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.     

Asymmetric total synthesis of four bioactive lignans using donor–acceptor cyclopropanes and bioassay of (−)- and (+)-niranthin against hepatitis B and influenza viruses

The asymmetric total synthesis of four lignans, dimethylmatairesinol, matairesinol, (−)-niranthin, and (+)-niranthin has been achieved using reductive ring-opening of cyclopropanes. Moreover, we performed bioassays of the synthesized (+)- and (−)-niranthins using hepatitis B and influenza viruses, which revealed the relationship between the enantiomeric structure and the anti-viral activity of niranthin.

The total synthesis of four lignans including (−)- and (+)-niranthin has been achieved utilizing cyclopropanes. Based on bioassays of the (+)- and (−)-niranthins using HBV and IFV, we speculated the bioactive site of niranthin against HBV and IFV.

Lignans are attracting considerable attention due to their widespread distribution in plants and their varied bioactivity.^1–5 For example, matairesinol,² dimethylmatairesinol,³ yatein,⁴ and niranthin⁵ are found in nature and exhibit e.g., cytotoxicity,^2b,d,3b,4b anti-bacterial,^2c anti-allergic,^3c anti-viral,^4d,5b,e anti-leishmanial,^5d and strong insect-feeding-deterrent activity.^4c Among these compounds, anti-viral compounds have received significant attention owing to the worldwide pandemic of coronavirus disease 2019 (COVID-19). Although niranthin exhibits anti-viral activity toward the hepatitis B virus (HBV),^5b,e the enantiomeric SAR (structure–activity relationship) for the anti-viral activity of niranthin has not been revealed so far. To examine the SAR for a pair of enantiomers, an independent asymmetric synthesis of both enantiomers is necessary. However, the alternative synthesis of (−)- or (+)-niranthin has not been reported.⁶ During our recent studies on the transformation of cyclopropanes,⁷ we have reported a reductive ring-opening of enantioenriched donor–acceptor (D–A) cyclopropanes and its application to an asymmetric total synthesis of yatein.⁷ⁱ As a further extension of this synthetic method, we disclose here the asymmetric total synthesis of (−)-dimethylmatairesinol, (−)-matairesinol, (+)-niranthin, and (−)-niranthin. Moreover, the results of bioassays using (+)-niranthin and (−)-niranthin against HVB and influenza virus (IFV) are described (Scheme 1).Open in a separate windowScheme 1Some examples of bioactive dibenzyl lignans. Scheme 2 outlines the enantioselective synthesis of optically active lactones 5a and 5b. Following our previous report,⁷ⁱ we attempted to synthesize the enantio-enriched bicyclic lactones 4a and 4b.Open in a separate windowScheme 2Enantioselective synthesis of key intermediates 5a and 5b.Initially, the cyclopropanation of enal 1 with dimethyl α-bromomalonate 2 using the Hayashi–Jørgensen catalyst afforded the desired optically active cyclopropylaldehydes 3a and 3b in good to high yield with high ee.^{7c,e,h–j,8,9} The reduction of the aldehydes to alcohols and subsequent lactonization with p-TsOH afforded lactones 4a and 4b in high yield with high ee. The optical purity of lactones 4a and 4b were determined using HPLC analyses on a chiral column, and the ee values of the enantioselective cyclopropanations were estimated based on these HPLC analyses. Next, treatment of bicyclic lactones 4a and 4b with hydrogen in the presence of a catalytic amount of Pd–C in AcOEt at 0 °C resulted in a regioselective reductive ring-opening to furnish benzyloxylactones 5a and 5b in good to high yield with high dr and high ee. In the hydrolysis step, debenzylation of the benzyloxyaryl group did not occur under these mild conditions, i.e., in AcOEt at 0 °C.⁷ⁱFor the α-benzylation of 5a and 5b to afford 6a–c, the corresponding substituted benzylhalides were necessary. 3-Methoxy-4-benzyloxybenzylbromide and 3,4-dimethoxybenzylbromide were easily prepared by known methods (for details, see the ESI^†); however, the preparation of 3,4-methylenedioxy-5-methoxybenzylbromide (16) required a modified procedure that involves the regioselective protection of the hydroxy group at the 3-position of 3,4,5-trihydroxybenzene (Scheme 3). The methylenedioxylation of gallic acid (10) during the first step resulted in a low yield of 11.¹⁰ Consequently, we successfully synthesized 12 using a cyclic boron-ester system.¹¹ Arylmethylbromide 16 was derived from ester 12 in two steps in good to high yield.Open in a separate windowScheme 3Preparation of substituted benzylbromide 16.Next, enolates were generated from lactones 5a and 5b using K₂CO₃ in DMF, and successfully attacked the benzylhalides on the less-hindered side to afford α-benzyl lactones 6a–c with excellent dr values (Scheme 4).^7i,12 The trans-α,β-disubstituted lactones 7a–c were obtained via the hydrolysis of the α-methoxycarbonyllactones 6a–c followed by decarboxylation. The transformation from the enol form to the keto form gave the thermodynamically favored trans products (7a–c) with excellent dr values.^7i,12 Finally, the debenzylation of 7a using a catalytic amount of Pd–C in methanol under a hydrogen atmosphere afforded matairesinol (7d) in 96% yield. Thus, the total syntheses of dimethylmatairesinol (7b) and matairesinol (7d) were achieved, and spectral data of these natural products were consistent with reported data.^2e,3d The absolute configuration of these compounds were determined using the known data of optical rotation values. The reduction of lactone 7c using LAH afforded diol 8 in 91% yield (Scheme 4). Subsequent dimethylation of the resulting diol 8 using NaH and MeI furnished (−)-niranthin in 89% yield with 95% ee.¹³ Spectral data of (−)-niranthin was also consistent with reported data.^5b,e,6 Following the total synthesis of (−)-niranthin, we also achieved the total synthesis of (+)-niranthin via an alternative enantioselective cyclopropanation using a different enantiomeric Hayashi–Jørgensen catalyst derived from d-proline instead of l-proline (Scheme 5).¹⁴Open in a separate windowScheme 4Alternative asymmetric total synthesis of dimethylmatairesinol, matairesinol, and (−)-niranthin.Open in a separate windowScheme 5Alternative asymmetric total synthesis of (+)-niranthin.(−)-Niranthin has been reported to exhibit anti-HBV activity.^5b,e Aiming to shed light on the relationship between its enantiomeric structure and activity, we performed a bioassay on the synthesized (−)- and (+)-niranthin against not only HBV, but also the influenza virus (IFV). The anti-HBV activity results are summarized in Fig. 1 and ​and2,2, while the anti-IFV activity is summarized in Fig. 3 (for details, see the ESI^†).Open in a separate windowFig. 1HBV-infection assay using (−)- and (+)-niranthin.Open in a separate windowFig. 2HBV-replication assay using (−)- and (+)-niranthin.Open in a separate windowFig. 3Growth-inhibition assay of IFV using (−)- and (+)-niranthin.Based on the assays using HBV-infected HepG2-hNTCP-C4 cells and HBV-replicating Hep38.7-tet cells, the amount of HBs antigen decreased in a concentration-dependent manner without apparent cytotoxicity. The 50% inhibition concentration (IC₅₀) in the HBV-infected cells was calculated to be 14.3 ± 0.994 μM for (−)-niranthin and 9.11 ± 0.998 μM for (+)-niranthin (Fig. 1), while the IC₅₀ in the HBV-replicating cells was calculated to be 16.2 ± 0.992 μM for (−)-niranthin and 24.2 ± 0.993 μM for (+)-niranthin (Fig. 2). These results show that (−)-niranthin and (+)-niranthin exhibit anti-HBV activity, and that there is no remarkable difference between the anti-HBV activity of both enantiomers. In contrast, based on the bioassay of (−)- and (+)-niranthins against IFV using MDCK cells, cytotoxicity of (−)-niranthin appears at >400 μM judging that cell viability without IFV is less than 80%, and (−)-niranthin inhibited IFV-infection to cells in a concentration-dependent manner on the concentration range of non-cytotoxicity, and exhibits anti-IFV activity at 200–400 μM judging that cell viability with IFV is over 50% (Fig. 3). However, (+)-niranthin does not exhibit anti-IFV activity, and similarly to (−)-niranthin, cytotoxicity appears at >400 μM. Thus, the anti-IFV activity between (−)- and (+)-niranthins is clearly different. Our findings suggest that the enantiomeric site in niranthin endows (−)-niranthin with more potent anti-IFV activity than (+)-niranthin. We speculated that the anti-HBV active site of niranthin might be a part of the molecular structure such as aromatic groups which are far from chiral centers. In contrast, anti-IFV active site of niranthin might be closer to the chiral centers (Scheme 6).Open in a separate windowScheme 6A speculation for the bioactive site of niranthin against HBV and IFV.     

Correction: A sensitive OFF–ON–OFF fluorescent probe for the cascade sensing of Al3+ and F− ions in aqueous media and living cells

Correction for ‘A sensitive OFF–ON–OFF fluorescent probe for the cascade sensing of Al³⁺ and F⁻ ions in aqueous media and living cells’ by Lingjie Hou et al., RSC Adv., 2020, 10, 21629–21635, DOI: 10.1039/D0RA02848G.

The authors regret that an incorrect version of Fig. 4 was included in the original article. The correct version of Fig. 4 is presented below.Open in a separate windowFig. 4The ESI-MS spectrum of Al³⁺–HNS complex.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Correction: Chrysomycins A–C,antileukemic naphthocoumarins from Streptomyces sporoverrucosus

Correction for ‘Chrysomycins A–C, antileukemic naphthocoumarins from Streptomyces sporoverrucosus’ by Shreyans K. Jain et al., RSC Adv., 2013, 3, 21046–21053, https://doi.org/10.1039/c3ra42884b.

The authors regret that incorrect versions of Fig. 6 and Fig. 7 were included in the original article. The correct versions of Fig. 6 and ​and77 are presented below.Open in a separate windowFig. 6Influence of compounds 1–3 on the nuclear morphology of human leukaemia HL-60 cells. The cells were treated with 1, 3 and 5 μM concentrations of these compounds for 24 h and stained with Hoechst 33258 for 40 min. The altered nuclear morphology and apoptotic bodies indicated by white arrows are seen in treated cells while the nuclei of the untreated cells were round and intact.Open in a separate windowFig. 7Phase contrast microscopy of compound-treated leukaemia HL-60 cells. Cells were treated with compounds 1–3 at 1, 3 and 5 μM for 24 h and visualized using a phase contrast microscope (Olympus1X72). The morphology of treated cells altered in a concentration-dependent manner, while the untreated cells remained healthy.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Light-up photoluminescence sensing of a nerve agent simulant by a bis-porphyrin–salen–UO2 complex

A luminescent bis-porphyrin–salen–UO₂ complex, showing a significant fluorescence light-up response upon reacting with DMMP (a simulant of nerve agents), is reported. The fluorescence change of this complex by excitation at 365 nm can be clearly observed with the naked eye, and this complex was successfully employed to construct a test paper to detect nerve agents.

The exposure of a nerve agent simulant to a fluorogenic sensor results in a significant increase in fluorescence response, allowing the construction of a paper test for the naked-eye detection of DMMP.

