about 510 nm and the full width at half maximum was about 170 nm. The low variation in emission intensity in the different pixels implies a small variation in thickness of the solution-coated layers. More specifically, if we assume a minimum diameter of 20 μm for an inkjetted electrolyte droplet, and a smallest inter-droplet distance of 10 μm, we attain a pitch of 30 μm, which corresponds to a high display resolution of 850 PPI. Finally, we draw attention to the herein presented static-display LEC that comprises solely air-stabile materials, and that we routinely fabricate the bilayer stack under ambient atmosphere.Open in a separate windowFig. 7(a) The patterned light emission from a bilayer LEC, with the emission pattern defined by the selected positions of the ink jetted electrolyte droplets. (b) The droplet diameter and pitch were 50 and 150 μm, respectively, and the device was driven at V = 3 V. The scale bar measures 300 μm.     

A stable tunnel-type NaGe3/2Mn1/2O4 anode for Na-ion batteries

Tunnel-type NaGe_3/2Mn_1/2O₄ was fabricated for anode of sodium ion batteries, delivering a discharge capacity of 200.32 mAh g⁻¹ and an ultra-low potential platform compared with that of pure Na₄Ge₉O₂₀ (NGO). The results of X-ray photoelectron spectroscopy (XPS) demonstrate that Ge redox occurs, and partial substitution of Mn effectively improves the Na-storage properties compared to those of NGO.

We investigated tunnel-type NaGe_3/2Mn_1/2O₄; the main structure is Na₄Ge₉O₂₀. NaGe_3/2Mn_1/2O₄ electrodes as anodes for sodium ions batteries deliver a discharge capacity of 200.32 mAh g⁻¹ and satisfactory capacity retention after 50 cycles.

In terms of the high abundance and ready availability of sodium, sodium-ion batteries (SIBs) have been generally regarded as a better alternative to lithium-ion batteries for power stations.^1–4 Hard carbon is widely recognized as one of the most attractive and ideal anode materials for SIBs.^5,6 However, the potential required for sodium ions to insert into hard carbon is very close to that for sodium plating, resulting in sodium dendrites, which raise safety concerns.^7,8 Moreover, the reaction of electrode materials with sodium through alloying or conversion mechanisms always results in serious volume changes in the process of sodium insertion and extraction.⁹ Therefore, insertion-type transition-metal oxides as anodes have attracted much attention owing to their suitable operating potentials and minor volume expansion.^10,11 Recently, embedded titanium/vanadium/molybdenum based oxides with layered structures have been studied as anode materials for SIBs,¹² such as layered Na₂Ti₃O₇,¹³ tunnel Na₂Ti₆O₁₃ (ref. 14) and spinel Li₄Ti₅O₁₂.¹⁵ In addition, post-spinel structured materials have been proposed, which show ultra-stable cycle performances via highly reversible sodium-ion insertion/desertion through large-size tunnels. Recently, in Zhou''s group, NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄ (ref. 17) have been prepared and they have been shown to possess robust cycle lifetimes (more than 10 000 cycles) and discharge plateaus of 0.84 V and 0.7 V, respectively. Meanwhile, in our group, Na_0.76Mn_0.48Ti_0.44O₂ has been developed, which holds an initial discharge capacity of 103.4 mAh g⁻¹, shows a superb rate capability and retains 74.9% capacity after 600 cycles.¹⁸ The large radius of the redox active metal center could optimize the tunnel size and thus boosting the electrochemical performance. It is also a big challenge to find further suitable active centers for insertion-type transition metal oxides as anodes of SIBs. Besides, a host of published reports have said that germanium-based materials can be used as alloy anodes for SIBs with highly reversible sodium storage properties and satisfactory ionic/electronic conductivity.¹⁹ However, it is unclear to us whether Ge could act as an active center in a transition-metal oxide anodes.In this work, we fabricated a tunnel-type NaGe_3/2Mn_1/2O₄ (NGMO) material. When used as the anode of SIBs, it delivers a sustained discharge capacity of 200.32 mAh g⁻¹. Compared with NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄,¹⁷ NGMO delivers a lower safety voltage of 0.36 V. Pure Na₄Ge₉O₂₀ (NGO) as a comparative sample, only exhibits a capacity of 24.8 mAh g⁻¹, which is far inferior to that of NGMO. During discharge and charge process, reversible redox reactions around Ge center occur, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The introduction of Mn in the NGMO improves the reversibility of the Ge redox performance.The structure of NGMO was carefully characterized by XRD, and Rietveld refinement was performed as depicted in Fig. 1. The main Bragg peaks of NGMO could be assigned to space groups of P1(2) and I4₁/a(88), which were fitted to give lattice parameters of a = 10.56/15.04 Å, b = 11.18/15.04 Å, and c = 9.22/7.39 Å, and a volume of 811.2/1672.2 ų, respectively. Na₄Ge₉O₂₀ has a typical tunnel structure, which consists of polymerized Ge/MnO₄ tetrahedra connected with Ge/MnO₆ octahedra. Four Ge/MnO₆ octahedra are connected together by sharing edges to form a tetrameric (Ge/Mn)₄O₁₆ cluster. Each cluster is connected to six GeO₄ tetrahedra, and adjacent clusters are connected by GeO₄ tetrahedra. Na atoms are located in the channels and have elongated Na–O bonds.¹⁹ This highly stable crystal structure can effectively accelerate the migration of sodium ions.²⁰Fig. 2 shows the low and high magnification scanning electron microscopy (SEM) images of NGMO, which is composed of particles of different sizes from 1 to 3 μm; the larger particles are the result of sintering at high temperature. SEM images of NGO with different magnifications are given in Fig. S1,^† and show that the average particle size of NGO is 1 μm.Open in a separate windowFig. 1The Rietveld refinement spectra of NGMO.Open in a separate windowFig. 2(a) and (b) SEM images of NGMO at different magnifications.The morphology and fine structure were studied by transmission electron microscopy (TEM). Fig. 3a and b show the low magnification TEM images. It can be seen from the images that NGMO has an irregular sheet-like morphology with particle sizes from 250 nm to 2 μm. As shown in Fig. 3c, the lattice spacing of the (200) plane is 4.55 Å. In the SAED pattern of Fig. 3d, the red line corresponds to the (020) plane in NGMO, and the lattice spacing is 13.100 Å. These results clearly demonstrate that NGMO exhibits good crystallinity. The corresponding energy dispersive X-ray spectroscopy (EDS) results, and Raman and infrared spectra (IR) are provided in Table S1 and Fig. S2.^† The results indicate that the atomic ratio of Na : Ge : Mn is close to 1 : 1.5 : 0.5 and that there is little sodium loss. The Raman and IR peaks in the high frequency region are attributed to stretching vibrations of Ge–O–Ge and the peaks between 600 and 400 cm⁻¹ are attributed to the bending vibrations of Ge–O–Ge in NGMO.Open in a separate windowFig. 3(a) and (b) Low resolution TEM images, (c) a HRTEM image and (d) a SAED image of NGMO.Galvanostatic electrochemical measurements were evaluated in a voltage range of 0.05–2.0 V, with the current density of 20 mA g⁻¹. Fig. 4a and b show the discharge and charge profiles of NGMO and NGO, respectively. Because of the formation of a solid electrolyte interface (SEI) layer in the initial cycle, the electrochemical behaviour tends stabilize in the second cycle, so voltage profiles are given from the second cycle; the first cycles of the discharge–charge curves of NGMO and NGO materials are given in Fig. S3.^† It can be seen intuitively that both NGMO and NGO have low voltage platforms, while NGMO has the smaller polarization. In Fig. 4a, we notice a reversible voltage profile in the second cycle with discharge capacity of 200.32 mAh g⁻¹ for NGMO. Only NGMO has a flat potential platform and delivers an ultra-low plateau potential. It can be seen from Fig. 4b that NGO shows a capacity of 24.8 mAh g⁻¹, obvious polarization at the 20^th cycle and increased capacity due to the surface side-effect. Fig. 4c indicates that the capacity retention of the NGMO electrode after 50 cycles is 86.2%, which is superior to that of NGO; the coulombic efficiency of NGMO is also provided in Fig. S4.^† To further understand the redox reactions along with the discharge/charge process in NGMO, Fig. 4d displays the differential capacity versus voltage (dQ/dV) curve. The clear anodic peak at 0.33 V and cathodic peak at 0.81 V correspond well with the redox reactions of NGMO.Open in a separate windowFig. 4Electrochemical performance: (a) and (b) voltage profiles of NGMO and NGO, respectively; (c) cycling performance; (d) dQ/dV profile of NGMO. The current density was controlled at 20 mA g⁻¹ over a voltage range of 0.05–2.0 V.The electrochemical impedance spectra (EIS) of fresh and cycled electrodes of NGMO and NGO, with a frequency range of 0.01 Hz to 100 kHz, are shown in Fig. 5. From Fig. 5a, it is obvious that the charge-transfer resistance of the fresh NGMO electrode is lower than that of NGO. This indicates that the migration of charges in the NGMO material occurs more easily than in NGO, which also facilitates the shifting of ions on the surface and inside of the electrodes of NGMO. In Fig. 5b, NGMO electrode in its 10^th cycle exhibits a smaller charge-transfer resistance than both NGO and the NGMO fresh electrode, indicating that the surface of NGMO more readily forms a stable SEI film.¹⁸ The EIS results were fitted by the model shown in Fig. S5.^† The fitting results are provided in Table S2.^† The resistances of the fresh and cycled electrodes of NGMO and NGO are composed of an internal resistance (R_s), the resistance of the surface film (SEI) (R_f; a small semicircle in the high frequency region), the resistance of the charge transfer (R_ct; another opposite semicircle in the middle frequency region), and the Warburg resistance (W; an oblique line in the low frequency region).²¹ Both the fresh and cycled electrodes of NGMO deliver lower charge transfer resistance than NGO. Meanwhile, the transfer resistance of the cycled NGMO electrode is lower than that of the fresh electrode and its slope at low frequency is higher than that of the fresh one (Table S2^†). All these results show that NGMO has lower resistance and better electronic/ionic conductivity than NGO.Open in a separate windowFig. 5Nyquist plots of (a) fresh NGMO and NGO electrodes, and (b) NGMO and NGO electrodes after ten cycles.The evolution of the chemical valence states of the 150-times discharged electrodes were observed by XPS and SEM as provided in Fig. S6.^† It is generally clear that the electrode surface was covered with a thick SEI layer after discharging. Ar plasma etching was used to obtain the internal information. The Ge 3d core-level of the discharged NGMO electrode with and without etching is shown in Fig. S7.^† Before etching, the peaks of the Ge 3d core-level could be fitted to Ge¹⁺ and Ge²⁺.²² After etching, (Fig. S7c^†), the peaks at 30.8 eV and 30.2 eV were also associated with Ge¹⁺ and Ge²⁺, respectively. This result indicates that the valence of Ge decreases as a whole and that there is no obvious difference between the etched and non-etched samples. The reversible redox reactions of Ge remain stable even after cycling. Meanwhile, the Mn 2p core-level spectra are shown in Fig. S7d–f.^† For the Mn 2p core level, owing to the spin orbit coupling, the valence states of Mn comprise two couples including Mn³⁺ and Mn²⁺ (Fig. S7e and f^†). The binding energies of Mn³⁺ are 642.37 eV and 654.04 eV, and the binding energies of Mn²⁺ are 640.71 eV and 652.18 eV. Similarly, after discharging, the binding energies of Mn³⁺ are 642.40 eV and 654.06 eV, and those for Mn²⁺ are 640.69 eV and 652.20 eV, indicating that there are no changes in Mn binding energies before and after etching. This is in good agreement with results in previous reports.^23,24 All results also show that a thin SEI layer has been formed, favoring ions transfer on the repeatedly cycled electrode. It can be seen from the refined XRD results that NGMO consists of Ge⁴⁺, Mn²⁺ and Mn³⁺, and combined with XPS analysis, the results show that the valence states of Mn does not change for the discharged NGMO electrode. Ge displays electrochemical activity in NGMO, and Mn exhibits good chemical stability in the framework.     

Crystal structure of a DNA duplex cross-linked by 6-thioguanine–6-thioguanine disulfides: reversible formation and cleavage catalyzed by Cu(ii) ions and glutathione

Herein, we determined the crystal structure of a DNA duplex containing consecutive 6-thioguanine–6-thioguanine disulfides. The disulfide bonds were reversibly formed and cleaved in the presence of Cu(ii) ions and glutathione. To our knowledge, this is the first reaction in which metal ions efficiently accelerated disulfide bond formation between thio-bases in duplexes.

The crystal structure of a DNA duplex cross-linked by 6-thioguanine–6-thioguanine disulfides has been solved.