In the last decades, the terroristic attacks and world conflicts have seen increasing use of unconventional weapons, leading to mass destruction.¹ The substances used are generally indicated as CBRN (chemical, biological, radiological, and nuclear) weapons. Among these, chemical warfare agents (CWAs) are organophosphate compounds that irreversibly bind and inhibit the enzyme acetylcholinesterase (AChE), thus preventing the message transfer mechanism of the nervous system, causing permanent health damages or death.² These nerve agents can easily aerosolise and vaporise when mixed with water and most other solvents and are mainly introduced in the human body through respiration and absorption by skin or eyes.The nerve agents are classified into two main groups: “G-series” (the oldest early 1900 and non-persistent) and V-series (more recent 1950s and persistent), although very recently, the “A-series or Novichok agents”, a new class of nerve agents, unfortunately deadlier than the previous series, has been developed. Hence, it is urgent to develop fast, easy, and highly sensing systems to detect in real time the presence of nerve agents, even at very low concentrations.³ In fact, the toxicity of the common nerve agents is in the range of ppm or sub-ppm (LC₅₀ values are 2 and 1.2 ppm for tabun and sarin, respectively, and even more low for soman and VX with LC₅₀ values of 0.9 and 0.3 ppm).⁴ Certainly, for safety reasons, CWAs cannot be used in common research laboratories. Thus, other substances, similar to the nerve agent for geometry and functional groups but less reactive, since they do not contain a good leaving group as observed in real warfare agents (e.g., F and CN), are used as simulants. In this context, dimethyl methylphosphonate (DMMP) has been recognized as one of the best simulants for G series nerve agents (Chart 1).⁵Open in a separate windowChart 1Chemical structures of bis-porphyrin–salen–UO₂ complex 1 and DMMP.In the last few years, several methods, including chemical sensors based on silicon nanoribbon field-effect transistors (SiNR-FETs),⁶ surface acoustic wave (SAW) devices,⁷ coordination complexes containing ligands,⁸ chemiresistive sensors,⁹ mesoporous silica material containing BODIPY derivatives,¹⁰ and gas chromatography-mass spectrometry, for the detection of CWAs have been developed.¹¹ More recently, two different methods for sensing CWAs and correlated simulants have been reported: (i) the covalent approach and (ii) a supramolecular approach. The former one is based on the reaction of the analyte with the sensor, leading to a measurable response (i.e., fluorescence emission and UV-Vis absorption).¹² This approach has few drawbacks, such as single detection for each sensor (sensors cannot be reused) and the low selectivity of the sensors due to false-positive responses.¹³ On the other hand, in the supramolecular approach, CWAs are recognised via non-covalent reversible interactions,¹⁴ involving different recognition sites (multi-topic) and allowing the reuse of sensors after the interaction with simulants.¹⁵During the last few years, metal–salen complexes have been used as hosts for the supramolecular detection of nerve gases. Atwood et al. reported an aluminium–salen complex for the detection of sarin and soman in an aqueous solution via ESI-MS experiments.¹⁶ Very recently, we were able to detect the DMMP simulant using uranyl or zinc metal–salen complexes via UV-Vis and/or luminescence spectroscopy techniques, exploiting a Lewis acid–base interaction between the metal centre and the phosphate group of the simulant.¹⁷ Unfortunately, many of these systems have shown a turn-off of fluorescence after interaction with the simulant. In fact, fluorogenic sensors represent a powerful tool for detecting CWAs because of the advantages of low cost, high sensitivity, and a real-time response.¹⁸In order to overcome this problem, we have designed and synthesized a new luminescent bis-porphyrin–salen–UO₂ complex 1, which is able to coordinate through uranyl ions to the P O group of DMMP,¹⁹ showing a fluorescence light-up response upon reacting with different concentrations of the simulant (Chart 1). In our strategy, the logical association of porphyrin derivatives, with their high stability and fluorescence properties, and uranyl–salen ligands, would lead to fluorogenic sensors that exhibit novel luminescence properties, which up to now were ruled out in uranyl–salen complexes due to the presence of uranyl metal ions. Moreover, complex 1 has been employed to construct a paper test for the naked-eye detection of DMMP. The targeted complex 1 was synthesized in three steps according to the sequence illustrated in Scheme 1.Open in a separate windowScheme 1Synthesis of bis-porphyrin–salen–UO₂ complex 1.The salicylic-porphyrin 2 was prepared by a statistical mixed-aldehyde condensation reaction according to the modified Lindsey procedure.²⁰ To a solution of benzaldehyde and pyrrole in CHCl₃, 4-hydroxyisophthalaldehyde was added in the presence of BF₃·Et₂O as a catalyst. The obtained porphyrinogen was then oxidized using 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The salicylic-porphyrin 2 was condensed with (1R,2R)-(+)-1,2-diphenylethylenediamine according to a classical literature procedure for salen synthesis,²¹ affording the bis-porphyrin–salen ligand 3, which was finally converted in the bis-porphyrin–salen–UO₂ complex 1 by uranyl acetate (overall yield 5.2%; see the ESI^†).With this complex in hand, we examined the detection performance towards DMMP. The spectroscopic characterization of complex 1 was conducted in CHCl₃. Fig. 1 shows the absorption and emission spectra of complex 1.Open in a separate windowFig. 1Absorption (left) and emission (right, λ_ex = 422 nm) spectra of complex 1 in CHCl₃ 1 μM.In particular, the UV-Vis spectrum shows the Soret band at 422 nm (ε = 3.1 × 10⁵ M⁻¹ cm⁻¹) and four Q-bands at 518 nm, 555 nm, 592 nm, and 650 nm, respectively. After excitation at 422 nm, two strong emission bands can be observed at 655 and 725 nm, relative to the π ← π* transition of the porphyrin core, in accordance with literature data for similar porphyrins.^20bΦ_F yield (Φ = 0.51) was estimated using N-butyl-4-butylamino-1,8-naphthalimide as the standard (Φ_F = 0.81).²²The sensing of DMMP was studied via fluorescence titrations in CHCl₃. The emission spectrum of 1 upon reacting with increasing amounts of DMMP showed a significant increase in the fluorescence signals at 625 nm and 725 nm (Fig. 2).Open in a separate windowFig. 2Fluorescence titration of complex 1 (1 μM in CHCl₃) with increasing amount of DMMP (0–5 eq.); the inset shows the intensity increase upon the progressive addition of DMMP equivalents.Job''s plot supports the 1 : 1 stoichiometry of the host–guest complex (see the ESI^†); thus, the binding constant value of log 6.54 ± 0.05 was calculated via a non-linear best fit,²³ monitoring the emission intensity at 655 nm and 725 nm, confirming the ability of sensor 1 to act as a light-up fluorescence host towards DMMP. According to LOD = 3σ/κ, the detection limit of complex 1 for DMMP was 7.23 ppb, which is much lower with respect to the lethal doses of the most common CWAs. Thus, the high binding constant value, the low LOD, and the high Stokes shift (>233 nm) represent ideal features for a practical sensor.To probe the selectivity of complex 1 towards DMMP, complex 1 was treated with numerous analytes, observing the change in the emission band. Complex 1 showed high selectivity towards DMMP over interferent substances, including air (real conditions, containing 24 000 ppm water, 400 ppm CO₂, 5 ppm NO, and 10 ppm CO), triethylphosphine, triphenylphosphine, and acetone (see the ESI^†). In addition, to verify the sensing properties of complex 1 also under competing conditions, to this solution, 1 eq. of DMMP has been added, leading to a significant increase in emission, thus showing hardly any interference with DMMP detection in the coexistence of DMMP and various analytes (Fig. 3).Open in a separate windowFig. 3Selectivity tests: normalized fluorescence responses of complex 1 (1 × 10⁻⁶ M in chloroform, λ_ex 422 nm) towards air (bubbled for 5 min), PPh₃ (2 ppm), (Et)₃P (2 ppm), acetone (2 ppm), and DMMP (1 ppm). Analytes are sequentially added up to the final addition of DMMP. Bars represent the final (I) over the initial (I₀) emission intensity at 625 nm.Prompted by the highly attractive features of complex 1, we used complex 1 as the main component to create a portable test paper for CWA detection. Thus, 45 μL of a 10 μM solution of 1 in CH₂Cl₂ was deposited onto the supports (filtration paper, 1.5 × 1.5 cm), and the solvent was removed. The supports were introduced into vials (23 mL of volume) containing different amounts of pure DMMP, and these systems were kept at 60 °C in order to vaporize the simulant. The DMMP vapours, interacting with the uranyl-complex via Lewis acid–base interactions, induced an increment in the fluorescence of complex 1 adsorbed on the paper upon excitation at 365 nm, which is more evident in the presence of a higher quantity of DMMP vapours, leading to significant colour changes from light blue to purple (Fig. 4). Moreover, complex 1 absorbed onto the filtration paper and was exposed to selected oxygenated molecules (e.g., acetone, ethanol, and Et₂O) did not show colour changes (see the ESI^†).Open in a separate windowFig. 4Qualitative tests of complex 1 absorbed onto the filtration paper and exposed to an increasing amount of DMMP vapours (2 μL, 20 μL, 50 μL, 70 μL, 100 μL, at 60 °C).Moreover, the system was found to be reversible. In fact, after exposure to air for 24 h at 60 °C, the device returns to the original condition without differences in terms of fluorescence with respect to the unused device, consequently confirming the non-covalent nature of the interactions between the sensor and the guest. Finally, the reusability of the device was tested for five cycles of absorption/desorption of DMMP without any loss in efficiency (Fig. 5).Open in a separate windowFig. 5Colour changes of complex 1 onto the paper support during cyclic DMMP detection/reactivation processes.These results indicate that the strategy we planned has led to the development of a simple test paper for the rapid detection of nerve agents.In summary, the new fluorescent sensor reported here shows a strong light-up response in the presence of the CWA simulant, with a high binding constant value and low limit of detection (ppb level), much lower with respect to the lethal dose of the most common CWAs. The presence of the porphyrin units is crucial for obtaining these emission properties. In addition, the paper tests confirmed the possibility of using this sensor for real applications due to the possibility to monitor the presence of a low concentration of CWAs also in the gas phase. The supramolecular approach allows restoring and reusing the initial device. Currently, we are working to obtain a quantitative detection of the simulant in gas phase by image elaboration.     

A solid-phase approach for the synthesis of α-aminoboronic acid peptides

A solid-phase synthesis of α-aminoboronic acid peptides using a 1-glycerol polystyrene resin is described. Standard Fmoc solid-phase peptide chemistry is carried out to construct bortezomib and ixazomib. This approach eliminates the need for liquid–liquid extractions, silica gel column chromatography, and HPLC purifications, as products are isolated in high purity after direct cleavage from the resin.

A solid-phase synthesis of α-aminoboronic acid peptides using a 1-glycerol polystyrene resin is described.