The reversible cross-linking of nucleic acids has been investigated for use in biochemical and medicinal studies. For instance, a cross-linking reaction through imine bonds that reversibly formed at lower temperatures and dissociated at higher temperatures has been reported.¹ Metal ion-mediated base pairs have recently attracted interest. In DNA and RNA duplexes, metal ions are placed between bases, and coordination bonds between metal ions and bases stabilize the duplexes.² Metal ion-mediated base pairs are reversibly formed at lower temperatures and dissociated at higher temperatures. Oligonucleotides with thiol tethers have been used to cross-link duplexes and hairpin structures by forming disulfide bonds.³ Disulfide bonds are reversibly formed by oxidization and dissociated by reduction. Disulfide bond formation between 4-thiouracil (^4SU) and 6-thiohypoxanthine (^6SH), and between ^4SU and 6-thioguanine (^6SG), has been reported.⁴ Disulfide bond formation between ^4SU and ^6SG in duplexes has been investigated^4f and applied for mechanistic studies of flap endonucleases.^4h In the reports, I₂ (an oxidizing reagent) was used to accelerate disulfide bond formation. In this paper, we report a novel crystal structure of a DNA duplex containing two consecutive cross-linked ^6SG–^6SG pairs. Notably, disulfide bond formation between 6-thioguanine bases in a duplex was accelerated in the presence of Cu(ii) ions.A DNA dodecamer (ODN-I) with a pseudo-self-complementary sequence d(CGCGAXXBCGCG) (X = ^6SG, B = 5-bromouracil) formed a duplex (duplex-I) consisting of C–G and A–B Watson–Crick base pairs and X–X pairs (Fig. 1). The B residue was incorporated in ODN-I to apply single-wavelength anomalous dispersion (SAD) method for crystal structure analysis.Open in a separate windowFig. 1A scheme for preparation of a DNA duplex containing 6-thioguanine–6-thioguanine disulfides.Thiobases, including 2-thiothymine (^2ST), 4-thiothymine (^4ST), and 6-thioguanine (^6SG), form metallo-base pairs.⁵ Duplexes containing ^2ST pairs and ^4ST pairs are stabilized in the presence of Hg(ii) and Ag(i) ions. The formation of the ^4ST–Ag(i)₂–^4ST pair in which two Ag(i) ions are placed between ^4ST bases was revealed by a crystal structure.^5b Additionally, metal ion binding of ^6SG has been reported.^5c,d,e Consequently, it is expected that duplex-I, which contains ^6SG–M–^6SG metallo-base pairs, could be formed by mixing metal ions and ODN-I. However, in the presence of Cu(ii) ions, we observed crystals of duplex-Iss including cross-linked ^6SG–^6SG pairs.Prior to crystallization, 2 mM ODN-1 was mixed with 2 mM CuCl₂ at room temperature. Single crystals of ODN-1 were obtained for a few days in a droplet prepared by merging 1 μl of ODN-1/Cu(ii) mixed solution and 1 μl of crystallization solution containing 50 mM MOPS (pH 7.0), 10 mM spermine, 250 mM ammonium nitrate, and 10% 2-methyl-2,4-pentanediol, which was equilibrated against 250 μl of 40% 2-methyl-2,4-pentanediol. In the crystal, two DNA fragments formed an antiparallel right-handed helix, as expected (Fig. 2a). The DNA duplex contains seven canonical Watson–Crick G–C and two A–B base pairs. At one end, two complementary residues, 5′-end C1 and 3′-end G12′, do not form Watson–Crick G–C base pairs, bulge out from the helix, and are involved in crystal packing contact (Fig. 2b). At the center of the duplex, two contiguous 6-thioG residues, X6 and X7, form disulfide-bonded base pairs with the X7′ and X6′ residues on the opposite strand, respectively (Fig. 2b and d). As a result, the DNA duplex is largely kinked at the center (Fig. 2d) where the minor groove of 6-thioG residues is widely exposed (Fig. 2c). A similar bent structure of a duplex containing an artificial disulfide pair has been solved by NMR spectroscopy.^3g Such structural disorders might be necessary for incorporating the disulfide pairs. Electron density maps clearly indicate the formation of a disulfide bond between the S6 atoms of the X6–X7′ and X7–X6′ base pairs (Fig. S1^†). In the X6–X7′ and X7–X6′ base pairs, two 6-thio-G residues align almost perpendicularly.Open in a separate windowFig. 2Secondary (a) and tertiary (b–d) structures of the DNA duplex containing two disulfide-bonded base pairs between 6-thio-G residues. X and B residues are 2′-deoxy-6-thioguanosine and 2′-deoxy-5-bromouridine, respectively. Views are from a phosphate-ribose backbone (b) and from the minor groove (c and d) of the two consecutive disulfide-bonded base pairs.To investigate disulfide bond formation in solution, solutions containing ODN-I′ in the presence of oxidation reagents, Cu(ii) ions and I₂, were analysed by high-performance liquid chromatography (HPLC) with a reverse-phase silica gel column. In ODN-I′, the B base in ODN-I was replaced by a T base. One minute after Cu(ii) ions were added to the ODN-I′ solution, a peak with a longer retention time was observed (Fig. 3B). The peak was separated and analysed by electron spray ionization time-of-flight mass spectrometry, and the result indicated the formation of duplex-I′ss. The addition of a large excess of I₂ did not induce the formation of duplex-I′ss (Fig. 3C), which differs from previous reports in which I₂ was successfully used for disulfide bond formation between 4-thiouracil and 6-thioguanine residues in DNA and RNA duplexes.^4f,g Also, duplex-I′ss was not generated by using KBrO₃ as an oxidation reagent in 24 h (Fig. S4^†).Open in a separate windowFig. 3HPLC profiles of solutions containing ODN-I′ and Cu(ii) ions and I₂. (A) A solution containing 20 μM ODN-I′ in 400 mM NH₄NO₃, 50 mM MOPS (pH 7.0). (B); 320 μM CuCl₂ was added to the solution. (C); 1 mM I₂ was added to the solution. Reactions were incubated at 4 °C.Glutathione have been used for cleave disulfide bonds and thioethers on nucleobases.⁶ Glutathione was added to the solution containing duplex-I′ss and the reaction was analyzed by HPLC. The peak for duplex-I′ss was immediately diminished and a peak for ODN-I′ was observed (Fig. S2c^†). In contrast, the addition of EDTA to a solution of duplex-I′ss did not alter the HPLC profile (Fig. S2b^†). Consequently, X–X pair formation (disulfide bond formation) was accelerated in the presence of Cu(ii) ions.As duplex-I′ss formed, the thiocarbonyl groups of the 6-thioguanine residues converted into disulfide groups; consequently, the absorbance at approximately 340 nm decreased (Fig. 4).⁷Open in a separate windowFig. 4Absorbance spectra of (A) duplex-I′ (4 μM), (B) duplex-I′ss (approximately 1 μM). (C) The spectra were overlapped. For easily compared, four times lager value of duplex-I′ss''s absorption is plotted.In conclusion, we determined the crystal structure of a DNA duplex containing consecutive 6-thioguanine–6-thioguanine disulfides. This is the first crystal structure of a nucleic acid duplex containing covalently linked bases through disulfide bonds. The DNA duplex is largely kinked at the disulfide base pairs where the minor groove of 6-thioG residues is widely exposed. The disulfide bonds were reversibly formed and cleaved in the presence of Cu(ii) ions and glutathione. Interestingly, oxidizing reagents such as I₂ and KBrO₃ did not accelerate disulfide bond formation. The arrangement of 6-thioguanine residues in the duplex structure may be related to their reactions. To our knowledge, this is the first reaction in which metal ions efficiently accelerated disulfide bond formation between thio-bases in DNA duplexes. Studies of disulfide bond formation of thio-bases (^2ST, ^4ST, ^6SG, etc.) in the presence of metal ions and metallo-base pair formations (interactions of thio–base pairs and metal ions) are currently in progress.     

Electrochemical preparation system for unique mesoporous hemisphere gold nanoparticles using block copolymer micelles

Gold nanoparticles (AuNPs) are widely used in various applications, such as biological delivery, catalysis, and others. In this report, we present a novel synthetic method to prepare mesoporous hemisphere gold nanoparticles (MHAuNPs) via electrochemical reduction reaction with the aid of polymeric micelle assembly as a pore-directing agent.

Mesoporous hemisphere Au nanoparticles using self-assembled micelles, for the first time, are demonstrated by using electrochemical reduction on a Ti substrate.

Gold (Au) is one of the most stable and versatile elements utilized in various fields, including catalysis, optics, and industrial purposes. Consequently, various shapes and sizes of AuNPs have been intensively studied to improve the performance of Au in different applications.^1–6 Previously, nanoporous or dendritic metal nanostructures, including Au nanostructures, have been synthesized by employing different reagents and conditions such as SH-terminated amphiphilic surfactant,⁷ pH controlling,⁸ and hard-templates.^9,10 The reported porous and dendritic Au nanostructures possess high surface areas and rich active sites, which in turn lead to highly enhanced catalytic activities.Recently, a soft-template method using self-assembled micelles or lyotropic liquid crystals as pore-directing agents has allowed the successful synthesis of mesoporous nanoparticles^11–13 and films^14–17 with different metal compositions. The metals with mesoporous structures demonstrate superior catalytic activity per weight or surface area over their nonporous bulk forms. Previously, our group reported a several-fold increase in the catalytic activity of mesoporous metals in reactions such as the methanol oxidation reaction (MOR),^14,15 ethanol oxidation reaction (EOR),^13,15–17 and nitric oxide reduction¹² as compared to their bulk nanoparticles and films. Such improvement in the catalytic activity of mesoporous structures is mainly attributed to their significantly larger surface areas, more exposed catalytically active sites, and increased durability against aggregation.Interestingly, nanoporous or mesoporous Au structures had been successfully synthesized by using a dealloying method¹⁸ and a hard templating method.⁹ Such methods, however, are a little complicated, and pore-directing templates often remain within the pores, thus leading to severe contamination. Using a thiol group is an alternative way to synthesize mesoporous Au nanospheres.⁷ A significant drawback of using a thiol group, however, is its strong chemical bonding with Au, thus becoming unable to be removed. The synthesis of mesoporous structures using self-assembled polymeric micelles as soft-templates, on the other hand, is a more facile method with fewer synthetic steps, and it is also known to be free of contaminations within the pores. Although a soft-templating method using polymeric micelles has been utilized for the preparation of mesoporous Au and Au-based alloy films towards surface-enhanced Raman scattering (SERS) signals,¹⁹ glucose sensing,^20,21 and MOR,²² the obtained morphologies have been limited to only films.Despite such apparent benefits arising from mesoporous structures and their synthesis using soft-templates, the synthesis of mesoporous AuNPs using soft-templates has not been achieved yet. It is mainly due to the physical and chemical properties of Au which make it extremely hard to form mesoporous structures. Herein, we adopt an electrochemical approach and the soft-template method to synthesize MHAuNPs successfully. As discussed above, we expect MHAuNPs to be highly efficient in various applications in medical diagnosis,²³ optical sensing,²⁴etc.In this report, MHAuNPs with different shapes and sizes are for the first time reported by changing various electrochemical deposition conditions such as applied potentials between electrodes and deposition times. Scheme 1 shows the schematic illustrations of the entire process of precursor preparation (Scheme 1a) and the MHAuNPs fabrication process (Scheme 1b), including the deposition and the detachment of the nanoparticles. The characterization methods implemented in this paper are mentioned in ESI.^†Open in a separate windowScheme 1(a) The process of Au precursor solution preparation and (b) fabricating MHAuNPs by electrochemical reduction.In a typical experiment, a p-doped silicon (Si) wafer was cleaned by using acetone, isopropyl alcohol, and deionized water (DIW) with sonication for 5 minutes, followed by nitrogen (N₂) gas blowing to dry the Si wafer. After the wet cleaning process, the Si surface was treated by oxygen (O₂) plasma for 5 minutes (Oxford Instruments PlasmaPro 80 Reactive Ion Etcher) to remove residual organic impurities. Then, 10 nm of titanium (Ti) layer and 100 nm of Au layer were deposited sequentially by electron beam evaporation (Temescal FC-2000 e-beam evaporator) at 10⁻⁶ torr. Commercially available Au etchant (Sigma-Aldrich) was used to etch the Au film to expose the Ti area (the left image in Scheme 1b). During etching, about 20 percent of Au area was left to be connected to the electrochemical work station, as drawn in Scheme 1b. In preparation of the Au precursor solution, 5 mg of poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO, the number of average molecular weight (M_w) for each block is 18 000 for PS and 7500 for PEO, respectively) was mixed in 1.5 ml of tetrahydrofuran (THF) followed by stirring at 300 rpm for 8 hours. Then, 0.75 ml of ethanol, 0.5 ml of HAuCl₄ aqueous solution (40 mM), and 1.25 ml of DIW were added sequentially. The solution was stirred for another 30 minutes at 200 rpm. The existing block copolymer micelles can be confirmed by TEM observation, and the average diameter is 25 nm, as shown in Fig. S1.^† For the electrochemical deposition, an electrochemical workstation (CH Instruments Inc. 660e) with three electrode system was used to deposit MHAuNPs on the Ti/Si substrate. After the deposition, the particles were carefully washed by chloroform, followed by a rinse using DIW to remove the residual micelles completely. To detach and collect MHAuNPs from the Ti/Si substrate, the substrate was soaked in ethanol and strongly sonicated for a few minutes (Scheme 1b).Fig. S2^† shows the details of the growth mechanism of MHAuNPs by different deposition times. At the initial stage (Fig. S2a^†), small nanoparticles are generated by reducing Au ions in the precursor solution throughout the substrate. Then, the seed starts growing and forming MHAuNPs as the deposition time increases (Fig. S2b–e^†). This similar growth mechanism is the same as the previous report.¹⁹ The high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) image (Fig. S2f^†) shows the mesopores inside the MHAuNPs are homogeneously generated. As-obtained MHAuNPs consist of a pure Au element without any impurities, as shown in Fig. S3.^† Fig. 1 and S4^† show scanning electron microscope (SEM) images of MHAuNPs deposited at different voltages from −0.2 V to −0.9 V vs. Ag/AgCl at high magnification and low magnification, respectively. Different deposition voltages lead to significant changes in the particle sizes but slight differences in the particle shapes. The size distributions of MHAuNPs and the plots of the average diameters of MHAuNPs by different deposition voltages are described in Fig. 2. The distribution graphs show the large sizes of particles, such as more than 1 μm in diameter, when the high voltage (−0.2 V vs. Ag/AgCl) is applied (Fig. 2a). The distribution becomes narrower upon the lower applied voltage. The average diameter-applied voltage plots in Fig. 2b show that the average particle size decreases from around 1.1 μm at −0.2 V to about 300 nm at −0.9 V. Thus when the lower deposition voltages are applied (i.e., the deposition rate is higher) (Fig. 1g–h), the smaller particles with a higher degree of size uniformity are obtained. The opposite trend is observed at higher deposition voltages (i.e., the deposition rate is lower) (Fig. 1a and b), at which the particles become larger and their size uniformity decreases. This trend is because the higher voltage allows only a limited number of seed particles to be deposited on the Ti/Si substrate, and each seed individually grows with no additional seed formation. Whereas the lower voltage can allow a higher number of seeds, leading to a uniform supply of electrons from the working Ti/Si electrode (Fig. S5^†). In addition, the lower deposition voltages make the particle shape more hemispherical in Fig. 1f–h.Open in a separate windowFig. 1The SEM images of MHAuNPs electrochemically deposited at (a) −0.2 V, (b) −0.3 V, (c) −0.4 V, (d) −0.5 V, (e) −0.6 V, (f) −0.7 V, (g) −0.8 V, and (h) −0.9 V for 500 s. The scale bars indicate 200 nm.Open in a separate windowFig. 2(a) Size distributions of MHAuNPs generated by different voltages and (b) the average diameter–the applied voltage plots. Fig. 3 shows the SEM images of MHAuNPs deposited at −0.2 V and for different deposition times from 250 s to 1000 s. Although longer deposition time does not change the number of MHAuNPs, it leads to the growth of MHAuNPs in lateral and vertical directions. Although the MHAuNPs grow more than about two or three times larger at long deposition time, the mesoporous formation does not seem to be changed, as shown in insets in Fig. 3. This point indicates that the deposition time is not the main factor affecting the formation of mesoporous structures as well as the number of particles (seeds), but it affects the sizes of particles.Open in a separate windowFig. 3The SEM images of MHAuNPs deposited at −0.2 V (vs. Ag/AgCl) for (a) 250 s, (b) 500 s, and (c) 1000 s. The scale bars indicate 10 μm. The insets in each figure are magnified SEM images of each condition (The scale bars in insets indicate 500 nm).In this report, 10 nm Ti layer on Si wafer plays an important key role in the formation of MHAuNPs, as previously mentioned in the experimental procedure. The use of the Ti substrate with low conductivity (ca. 2.38 × 10⁶ S m⁻¹), which is about only 5.8% in comparison with that of Au (ca. 4.10 × 10⁷ S m⁻¹), is not common in the electrochemical plating research field.^25–30 Most of the papers on mesoporous metal structures synthesized by electrochemical deposition have utilized Au or Pt substrates due to its chemical stability and high electrical conductivity.^14–17 Fig. S6^† shows the amperometry (i–t) curves during the deposition of MHAuNPs (black dots) on a Ti/Si substrate and mesoporous gold films (red dots) on an Au substrate at the same deposition condition. As shown in Fig. S6,^† around 1/7 times less current flows on the Ti/Si substrate throughout the deposition time. This low current density on the Ti/Si substrate is one of the factors for fabricating MHAuNPs. Low current density causes the formation of a few particles (i.e., seeds) at the initial stage of the deposition and leads to seed growth in a few places, as explained in Fig. S2.^† Furthermore, the use of Ti/Si substrates affects the bottom parts of MHAuNPs to become an arch shape. Only edges of MHAuNPs attach onto the Ti/Si substrates, as shown in Fig. 4. This attachment is because the interaction between the deposited MHAuNPs and the Ti substrate surface (probably, the Ti surface can be partially oxidized, forming TiO_x) is very weak. Therefore, the deposited MHAuNPs can be easily detached from the Ti/Si substrates by sonicating the substrates in solvents (Scheme 1b). The collected MHAuNPs in a solvent are obtained as colloidal particles as shown in Fig. S7.^† Such interesting hemispherical mesoporous nanoparticles have advantages to electrocatalytic activities in comparison to spherical mesoporous metals.³¹ The method using a Ti/Si substrate as a working electrode can be repeatedly implemented with one substrate and without change of the precursor solution, thus it can be effective for mass production in the future.Open in a separate windowFig. 4The SEM image of MHAuNPs deposited at −0.6 V. The arrow shows that the bottom of the MHAuNPs is an arch.Finally, surface-enhanced Raman scattering (SERS) effects on MHAuNPs were investigated by using an adsorbate called rhodamine 6G (R6G), as shown in Fig. S8.^† The resulting MHAuNPs at all conditions (−0.3 V, −0.6 V, and −0.9 V) show substantially strong SERS intensity (Fig. S8a^†), while Ti/Si and Si substrates without MHAuNPs show noise level of intensity. To further investigate enhancement factor (EF) and limit of detection (LoD), various concentrations of R6G with MHAuNPs fabricated at −0.9 V were used for the SERS studies (Fig. S8b^†). The main peak of SERS is 1363 cm⁻¹, and it disappears from less than 10⁻⁶ M concentration, while the 1183 cm⁻¹ peak still exists at 10⁻⁸ M (Fig. S8c^†). The maximum EFs at 1363 cm⁻¹ (10⁻⁶ M) and 1183 cm⁻¹ (10⁻⁸ M) are 1.5 × 10⁴ and 3.1 × 10⁶, respectively (Fig. S8d^†). Transmission electron microscope (TEM) images in Fig. S9^† show the detailed particle structures and the electron diffraction (ED) pattern confirmed the crystal structure is the face-center cubic (FCC) structure. The sharp surface structures and the pores on MHAuNPs provide abundant hot spots that have been reported as the origin that enhances SERS intensity owing to the plasmon resonances.^19,32 Besides, the high density of small-sized MHAuNPs (Fig. S5 and S10^†) boosted higher SERS intensity.In conclusion, we have synthesized MHAuNPs by using 10 nm Ti-coated Si substrates as a working electrode on a Si wafer and electrochemical deposition using self-assembled polymeric micelles as pore-directing agents. The low current generates Au seeds at only a few places, and it acts as the points that MHAuNPs start growing. The particle shapes and sizes can be controlled by changed applied voltages and deposition times. The lower voltages make small particles and the great hemispherical AuNPs with mesoporous architecture. The long-time deposition does not affect any mesoporous formation, but the particle shape and size. Besides, the low affinity between Au and Ti (probably, oxidized layer) results in the arch on the bottom of MHAuNPs, which helps the particles detached from the substrates easily. These results indicate that different thicknesses and compositions of working electrodes can provide different metal deposition phenomena, which can bring out unique shaped particles with mesoporous architectures in the future.     

Discovering a new MgH2 metastable phase

Formation of a new metastable fcc-MgH₂ nanocrystalline phase upon mechanically-induced plastic deformation of MgH₂ powders is reported. Our results have shown that cold rolling of mechanically reacted MgH₂ powders for 200 passes introduced severe plastic deformation of the powders and led to formation of micro-lathes consisting of γ- and β-MgH₂ phases. The cold rolled powders were subjected to different types of defects, exemplified by dislocations, stacking faults, and twinning upon high-energy ball milling. Long term ball milling (50 hours) destabilized β-MgH₂ (the most stable phase) and γ-MgH₂ (the metastable phase), leading to the formation of a new phase of face centered cubic structure (fcc). The lattice parameter of fcc-MgH₂ phase was calculated and found to be 0.4436 nm. This discovered phase possessed high hydrogen storage capacity (6.6 wt%) and revealed excellent desorption kinetics (7 min) at 275 °C. We also demonstrated a cyclic-phase-transformation conducted between these three phases upon changing the ball milling time to 200 hours.

Effect of mechanically-induced cold-rolling followed by high energy ball milling on cyclic phase transformation.

The worldwide interest in MgH₂ is attributed to the natural abundance of Mg metal, and its capability to store hydrogen up to 7.60 wt% (0.11 kg H₂ L⁻¹).^1–3 Among the metal hydride family, MgH₂ has been considered as a promising candidate for solid-state hydrogen storage.^4–6 Despite the attractive properties of MgH₂, and the simplicity for producing the compound on an industrial scale at ambient temperature via a reactive ball milling (RBM) technique,^7,8 MgH₂ is a very stable compound, and possesses slow kinetics of hydrogenation and dehydrogenation under 300 °C.⁹ Since the 1990s efforts have been made in order to destabilize MgH₂ and improve its hydrogenation and dehydrogenation kinetics upon doping with a long list of catalysts.^10–15 Most of the used catalysts showed significant enhancement of the kinetics behavior for MgH₂, indexed by a decreasing decomposition temperature and a speed-up of its kinetics behavior.^16–20 In spite of the beneficial effects obtained upon adding such foreign catalytic agents, they always lead to a dramatic decrease of the hydrogen storage capacity of MgH₂.^21,22Apart from doping MgH₂ powders with catalysts, it has been experimentally demonstrated by some authors that changing the crystal structure of stable β-tetragonal MgH₂ phase to a less stable phase of γ-orthorhombic MgH₂ led to improve the gas uptake/release kinetics and decrease the hydrogenation temperature without drastic decreasing of the storage capacity.^23–26 The β-to-γ phase transformations can be attained via severe plastic deformation (SPD)²⁷ at ambient temperature by different approaches such as high-energy ball milling (HEBM),²⁸ cold rolling (CRing),²⁹ equal channel angular pressing (ECAP),³⁰ and high pressure torsion (HPT).^22,31 A common result of these employed techniques is the formation of nanocrystalline phase along with introducing high intensity defects, leading to increase of grain boundaries density. Presence of these defects in the lattice leads to create nucleation points for hydrogenation, where existence of large number of grain boundaries assists fast diffusion pathways for hydrogen.The present work has been addressed in part to study the effect of HEBM on the structure and decomposition properties of CRed MgH₂ powders. Moreover, we aimed to investigate experimentally the possibility of formation a new metastable MgH₂ phase rather than the reported g-phase and theoretically calculated δ and ε phases³² upon long term of milling.For the purpose of the present study, 5 g Mg (∼80 μm, 99.8 wt%) powder was balanced inside a He gas atmosphere-glove box and sealed together with fifty hardened steel-balls (11 mm in diameter), using ball-to-powder weight ratio as 40 : 1. The vial was then evacuated to the level of 10⁻³ bar and then filled with 50 bar of H₂. The RBM process was carried out at room temperature, using planetary-type HEBM. After 25 hours (h) of RBM, the vial was open inside the glove box to discharge the powders. The powders were charged and sealed in a stainless steel (SUS304) tube (0.8 cm diameter and 20 cm length) inside the glove box. The tube contained MgH₂ powders were severely CRed for different number of passes (1 to 200 passes), using two-drum type manual cold roller (11 cm wide × 5.5 cm rollers diameter). The as-CRed powders for 200 passes were then HEBMed under hydrogen gas for different milling time, in the range between 3 h to 50 h. All the samples were characterized by means of X-ray diffraction (XRD) with Cu radiation, field-emission high-resolution transmission electron microscope (FE-HRTEM) equipped with energy-dispersive X-ray spectroscopy (EDS), field emission scanning electron microscope (FE-SEM)/EDS, and differential scanning calorimeter (DSC). The absorption/desorption kinetics were investigated via Sievert''s method in different temperatures under hydrogen gas pressure in the range between 200 mbar to 8 bar.The XRD patterns of MgH₂ powders obtained after 25 h of RBM is shown in Fig. 1a. Bragg peaks corresponding to starting hcp-Mg powders were hardly seen and replaced by sharp diffracted lines related to γ- and β-MgH₂ phases, as elucidated in Fig. 1a. The SEM observations indicated the powders tendency to agglomerate, forming large aggregates upon CRing for 25 passes (Fig. 2a). Increasing the CRing passes to 200 times led to grain refinement, as implied by the broadening in the Bragg-peaks presented in Fig. 1b. At this stage, micro-bands with thickness of 143 μm were developed as a result of cold working generated during CRing process, as shown in Fig. 2b. The FE-HRTEM image of as-CRed powders for 200 passes indicated the development of lattice imperfections (e.g. stacking faults and deformation twins). This is implying the mechanically-induced SPD, as elucidated in ESI, Fig. S1.^†Open in a separate windowFig. 1XRD patterns of MgH₂ powders obtained after 25 h of RBMing (a), and then CRed for 200 passes (b). XRD pattern of CRed powders for 200 passes and then HEBMEd for 50 h is presented in (c).Open in a separate windowFig. 2FE-SEM micrographs taken at accelerated voltage of 1 kV of MgH₂ powders obtained after 25 h of RBMing and then CRed for (a) 25, and (b) 200 passes.In order to study the effect of HEBM on the stability of cold-rolled MgH₂ aggregates, the powders were charged into tool steel vial and ball milled under 50 bar of H₂ for different milling time. The XRD pattern of CRed MgH₂ powders obtained after 200 CR passes and then HEBMed for 50 h is displayed in Fig. 1c. Obviously, the Bragg peaks corresponding to γ- and β-MgH₂ phases were completely vanished and replaced by new Bragg peaks of unreported phase, appeared at scattering angles (2θ) of, 35.034, 40.584, 58.758, 70.115, and 73.868 (Fig. 1c). XRD analysis indicated that this discovered MgH₂ phase has face centered cubic structure (fcc) of space group, Fm$\bar{3}$m(225). The lattice parameter (a_o) of this phase, calculated from (111) was 0.44361 nm. The as-synthesized fcc-MgH₂ powders had fine spherical particles with sizes distributed in the range between 0.25 μm to 1 μm, as displayed in Fig. 3.Open in a separate windowFig. 3FE-SEM micrograph of MgH₂ powders obtained after 25 h of RBM, CRed for 200 passes and finally HEBMed for 50 h.