α-Aminoboronic acids are currently being investigated for their utility as reversible covalent inhibitors in a diverse range of therapeutic applications (Fig. 1).¹ These compounds'' Lewis acidity enables the formation of stable tetrahedral adducts with nucleophilic residues in biological targets (Fig. 2). In 2003, the first boronic acid drug, bortezomib, was approved for the treatment of multiple myeloma.² Ixazomib, a related α-aminoboronic acid inhibitor, was later approved in 2015 for the same indication.³Open in a separate windowFig. 1α-Aminoboronic acids featured in various drug discovery programs. Flaviviral protease inhibitor (1), HCV protease inhibitor (2), LepB inhibitor (3).Open in a separate windowFig. 2Yeast 20S proteasome in complex with bortezomib (PDB ID 2F16). The boronic acid forms a stable tetrahedral adduct with the N-terminal threonine (Thr1).Peptidic α-aminoboronic acids, such as bortezomib and ixazomib, have traditionally been assembled using standard peptide coupling techniques,⁴ wherein an α-aminoboronic ester is introduced onto a pre-constructed peptide and is subsequently deprotected to unmask the boronic acid. Metal-catalyzed decarboxylative borylation strategies have also been reported for the preparation of α-aminoboronic acid peptides.⁵ This approach provides direct access to these compounds from their parent peptide constructs but sacrifices stereochemical integrity.Regardless of the method, α-aminoboronic acid/ester peptides are difficult to prepare for a number of reasons.⁶ First, the C–B bond can be oxidatively labile.⁷ Second, α-aminoboronic acids and esters containing an unsubstituted α-amino group can undergo a spontaneous 1,3-rearrangement (Scheme 1, A); this process can be minimized or suppressed entirely if the amino group is rapidly acylated or protonated.⁸ Third, boronic esters can be hydrolytically labile, especially at low pH (Scheme 1, B).^6,9 Therefore, any multistep approach must entail careful extractive workups and purifications to ensure that the ester remains intact.Open in a separate windowScheme 1General considerations for preparing α-aminoboronic acid peptides.While solid-phase peptide synthesis (SPPS) has become a standard method for the construction of peptides,¹⁰ this technology has remained underexplored for the preparation of α-aminoboronic acid peptides.¹¹ An approach of this type could eliminate liquid–liquid extractions and HPLC purifications and could enable high-throughput access to this class of compounds. To the best of our knowledge, there has only been one report of C-terminal SPPS to generate α-aminoboronic acid peptides (Scheme 2).¹² Although this study provides a critical conceptual foundation, the approach it describes lacks the simplicity of a traditional solid-phase approach, requiring a complex 8-step synthesis to prepare resin-bound α-aminoboronic ester 6 for SPPS. This limitation may preclude its use as a general strategy for the preparation of α-aminoboronic acid peptides.Open in a separate windowScheme 28-Step preparation of a resin-bound α-aminoboronic acid for C-terminal SPPS.We sought to identify an approach that could enable access to resin-bound α-aminoboronic acids for SPPS in a limited number of steps using the emerging supply of commercially available α-aminoboronic acid building blocks. The Klein group recently described the use of a 1-glycerol polystyrene resin that could be used for Fmoc SPPS to construct boronic acid-containing peptides.^13–15 These results prompted us to explore the use of this resin for preparation of α-aminoboronic acid peptides, specifically bortezomib and ixazomib.Considering the unique reactivity of α-aminoboronic acids, we needed to devise a concise loading strategy that would suppress the potential for C to N boron migration. This required the amine to remain protonated or acylated throughout the loading process. These considerations lead to the design of a two-step loading protocol (Scheme 3). Commercially available boroleucine pinanediol ester 7 was hydrolysed with aqueous HCl. The boroleucine salt (8) was isolated in quantitative yield, free of pinanediol impurities, after a simple liquid–liquid extraction. The crude boroleucine salt was then shaken with the 1-glycerol polystyrene resin (loading capacity 0.6 mmol g⁻¹),¹³ Fmoc chloride, and N,N-diisopropylethylamine to provide resin-bound Fmoc-protected boroleucine 9.Open in a separate windowScheme 32-Step protocol for loading boroleucine onto a 1-glycerol polystyrene resin.With the C-terminal α-aminoboronic acid resin in hand, we used standard Fmoc SPPS coupling techniques¹⁶ to synthesize bortezomib (Scheme 4). Fmoc deprotection (piperidine, DMF) and amide coupling (Fmoc-Phe-OH, TBTU, N,N-diisopropylethylamine, DMF) delivered intermediate 10. A subsequent Fmoc deprotection/coupling sequence with pyrazinecarboxylic acid produced resin-bound bortezomib 11. Hydrolysis of the resin bound peptide was accomplished with gentle shaking in a THF/water mixture.¹³ Filtration and concentration delivered bortezomib (7-steps from boroleucine pinanediol ester 7) in 54% yield and in >95% purity.Open in a separate windowScheme 4Solid-phase synthesis of bortezomib.The synthesis of ixazomib (Scheme 5) was accomplished in an analogous manner. Fmoc deprotection (piperidine, DMF) and amide coupling (Fmoc-Gly-OH, TBTU, N,N-diisopropylethylamine, DMF) delivered intermediate 12. The deprotection/coupling sequence was repeated with 2,5-dichlorobenzoic acid to generate resin-bound ixazomib 13. Finally, boronic ester hydrolysis (THF/water) provided ixazomib in 49% yield and in >95% purity.Open in a separate windowScheme 5Solid-phase synthesis of ixazomib.     

Oxa-Michael-based divergent synthesis of artificial glutamate analogs

Herein we report stereoselective generation of two skeletons, 1,3-dioxane and tetrahydropyranol, by oxa-Michael reaction as the key reaction from δ-hydroxyenone. The construction of the 1,3-dioxane skeleton, achieved through hemiacetal formation followed by oxa-Michael reaction from δ-hydroxyenone, was exploited to access structurally diverse heterotricyclic artificial glutamate analogs. On the other hand, formation of a novel tetrahydro-2H-pyranol skeleton was accomplished by the inverse reaction order: oxa-Michael reaction followed by hemiacetal formation. Thus, this study succeeded in showing that structural diversity in a compound collection can be acquired by interchanging the order of just two reactions. Among the skeletally diverse, heterotricyclic artificial glutamate analogs synthesized in this study, a neuronally active compound named TKM-50 was discovered in the mice in vivo assay.

By interchanging the order of reactions, two types of skeletons were created and a neuroactive artificial glutamate analog was developed.

Ionotropic glutamate receptors (iGluRs) mediate the majority of the excitatory neurotransmissions such as learning, memory, and nociception in the mammalian central nervous system (CNS).¹ To study and control the function of iGluRs, specific glutamate analogs have been developed in natural product chemistry² and in medicinal chemistry.^3,4 IKM-159 (Fig. 1A) is an artificial glutamate analog designed and developed based on dysiherbaine^5,6 and kainic acid⁷ in our laboratories as an antagonist selective to (S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA)-type iGluR.^8–12 The AMPA receptor consists of four subunits: GluA1, GluA2, GluA3, and GluA4.³In vitro, IKM-159 selectively inhibits the GluA1/GluA2 heterodimer and GluA4 homodimer.¹⁰In vivo, IKM-159 inhibits voluntary action of mice for 50 min to several hours upon intracerebroventricular injection. The potency and selectivity of IKM-159 are, however, not very satisfactory to selectively modulate the function of AMPA-type iGluR. As an attempt to improve the biological profiles of IKM-159, we have been studying its structural modification.Open in a separate windowFig. 1Background (A) and summary (B) of this work. (A) AMPA-type iGluR antagonist IKM-159. (B) Artificial glutamate analogs 1 and 2, generated by oxa-Michael-based transformations (this work). The gray circle in 1 denotes the position for the structural diversity.From the first-generation studies on structure–activity relationships (SARs) of IKM-159, it had been shown that the ring size and the heteroatom of the C-ring were important for neuroactivity of IKM-159.^10,13 We then studied the second-generation SAR on the oxa analogs generated by a Prins-Ritter three-component coupling strategy, although all analogs were found to lose the original neuronal activity of IKM-159.¹⁴ Herein, we report our continuous effort along this line employing the homoallylic alcohol such as 5 and 7 (see Scheme 2) as the common intermediates.¹⁵Open in a separate windowScheme 2Preparation of the common intermediate 7 (racemate).One of the strategies in this work is the thermodynamically controlled, stereoselective formation of 1,3-dioxane (1 in Fig. 1B) by hemiacetal formation followed by oxa-Michael reaction from δ-hydroxyenone derivative that we recently developed (Scheme 1).¹⁶ The other strategy is the novel stereoselective formation of tetrahydropyranol (2 in Fig. 1B) by the inverse reaction order; oxa-Michael reaction followed by hemiacetal formation (see Scheme 5). Thus, this study succeeded in showing that structural diversity in a compound collection can be acquired by interchanging the order of just two reactions; hemiacetal formation and oxa-Michael reaction. Among the skeletally diverse, heterotricyclic artificial glutamate analogs thus synthesized, a compound named TKM-50 (1ar) was discovered to be neuronally active in the mice in vivo assay.Open in a separate windowScheme 1Our recent work regarding stereoselective 1,3-dioxane formation.¹⁶ For clarity and comparison, enantiomers of the reported compounds are shown in this scheme.Open in a separate windowScheme 5The heterotricyclic artificial glutamate analog 2, constructed by intermolecular oxa-Michael reaction of MeOH followed by acetalization.The substrate used for the 1,3-dioxane formation was prepared from the known dimethyl ester 5 (Scheme 2).¹⁷ Exposure of dimethyl ester 5 to hydrochloric acid (6 M) at 65 °C provided dicarboxylic acid 6.¹⁷ Without purification, dicarboxylic acid 6 was treated with BnBr and Cs₂CO₃ to furnish the common intermediate 7 in 72% yield (2 steps).¹⁸The alkene 7 was subjected to cross metathesis with methyl vinyl ketone (8) mediated by Hoveyda-Grubbs second generation catalyst (9)¹⁹ to provide enone 10 in 82% yield (Scheme 3). Upon exposure to paraformaldehyde as an equivalent of formaldehyde and 1,3,5-trioxane¹⁶ in the presence of MsOH, 1,3-dioxane ring formed smoothly by oxa-Michael reaction to give rise to desired (7R*)-heterotricycle 11r and the (7S*) epimer 11s (structure not shown) in the ratio of >9 : 1, as well as the N-hydroxymethylated product 12r (see Scheme 3) and the (7S*) epimer 12s (structure not shown). Since we had found that alkaline hydrolysis is of use to remove the N-hydroxymethyl group, the mixture of hemiaminals (12r/12s) and free amides (11r/11s) was treated with ammonium hydroxide²⁰ to obtain free amide 11r in 73% isolated yield (2 steps), and free amide 11s in 10% yield (estimated by NMR, 2 steps). The formation of 1,3-dioxane ring of 11r was determined by the HMBC correlations (Fig. 2A), and the stereochemical configuration was established by a ³J_H,H value and NOESY correlations denoted in Fig. 2B. Both configuration and conformation of 11r are identical to those we observed recently in the simple case (3 → 4, see Scheme 1),¹⁶ showing that the 1,3-dioxane formation in this study is also thermodynamically controlled (see below for the mechanism).Open in a separate windowScheme 3Stereoselective 1,3-dioxane formation leading to heterotricyclic artificial glutamate analog 1ar.^{a a}dr denotes the diastereoselectivity in the 1,3-dioxane formation.Open in a separate windowFig. 2Structure analysis of 1,3-dioxane 11r. (A) HMBC correlations in 11r indicates formation of the 1,3-dioxane ring. (B) Small ³J_H,H value and NOESY correlations show the configuration and the conformation of 11r.The proposed mechanism for the 1,3-dioxane formation is shown in Scheme 4A. Reaction of alcohol 10 and paraformaldehyde would form hemiacetal intermediate A under acidic conditions, which, then undergoes intramolecular oxa-Michael reaction to give 11r and 11s. Since the second conjugate addition is generally a thermodynamically controlled, reversible process, production of more stable (7R*) isomer 11r predominated over the (7S*) epimer 11s, as discussed also in our preliminary study.¹⁶ It should be also noted here that, in that preliminary study employing a simple substrate, the (7S) epimer had not been obtained.¹⁶ Generation of the less stable (7S*) epimer 11s in this study would be due to unfavorable steric interactions between the acetyl group and the benzyl ester on the near side in 11r (Scheme 4B), that make the energy difference between the two diastereomers (11r and 11s) smaller.Open in a separate windowScheme 4The plausible mechanism of 1,3-dioxane ring formation. (A) Stepwise mechanism that consists of hemiacetal formation followed by intramolecular oxa-Michael reaction. (B) The steric repulsion included in the stable isomer 11r.Then two benzyl groups of 11r were removed by hydrogenolysis²¹ to cleanly provide glutamate analog 1ar ((2R*,7R*)-TKM-50) in 86% yield (Scheme 3).With the same reaction sequences for 1ar (Scheme 3), two more analogs 1br and 1cr were furthermore synthesized (Fig. 3). The marked decrease in diastereoselectivity in these oxa-Michael reactions (see Fig. 3) suggests that the steric repulsion between the pentyl/methoxyphenyl group and the benzyl ester on the near side is extremely large. The minor (7S*) diastereomers obtained in these oxa-Michael reactions were also isolated and deprotected to give 1bs and 1cs (see the ESI^†), which were subjected to in vivo assay (see below).Open in a separate windowFig. 3Other 1,3-dioxane analogs synthesized by the intramolecular oxa-Michael reaction.^{a a}dr denotes the diastereoselectivity in the 1,3-dioxane formation.We also found that another skeleton can be constructed from δ-hydroxyenone being used for 1,3-dioxane formation, under alkaline hydrolytic conditions. Thus, as shown in Scheme 5, the δ-hydroxyenone 13 derived from homoallylic alcohol 5 by cross metathesis was selectively transformed into cyclic hemiacetal 2 in 53% yield (1 M LiOH in water, MeOH, rt). In this transformation, dimethyl ester and δ-hydroxyenone moiety independently suffer hydrolysis and cyclization, respectively, to generate glutamate analog 2 efficiently. The configuration of 2 was determined by combined analysis of NMR and DFT calculation (see the ESI^†).²²The plausible mechanism for the hemiacetal formation is shown in Scheme 6. In view of the fact that the hydroxy and carbonyl groups are located apart in 13, the six-membered-ring formation should take place after saturation of the trans-alkene. It is, therefore, supposed that oxa-Michael reaction of MeOH to enone 13 first generates saturated ketone Cvia enolate B.²³ Under alkaline conditions, the alkoxide C intramolecularly attacks carbonyl group to give rise to hemiacetal 2. Considering the fact that oxa-Michael reaction and the acetalization are thermodynamically controlled, reversible processes, energetically favorable diastereomer 2 would have been obtained predominantly (see the ESI^† for discussions on thermodynamic stability of 2). A related example had been reported in 1992 by Shing et al.²⁴Open in a separate windowScheme 6The plausible mechanism for hemiacetal formation under alkaline conditions.Behavioral activities of all six compounds upon intracerebroventricular (i.c.v.) injection were evaluated in mice (Fig. 4).²⁵ Injection of 1ar (TKM-50, 50 μg per mouse) resulted in loss of voluntary motor activity for 10 min after injection and then ataxia-like motions were recorded, thus annotated as hypoactive. The hypoactivity observed for 1ar (TKM-50) is thus somewhat weaker than IKM-159 which causes loss of mice spontaneous activity for up to 4 h.¹² Other congeners, however, did not cause any noticeable behavioral changes at the same dose tested.Open in a separate windowFig. 4The in vivo activities on mice.     