Construction of fluorescence active MOFs with symmetrical and conformationally rigid N-2-aryl-triazole ligands

1,4-Bis-triazole-substituted arene (NAT) was designed and synthesized for the construction of metal organic frameworks. Unlike the tri-phenyl analogs, which give a twisted conformation between three benzene rings due to the A-1,3 repulsion, the NAT-ligand gave the energetically favored co-planar conformation with the strong fluorescence emission. With this ligand, two new MOFs, NAT-MOF-Cd (2,3,4-c) and NAT-MOF-Cu (4-c), were successfully obtained with the structure confirmed by X-ray. With the six-coordinated Cd(ii) cluster, an interesting metal–ligand coordination and H-bonding hybridized porous polymeric structures were observed. In contrast, a typical Cu(ii) paddle wheel coordination was obtained with NAT and Cu, giving a new MOF structure with moderate stability in aqueous solution from pH 1–11 for 24 hours, which suggests a promising future for applications in fluorescence sensing and photocatalysis.

1,4-Bis-triazole-substituted arene (NAT) was designed and synthesized for the construction of fluorescent metal organic frameworks.

With large surface areas, diverse geometries, tunable pore sizes, and accessible functional sites, metal organic frameworks (MOFs) have shown exciting applications in various research areas, including gas molecule storage and separation, chemical catalysis, molecular sensing, luminescence, drug delivery, optical/electronic devices, etc.¹ From the design perspective, the geometry of the ligand is crucial for the overall network construction and its function.² Over the past decade, our group has been focusing on the investigations of 1,2,3-triazole derivatives for applications in chemical synthesis and material preparation.³ These efforts have led to the discovery of interesting new functions associated with certain triazole analogs. One particularly interesting building block is the N-2-aryl-1,2,3-triazole (NAT) moiety recently reported from our lab.⁴ Unlike the ubiquitous N-1 isomers, which were prepared from CuAAC (click chemistry), the NATs have shown excellent co-planar conformation between triazole and arene rings.⁵ As a result, NAT gives strong fluorescence while N-1 isomer shows almost no emission.⁶ Both the unique structure and interesting photo activity makes NAT interesting molecular building blocks in materials development.⁴ Herein, we report two new NAT based MOFs with moderate water (acid/base) stability and fluorescence emissions.Our interest in NAT-MOF synthesis was initiated by the realization of twisted conformation associated with the poly-arene system. It is well-known that spacing linkers are important in MOF design to alter pore size.⁷ Considering the required structure rigidity often required for stable MOF structure, poly-arenes are one very popular linker motif.⁸ However, as shown in Scheme 1A, the typical bi-aryl molecule could not adopt co-planar conformation due to the A-1,3 repulsion between the ortho-protons on adjacent benzene rings.⁹ As a result, two conformations could be adopted: (A) twisted (all three rings twisted) and (B) orthogonal (rings 1 and 3 are parallel and twisted the same angle with ring 2).¹⁰ Overall, it is impossible to have all three aryl rings lining up to form a planar conformation, which could be crucial to open the window for effective π–π stacking. In contrast, with a coplanar conformation, NAT could serve an interesting and ideal new ligand for the extended poly-arene linker MOF construction. Besides the structure novelty, two other important concerns for MOF construction are material stability and practical ligand synthesis for potential applications.¹¹ With low electron density, 1,2,3-triazole is very stable toward oxidative conditions.¹² Moreover, the N-2-substitution successfully avoid potential triazole ring opening through N₂ extrusion.¹³ We then put in our efforts to develop a practical synthesis of the triazole-arene ligands. After evaluating various protecting groups and reaction sequences, a general route was developed as shown in Fig. 1.Open in a separate windowScheme 1Tri-aryl linker in MOF construction: twisted conformation.Open in a separate windowFig. 1Synthesis of NAT ligands: (a) PMBBr (1.1 equiv.), K₂CO₃ (4 equiv.), MeCN, 60 °C, 96%; (b) p-B(OH)₂–C₆H₄–COOMe, 5% Pd(PPh₃)₄, K₂CO₃ (4 equiv.), 1,4-dioxane : H₂O(1 : 1), 100 °C, 97%; (c) TFA, 120 °C, 54%; (d) 1,4-phenylenediboronic acid (0.5 equiv.), Cu(OAC)₂ (1.5 equiv.), pyridine (2 equiv.), THF, 1 atm O₂, 70 °C, 50%; (e) KOH, MeOH : THF (1 : 1), 100 °C, 90%.The synthesis starts from commercial available 4,5-dibromo-triazole 1. Protecting triazole with PMB group followed by the Suzuki coupling gave 4,5-diaryl triazole in excellent overall yields. Deprotection of PMB group gave NH triazole 2, which was applied to copper mediated Ullman coupling to afford the tetra-esters. Saponification followed by recrystallization gave the tetra-acid NAT ligand. Overall this route could afford the desired NAT ligand in gram scale with five linear steps. The resulting NAT tetra-acid is fluorescence active both in solution and in a solid state as expected. Notably, this route could be easily applied to the coordination with other linkers (besides benzene) for the coordination of new functional groups into the central arene, which is currently under investigation in our lab. With tetra-acid NAT available, we explore the possibility of applying them in MOF construction through coordination with two typical cations, Cd²⁺ from group 12 and Cu²⁺ from group 11. Fortunately, both complexes were successfully obtained under typical MOF preparation conditions (see details in ESI^†). The X-ray crystal structures of both MOFs were successfully obtained.As shown in Fig. 2, NAT-MOF-Cd was prepared through solvothermal condensation with NAT ligand and Cd(NO₃)₂ in the mixture of DMF, EtOH, water, and HNO₃ (5 : 1 : 1 : 1) at 85 °C. The needle-like transparent crystal was obtained. X-ray crystal structure reveals the typical 7-coordinated Cd cluster from three carboxylates and one DMF (Fig. 2A). Interestingly, as a tetra-acid ligand in coordination with a six-coordinated Cd cluster, the NAT ligand shows two different coordination patterns. For NAT-1, both carboxylates on each end of the ligand coordinated with Cd²⁺, forming cycle I (9.5 Å × 13.7 Å) through binding with neighboring NAT-1. On the other hand, only one carboxylate on each side of NAT-2 ligand coordinates with Cd²⁺ (to satisfy the overall six-coordination Cd cluster). The NAT-2 binding with Cd-SBU not only links the cycle 1 but also produces metallocycle 2 with a larger size (23.2 Å × 33 Å). As expected, both NAT-1 and NAT-2 show good co-planar conformation with the dihedral angle <30° in both cases, which highlight the unique structural feature of NAT over poly-arenes. While the cycle-1 and cycle-2 form large network extension, it is overall a 2D layer structure (Fig. 2B). However, with the un-coordinated free COOH in NAT-2, a layer-by-layer hydrogen bonded metal organic framework was achieved. As a result, the overall 3D packing framework is composed of layer-by-layer H-bonding and vertical six-coordinated Cd(ii) cluster MOF along the b axis as shown in Fig. 2C. The H-bond between COOH of each layer (d_OH⋯O = 2.623 nm) is confirmed and perfectly lining up the open cavity, forming cylindrical pores in the vertical direction (Fig. 2D).Open in a separate windowFig. 2(A) coordination environments of both ligand and Cd(ii) ions; (B) two coordination patterns of NAT-1 and NAT-2; (C) 3D packing view of NAT-MOF-Cd along b axes with fitted pores; (D) layer-by-layer H-bonded framework of NAT-MOF-Cd.The topology of this new NAT-MOF-Cd is calculated to be 2,3,4-c net with stoichiometry (2-c)(3-c)2(4-c) while the point (Schlafli) symbol is {4·8²}2{4²·8²·10²}{8}. The PXRD of the crystal structure was uniform with simulated data (Fig. S3a^†). FT-IR of NAT-MOF-Cd (Fig. S5a^†) showed the disappearing of 2990 cm⁻¹ adsorption associated with carboxylic acid and the appearance of 1390 cm⁻¹ and 1590 cm⁻¹ signal associated with the symmetric and asymmetric stretching of carboxylates. Thermogravimetric analyses (TGA) revealed the good thermal stability of NAT-MOF-Cd up to 351 °C (no decompositions) and completely collapsed at around 456 °C (Fig. S6a^†).The successful synthesis of NAT-MOF-Cd confirmed our hypothesis that fluorescence active NAT ligand could be used as a novel co-planar linker to coordinate with secondary building units (SBU) for porous MOF construction. The three-dimensional H-bonded MOF structure is interesting molecular architecture. However, it raised the concern whether it will be stable in an aqueous solution where many applications are taking place. Soaking NAT-MOF-Cd in water for only 30 minutes caused a total collapse of MOF structures confirmed by PXRD (Fig. S4^†). To achieve more rigid MOF structures with this NAT, we put our attention to Cu²⁺. It is well known that Cu(ii) cations could coordinate with four carboxylates to form a paddle wheel SBU. The good match of tetra-acid and four-coordinated Cu makes NAT ligand ideal for MOF construction via Cu-SBU. After screening various conditions, we were pleased to identify the combination of NAT with Cu(NO₃)₂·3H₂O (1 : 2) in a mixture of DMF, EtOH, water, and AcOH (5 : 1 : 1 : 1) as the optimal conditions for MOF synthesis. The FT-IR spectra (Fig. S5b^†) of resulting MOF revealed the disappearance of carboxylic acid groups around 2983 cm⁻¹ and the symmetric and asymmetric stretching of carboxylate groups at 1394 cm⁻¹ and 1581 cm⁻¹. The cubic-shape blue crystal was obtained with the structure confirmed by X-ray.As revealed by X-ray, the NAT-MOF-Cu is constructed with a typical Cu(ii) paddle wheel SBU (Fig. 3A). Each paddle wheel is coordinated with carboxylates from four NAT with two DMF bound on the c direction (Fig. 3B). Similar to NAT-MOF-Cd, the ligand in NAT-MOF-Cu shows good co-planar conformation with the dihedral angle <30°. The overall packing structure with pores along the c axis of NAT-MOF-Cu is shown in Fig. 3C. Meanwhile, the topology of NAT-MOF-Cu is calculated to be 4-c net with the point (Schlafli) symbol of {4⁴·6²}. The PXRD pattern was consistent with the simulated curve of the crystal structure (Fig. S3b^†). Thermalgravimetric analyses confirmed the thermal stability of NAT-MOF-Cu that it could behave intact as crystal scaffold until 354 °C before completely decomposed at around 551 °C (Fig. S6b^†).Open in a separate windowFig. 3(A) Paddle wheel SBU of NAT-MOF-Cu; (B) coordination environments of NAT-MOF-Cu; (C) 3D packing view of NAT-MOF-Cu along c axes; (D) CO₂ sorption and pore volumes of NAT-MOF-Cu at 195 K.The porosity of both NAT-MOFs was identified by CO₂ adsorption at 195 K. The framework behaves reversible type-I isotherm adsorption features, in which gas molecules present sharp adsorption at relatively low pressure (P/P₀ < 0.1) and reach to a plateau at 131 cm³ g⁻¹ with NAT-MOF-Cu (Fig. 3D). The Brunauer–Emmett–Teller (BET) and Langmuir surface area were calculated to be 309 m² g⁻¹ and 436 m² g⁻¹ for NAT-MOF-Cu because of large pore volumes from Horvath-Kawazoe calculation mode and regular stacking. Gas adsorptions of CO₂ and N₂ for NAT-MOF-Cu at 273 K were also obtained (Fig. S7b^†). The maximum adsorption of CO₂ was 43 cm³ g⁻¹ and the value of CO₂/N₂ selectivity (Fig. S7d^†) was obtained as 44.2 by ideal solution adsorbed theory (IAST) with a good correlation factor (R² > 0.999). Gas adsorption including BET surface area (37 m² g⁻¹) and CO₂/N₂ selectivity (8.5) at 273 K of NAT-MOF-Cd was also measured. Detailed figures are provided in ESI (Fig. S7 and S8^†).The stability of both NAT-MOFs was evaluated. As demonstrated previously, with the layer-by-layer H-bond, NAT-MOF-Cd is not stable in protic solvents. In contrast, the NAT-MOF-Cu showed excellent stability. First, immersing NAT-MOF-Cu in various organic solvents, including MeOH, EtOH, THF, MeCN, DCM, m-xylene, and 2-propanol, at room temperature for 24 h showed no crystal decomposition based on crystal PXRD (Fig. 4A). Furthermore, stability in aqueous solution under different pH was also evaluated by soaking the MOF in the aqueous solutions. NAT-MOF-Cu showed almost no decomposition in aqueous media with pH from 1 to 11 (Fig. 4B). Further increasing to pH = 13 or reducing to pH = 0 did give complex decomposition over time. However, the ability to survive aqueous solution over a large pH range showed the good stability of this new NAT-MOF-Cu porous materials.Open in a separate windowFig. 4Solvent stability tests of NAT-MOF-Cu in (A) organic solution and (B) aqueous solution.In general, the formation of carboxylate MOF complexes could enhance ligand fluorescence intensity due to the locked conformation that prevents undesired excitation state relaxation. NAT-MOF-Cd showed enhanced fluorescence (Φ = 26%) compared with NAT ligand (Φ = 6.7%) in solid state (Fig. 5). Interestingly, NAT-MOF-Cu gave significant fluorescence quenching with the resulting MOF showed almost no emission (Φ < 0.1%). This result suggested plausible electron or charge transfers in the excitation state with this new MOF material. The exact mechanism is currently under investigation along with the potential application of this new photoactive MOF as sensor or photocatalysts. The results will be reported in due course. Nevertheless, the intrinsic photoactivity along with the co-planar conformation makes NAT a promising new ligand system for future porous material construction.Open in a separate windowFig. 5Solid state fluorescence emission of NAT and NAT-MOFs.In summary, we designed and synthesized 1,4-bis-triazole-substituted arene (NAT) as a new ligand for the construction of metal organic frameworks. Compared with poly-arenes system, NAT adopts co-planar conformation with the dihedral angle <30°. NAT-MOF-Cd (2,3,4-c) was formed by six-coordinated Cd(ii) clusters with interesting H-bonded MOF framework while NAT-MOF-Cu (4-c) was obtained with Cu(ii) paddle wheel SBU. NAT-MOF-Cu showed reasonable stability in wide pH range (1–11) and organic solvent for over 24 h. Photoluminescence properties were observed upon the formation of NAT-MOF, suggesting potential applications of this photoactive porous material through the new ligand design.     