Correction: Efficient removal of cobalt from aqueous solution using β-cyclodextrin modified graphene oxide

Correction for ‘Efficient removal of cobalt from aqueous solution using β-cyclodextrin modified graphene oxide’ by Wencheng Song et al., RSC Adv., 2013, 3, 9514–9521.

The authors regret that Fig. 1 and ​and33 were incorrect in the original article. The SEM images of both GO and β-CD, and the Raman spectra of both, were confused with other samples. The correct versions of Fig. 1 and ​and33 are presented below.Open in a separate windowFig. 1SEM images of (a) GO and (b) β-CD-GO.Open in a separate windowFig. 2Raman spectra of GO and β-CD-GO.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Dimers of pyrrolo-annelated indenofluorene-extended tetrathiafulvalenes – large multiredox systems

Novel scaffolds of indenofluorene (IF)-extended tetrathiafulvalenes (TTF) were synthesized starting from a new pyrrolo-annelated IF-TTF monomer. Rigid para- and meta-phenylene linked dimers were obtained via N-arylation reactions of the monomer, and their optical and redox properties were elucidated by UV-Vis absorption spectroscopy and cyclic and differential pulse voltammetries.

A new pyrrolo-annelated indenofluorene-extended tetrathiafulvalene building block was developed and employed in N-arylation reactions for construction of redox-active dimeric scaffolds.

Tetrathiafulvalene (TTF) is a redox-active unit that reversibly undergoes two sequential one-electron oxidations, forming first a radical cation (TTF⁺) and subsequently a dication (TTF²⁺) containing two aromatic 1,3-dithiolium rings, and it is due to these redox properties that it is an attractive unit for materials and supramolecular chemistries.¹Extension of the conjugated system, leading to so-called extended TTFs, has successfully been used as a tool to finely tune the redox properties and geometries of the various redox states.² For example, introduction of an indeno[1,2-b]fluorene (IF) core³ has provided indenofluorene-extended TTFs of the general structure IF-TTF shown in Fig. 1. X-Ray crystallographic and computational studies reveal that all three redox states (0, +1, +2), generated in sequential and reversible steps, take a fully planar structure, and spectroelectrochemical studies have shown that the individual redox states exhibit significantly redshifted absorptions relative to those of TTF, TTF⁺, and TTF²⁺, respectively.²Open in a separate windowFig. 1Structures of cis/trans-isomeric TTFs, mono-pyrrolo-TTF (MP-TTF), indenofluorene (IF) and an indenofluorene-extended TTF (IF-TTF).Recently, we developed synthetic protocols for linking together two IF-TTF units via anchoring at a peripheral position of each dithiafulvene unit.⁴ Such dimers unfortunately exist as unseparable mixtures of cis and trans isomers (cf., the disubstituted TTFs shown in Fig. 1), and to avoid this problem of isomerism we decided to develop a synthetic protocol for fusing a pyrrole unit to one of the dithiole rings of IF-TTF as in target molecule 1 shown in Fig. 2. Indeed, the related mono-pyrrolo-annelated TTF (MP-TTF, Fig. 1) and bis-pyrrolo-annelated TTF have proven important as versatile π-donor building blocks in macromolecular and supramolecular chemistry.⁵Open in a separate windowFig. 2New pyrrolo-annelated IF-TTF mono- and dimers.Dimerization of two units 1via its nitrogen atom and a suitable linker would prevent formation of isomers. As linkers we decided to explore rigid phenylene units as in target molecules 2 (para-phenylene bridge) and 3 (meta-phenylene bridge) shown in Fig. 2. Previously prepared cis/trans isomeric IF-TTF dimers had flexible linkers and showed intramolecular associations upon oxidation,⁴ which would be prevented by these rigid linkers. Moreover, intermolecular interactions are to a large extent prevented by the peripheral tert-butyl substituents that were chosen as substituent groups to enhance solubility of the dimers.Synthesis of 1 proceeds according to Scheme 1, employing the known diketone 4,⁴ the N-tosyl-protected pyrrolo-annelated 1,3-dithiole-2-thione I^5a and the phosphonate ester II^3b as precursors. A phosphite-mediated coupling between 4 and I was carried out to give mono-olefinated product 5 in a yield of 61%. This compound was next subjected to a Horner–Wadsworth–Emmons olefination with compound II, deprotonated by sodium hexamethyldisilazide (NaHMDS), providing the tosyl-protected mono-pyrrolo IF-TTF6 in good yield (76%). This compound was subsequently deprotected using NaOMe to give in almost quantitative yield the monopyrrolo IF-TTF1 with a pyrrole N–H unit available for further reactions.Open in a separate windowScheme 1Synthesis of monomer 1 by stepwise olefination reactions. NaHMDS = sodium hexamethyldisilazide.With monomer 1 in hand, N-arylation reactions with 1,4- and 1,3-diiodobenzene, catalyzed by an excess of CuI and (±)-1,2-trans-diaminocyclohexane, were conducted in THF at reflux (Scheme 2). This procedure, previously applied for N-arylation of pyrrolo-annelated TTFs,⁶ yielded dimers 2 and 3, respectively. The procedure worked best for 1,3-diiodobenzene. For the arylation reaction with 1,4-diiodobenzene it was observed that the first arylation progressed rather willingly, as the mono-arylated product 7 could be isolated in 91% yield after only 3 hours. Only when compound 7 was subjected to significantly longer reaction time, however, formation of dimer 2 was observed, and the compound was isolated in 9% yield after 18 hours of reaction time. This result indicates that the substitution of an iodide on the benzene ring with one IF-TTF unit, in the para position, decreases the reactivity of the second iodide significantly, possibly due to the strongly electron-donating character of the pyrrolo-TTF. Albeit inconvenient in the current work, it could be a potential advantage for stepwise construction of unsymmetrical scaffolds. For the corresponding reaction with 1,3-diiodobenzene smooth formation of the dimer 3 was observed, and this product was isolated in 23% after 16 hours, while no mono-arylated intermediate could be isolated.Open in a separate windowScheme 2Synthesis of dimers 2 and 3, and of mono-arylated species 7 and 8.Synthesis of a trimer consisting of three IF-TTF units around one central benzene ring was also attempted by coupling of monomer 1 and 1,3,5-triiodobenzene. However, when employing the conditions proven successful for synthesis of the dimers, no reaction was observed; instead, 86% of the starting material was re-isolated. Nevertheless, when conducting the reaction in a sealed vial and heating to 100 °C, full conversion of monomer 1 was observed. However, the isolated products were mono-arylated monomer 8 and previously isolated dimer 3, and not the desired trimer. This result signals again that the reactivity of the iodides in the arylation reaction decreases upon introduction of IF-TTF units. Upon elevated pressure, as applied in the attempt to achieve the desired trimer, a competing reaction by which the iodides are substituted for hydrogen atoms is observed to exceed the arylation reaction. The mono-arylated compound 8 was used as a reference compound in subsequent studies of the synthesized dimers.The photophysical properties of monomers 1 and 8 as well as dimers 2 and 3 were investigated by UV-Vis absorption spectroscopy in CH₂Cl₂ at 25 °C (Fig. 3). Absorption maxima and extinction coefficients are listed in Scheme 1).^3a Expansion of the π-system with a benzene ring (compound 8) had little effect on the longest-wavelength absorption maximum (471 nm). Linking the monomeric units by phenylene linkers, dimers 2 and 3, did not change the longest-wavelength absorption maxima significantly either, but the intensity of the absorption was expectedly doubled (or slightly more than doubled).Open in a separate windowFig. 3UV-Vis absorption spectra of monomer 1 (dashed line), monomer 8 (dot dash dot) and dimers 2 (dotted line) and 3 (full line) in CH₂Cl₂ at 25 °C.Absorption maxima (λ_max) and extinction coefficients (ε) in CH₂Cl₂ at 25 °C, and oxidation potentials (from DPV) E_oxvs. Fc/Fc⁺ in 1 : 1 CH₂Cl₂/C₆H₅Cl, for compounds 1, 2, 3 and 8

A greener approach towards the development of graphene–Ag loaded ZnO nanocomposites for acetone sensing applications

We report a facile, green synthesis of graphene/Ag/ZnO nanocomposites and their use as acetone sensors via a medicinal plant extraction assisted precipitation process. The choice of plant extract in combination with metal nitrates led to self-sustaining colloid chemistry. Along with the green synthesis strategy, structural, morphological and gas sensing properties are described.