A polycarboxylic chelating ligand for efficient resin purification of His-tagged proteins expressed in mammalian systems

We describe the synthesis of a novel polyamino polycarboxylic ligand, its ability to coordinate metal-ions and attachment to a solid support designed for protein purification through Immobilised Metal-ion Affinity Chromatography (IMAC). The resin was found to be highly efficient for purification of His-tagged HCV E2 glycoproteins expressed in 293T mammalian cells.

We describe the synthesis of a novel polyamino polycarboxylic ligand, its ability to coordinate metal-ions and attachment to a solid support designed for protein purification through Immobilised Metal-ion Affinity Chromatography (IMAC).

Protein purification has become a major research concern due to the increasing market of recombinant proteins and the necessity to provide highly pure proteins in good yields.¹ Immobilised Metal-ion Affinity Chromatography (IMAC) represents one of the most attractive choices due to several advantages including competitive costs, simplicity of the experimental procedures and small tag protein modifications that do not usually interfere with protein activity.² IMAC consists of immobilising transition metal ions (Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) on a solid matrix considering the ability of metal-ion complexes to interact with protein tags containing donor atoms such as oxygen, nitrogen and sulfur.³ A common tag is polyhistidine i.e. four to eight histidine residues, which can be easily incorporated into proteins. This small tag yields a very strong interaction with metal ions and, in most cases, does not significantly affect protein functions and can be conveniently removed by proteolysis.⁴ The solid matrices used for such applications are usually functionalised-saccharide polymers on which organic chelators are grafted, such as iminodiacetic acid (IDA, I) or nitrilotriacetic acid (NTA, II) (Fig. 1). The most known used system in IMAC technique is the one formed by Ni–NTA complexes and His-tagged proteins.^4,5 This interaction is reversible, therefore captured proteins are eluted by adjusting pH or by using an imidazole containing buffer. Depending on the composition of the buffer, the nature of the column-bound molecules changes with the coordination sphere of the immobilized metal ions. Thus, IMAC is highly suitable for the extensive purification of His-tagged proteins, achieving purities of up to 95%, in high yields. Chelating ligands covalently attached to solid matrices are very important and are usually multidentate organic ligands that bind to metal ions. The most encountered ligands are tridentate (i.e. IDA, I) and tetradentate (i.e. NTA, II); nevertheless, intensive studies for discovering new ligands that could generate proper matrices for maximizing protein purification yields are present in the literature.^3b,6 Most of these ligands take the form of acyclic or cyclic polyamino polycarboxylic acids, which have been often called bifunctional chelating agents, also useful for a wide range of applications involving biomolecules labelling.⁷ A new type of chelating agents holding numerous coordinating sites, prone to lanthanide coordination and formation of luminescent stable complexes, such as compounds of type III (Fig. 1), has been increasingly developed for MRI applications or for naked eye visualization of labelled proteins.⁸Open in a separate windowFig. 1Structures of various polyamino polycarboxylic chelating agents and structure of compound 1 used in this work.In this context, we describe herein synthesis of a new functional chelating agent 1 (Fig. 1), its ability to form metal complexes and utility for preparation of a new solid matrix to be employed in affinity chromatography and proteins purification. Similar compounds with 1 have been previously investigated as ligands for metal-ion complexes, bearing various substituents in para-position relative to the phenol group and information is readily available regarding stoichiometry, electronic and magnetic properties of complexes with Cu(ii)⁹ and lanthanides.¹⁰Synthesis of the key intermediate 3 (Scheme 1) was achieved in a two steps sequence starting from p-hydroxybenzoic acid by bromomethylation and subsequent alkylation with tert-butyl iminodiacetate, in 50% global yield. Previously reported¹¹ synthetic approaches of such compounds refer to Mannich reactions^11a–c of the corresponding phenols using paraformaldehyde and secondary amines or hydroxymethylation of the phenols followed by bromination with hydrogen bromide.^11d However, in most cases, the Mannich monosubstituted product predominates, while the hydroxymethylation reactions in basic medium tend to provide polymerisation products, especially in presence of electron-withdrawing substituents. Thus, direct bromomethylation in acidic conditions was found to be the best option, due to the cleanliness of the reaction procedure, as it occurred free of by-products (i.e. polymers). N-Alkylation proceeded smoothly and the esters was conveniently obtained. With compound 3 in hand, we performed deprotection of the tert-butyl ester using 25% TFA in dichloromethane to afford compound 1. We monitored the reaction progress by RP-HPLC (ESI, Fig. S1^†) and observed that complete deprotection occurred overnight. Compound 1 is water soluble and the recorded UV-vis spectra indicated an absorption maximum at λ_max = 253 nm (ε = 7591.6 L mol⁻¹ cm⁻¹, ESI, Fig. S2^†).Open in a separate windowScheme 1Synthesis of the compounds: (a) (CH₂O)_n, 48% aq HBr, 60–80 °C, 6 days, 76%; (b) NH(CH₂CO₂tBu)₂, NaHCO₃, MeCN, rt, overnight, 70%; (c) 25% TFA in CH₂Cl₂, overnight, 94%.Ligand 1 was investigated for the complexation properties with various metal ions and we performed a screening among Ni(ii), Zn(ii), Cu(ii), Fe(ii), Yb(iii), Er(iii), Nd(iii) (ESI, Fig. S3^†). The absorption spectra of the mixtures between the ligand and the metal ions suffered a general shifting of the absorption maxima towards higher wavelengths indicating formation of metal-ion complexes.We were particularly interested in formation of the complexes with Zn(ii) and Ni(ii) ions that were studied by UV-vis in order to establish the ligand to metal stoichiometry. Formation of the complex with Zn(ii) was followed by addition of increasing amounts of the metal ions in aqueous solutions of compound 1, in basic medium ensured by triethylamine (TEA) (see ESI, Fig. S4^†). The experiments revealed a 1:2 ligand to metal stoichiometry and the association constants calculated using the on-line tool "supramolecular.org"¹² were K₁₁ = 3.66 × 10⁵ ± 243 M⁻¹ and K₁₂ = 4.19 × 10⁶ ± 233 M⁻¹.The UV-vis absorption spectra of the complex resulted between compound 1 and Ni(ii) ions revealed multiple bands, specific to green complex formation, both in solution (Fig. 2) and solid state (Fig. 4). Thus, the absorption band in the visible region, assigned to phenoxo-to-Ni(ii) ligand-to-metal charge-transfer (LMCT) band has λ_max = 381 nm and the large band between 600–800 nm corresponds to Ni(ii) d–d transitions, with maximum at λ_max = 651 nm.⁹Open in a separate windowFig. 2The absorption spectra of the Ni(ii) complex in aqueous solution.Open in a separate windowFig. 4Solid UV-vis spectra of Ni(ii) complex derived sepharose.UV-vis titration experiments (Fig. 3) of compound 1 with Ni(ii) revealed absorbance modification at λ_max = 282 nm. Data processing (ESI, Fig. S6^†)¹² confirmed a ligand to metal ions stoichiometry of 1:2, yielding high values for the association constant (K₁₁ = 1.33 × 10⁹ ± 1000 M⁻¹ and K₁₂ = 2.7 × 10⁶ ± 127 M⁻¹) only ten times smaller than the value of Ni–NTA complex (K_D = 1.8 × 10⁻¹¹ M).^6g The values of the association constants are consistent with previous data for similar polyamino polycarboxylic acids¹³ with the corresponding metal-ions.Open in a separate windowFig. 3UV-vis spectra during Ni²⁺ titration of compound 1.Once compound 1 characterised, we moved further to prepare a solid matrix able to afford His-tagged proteins purification. For this, we used commercial Thermo Scientific™ CarboxyLink™ Coupling Gel (Immobilized Diaminodipropylamine) which is a crosslinked beaded agarose support functionalised with diaminodipropylamine residues, able to react with carboxyl groups. Attachment of compound 3 was performed using peptide coupling conditions after solvent exchange with DMF (N,N-dimethylformamide), using PYBOP (benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluoro phosphate) as coupling reagent, DIPEA (N,N-diisopropylethylamine) as base and two equivalents of 1 relative to the resin loading. Qualitative check of the coupling was performed using ninhydrin test. i.e. colourless solution of the mixture of resin with ninhydrin, indicating complete block of the amino groups (see ESI^† for details regarding experimental procedure). Loading of Ni(ii) ions was further performed by prior solvent exchange with water and treatment of the resin with lithium hydroxide to ensure deprotection of the tert-butyl esters. Finally, the resin was thoroughly washed with water in order to remove unbounded metal-ions.We analysed the functionalised solid support by UV-vis (Fig. 4), Fourier-transform infrared spectroscopy (FTIR) and RAMAN spectroscopy. Thus, the UV-vis spectrum of a sample containing a mixture of sepharose and Ni(ii) ions displayed the d–d transition band, specific to the Ni(ii) ions, while the UV-vis spectrum of the resin containing the complex had a similar profile with the spectrum of the isolated complex, displaying a band at λ_max = 350 nm and indicating the presence of the nickel complex, due to the band at λ_max = 665 nm (Fig. 4).The FTIR of the Sepharose grafted with 3 showed specific bands assigned to stretching of the carboxyl and amide groups at 1719 and 1640 cm⁻¹, respectively, suggesting successful modification of the sepharose resin. Presence of the Ni(ii) complex of 1 was confirmed by the two bands with maxima at 1602 and 1397 cm⁻¹ that were correlated with the carboxylate asymmetric and symmetric vibrations, concomitant with decrease of the initial carbonyl stretching band; moreover, the difference of 205 cm⁻¹ between the bands of the carboxylates proved coordination of Ni(ii) to ligand 1.¹⁴ Additional bands at 532 and 439 cm⁻¹ were attributed to Ni–N and Ni–O respectively, confirming presence of the metal ion complex.¹⁵ The acquired spectra and FTIR assignment table are presented in ESI (see Fig. S6^†). RAMAN spectra also confirmed formation of the Ni(ii) complex (see Fig. S6^†).Next, the retention capacity of the resin for His-tagged proteins was evaluated. We started testing a pure recombinant protein expressed in E. coli, a frequently used prokaryotic expression system.¹⁶ The protein to be expressed, namely S100 calcium binding protein B (S100B), was fused in the N-terminus with a hexahistidine tag having a molecular mass of 13.8 kDa. An IMAC experiment was performed using equal amounts of the following resins: commercial amino-sepharose (S), sepharose resin derivatized with 1 (S–1), commercial Ni-NTA resin (NiNTA), sepharose derivatized with Ni(ii) complex of 1 (S–1–Ni). Resins were incubated with approximately 400 μg of HisS100B protein. After washing and performing a specific elution with imidazole, bound (B) and unbound (UN) protein fractions were resolved by SDS-PAGE and visualized by Coomassie Blue staining (Fig. 5). Binding of HisS100B was specific for S–1–Ni and NiNTA resins with no visible non-specific binding to the control resins. Binding efficacy was comparable for S–1–Ni and NiNTA since the bound and unbound fractions of HisS100B are similar for the two resins.Open in a separate windowFig. 5Pulldown of recombinant HisSB100. IMAC was performed using 400 μg of HisS100B purified protein on commercial amino-sepharose (S), sepharose resin derivatized with 1 (S–1), commercial Ni–NTA resin (NiNTA), sepharose derivatized with Ni(ii) complex of 1 (S–1–Ni). The eluates (B) and 1/30 of the supernatants (UN) were separated by SDS-PAGE and stained with Coomassie Blue. 60 μg of purified HisS100B were loaded as the reference input.Then, the resin binding efficacy and specificity was tested in a complex lysate of E. coli expressing HisS100B. After protein expression induction, the cell pellet was sonicated in lysis buffer and the soluble protein fraction was further processed. A pulldown assay was further performed for the soluble fraction similarly to the pure protein experiment (Fig. 6). HisS100B interacted minimally with the control resin S–1. Both S–1–Ni and NiNTA pulled down HisS100B, while S–1–Ni interacted non-specifically with more prokaryotic proteins compared to NiNTA. A higher affinity of the interaction between S–1–Ni and histidine or clusters of histidine residues on specific prokaryotic proteins could explain the difference between resins specificity.¹⁷ To assess the stability of the resin we repeated the pull-down for three times without an obvious loss of the binding efficacy as estimated by Coomassie Blue staining of SDS-PAGE gels in Fig. S10.^† The binding capacities of the two resins were determined as described in the ESI.^† The binding capacities of S–1–Ni (409 ± 42 μg ml⁻¹ resin) and NiNTA (412 ± 78 μg ml⁻¹ resin) were not significantly different. Next, we tested the affinity purification of HisRAGE, a protein with a higher molecular mass (40k Da apparent MM). As shown in Fig. S11,^† the S–1–Ni pulled down HisRAGE as efficiently as NiNTA, along with non-specific prokaryotic proteins.Open in a separate windowFig. 6Purification of recombinant HisS100B from E. coli BL21 cell lysate using commercial amino-sepharose (S), sepharose resin derivatized with 1 (S–1), commercial Ni–NTA resin (NiNTA), sepharose derivatized with Ni(ii) complex of 1 (S–1–Ni). 500 μg of clarified lysate was used for HisS100B pull down. Clarified lysate (input), the eluates (B), and 1/30 of the supernatants (UN) were separated by SDS-PAGE and visualized with Coomassie Blue staining.Proteins with higher molecular masses and post-translational modifications (PMTs) are difficult and sometimes impossible to express in prokaryotic expression systems. Thus, eukaryotic expression systems (yeast, insect, mammalian) remain the main choice for producing proteins with high molecular mass, disulphidic bonds, glycosylation or phosphorylation.¹⁸ Mammalian cell lines such as CHO and 293T are used extensively for recombinant protein purification for industrial or academic purposes.¹⁹ IMAC is a popular protein purification technique with reasonable costs, yields and protein purity. However, if secreted proteins are to be purified from mammalian cell culture media, animal serum proteins compete with secreted recombinant proteins during IMAC resin binding.²⁰ Thus, chemically defined, serum-free media are used which are often much more expensive and incompatible with classic production adherent cell lines. Therefore, we decided to test the capacity of S–1–Ni in the context of a soluble glycoprotein secreted in a serum rich cell culture media. We chose Hepatitis C Virus E2 glycoprotein which is a type I transmembrane glycoprotein with 11 glycans.²¹ HCV E2 ectodomain (50 kDa) was expressed in 293T cells cultured in media with 2% or 10% fetal bovine serum (FBS). Following the pulldown procedure, the protein was eluted, resolved in an SDS-PAGE gel and detected by western blot. As shown in Fig. 7, the protein was abundantly expressed in the cell lysate. In the cell culture media, a higher molecular mass species was secreted suggesting glycan processing to complex structures, as previously reported.²²Open in a separate windowFig. 7Purification of recombinant HCV E2s protein. 293T cells were transfected with a plasmid coding for HCV E2 ectodomain (AB154191_E2_C1b, aa 384_656) (S) or pcDNA3.1 empty vector (C). 48 h post-transfection, the clarified media was subjected to pulldown using either NiNTA or S–1–Ni resin. Cell lysates, eluates (B), and 1/30 of the supernatants (UN) were separated by SDS-PAGE and HCV E2s was detected by western blot.Strikingly, S–1–Ni was able to pull down E2s both in presence of 2% and 10% FBS while there was no protein detected in NiNTA eluates. A possible explanation is a difference in the binding affinity of the His tag to the two resins in the context of E2s and high serum protein concentration. As noticed for the prokaryotic proteins purification, the S–1–Ni interacted non-specifically with the bovine serum albumin present in high concentrations in the input media (3.7 mg ml⁻¹) (Fig. S9^†). In these pulldown conditions the specific binding capacity was estimated at 200–300 ng ml⁻¹ of S–1–Ni and 10–20 ng ml⁻¹ of NiNTA for the two serum concentrations. Although further work is required to evaluate the general use of the new resin for protein purification in the presence of high concentrations of serum, our work paves the way for designing new chelators which could enable a cost efficient purification of proteins with complex structural elements produced in mammalian expression systems.     