We report a facile, green synthesis of graphene/Ag/ZnO nanocomposites and their use as acetone sensors via a medicinal plant extraction assisted precipitation process.

Nature is a self-made-laboratory of her own, which offers insight and ways to develop advanced nanomaterials for a variety of applications. Today''s environmental pollution scenario is demanding green approaches, and hence green routes are receiving more attention and popularity. These methods are cost effective, environmentally compatible (non-toxic and pollution free), and involve syntheses at ambient conditions. Importantly, the obtained products from green synthesis routes are biocompatible and free from toxic stabilizers. Fig. 1 illustrates a green route approach for developing various nanoparticles using microorganisms, plants and others.^1–5 Amongst the various green routes, plant extracts are a very promising and facile strategy of developing varieties of nanomaterials. The extracts include leaves, flowers, fruits, stems, and roots.^6,7 Within the large family of metal oxide nanoparticles, ZnO based nanocomposites are being used in various applications such as electronics, communication, sensors, cosmetics, environmental protection, biology and medicinal industry.^8–15 ZnO is an interesting material from several points of view. It is one of the few oxides that shows quantum confinement effects in an experimentally accessible size range.¹⁶ Doped ZnO is a well-known transparent conductor.^17,18 ZnO and its composites can be considered as workhorse for understanding the various modern application fields. Speaking about gas sensing application (and particularly detection of acetone), acetone is widely used in industries and laboratories, as a common reagent. It is harmful to health and a biomarker for diagnosis of type-I diabetes. Diabetes as the disease causes increase in the concentration of acetone in human breath which is higher than 1.8 ppm for type-I diabetes patients.^19,20 Monitoring acetone and thereby other VOCs in breath is a promising and expanding field. Techniques such as solid-phase micro-extraction, mass spectrometry coupled gas/liquid chromatography, selected ion flow tube mass spectrometry have provided highly selective analysis of VOCs in breath.^21,22 Nonetheless the aforementioned analytical techniques are highly sensitive and selective for diagnosis of diabetes mellitus, they are expensive and the issue of portability is particularly important considering that diabetes mellitus should be monitored and diagnosed in real-time for daily healthcare purposes.²³ It is highly desirable to develop convenient and effective techniques for sensing acetone. Therefore, many researchers have attempted to develop highly sensitive and selective semiconductor-type acetone-sensing materials and devices by utilizing various types of metal oxides for the detection of acetone vapor at different concentration levels.^24–26 With this regards, one of the more promising sensitive and selective metal oxide gas sensing candidates to emerge is zinc oxide (ZnO) for acetone sensing.^27,28Open in a separate windowFig. 1Schematic illustration of possible green chemistry through various means.In the present investigation, we demonstrate the new approach of developing graphene/Ag/ZnO nanocomposites through medicinal plant extract, thereby their gas sensing proficiency towards reducing gases and volatile organic compounds (VOCs). The purpose of using Ag is to facilitate the spill-over mechanism, whereas graphene is used to enhance the electrical conductivity and surface area, needed for better gas sensing property.The chemicals zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) purum grade, silver nitrate (AgNO₃) trace metal basis, and graphene oxide (GO) used in a typical synthesis, are bought from Sigma-Aldrich. The ayurvedic medicinal powder was purchased from proprietary brand (Patanjali, India) local supplier. The detailed medicinal content with their wt% are highlighted in the ESI (ESI-1^†). Double distilled water (DW) was used for the extract preparation and complete synthesis process. In a typical synthesis, 10 g of medicinal powder was dissolved in 100 mL of distilled water. The mixture was stirred for 3 h at 50 °C and allowed to cool down to room temperature. The extract was obtained by centrifuging the mixture at 2000 rpm for 30 min. The radial acceleration caused the particles to settle down at the bottom of the tube, with supernatant at the top. The dark brown colored supernatant (extract) was then transferred into a separate beaker, and maintained separately for the green synthesis of ZnO (Fig. 2a).Open in a separate windowFig. 2Complete synthesis process of developing Ag/ZnO by green route (a) extract preparation, (b) synthetic protocol, and (c) complete process in general. Fig. 2b contains a simplified schematic of the preparation of the various sample groups: (i) pristine ZnO, (ii) Ag/ZnO, and (iii) GO-Ag/ZnO. The complete process along with film fabrication and gas sensing analysis is highlighted in Fig. 2c. In part-i, pristine ZnO nanoparticles were prepared by dissolving Zn(NO₃)₂·6H₂O (29.74 g, 1 M) in 100 mL of distilled water, followed by the slow and dropwise addition of supernatant extract (100 mL) in a volume equivalence. This mode of addition of extract is preferred because rapid addition results in non-homogeneous reaction mixture, and thereby bigger particle formation. In part-ii, Ag/ZnO nanoparticles were prepared by replacing an amount of zinc nitrate corresponding to mol fraction of Ag (0.5–2 mol%), followed by similar steps as in part-i. Finally, GO/Ag/ZnO nanoparticles (Part-iii) were synthesised by adding 10 mg of GO in the reacting mixture. Prior the addition of supernatant, GO was ultrasonicated in 10 mL DW for 15 min, in order to well disperse the GO in the reaction medium. So as to make the concentration balance, 90 mL of DW was taken to dissolve both the nitrates. The rest of the synthetic protocol was kept identical to the one used in aforementioned parts (i & ii). The labels were made as Z1 to Z6 for the samples-Pristine ZnO, 0.5–2 mol% Ag loaded ZnO, and GO loaded 1 mol% Ag–ZnO, respectively.The obtained precipitate was dried at 50 °C for 3 h and sintered at 400 °C for 2 h in air. The TGA-DTA analysis is discussed in ESI (ESI-8^†). Using screen printing technique, the thick films of the respective samples were developed on the alumina substrates, and sintered at 400 °C for 1 h in air to remove the added binders from the thixotropic paste. The details of thixotropic paste formation and thereby thick film formation are described in ESI (ESI-2).^†The XRD signatures of -pristine, -reference, and -Ag loaded ZnO highlighted in Fig. 3. The samples show wurtzite ZnO structure corresponding to JCPDS file no. 36-1451. The plane reflections of the samples at various Ag doping are highlighted in Fig S2.^† The XRD shows formation of highly oriented peak along (002) plane indicating highly crystalline material. The crystallites size of 53.23 nm was determined using FWHM for most intense peak (002), using Scherrer formula.²⁹D = 0.9λ/β cos θ1where ‘β’ is FWHM (in radians), ‘θ’ the angle of reflection and ‘λ’ the wavelength of X-ray radiation used.Open in a separate windowFig. 3XRD patterns for Z1, Z3, Z6 with standard ZnO pattern.SEM images (Fig. 4) showcase loosely bounded grains/aggregates (20–50 nm) with lot of empty spaces. Such structures facilitate copious channels for effective gas diffusion, required for better gas sensing ability.³⁰ From BET analysis (Fig. 5 and S3^†), the obtained isotherms are of type-IV and H3, which are typical for mesoporous material. The hysteresis loops overlap with one another in Ag loadings and pristine ZnO, indicating the lower absorption of N₂ and thereby comparatively lower surface area. An improvement in the surface area has custom tailored with the addition of GO into ZnO matrix. The blue isotherm of GO/Ag/ZnO clearly highlights an improved absorption values than the rest of the samples, indicating the larger surface area. The surface area and pore size values are tabulated in the Table S1.^† The elemental composition that resulted from EDAX analysis (Fig. 5b) shows the presence of Zn, Ag and O elements according to atomic ratios taken in the initial precursors. Its elemental mapping illustrates (Fig. 5c) the uniform distribution of Ag in the ZnO matrix. The corresponding elemental mappings of pristine and ZA-1/GO samples are highlighted in Fig. S4 and S5.^†Open in a separate windowFig. 4SEM microscopy images of all the developed samples.Open in a separate windowFig. 5(a) N₂ isotherms of Z1, Z3, and Z6 samples, (b & c) EDAX and elemental mapping of Z3 sample.     

High-contrast mechanochromic benzothiadiazole derivatives based on a triphenylamine or a carbazole unit

Four triphenylamine or carbazole-based benzothiadiazole fluorescent molecules have been successfully synthesized and characterized. Interestingly, the donor–acceptor (D–A) type luminogens 1, 2, 3 and 4 showed different solid-state fluorescence. Furthermore, the four compounds exhibited reversible high-contrast mechanochromism characteristics.

Four triphenylamine or carbazole-based benzothiadiazole dyes were synthesized. Interestingly, the four dyes exhibited high-contrast mechanochromism characteristics.