An aluminium fluorosensor for the early detection of micro-level alcoholate corrosion

The detection of the dry alcoholate corrosion of aluminium is vital to design a corrosion resistive aluminium alloy for the storage and transportation of biofuel (methanol or ethanol). By synthesizing an Al³⁺ fluorescent probe operable in an alcoholic medium, we quantified the alcoholate corrosion in terms of the fluorometrically estimated soluble alkoxide (Al(OR)₃) generation under nitrogen atmosphere. With time, a linear increase in corrosion with specific aluminium dissolution rate constants ∼2.0 and 0.9 μg per day per cm² were estimated for aluminium and Al-7075 alloy, respectively. During open atmosphere monitoring, the adsorbed moisture converted small extent of Al(OR)₃ to the insoluble Al(OH)₃ at the alloy surface which retarded the alcoholate corrosion appreciably.

The monitoring of aluminium alcoholate corrosion using a fluorescent probe operable in alcohol.

Switching over from conventional fossil fuel to biofuel is of current interest owing to the maximum utilization of eco-friendly non-conventional energy.¹ Commercially produced less polluted biofuels such as methanol and ethanol, mixed with fossil fuels have an acceptable performance capacity for the gasoline engine.² Moreover, in comparison to the gasoline, methanol and ethanol have much higher octane rating or compression ratio to resist the knocking for better thermal efficiency.³ Since most of the fuel tanks/pipes are made of aluminium or its alloys owing to its high strength-to-density ratio, the aluminium corrosion due to the formation of alkoxide (alcoholate or dry corrosion) during storage or even transportation of such bio-alcohols may cause leakage in the fuel tanks and in worst cases enough threat is speculated for fire and explosion.⁴ Mechanical overloads, alloy impurities even at elevated temperatures are further contenders for accelerating the alcoholate corrosion.⁵ However, a prolong exposure to the moisture retards the alcoholate corrosion by forming a protective layer of hydrated aluminium oxide in the metallic surface but moisture impurity in the fuel may damage the gasoline engine.⁶ Hence, a maintenance optimization is crucial in critical engineering disasters by detecting alcoholate corrosion as in its nascent state with minimizing the chance of water contamination.^6,7Several electrochemical and mechanical methods have been exploited for decades to propose aluminium alcoholate and other corrosions;⁶ yet the early detection of the alcoholate corrosion is still a challenging task due to the lack of sensitive analytical methods.^6,8 Here, the fluorescence technique may act as a better alternative owing to its simplicity and high sensitivity.⁹ Till date, a large number of fluorescent probes for Al³⁺ have been exploited in the biological or environmental domain,¹⁰ but has never focused on alcoholate corrosion studies. Based on this requirement, we synthesized a fluorescent probe, namely HMBDC ((6Z)-6-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylideneamine)-2H-chromen-2-one), to detect alcoholate corrosion with μg-level detection ability along with its retarding signature in the presence of moisture in a judicious way. Such novel method may lead to an early detection of alcoholate corrosion in a simpler way.The non-fluorescent phenolic Schiff-base molecule containing a coumarin moiety (HMBDC) was prepared by condensing an equimolar mixture of 6-amino coumarin (6-ACO) and 2-hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde (HHMB) in dry ethanol (Scheme 1 and Fig. S1^†) (c.f. ESI^† for details). Among various organic solvents, the interaction of HMBDC with Al³⁺ was observed only in the alcoholic medium according to the UV-vis studies (Fig. S2^†). In methanol, the absorption intensity at ∼353 nm for HMBDC (5 μM) decreased gradually with the continuous addition of Al(NO₃)₃ until saturated at ∼8-equiv., giving rise to a new peak at ∼406 nm, where an isosbestic point at ∼384 nm assures the formation of Al³⁺/HMBDC complex (1) (Fig. 1A). Upon optimization of the complex formation affinity in various ethanol/methanol mixed media, highest reactivity with the lowest saturated Al³⁺ concentration (∼5 equiv.) compared to that obtained in pure methanol was observed in a 4 : 1 methanol/ethanol-mixed medium (Fig. S3^†). Most probably, more effective H-bonding interaction of the dimeric ethanol/methanol¹¹ with 1 induces greater complex (1) stability, although the complex formation reactivity was much less in pure ethanol compared to the methanol medium (Fig. 1 and S2^†).Open in a separate windowScheme 1Synthesis of HMBDC and its complexation with Al³⁺ in an alcohol solvent.Open in a separate windowFig. 1(A) UV-vis absorption and (B) fluorescence spectra of HMBDC (5 μM) in the presence (red) and absence (black) of increasing concentration of Al(NO₃)₃ (0–40 μM) in anhydrous methanol at 25 °C. The intensity changes with increasing Al³⁺ concentrations are indicated by arrows. (C) Al³⁺ concentration dependent relative increase in the fluorescence intensity with respect to its absence in methanol (red) or methanol/ethanol (4 : 1) (blue). (D) Fluorescence intensity ratios in the presence and absence of various ions or mixture of ions in the mixed solvent (25 μM each; blue) or methanol (40 μM each; other colors) or are shown by bar-diagram.In spite of the stronger H-bonding interactions of 1 with water compared to the methanol or ethanol, a complete dissociation of 1 in the presence of 20% (v/v) water in methanol (Fig. S4^†) suggests that, in addition to the solvent assisted H-bonded structural stability of 1, alcohol molecule may also participate in the coordination with Al³⁺ to form 1. Indeed, the possible methanol coordination is reflected in the ESI-MS⁺ analysis (Fig. S5B^†). In addition, the Job''s plots in the absorption studies showed that the HMBDC formed 1 : 1 stoichiometric complex with Al³⁺ (Fig. S6^†). To elucidate the probable structure of 1, we carried out the DFT-based theoretical calculation by considering the 1 : 1 stoichiometric Al³⁺/HMBDC complex with or without methanol coordination. A stable structure of 1 was obtained when the oxygen atom of the methanol molecule coordinates with Al³⁺ and other two coordination sites of Al³⁺ are occupied by the phenolic-oxygen and imine-nitrogen of HMBDC (Scheme 1, Fig. 2 and S7^†). Facile coordination of those hard donor sites of HMBDC towards harder Al³⁺ is susceptible towards alcohol assisted stabilization of 1. The UV-vis absorbance at ∼402 nm for 1 computed from the time-dependent DFT (TD-DFT) calculations in methanol medium, where the HOMO (90) → LUMO (92) excitation nicely matched with the experimental absorbance at ∼406 nm (Fig. 1 and ​and2).2). However, monitoring of the ¹H-NMR peak characterized for aldimine proton is a useful strategy to identify the bonding of the imine-N to Al³⁺.¹² We observed that the aldimine proton peak intensity for HMBDC in CD₃OD was quenched to a great extent with a considerable down-field shift from 8.80 to 8.88 ppm in the presence of Al³⁺ (Fig. S8^†); the down-field shift is expected owing to the imine-N and Al³⁺ coordination, but intensity quenching does not follow the previous trend in the aprotic polar medium.¹² The generation of a partial positive charge at the N-centre upon its binding with the Al³⁺ may enhance the acidity of the aldimine proton to become labile for participating in the H/D exchange in a protic medium (CD₃OD), as reported previously for other allied systems.¹³ These results strongly suggest the imine-N and Al³⁺ bonding in 1. On the other hand, Al³⁺ induced large decrease in the IR intensity at ∼3300 cm⁻¹ for phenolic-OH also supports the phenoxide coordination (Fig. S9^†).Open in a separate windowFig. 2Frontier molecular orbital profiles including various UV-vis absorption parameters of HMBDC (left panel) and HMBDC/Al³⁺ complex (right panel) based on TD-DFT (B3LYP/6-31G(d)).The electronic distribution in the molecular orbital diagram (MO) of the HMBDC evaluated from the DFT calculation showed an intra-molecular photo-induced electron transfer (PET) from coumarin to the HHMB moiety, which makes the HMBDC non-fluorescent (Fig. 2). Al³⁺ induced an instantaneous increase in the fluorescence intensity for HMBDC (5 μM) in the alcoholic medium (methanol/ethanol or their mixture) due to the formation of 1 (Fig. 1B and S10^†). A gradual fluorescence intensity increase at ∼506 nm (λ_ex = 406 nm) of ∼30-fold for 8 equiv. of Al³⁺ and ∼40-fold for 5 equiv. of Al³⁺ was observed in the methanol and 4 : 1 (v/v) methanol/ethanol medium, respectively (Fig. 1 and S10B^†). According to the HOMO and LUMO electronic distributions for 1 in the DFT studies, the PET process in HMBDC was highly restricted upon its binding with Al³⁺ in 1, causing for the large increase in the fluorescence intensity (Fig. 2). However, the better fluorescence response (lower intensity-saturated Al³⁺ concentration and larger intensity increase) in the mixed medium than pure methanol may be associated with greater stability of 1, as described in the previous section (Fig. S2^†). The fluorescence intensity increase remains invariant using other soluble Al(iii)-salts (Fig. 1D and S11^†), which eliminates the role of counter anions for the increasing intensity. To ascertain the Al³⁺ selectivity, we performed similar fluorescence studies with other potentially interfering cations but failed to produce any noticeable fluorescence (Fig. 1D and S12^†). However, a linear intensity increase with the increase in the concentration of Al³⁺ up to 6 equiv. in methanol and 4 equiv. in the 4 : 1 methanol/ethanol mixed medium can be useful for a ratiometric detection of unknown concentration of Al³⁺ (Fig. 1C), where the limit of detection¹⁴ (LOD) of Al³⁺ with HMBDC in the methanol medium was found to be ∼0.5 μM (c.f. details in ESI^†). Most importantly, HMBDC recognized Al³⁺ selectively from the mixture of various other cations, and also in presence of other soluble Al(iii) salts, particularly, aluminium alkoxide (ethoxide) with similar accuracy (Fig. 1D and S12^†). Therefore, the Al(iii) sensing ability for an alcoholate corrosion with an aluminium alloy must not be perturbed due to the interference of other leached cations.The dry alcoholate corrosion of aluminium or its alloy while forming soluble alkoxide (Al(OR)₃) can be detected upon incubation in an anhydrous alcoholic medium. However, under a condition of prolonged incubation, the contamination of trace amounts of moisture may also trigger the conversion of Al(OR)₃ to Al(OH)₃, followed by the hydrated alumina (Al₂O₃·xH₂O) coating on the metallic surface.⁶ The formation of hydrated alumina can also be possible via the decomposition of Al(OR)₃.⁶ To characterize the alcoholate corrosion as an exclusive process to the maximum limit, we minimized those wet-processes by allowing the corrosion under inert conditions. A previously grazed aluminium-sheet (dimension ∼3.5 × 1.5 × 0.2 cm³; surface area ∼12.5 cm²) was incubated for 18 days in 100 mL anhydrous methanol or methanol/ethanol (4 : 1) mixed solvent under nitrogen atmosphere by purging nitrogen every 24 h, where the small change in the solution volume if required was adjusted by injecting an appropriate amount of the nitrogen-saturated anhydrous solvent. The amount of Al(OR)₃ (R = -Me, -Et) generated in the medium was estimated by monitoring the HMBDC (5 μM) fluorescence. After 10-fold dilution of the medium with the parent solvent, the amount of Al(OR)₃ formed or the alcoholate corrosion was estimated in every 3 days interval according to the amount of Al³⁺ obtained from the time-dependent fluorescence responses (Fig. S13^†) as per the linear calibration plots in Fig. 1C multiplied by the dilution factor. A linear increase in the normalized fluorescence intensity from ∼3.5 to 16.8 and ∼7.3 to 36.1 was observed with an increase in the incubation time period from 3 day to 18 day for methanol and methanol/ethanol (4 : 1) media (Fig. 3A and S13^†), respectively, which correspond to the linear increase in the Al³⁺ amount in the medium from ∼3.2 to 16.6 μmol for either solvents (Fig. 3C). Indeed, the weight-loss of ∼0.47 mg i.e., ∼17.5 μmol was found to be closely similar with that of the increase in Al³⁺, revealing that not only the dry corrosion leads to the generation of Al³⁺ (Al(OR)₃) as the only product, but also HMBDC is highly effective for an accurate estimation of the alcoholate corrosion. In addition, the nice correlation between the weight-loss and Al(OR)₃ amount also reveals that the decomposition of alkoxide into insoluble alumina is negligibly small during the whole corrosion time-course.Open in a separate windowFig. 3(A and B) Extent of the fluorescence intensity increase due to corrosion-induced leached Al³⁺ (F(x)/F(0)) of HMBDC (5 μM) and (C and D) amount of Al³⁺ in the corrosion medium according to fluorescence response are plotted with various incubation times of pure aluminium sheet or its alloy (Al-7075) in different mediums/atmosphere conditions: nitrogen atmosphere in methanol (red) and methanol/ethanol (4 : 1) (blue); open atmosphere in methanol (green) and methanol/ethanol (4 : 1) (purple). The data at nitrogen conditions are only fitted linearly. (A and B) The fluorescence intensity of HMBDC (5 μM) were monitored after the 10-fold dilution of the corrosion medium with the same solvent. (C and D) The amount of Al³⁺ estimated as the amount obtained from the normalized intensity with comparing the linear plots in Fig. 1C multiplied by the dilution factor 10. The actual amount of alcoholate corrosions for the mixed medium under open atmosphere are depicted by solid circle (purple).However, under open atmospheric conditions maintained by air purging (average relative humidity ∼70%; average temperature 28 °C) in every 24 h interval while maintaining other similar experimental conditions and analysis protocol, the specific corrosion rate (∼2.0 μg per day per cm²) up to 12 days, was found to be closely similar to that detected under the nitrogen atmosphere (Fig. 3C and S13^†). The results also indicate that the early stage of the alcoholate corrosion process (at least up to 12 days) for pure aluminium is not affected significantly by the atmospheric moisture content, although the final corrosion amount after 18 days incubation in normal atmosphere was slightly lower (∼84%) for the mixed medium compared to that obtained for pure methanol (Fig. 3C). The decrease in the Al(OR)₃ amount can be affected by two processes: (a) Al(OR)₃ to insoluble Al(OH)₃ conversion due to the adsorbed moisture; (b) actual retardation of the corrosion rate due to the surface deposition of Al(OH)₃. The extent of the conversion of Al(OR)₃ to Al(OH)₃ in the corrosion medium under the open air condition can be assessed by estimating the fluorescence intensity at every 3 day time interval in the absence of aluminium sheet (from day-3 to day-18) with the addition of same amount of Al(OEt)₃ (3.2, 5.7, 8.0, 11.7, 14.2 and 16.4 μmol (final added amount) at day 0 (beginning of day 1), 3, 9, 12 and 15, respectively, in 100 mL mixed medium) as that of the alkoxide amount detected due to the corrosion under nitrogen condition (Fig. S14^†). In comparison to the actual added Al(OEt)₃, any decrease in the Al(OEt)₃ amount upon such incubation should be added with the corrosion induced formation of Al(OR)₃ amount under nitrogen condition for respective time interval to obtain the actual alcoholate corrosion. The actual corrosion was found to be slightly higher than that estimated from the corrosion-induced Al(OR)₃ formation (Fig. 3C, solid symbol). According to the LOD of Al³⁺, the detection of the alcoholate corrosion amount as minimum as ∼0.1 μg mL⁻¹ can be possible by monitoring the fluorescence response of HMBDC.Alcoholate corrosion in a widely used aluminium alloy, Al-7075 (composition: Al, 90%; Zn, 5.5%; Mg, 2.5%; Cu, 1.5 and Si, 0.5%) was also studied. The previously grazed alloy sheet with same dimension and surface area as that of the pure aluminium sheet was incubated in 100 mL anhydrous methanol or 4 : 1 methanol/ethanol under nitrogen as well as normal atmospheric conditions. The amount of the alcoholate corrosion in every 3 days interval up to 30 days was estimated by evaluating the fluorescence response of HMBDC (Fig. 3B and S15^†). In comparison to the pure aluminium sheet, the increase in corrosion from ∼1.5 to 4.0 μmol evaluated from the increase in the normalized fluorescence intensity (1.65 to 5.90 in methanol; 2.64 to 10.40 in methanol/ethanol (4 : 1) mixture) with the increase in the incubation time from day-3 to day-12 follows a similar linear relation regardless of the solvent compositions and atmospheric conditions (Fig. 3B and D), while the intrinsic rate of corrosion ∼0.95 μg per day per cm² was more than 2-fold slower (Fig. 3C and D). The lower rate constant value for the alloy compared to pure aluminium indicates that the contamination of other metals in the alloy resists the early stage alcoholate corrosion process. However, under normal atmospheric condition, the corrosion amount vs. time relation deviates from the linearity after 12 days. Importantly, after 30 days of incubation, a large reduction in the Al(OR)₃ amount from ∼11.38 to 6.64 μmol was estimated for the mixed medium, but the change was only from ∼13.20 to ∼12.33 μmol for pure methanol (Fig. 3D). By determining the hydration-induced conversion amount of Al(OR)₃ to Al(OH)₃ according to the procedure, as described before (Fig. S16^†), the actual alcoholate corrosion was found to decrease from ∼11.38 to 7.70 μmol by changing the condition from nitrogen to open atmosphere after 30 days (Fig. 3D, solid symbol). Our study reveals that in comparison to pure methanol, the formation of Al(OH)₃ under open atmospheric condition retards the alcoholate corrosion largely due to the presence of more hygroscopic ethanol.¹⁵ The deposition of Al₂O₃·xH₂O onto the alloy-surface is responsible for resisting the further alcoholate corrosion⁶ (Fig. 3D). In fact, the generation of more surface pits owing to the higher extent of the alcoholate corrosion in methanol over the mixed medium was also detected by naked eye (Fig. S17^†). The surface morphology in the SEM studies showed that the alloy surface was little bit smoother after the corrosion in the mixed medium (Fig. S18^†), justifying our proposition for the surface deposition of Al₂O₃·xH₂O. On the other hand, cyclic voltammetric studies in the corrosion medium exposed to normal atmospheric conditions identified an irreversible cathodic peak at ∼−0.7 V due to the formation of insoluble Al(OH)₃ in addition to the conversion from Al to Al³⁺, but such irreversible peak was not observed for the medium exposed to nitrogen (Fig. S19^†). Moreover, the formation of white gelatinous precipitate of Al(OH)₃ in the mixed medium was clearly visible by naked eye under normal atmospheric conditions (Fig. S17B^†). All those results strongly support that the initiation of the wet-process by forming Al(OH)₃ inhibits the alcoholate corrosion rate.In conclusion, a phenolic Schiff-base consisting of a coumarin unit as a fluorescent sensor for Al³⁺ operable only in the alcoholic medium is synthesized to monitor dry alcoholate corrosion. The photo-induced electron transfer process in the probe molecule exhibits Al³⁺ induced large increase of fluorescence intensity, lifted by its complexation with Al³⁺, which was further stabilized by the coordination and H-bonding interaction with the solvent molecule. The alcohol specific complex formation and subsequent fluorescence generation was suitably tuned to monitor the alcoholate corrosion by fluorometrically estimating aluminium alkoxide formation with a sensitivity of ∼10 μg L⁻¹. However, the simultaneous participation of small extent of the wet-process (Al(OR)₃ to Al(OH)₃ conversion) and its deposition in metal surface, particularly for the alloy, inhibits the dry alcoholate corrosion. The alloy specific detection of the early stage alcoholate corrosion is in progress to obtain suitable material useful as a biofuel container.     

Experimental review of PEI electrodeposition onto copper substrates for insulation of complex geometries

Polyetherimide (PEI) was used for coating copper substrates via electrophoretic deposition (EPD) for electrical insulation. Different substrate preparation and electrical field application techniques were compared, demonstrating that the use of a pulsed voltage of 20 V allowed for the best formation of insulating coatings in the 2–6 μm thickness range. The results indicate that pulsed EPD is the best technique to effectively coat conductive substrates with superior surface finish coatings that could pass a dielectric withstand test at 10 kV mm⁻¹, which is of importance within the EV automotive industry.

Electrophoretic deposition relying on electrodeposition of charged polymers via modulated electrical fields is reported. Superior surface finishes that could pass a dielectric withstand test at 10 kV mm⁻¹ were obtained for pulsed potentials at 20 V.