Stimuli-responsive materials receive much attention currently due to their academic importance and potential applications in optoelectronic devices and fluorescent sensors,^1–7 especially organic smart materials whose solid-state luminescence can be tuned by external stimuli.^8–13 Mechanochromic fluorescence materials, as a class of smart materials, are also receiving increasing attention.^14–20 To date, a number of mechanofluorochromic organic molecules have been reported.^21–24 In contrast, examples of high-contrast mechanochromic luminescence materials are still inadequate. Indeed, many traditional organic materials are aggregation caused quenching (ACQ)-active, and these materials are weakly emissive or nonluminescent in the solid state due to the presence of strong intermolecular electronic interactions in their aggregated state, which promotes the formation of exciplexes and excimers.^25–27 Obviously, the ACQ effect is unbeneficial to gain high-contrast mechanofluorochromic materials.^28–30 It is no doubt that mechanochromic molecules with a bright solid-state fluorescence emission are easier to achieve high-contrast mechanofluorochromic phenomenon. Therefore, the corresponding highly emissive smart luminophors have attracted considerable attention.^31,32In general, the emission characteristics of mechanochromic luminescence materials depend strongly on their molecular structures and intermolecular interactions.^33–35 Therefore, it is an effective method for the realization of mechanofluorochromic materials to change the morphological structures by means of external mechanical stimulus.³⁶Benzothiadiazole-based derivatives are regarded as attractive candidates for the organic π-conjugated fluorescent dyes owing to their strongly electron-withdrawing feature.^37–41 Meanwhile, the benzothiadiazole unit is also advantageous to the construction of donor–acceptor (D–A) type molecules, which have emerged as a significative class of optical materials finding potential value in some areas such as in fluorescent sensors and displays.^42,43 Motivated by the fact that triphenylamine or carbazole fluorogen has been broadly applied in the field of emissive materials,^44,45 we attempted to link one triphenylamine or carbazole group to one benzothiadiazole moiety. As a result, we have obtained four D–A type fluorescent molecules on the basis of a combination of the electron-donating triphenylamine or carbazole unit and the electron-accepting benzothiadiazole unit (Fig. 1). Compound 1, 2, 3 or 4 contains rotatable aromatic rings, and thus their molecular structures are nonplanar, which is advantageous to the radiative decay in the aggregated state. Indeed, compounds 1, 2, 3 and 4 showed bright solid-state fluorescence with different emission colors. In addition, we found that the D–A type luminogens 1, 2, 3 and 4 applying the triphenylamine or carbazole moiety as an electron donor and the benzothiadiazole moiety as an electron acceptor exhibited various mechanochromic fluorescence characteristics with good reversibility. Furthermore, luminogen 1 showed mechanofluorochromic behavior involving color change from orange to rare red.Open in a separate windowFig. 1Molecular structures of the compounds 1, 2, 3 and 4.To investigate the solid-state fluorescence behaviors of compounds 1, 2, 3 and 4 in detail, the corresponding solid-state emission spectra were studied initially. As shown in Fig. 2, the fluorescence spectrum of triphenylamine-containing benzothiadiazole derivative 1 exhibited one emission band with the λ_max at 575 nm, and the fluorescent molecule exhibited strong orange luminescence with the fluorescence quantum yield (Φ) of 7.13%, and triphenylamine-containing compound 2 exhibited strong yellow luminescence (Φ = 7.43%) with the λ_max at 567 nm. In contrast, the emission spectrum of carbazole-based benzothiadiazole derivative 3 exhibited one emission band with the λ_max at 504 nm, and the luminogen exhibited bright green fluorescence with the quantum yield of 16.10%, and carbazole-based compound 4 also exhibited bright green fluorescence (Φ = 16.53%) with the λ_max at 498 nm. Therefore, the photoluminescence (PL) behaviors of compounds 1, 2, 3 and 4 could be adjusted via introducing various fluorogens containing triphenylamine and carbazole. In addition, the fluorescence lifetimes of 1, 2, 3 and 4 were also measured. As shown in Fig. 3, the average lifetime of fluorescent molecule 1 was 0.82 ns, the average lifetime of 2 was 1.56 ns, the average lifetime of 3 was 3.00 ns, and the average lifetime of 4 was 1.41 ns.Open in a separate windowFig. 2Solid-state emissive spectra of the compounds 1, 2, 3 and 4, and the related fluorescence images under 365 nm UV light.Open in a separate windowFig. 3(a) Time-resolved luminescence (575 nm) of solid sample 1. Excitation wavelength: 365 nm. (b) Time-resolved luminescence (567 nm) of solid sample 2. Excitation wavelength: 365 nm. (c) Time-resolved luminescence (504 nm) of solid sample 3. Excitation wavelength: 365 nm. (d) Time-resolved luminescence (498 nm) of solid sample 4. Excitation wavelength: 365 nm.Subsequently, the mechanochromic fluorescence characteristics of compounds 1, 2, 3 and 4 were investigated. As shown in Fig. 4, the solid sample of luminogen 1 showed a bright orange fluorescence. Interestingly, the orange luminescence was changed to the red luminescence with the λ_max at 593 nm upon treating with mechanical force stimulus. Furthermore, the initial orange emission could be restored after treatment of the ground compound 1 with fuming dichloromethane for 1 min. Therefore, 1 showed reversible high-contrast mechanofluorochromic behavior with color change from orange to red, which is a relatively rare color conversion among all mechanochromic fluorescence phenomena.Open in a separate windowFig. 4(a) PL spectra of solid sample 1 at different conditions. Excitation wavelength: 365 nm. (b) Fluorescence image of the unground sample 1 under 365 nm UV light. (c) Fluorescence image of the ground sample 1 under 365 nm UV light. (d) Fluorescence image of the ground sample 1 after treatment with dichloromethane under 365 nm UV light.Similarly, as shown in Fig. 5, compound 2 also showed reversible high-contrast mechanochromic fluorescence behavior. Moreover, the reversible mechanochromic fluorescence of 1 or 2 could be repeated four times between the orange or yellow and red or orange emissions without obvious changes by alternating grinding and dichloromethane treatments. To date, this mechanochromic luminescence conversion of some reported mechanochromism compounds with superior performance is also repeated three or four times,^46–48 and thus the reversibility of the mechanochromic fluorescence effect of 1 or 2 is good (Fig. 6). On the other hand, as shown in Fig. 7, when sample 3 were ground in an agate mortar with a pestle, the green emission was changed to the yellow-green fluorescence with the λ_max at 533 nm. Moreover, the yellow-green emission could also revert to the original green emission after a 1 min treatment of the ground powder with fuming dichloromethane vapor. Furthermore, as shown in Fig. 8, compound 4 also showed similar mechanochromic fluorescence behavior.Open in a separate windowFig. 5(a) PL spectra of solid sample 2 at different conditions. Excitation wavelength: 365 nm. (b) Fluorescence image of the unground sample 2 under 365 nm UV light. (c) Fluorescence image of the ground sample 2 under 365 nm UV light. (d) Fluorescence image of the ground sample 2 after treatment with dichloromethane under 365 nm UV light.Open in a separate windowFig. 6(a) Repetitive experiment of mechanofluorochromic effect for compound 1. (b) Repetitive experiment of mechanofluorochromic effect for compound 2.Open in a separate windowFig. 7(a) PL spectra of solid sample 3 at different conditions. Excitation wavelength: 365 nm. (b) Fluorescence image of the unground sample 3 under 365 nm UV light. (c) Fluorescence image of the ground sample 3 under 365 nm UV light. (d) Fluorescence image of the ground sample 3 after treatment with dichloromethane under 365 nm UV light.Open in a separate windowFig. 8(a) PL spectra of solid sample 4 at different conditions. Excitation wavelength: 365 nm. (b) Fluorescence image of the unground sample 4 under 365 nm UV light. (c) Fluorescence image of the ground sample 4 under 365 nm UV light. (d) Fluorescence image of the ground sample 4 after treatment with dichloromethane under 365 nm UV light.As can be seen in Fig. 9, the reversibility of the mechanofluorochromic behavior of compound 3 or 4 is also excellent. Next, the powder X-ray diffraction (XRD) patterns were studied in order to ensure the morphological characteristics. As can be seen in Fig. 10, the XRD patterns of compound 1 or 2 exhibited a number of sharp reflection peaks, suggesting that the unground compound 1 or 2 was crystalline in nature. However, the ground powder sample became amorphous, with a lack of sharp diffraction peaks. Therefore, the change in fluorescence of compound 1 or 2 could be attributed to the conversion from a crystalline state to an amorphous state. On the other hand, when the ground sample was exposed to dichloromethane vapor for 1 min, the sharp and intense peaks reappeared, indicative of the recovery of the crystalline nature. As presented in Fig. 11, the structural transition of the powder sample of compound 3 or 4 was similar to that of 1 or 2. Based on the above mentioned analysis, the powder XRD results demonstrated that the interesting mechanochromic fluorescence characteristics of compounds 1, 2, 3 and 4 were ascribed to the switchable morphology transition between the crystalline state and the amorphous state.Open in a separate windowFig. 9(a) Repetitive experiment of mechanofluorochromic effect for compound 3. (b) Repetitive experiment of mechanofluorochromic effect for compound 4.Open in a separate windowFig. 10(a) Powder XRD patterns of compound 1 in different solid states. (b) Powder XRD patterns of compound 2 in different solid states.Open in a separate windowFig. 11(a) Powder XRD patterns of compound 3 in different solid states. (b) Powder XRD patterns of compound 4 in different solid states.In conclusion, in this work, four triphenylamine or carbazole-based benzothiadiazole fluorescent molecules were successfully synthesized. The compounds 1, 2, 3 and 4 belonged to the highly solid-state emissive donor–acceptor (D–A) type luminescent molecules. It is noteworthy that the four D–A type luminogens exhibited high-contrast mechanofluorochromic characteristics. Furthermore, the reversibility of their mechanochromic phenomena is good. The results of powder XRD experiments confirmed that this switchable morphology transformation is responsible for the reversible mechanochromic fluorescence characteristics of 1, 2, 3 and 4. This work is valuable for designing high-contrast mechanochromic materials involving red light-emitting feature.     

Correction: High iodine adsorption performances under off-gas conditions by bismuth-modified ZnAl-LDH layered double hydroxide

Correction for ‘High iodine adsorption performances under off-gas conditions by bismuth-modified ZnAl-LDH layered double hydroxide’ by Trinh Dinh Dinh et al., RSC Adv., 2020, 10, 14360–14367, DOI: 10.1039/D0RA00501K.

The authors regret that the reference for Fig. 1 was omitted. The reference has been added below.Open in a separate windowFig. 1Schematic of the device for the iodine adsorption experiments in static air.¹The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

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.     

Polymer–carbon dot hybrid structure for a self-rectifying memory device by energy level offset and doping

A strategy for self-rectifying memory diodes based on a polymer–carbon dot hybrid structure, with a configuration of rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al, has been proposed. The fabricated device exhibits a rectification of 10³ in the rectification model and an ON/OFF current ratio of 121 in the memory model. The rectifying behavior was attributed to an energy level offset between the electrodes and the bilayer polymers and the memory effect was induced by carrier trapping of carbon dots within the polymers.

A strategy for self-rectifying memory diodes based on a polymer–carbon dot hybrid structure, with a configuration of rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al, has been proposed.