Electrophoretic deposition (EPD) is a useful and scalable technique for the production of electrically insulating coatings on conductive substrates having irregular shapes, while at the same time allowing a precise control of the coating thickness.^1,2 EPD relies on the preparation of an aqueous colloidal suspensions based on charged polymers that are deposited as particles onto an electrically conductive substrate by an induced electric field.³ In contrast to other electrical field induced deposition methods such as electroplating, EPD does not require the suspension to have high electrical conductivity, allowing for small power losses due to the total current being used for the coating formation.⁴ Advantages also include that more complex shapes/parts can be coated due to the emulsion being able to reach into difficult geometries, consequently covering areas in hard to reach segments that otherwise would remain uncoated in a dip-coating procedure.^1–4To promote high electrophoretic movement of the polymer particles and homogenous formation of the polymer coatings, both high polymer charge and high polymer colloidal suspension concentration are required in the coating system.^5–7 These factors are considered the most important in the selection of polymer for the EPD systems.⁴ The control over the coating quality has shown, however, to rely on several more parameters, including deposition time, electric field strength, suspension viscosity, applied voltage, etc.⁷ To enhance the polymer charge and solubility, functional groups have been added to the polymer structure and/or a mixture of solvents have been utilized, respectively.^8,9 Furthermore, the thermal stability of the selected polymers needs to be considered to withstand the temperature build-up in the conductive substrate (as an applied coating) during regular operations, e.g., in the automotive parts. Here, polyether ether ketone (PEEK) and polyetherimide (PEI) have shown to be useful in terms of producing homogenous coatings and adequately allowing for thermal post-treatments performed. These post treatments are made to ensure uniform concealing and reformation of the insulating polymer characteristics, i.e., post-coating deposition.^10–12 Among the two, the PEI is the most interesting as it is the least expensive.The EPD of PEI requires a quaternization reaction resulting in protonated PEI (qPEI) for the formation of a charged polymer emulsion suspension and subsequent effective electrophoretic deposition of the PEI onto the conductive substrate.^13,14 The PEI''s imide group undergoes a ring-opening reaction by the addition of 1-methylpiperazine, leading to the formation of an amide group in the PEI structure (Fig. 1a and S1^†). The process is followed by acid protonation of lactic acid, and charge neutralization of the amine group, which in turn allow for the preparation of the polymer suspensions (emulsion) required for the EPD process (Fig. 1a). The inset in Fig. 1a shows the resulting polymer emulsion in the round bottom flask. After the EPD emulsion coating was carried out on the substrate, the coated substrate was treated at high temperature to allow the re-imidization of the PEI, producing the insulating coating (Fig. 1a). A challenge in this context is the possible H₂ gas formation on the cathode electrode, from one of the half reactions in the electrolysis of water, and occasional formation of bubbles/voids in the formed PEI coatings.¹⁵ Therefore, alternating currents (AC) and pulsed direct currents (DC) were evaluated as process alternatives to improve the coating properties.¹⁶ However, to our best knowledge, enlightening comparisons between different voltages, current set-ups, surface substrate preparations, and charged polymer suspension formation have previously not been cross-examined, and compared, in terms of finding optimal conditions for generating useful EPD conditions and formation of uniform insulating coatings.Open in a separate windowFig. 1The electrophoretic deposition process from PEI to qPEI followed by re-imidization to PEI (a), and FTIR transmission spectra of the different samples (b). The coated PEI material was scrapped off prior to the FTIR analysis.In this work, copper substrates with different surface treatments (polishing and sandblasting, Fig. S2^†) and acidic cleaning (HCl and HNO₃), were used for the EPD of qPEI under different potentials (2–20 V), see Fig. 2 and S3.^† The selected potentials were obtained from screening useful conditions with associated electrical field strengths, as well as motivated from previous works.^17,18 The qPEI suspension formation was evaluated in terms of lactic acid/water content and mixing temperature of the solution after the addition of water (Table S1^†). The coated copper substrates with qPEI were always identically heat treated to re-imidize the qPEI forming the electrically insulating PEI layer (Fig. S4 and S5^†). To prepare the qPEI, PEI (20 g) was dissolved in an NMP/acetophenone solution and 1-methylpiperazine was added. The solution was treated at 110 °C (2 h), which promoted the ring-opening of the imide ring in the PEI, from the solvated state (sPEI) to the fully protonated state (qPEI) (Fig. 1a and S1^†). The changes in the FTIR spectra for the corresponding sPEI and qPEI is shown in Fig. 1b. The PEI stretching bands at 1780 and 1720 cm⁻¹ (imide carbonyl of the five-member ring) and 1360 cm⁻¹ (carbon–nitrogen vibration of imide five-member ring) were considerably reduced in the sPEI sample, and almost absent in the qPEI sample. Additionally, the strong band observed at 1670 cm⁻¹ (–C O vibration from amide) demonstrated the successful formation of qPEI (Fig. 1b).¹⁴ The band at 1360 cm⁻¹ and the broad shoulder observed for the band at 1670 cm⁻¹ was ascribed to the characteristics bands of acetophenone (Fig. S6^†). The presence of the bands at 1780, 1720 and 1548 cm⁻¹ confirmed the re-imidation of the qPEI forming PEI coating, see Fig. 1b.Open in a separate windowFig. 2Current density decay on the copper substrate during the EPD process using different constant voltages and substrate pretreatments (a), compassion between constant and pulse voltage 20 V (b), and initial current density response when qPEI suspensions of different storage time were used (c). The result in “c” are the average of triplicates and the error bars the standard deviation.The formation of a qPEI polymer suspension was strongly relying on the combined composition of lactic acid, acetophenone, and water, as well as the suspension temperature. The suspensions formed by 0.35 wt% lactic acid, 79.65 wt% acetophenone, and 20 wt% water, mixing temperature = 90 °C, (Suspension 1, Table S1^†), resulted in solid lumps being formed at the bottom of the reactor (Fig. S7a^†). Increasing the water content to 46 wt% on the expense of the combined amount of lactic acid and acetophenone (Suspension 2, Table S1^†) resulted in a more homogenous suspension without apparent particle agglomeration at room temperature if rapid stirring was applied (Fig. S7b^†). An increase of the lactic acid content to 1.3 wt% and water content to 67.5 wt% (Suspension 3, Table S1^†) led to an unstable suspension that immediately phase separated, unless the suspension was stirred rapidly using mechanical stirring (Fig. S7c and d^†). To stabilize the emulsion, the mixing temperature was decreased to 65 °C and the emulsion turned into a milky, highly viscous liquid (Suspension 4). At lower temperatures (23 °C or lower), a milky suspension with lower viscosity was formed (Suspension 5) that was stable for up to 48 h before any phase separation was observed (Fig. S8^†). The pH of this optimized suspension was 4.6 and showed an electrical conductivity of 126 mS m⁻¹. The addition of water to the qPEI was also found necessary to carry out as slow as possible to avoid excessive foam formation in the making of the optimized suspension, see further Fig. S9.^†The EPD process on the copper substrate using Suspensions 1 and 2 did not produce a coating layer on the substrate. Suspension 1 yielded a suspension with lumps formed whereas Suspension 2 appeared more homogenous (Table S1^†) but no coating was formed, possibly due to limited ionic movement, which is a key factor/requirement for producing homogenous EPD coatings.²Trials using Suspensions 3 and 4 for the EPD process revealed that an increased lactic acid content facilitated the formation of the coating, probably due to improved ionic movement, however none of these solutions produced homogenous coatings. Suspension 5 resulted in evenly covered copper substrates (Fig. S5^†). The pH of the qPEI Suspension 3 was 8.6 ± 0.1 while for Suspension 5 was 4.6 ± 0.1. The difference in the pH for the aforementioned suspensions (both having the same lactic acid/water composition, Table S1^†) could result from different zeta potential values, where very low zeta potential values have been previously showed to have a negative effect on the ionic stability/mobility in colloidal dispersions.² Suspension 5 was therefore used for the further EPD of the copper substrates due to its low viscosity, low mixing temperature, and ability to form homogenous qPEI coatings on the substrate (Table S1 and Fig. S5^†). Fig. 2a shows the current density decay of the qPEI suspension during the EPD process for representative samples. The decrease in conductivity shown in Fig. 2a represents the coverage of the copper substrate (electrode) with the insulating qPEI layer. It should be noted that the decrease in current density was not ascribed to a depletion of qPEI in the solution/suspension. The same suspension could be used several times without an evident decrease in the initial current density. Fig. 2a shows that a sandblasted (Sbl) substrate coated at 7 V (constant voltage) resulted in double initial current density compared to the non-sandblasted substrate, independently of the pre-acid treatment used (7 V HCl Sbl – 7 V HNO₃ Sbl vs. 7 V HCl – 7 V HNO₃, respectively). The sandblasting process increased the surface area of the substrate, which was suggested to have favoured a higher exposure of the substrate in the qPEI suspension. Despite the increase in surface for the sandblasted samples, the current density equilibrium was always reached within less than 15 s, similarly to the non-sandblasted samples (Fig. 2a). The lower voltages in Fig. 2a were not further explored due to inefficient formation of the coatings and lack of electric field gradient in the suspension. A higher voltage of 20 V was instead considered but due to the unwanted half reaction, the pulsed voltage setting was explored. Although the use of 20 V pulsed voltage showed similar initial current density, the pulsed case presented a lower current density decay as compared to the constant 20 V settings, and the current density equilibrium was reached beyond 30 s for the pulsed sample (Fig. 2b). Fig. 2c shows the effect of keeping the qPEI suspension stored for extensive time (2 weeks) on the initial current density (prior to initiating the EPD process). The observed decrease in the initial current density for both 7 V and 20 V EPD treatment suggests that the number of charged particles in the suspensions decreased with longer storage times, which was a consequence of the qPEI emulsion aggregation. Accordingly, the initial current density obtained after storing the suspension for 2 weeks and using constant 20 V was similar to 7 V when used fresh (ca. 0.012 mA cm⁻², Fig. 2c). The result show that for a production of qPEI substrate coating by EPD, the quality/thickness of the polymer coating is affected by the qPEI suspension storage time, but this may not be problematic as long as the voltage of the EPD is used to compensate for the reduced total charge of the emulsion. Fig. 3a shows the result of an unsuccessful EPD coating when using the heterogeneous Suspension 3 (Table S1 and Fig. S7c, d^†). The uneven coverage was revealed in the SEM micrographs, and the oxidation of the copper during the thermal treatment (re-imidization step) is visually observed in the image as a yellow/green coat (Fig. 3a). The sandblasting of the copper substrate in combination with coating from Suspension 5 (7 V HCl Sbl) is shown in Fig. 3b. The micrograph of the surface shows a homogenous and complete PEI coverage. No defects from the thermal treatment of the qPEI coated layer (to produce PEI by re-imidization, Fig. 1a) were observed (Fig. 3b). It is noteworthy that the PEI layer coated the rough sandblasted copper substrate perfectly around the periphery of the copper substrate, demonstrating the ability of the EPD process to evenly coat substrates of complex profiles.Open in a separate windowFig. 3Copper substrate after the EPD process and thermal treatment (qPEI re-imidization) and SEM micrographs of the PEI coatings of samples coated (constant voltage) at 7 V HCl – Suspension 3 (a), 7 V HCl Sbl (b), 7 V HCl (c), and 7 V HCl coating layer after being scratched with a scalpel (d). The coatings in (b) and (c) firmly attached to the copper substrates. Fig. 3c shows a copper substrate that was coated identically as Fig. 3b with Suspension 5, but in absence of sandblasting (7 V HCl). The microscopy revealed the smooth and evenly coated substrate obtained, with copper imperfections revealed through the coating (see photo Fig. 3c, left). A micrograph of the same coated 7 V HCl sample display how a scratch on the surface (with a scalpel after the coating procedure) appeared (Fig. 3d, left). The PEI coating layer had a thickness of about 3–4 μm on the copper surface and the coatings was revealed as firmly attached to the copper substrates, as seen from the encircled area shown with higher resolution imaging (Fig. 3d, right). Nano-sized cracks are observed on the coating close to the cut edge (Fig. 3d, right), which resulted from the plastic deformation of the PEI film coating. The results demonstrate the importance of using a homogenous suspension for obtaining well-distributed polymer and properly coated conductive substrates.A typical defect observed in the EPD coated samples with the post-thermal treatment converting the qPEI to PEI (re-imidized) was the formation of bubbles in the coating film, see Fig. 4a. The effect was ascribed to volatiles formed at the surface of the copper as a consequence of hydrolysis occurring during the EPD process, leaving remains of gas (H₂/O₂) and/or hydroxyl groups that during the thermal treatment allowed condensation of water and resulted in poor consolidation of the coating.^15,19 The formation of these bubbles was observed for all the samples after the drying of the coated substrate when using constant voltage for the EPD (60 °C oven for 17 h). On the contrary, the use of pulsed voltage (20 V) favoured the preparation of a smooth coatings on the substrate under the same drying conditions (60 °C oven for 17 h, Fig. 4b). Here, the presence of defects was limited to the edge of the sample (Fig. 4b). It is suggested that the pulsed deposition limited the hydrolysis reactions in the qPEI suspension due to the overall shorter time with applied potential, reducing the formation of hydrolysed fractions trapped between the coating and the substrate.^17,18,20 However, more extensive drying of the coated copper substrate (60 °C oven for 44 h), also resulted in smooth coatings with no apparent defects, both for the constant 20 V and pulsed 20 V depositions (Fig. 4c and d). The longer drying times accordingly favoured the evaporation of the formed hydrolysed species and facilitated the release of entrapped volatiles. In industrial production, long drying times often represents excessive energy costs. The pulsed DC voltage is therefore suggested as a useful condition for generating smoothly coated substrates in short drying times. A stepwise build-up of the coating also enables otherwise entrapped volatiles to dissipate between each pulse when voltage is absent, resulting in a more homogenous morphology. The cross-section of the coated substrates (prepared via constant or pulsed voltage deposition) are shown in Fig. 4c and d, respectively. The EDS mapping of the coatings display the carbon base of the polymer, whereas the thickness could be established to around 4–6 μm.Open in a separate windowFig. 4Shows the copper substrate coated without (a) and with (b) pulsed EPD conditions at 20 V, after for 17 hours of thermal post treatment at 250 °C. Images (c) and (d) show the same substrates after 44 hours 250 °C heat treatment, with accompanying SEM and EDS mapping insets; without pulsed 20 V deposition and with pulsed 20 V deposition, respectively. (e) Schematic representation of the electrical insulation resistance test setup. Note: it is not drawn to scale, as the coating is about 1/1000 the thickness compared to the size of electrode.The DSC and TGA analysis performed on the removed PEI from the copper substrates showed that no significant difference in thermal properties resulted from carrying out the EPD process, i.e. when comparing the thermal characteristics of the virgin PEI before the quaternization reaction and the final PEI from the coating (Fig. S10a and b^†). A slight decrease in the degradation temperature for the PEI retrieved from the coating layer as compared to the raw PEI (528 °C and 541 °C, respectively, Fig. S11 and S12^†) was however observed. Although this could indicate some molecular changes in the PEI polymer on the coating and/or traces of low molecular weight fractions such as lactic acid (Fig. S12a and b^†), these temperatures are well above the service temperature of electrically conductive copper cables (even considering heat build-up).As an example of the insulating characteristics of the PEI coating, the electrical insulation of the coated and re-imidized samples, i.e. 7 V HCl sample (constant voltage), were evaluated using an insulation resistance test at a temperature of 20 °C (see Fig. 4e). The top electrode had a polished surface with a radius of 2 mm and the electrode was kept at a positive potential of 50 V with reference to the negative sample. For automotive applications, this voltage level may fit for VCA-systems (below 60 VDC).²¹ Considering that the coating thickness was on average ca. 5 μm, these conditions yielded a substantial electric stress of 10 kV mm⁻¹ during the test. A Keithley 2450 source-measure-unit (SMU) was used to apply the voltage and to measure the corresponding current and resistance. Non-coated copper substrates showed a resistance of less than 0.1 ohm, essentially a short circuit (the SMU was current limited). The coated samples on the other hand showed an insulation resistance greater than 10⁹ ohm, illustrating the effectiveness of the EDP method and the re-imidization process in forming an insulating polymeric coating.