Polymer-based resistive switching memory devices, as ideal candidates for future emerging memory devices, have attracted a great deal of attention due to their simple structure, tunable properties, high-density integration, low-power consumption, facile fabrication process, and low-cost potential.^1–4 Generally, a cross-bar architecture in memory arrays has been designed to achieve high-density data storage.^5–8 However, the sneaking current issue, which is caused by a cross-talk effect in the cross-bar array, results in a misreading of a cell in a high resistance state (HRS) when the neighboring cells are in a low resistance state (LRS).^9–13 To alleviate the sneaking current issue, great effort has therefore been devoted to the search for a memory device with a rectifying effect. For example, the architecture of one diode-one resistor (1D1R) or one transistor-one resistor (1T1R) can improve reading accessibility in an integrated memory array structure.^11,12,14,15 However, they still suffer from limitations of complex device structure, fabrication process, low yield and high energy consumption,¹⁶ which could be circumvented by creating a self-rectifying memory device with a simple sandwich architecture and solution process fabrication.Self-rectifying memory devices with metal/insulator/metal structure have risen as an important class of memory technology in high density data storage. Numerous transition metal oxide materials, e.g., Cr-doped SrTiO₃,¹⁶ Pr_0.7Ca_0.3MnO₃ (PCMO),¹⁷ ZrO₂,¹⁸ TiO₂,¹⁹ HfO_2−X,^20,21 and TaO_X,²² and Si-based materials, e.g., a-Si^23,24 and Si₃N₄,²⁵ can serve as the insulator layer and exhibit excellent self-rectifying memory features such as short switching time, large resistance ratio, and good retention ability. Unfortunately, most devices are fabricated by traditional film plating technology such as pulsed laser deposition, electron beam deposition, sputtering deposition and thermal evaporation, leading to very complicated process. Nowadays, polymer rectifying devices by energy level offset^26,27 and memory devices by doping method^28,29 have been widely fabricated, with the merit of solution process. Therefore, combining energy level offset and doping technique in solution processed polymer diodes are anticipated to achieve self-rectifying memory performance.In this letter, we reported a solution processed polymer–carbon dots hybrid structure for self-rectifying memory device by energy level offset and doping method, with a configuration of reduced graphene oxide (rGO)/poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS)/carbon dots/poly(2-methoxy-5(2′-ethyl)hexyloxy-phenylenevinylene) (MEH-PPV)/Al. Bilayer PEDOT : PSS and MEH-PPV are used as the rectifying active layers due to their energy level offset with electrodes. Carbon dots are doped as the memory active layer due to their carrier trapping behavior within polymers. rGO film as bottom electrode and Al as top electrode are fabricated by thermal annealing and thermal evaporation, respectively. The fabricated device in a 6 × 6 cross-bar array exhibits rectifying function with a rectifying ratio of 10³, and also possesses stable memory effect with a minimum ON/OFF current ratio of 121. Our strategy is promising for preparation of other polymer–nanoparticle hybrid structures for self-rectifying memory devices.The self-rectifying memory devices were fabricated basing on a facial solution-based process as shown in Fig. 1. The patterned rGO electrodes with a square resistance of 1 kΩ sq⁻¹ were fabricated from solution-processed GO films via an oxygen-plasma etching approach.^30,31 A 30 nm-thick PEDOT : PSS layer was spin-coated on rGO electrode surface, then treated with 20 W oxygen-plasma for 20 s, followed by thermal annealing in air at 120 °C for 20 min. The carbon dots aqueous with a concentration of 3 mg ml⁻¹ was spin-coated onto PEDOT : PSS layer at 3000 rpm and subsequently annealed at 100 °C for 20 min. Subsequently, a 30 nm-thick MEH-PPV layer was spin-coated and then annealed in a N₂ gas environment at 70 °C for 30 min. Finally, 6 Al lines of 500 μm in width were deposited perpendicularly to rGO lines, through a shadow mask via thermal evaporation. Electrical properties of the as-fabricated devices were investigated using a semiconductor parameter analyzer (Keithley 4200) in the ambient environment.Open in a separate windowFig. 1Schematic diagrams of the fabrication process for rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al self-rectifying memory devices.To illuminate that the memory effect is caused by the carrier trapping effect of carbon dots, the rectifying device without carbon dots was fabricated and characterized (Fig. 2(a)). The current–voltage (I–V) characteristics of the rectifying diode device are shown in Fig. 2(b). Basing on optimal fabrication conditions, the rectifying diode device with the optimized structure exhibited a maximum rectification ratio (RR) of ∼10³ following RR = |I_forward/I_reverse (here – I(V−)/I(V+))|.³² This rectifying effect is attributed to the Schottky barrier between Al electrode and MEH-PPV layer when a negative bias is applied to the rGO electrode. As is shown in the energy band diagram (Fig. 2(c)), the work function of rGO and Al is 4.8 eV and 4.3 eV, respectively. The highest occupied molecular orbital (HOMO) of PEDOT : PSS and MEH-PPV occurs at 5.2 and 5.1 eV, respectively. In this case, during the positive voltage sweep, holes from the rGO electrode can be efficiently injected into the HOMO of PEDOT : PSS layer with a 0.4 eV barrier height. On the other hand, when applying a negative voltage, it''s difficult to realize the injection of holes from Al electrode to the LUMO of MEH-PPV layer due to a large barrier height up to 0.8 eV.Open in a separate windowFig. 2(a) Schematic structure, (b) typical I–V characteristics and (c) energy band diagram of the rectifying device. (d) Schematic structure, (e) typical I–V characteristics (the arrows represent the sweep directions) and (f) energy band diagram of the rectifying memory device.To obtain the self-rectifying memory effect, carbon dots are introduced as the memory active layer due to their carrier trapping behavior within polymers (Fig. 2(d)). Carbon dots were prepared by pyrolyzing citric acid as described elsewhere.³³ The UV-vis absorption and PL spectra of carbon dot are demonstrated in Fig. 3(a), from which an apparent UV-vis absorption band centered at 335 nm is observed and the maximum PL emission occurs at a wavelength of 445 nm, suggesting their semiconductor properties. High quality TEM image shows that carbon dots have a diameter of ∼4 nm (Fig. 3(b)), suggesting their strong ability to capture charges. Moreover, current–voltage (I–V) characteristics of the self-rectifying memory device are investigated and demonstrated in Fig. 2(e). When a negative bias was initially applied to the rGO bottom electrode, the device exhibited a high resistance state (HRS, namely OFF state). By applying a low positive voltage, the holes injected from rGO electrode were captured by carbon dots, the carbon dots served as trap centers due to the boundary and quantum confinement effect.³⁴ With the increase of sweep voltage, the injected carriers increase rapidly and the traps were nearly filled. When the power supply approaches the threshold voltage, the traps were filled completely, the device underwent a resistive switching from HRS to low resistance state (LRS, namely ON state). The device maintained a LRS with trap filling when the voltage swept from 4 to 0 V. The device exhibits rectifying effect at LRS due to the Schottky effect coming from the bilayer''s energy level offset with electrodes. When the power was turned off, the captured charges might be released from the trap centers due to the shallow traps of carbon dots and the device returned to HRS.Open in a separate windowFig. 3(a) UV-vis absorption and PL spectra of the carbon dots. (b) TEM image of the carbon dots.The performance of the self-rectifying memory devices was evaluated under ambient conditions. Fig. 4(a) shows the typical I–V characteristics of the self-rectifying memory device under positive voltage sweep. The device could not maintain in the ON state steadily and it relaxed to the OFF state as once the power was removed, suggesting its volatile property. Fig. 4(b) shows the statistical distribution of ON-/OFF-state current (measured at 3 V) of the operative memory cells. The distribution of OFF- and ON-state current values lays within two orders of magnitude, the maximum currents at ON state and OFF state are about 10⁻⁴ and 10⁻⁷ A, respectively, with a ON/OFF current ratio of 10² to 10³ : 1, reducing the misreading probability during the operation process. We also measured 40 randomly selected memory cells to evaluate the uniform distribution of switching threshold voltages, as shown in Fig. 2(c). The memory cells demonstrate an average value nearly 3 V of switching voltage. The retention time of the ON and OFF state with a continuous 3 V is measured in Fig. 4(d). The ON state can be maintained by applying a refreshing pulse of 4 V every 5 s, a slightly degradation was observed at the beginning and underwent stable with an ON/OFF current ratio of 30. The curves of first sweep in Fig. 4(e) and (f) indicated that both devices with the same structure basing on rGO electrode and ITO electrode can realize the self-rectifying memory effect, even though the former one exhibits higher endurance stability. After 100 consecutive voltage sweeps, the rectifying ratio of rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al device still maintains 10² (Fig. 4(e)). Simultaneously, the ON/OFF ratio of the device at 3 V remained approximately 36. In contrast, the phenomenon of rectifier and memory gradually disappeared after six consecutive voltage sweeps for ITO based devices (Fig. 4(f)). This performance degradation was caused by the unstable property of the ITO/PEDOT : PSS interface in ambient environment. The acidic PEODT : PSS solution can etch ITO during the polymer spin-coating process, and hydrolysis of deposited PEDOT : PSS via moisture absorption can also etch ITO.³⁵ On the contrary, rGO electrodes are physically, chemically and electrically stable in ambient environment, guaranteeing a higher endurance stability of self-rectifying memory devices with rGO electrode.Open in a separate windowFig. 4(a) Typical I–V curves of the rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al device. (b) Statistical distribution of the ON-/OFF-state currents measured at 3 V. (c) Statistics histograms of switching voltages of the rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al devices from 40 memory cells. (d) Retention time for the rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al device under a continuous positive bias stress. (e) Cycle endurance test of the rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al device. (f) Cycle endurance test of the ITO/PEDOT : PSS/carbon dots/MEH-PPV/Al device. The arrows represent the sweep directions.In summary, a self-rectifying polymer memory device with the configuration of rGO/PEDOT : PSS/carbon dots/MEH-PPV/Al has been designed and fabricated through solution process. The memory effect of the as-fabricated device is attributed to the carrier trapping effect of carbon dots within polymers and the corresponding rectifying characteristic comes from the bilayer''s energy level offset with electrodes. The self-rectifying memory device exhibits a maximum rectification of 10³ in rectify model and a minimum ON/OFF current ratio of 121 in memory model. Moreover, the devices show high endurance stability of self-rectifying memory effect with rGO electrode compared to that with ITO electrode. Importantly, the solution process fabrication make this device extremely simple. Because of the self-rectifying memory feature, the simple devices have great potential application in cross-bar structure memory for high-density data storage.     

The Baeyer–Villiger rearrangement with metal triflates: new developments toward mechanism

Based on MS analysis, the mechanism of the Baeyer–Villiger oxidation of cyclic ketones with hydrogen peroxide using metal triflates (Ga(OTf)₃ and Er(OTf)₃) as catalysts was proposed. In the case of cyclohexanone as a substrate, dimeric, trimeric and tetrameric peroxide structures were detected.

New insight into the mechanism of Baeyer–Villiger oxidation with H₂O₂ and metal triflates was presented.

The Baeyer–Villiger oxidation (BV) of cyclic ketones is a convenient method of lactone preparation. Lactones are specialty chemicals which are used, among others, in the pharmaceutical, flavour and fragrances, and agrochemical industries.¹ Typical oxidants in the BV reaction are highly reactive organic peracids. However, due to their numerous disadvantages, such as sensitivity to shock and temperature, generation of corrosive waste, and hazardous storage and handling, attention is turned to more environmentally friendly hydrogen peroxide.² H₂O₂ is safer than peracids, in particular in concentrations lower than 70%. The use of H₂O₂ in oxidation reaction results in the formation of water as a by-product.² Nevertheless, hydrogen peroxide is kinetically inert and it is necessary to activate it.^1–3 One of the types of catalysts used for hydrogen peroxide activation are Lewis acids,⁴ which are characterized by high activity and selectivity, however, they suffer from poor stability in an aqueous environment. Tin-containing zeolites are one of the most promising Lewis acidic catalysts for the BV reaction. Their advantages include both their high activity and stability during recycling.⁵Another interesting proposition to answer the issue concerning hydrolytic stability of Lewis acidic catalysts for BV oxidation is the use of metal trifluoromethanesulfonates (triflates). Some of them exhibit both relatively high hydrolytic stability and Lewis acidity.^6,7 Berkessel et al. demonstrated that rare earth triflates, especially Sc(OTf)₃, were extremely active in BV oxidation as catalysts using hydrogen peroxide as an oxidant. However, these studies were presented only for the oxidation of very reactive cyclobutanones.⁸In our earlier work, silica-bound gallium(iii) triflate was used for the oxidation of 2-adamantanone yielded unexpected 95% of lactone with 99% selectivity.⁹ Encouraged by this result a number of metal triflates, with tin(ii) triflate as the most active, were proved to be active catalysts in Baeyer–Villiger oxidation of 2-adamantanone, giving full conversion of ketone after short reaction time (20 minutes using 0.1 : 0.2 : 2.0 molar ratio of Sn(OTf)₂ : ketone : 30 wt% H₂O₂).¹⁰Although the BV oxidation was discovered in 1899, attempts to establish the mechanism took the next fifty years.¹ A generally accepted mechanism of the BV oxidation of carbonyl compounds with peracid assumes that in the first stage an attack of peracidic nucleophilic oxygen on the carbonyl carbon of the ketone occurs leading to a formation of tetraedric intermediate product, called Criegee adduct or intermediate. The second stage involves an 1,2-anionotropic rearrangement. This mechanism was confirmed by von E. Doering by a labeling experiment with benzophenone-O¹⁸.¹¹ In case of using peracids as oxidants the presence of catalyst is optional.² When using hydrogen peroxide as the oxidant the addition of catalyst is required. Depending on the catalyst used various BV oxidation pathways are proposed. Catalysts can activate the ketone or oxidant, or both of them, and usually one of the activation methods dominates (Fig. 1).²Open in a separate windowFig. 1Types of catalytic activation in the BV oxidation with hydrogen peroxide: (a) electrophilic activation of a ketone by the Lewis acid; (b) electrophilic activation of the Criegee adduct by the Lewis acid; (c) nucleophilic activation of the Criegee adduct by the Brønsted acid; (d) nucleophilic activation of hydrogen peroxide by the Lewis acid; (e) electrophilic activation of hydrogen peroxide by the Lewis acid.² M – Lewis acid, B – Brønsted acid.Metal triflates are generally considered as Lewis acids, however some of them can partially hydrolyze to triflic acid under the influence of water. Hence, they may participate in the activation of the ketone, the Criegee intermediate or hydrogen peroxide, both by Lewis or Brønsted sites.Herein, we decided to take a deeper look at the BV oxidation mechanism with hydrogen peroxide using selected metal triflates. The studies shed light for the role of metal triflates in the non-classical approach of lactones formation concerning the high-energy compounds such as peroxides as intermediate compounds.In the preliminary studies, two model metal triflates which significantly vary in hydrolytic stability: gallium(iii) triflate and erbium(iii) triflate were used to compare their stability in the presence of water. Test based on the reaction with retinyl acetate was conducted (Fig. 2). In this test, the presence of Brønsted acid in the test sample is confirmed by the formation of retinyl cation (blue color) from retinyl acetate (yellow) and detected by UV-Vis spectroscopy.¹² Brønsted acid (triflic acid) in the studied samples of metal triflate may be formed by slow hydrolysis in the presence of traces of water. Therefore, triflic acid was used as a benchmark for these studies.Open in a separate windowFig. 2Retinol carbocation formation.¹²In order to limit the water content in UV-Vis tests, anhydrous nitrobenzene was used as a solvent. Results indicates (Fig. 3) that triflate with rare earth metal Er(OTf)₃ was characterized by high hydrolytic stability and the presence of retinol carbocation was not observed while Ga(OTf)₃ underwent partial hydrolysis as only part of the retinol acetate was converted to carbocation during tests. Due to the high reactivity of triflic acid, its amount used for the analysis was three times lower than that of metal triflates, which were used in the same molar amount.Open in a separate windowFig. 3UV-Vis spectroscopy of reaction mixtures consisting of metal triflates or triflic acid with retinyl acetate.In the case of triflic acid, all retinol acetate was transformed into retinol carbocation. The obtained results confirmed that Er(OTf)₃ is hydrolytically stable Lewis acid comparing to Ga(OTf)₃ which can undergo hydrolysis.Next, the BV oxidation reactions of cyclic ketones of varied reactivity were carried out in the presence of two molar excess of 60 wt% aq. H₂O₂ and Er(OTf)₃ or Ga(OTf)₃. The following cyclic ketones were selected: cyclobutanone, 2-adamantanone, norcamphor, 2-methylcyclohexanone and cyclohexanone. Secondary groups in cyclic ketones are more prone to migrate than primary alkyl groups in BV oxidation. Therefore, norcamphor, 2-adamantanone and 2-methylcyclohexanone are more reactive then cyclohexanone which is moreover non-strained and hardly reactive in BV oxidation. Very reactive, strained cyclobutanone is readily oxidized with H₂O₂.Results shown in

Photocatalytic degradation of organic pollutants through conjugated poly(azomethine) networks based on terthiophene–naphthalimide assemblies

A conjugated poly(azomethine) network based on ambipolar terthiophene–naphthalimide assemblies has been synthesized and its electrochemical and UV-vis absorption properties have been investigated. The network has been found to be a promising candidate for the photocatalytic degradation of organic pollutants in aqueous media.

A conjugated two-dimensional poly(azomethine) network based on ambipolar terthiophene–naphthalimide assemblies has been synthesized and its electrochemical and UV-vis absorption properties have been investigated.

Due to the rapid growth of urbanization and intensive industrialization, pollution has evolved into a serious concern that produces a great negative impact on human health and the environment.^1,2 Therefore, many efforts are currently devoted to addressing environmental remediation through the degradation and removal of hazardous contaminants.^3–5 In this regard, photocatalysis has been identified as a suitable approach for environmental remediation given that it is an energy efficient technique that does not require chemical input and does not produce sludge residue.⁶ In recent years, organic semiconducting polymers have evolved into a new type of metal-free and heterogeneous photocatalyst suitable for solar-energy utilization.⁷ The modularity of organic polymers allows the efficient tunning of their electronic and optical properties by bottom-up organic synthesis through the choice of suitable monomeric building blocks.^8–10 Within this context, there is a growing demand for new organic polymeric semiconductors carefully designed to have suitable energy levels of the frontier orbitals, an appropriate bandgap and good intrinsic charge mobility.¹¹For the design of suitable polymeric semiconductors for photocatalysis, it is not only important that the photocatalysts absorb light in the visible light range but also an efficient dissociation of the photogenerated charge carriers is required. The combination of electron-poor acceptor (A) and electron-rich donor (D) moieties in the polymer structure may prevent a fast recombination process following photoexcitation.¹² In addition, it has been found that polymers networks bearing conjugated moieties may exhibit π-stacked columns that can facilitate charge transport.¹³In this respect, molecular and polymeric materials based on the combination of oligothiophene^14,15 and naphthalimide moieties^16,17 connected through conjugated linkers have shown to be very effective in order to efficiently tune their frontier orbital levels and produce tunable organic semiconductors with good charge transport properties.^18–24 As an example, in Fig. 1 is depicted the structure of NIP-3T, an ambipolar organic semiconductor, for which the one-electron HOMO–LUMO excitation consists of the displacement of the electron density from the HOMO, primarily localized on the oligothiophene fragment, to the LUMO, localized on the naphthalimide unit.²⁵Open in a separate windowFig. 1(a) Monomer containing an electron donor terthiophene system directly conjugated with an electron acceptor naphthalimide moiety through a conjugated pyrazine linker (NIP-3T). (b) HOMO and (c) LUMO computed orbital topologies for NIP-3T.²⁵Among conjugated polymers, poly(azomethine)s have found application as organic semiconductors in heterogeneous photocatalysis because of their π-conjugated system and suitable band levels matching the redox window of water.²⁶ The incorporation of D–A monomeric assemblies into poly(azomethine) networks represents an efficient strategy to obtain ambipolar polymeric networks with tunable frontier orbital levels for photocatalytic applications. Thus, in this communication we report the synthesis of a novel donor–acceptor poly(azomethine) network (NIP3T-ANW, Scheme 1) based on NIP-3T monomers. The potential of this system as photodegrading agent for the elimination of contaminant organic dyes in aqueous media is also explored.Open in a separate windowScheme 1Schematic representation of the synthesis of NIP3T-ANW.The synthesis of the macromolecular poly(azomethine) network NIP3T-ANW is acomplished through Schiff-base reactions between trigonal monomers endowed with amine functionalities (TAPB,²⁷Scheme 1) and linear naphthalimide–thiophene-based monomers endowed with complementary aldehyde functional groups (NIP3T2CHO,²⁴Scheme 1). Typically, both monomers were dissolved in an o-dichlorobenzene/n-butanol/acetic acid (1 : 1 : 0.1) mixture, which was then heated at 120 °C under solvothermal reaction conditions for 72 h. A black solid was obtained which was insoluble in common solvents such as water, acetone, THF, toluene or chlorinated solvents like dichloromethane or chloroform. The obtained solid was washed several times with THF to remove the starting materials and low-molecular weight by-products. After drying under vacuum, a black solid was obtained. The yield, as determined by weight, was 98%.To investigate the chemical nature of the material, as well as to determine the conversion of the functional groups after the reaction, we have employed attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy (Fig. 2). The bands arising from the NH₂ stretching (3000–3400 cm⁻¹) and NH₂ deformation (1650 cm⁻¹) vibrations of the primary amine group of TAPB and the signals from the aldehyde groups of NIP3T2CHO around 2870 (C–H stretching) and 1663 cm⁻¹ (C O stretching) are virtually absent in the NIP3T-ANW spectrum. In addition, a prominent new band is found at 1573 cm⁻¹, which can be assigned to the C N stretching vibration of the imine linkages within the newly formed poly(azomethine) network.^28–30Open in a separate windowFig. 2(a) IR spectra of NIP3T2CHO (blue), TAPB (red) and NIP3T-ANW (black). (b) Solid-state ¹³C CP-MAS NMR spectrum of NIP3T-ANW.Solid-state ¹³C cross-polarization magic angle spinning NMR (¹³C CP-MAS NMR) spectrum (Fig. 2) reveals the characteristic imide signals of the 1,8-naphthalimide moiety at 164.4 ppm, as well as the signal corresponding to the imine carbon at 154 ppm and a signal at 148 ppm which can be assigned to the aromatic carbon neighbouring the nitrogen of the C N group. The absence of the sp² carbons from the NIP3T2CHO ²⁴ aldehyde functionalities above 180 ppm satisfactorily confirms the condensation between the aldehyde and the amine derivatives.Due to the rigidity and geometry of the building blocks, the imine linkers could be ideally generated in such a way that result in a canonical layered hexagonal structure³¹ as predicted by theoretical calculations (Fig. S1 and S2^†). However, in the actual framework, X-ray diffraction (XRD) measurements indicate that the material is mainly amorphous with only some ordered regions, as indicated by the good agreement between the weak and broad diffraction peaks observed at 2θ values larger than 3 and those predicted by calculations for the ideal canonical layered hexagonal structure (Fig. S3^†). In this regard, NIP3T-ANW was submitted to an exfoliation process following a previously described protocol³² for the exfoliation of two-dimensional polymers (see ESI^† for details). The exfoliated material was analysed by dynamic light scattering (DLS) showing a monomodal size distribution of ca. 400 nm (Fig. S4^†) and transmission electron microscopy (TEM) reveals a sheet-like structural aspect (Fig. S5^†).Thermogravimetric analysis of the poly(azomethine) network NIP3T-ANW shows that the degradation starts at around 450 °C and only 40% weight loss is observed at 700 °C (Fig. S6^†). This thermal stability is significantly higher than that observed for NIP3T (Fig. 1), the analogous molecular system based on terthiophene connected with naphthalimide through pyrazine for which the degradation starts at 200 °C (Fig. S6^†).^22,25In photocatalysis mediated by semiconductors, electron–hole pairs (excitons) are generated after light absorption and afterwards dissociate into free charge carriers that can be utilized for redox reactions^33,34 such as CO₂ fixation,³⁵ water splitting³⁶ or organic mineralization.³⁷ Some of the crucial factors that make the photocatalytic process favourable are the levels of conduction and valence as well as the width of the band gap.^38–40 Thus, in order to characterize these parameters, the electrochemical and optical properties of NIP3T-ANW have been analysed.The electrochemical properties of NIP3T and NIP3T-ANW were studied by cyclic voltammetry (Fig. 3). Both materials show ambipolar redox behaviour in which the reversible reduction processes are characteristic of the naphthalimide unit, while the oxidation processes can be ascribed to the conjugated oligothiophene moiety.^19–22,24 For NIP3T-ANW, the first reversible reduction wave (−1.29 V) is shifted to less negative values in comparison with NIP-3T (−1.41 V). On the other hand, the first oxidation half wave potential for NIP3T is observed at +0.41 V and for NIP3T-ANW at +0.44 V. These shifts agree with the electron acceptor ability of the imine linker.Open in a separate windowFig. 3(a) The UV-vis DRS spectra of NIP3T and NIP3T-ANW. (b) Cyclic voltammetry of the NIP3T monomer and the corresponding polymer.The absorption spectrum of NIP3T-ANW as determined by UV-vis diffuse reflectance spectroscopy (UV-vis DRS, Fig. 3) shows a strong absorption in all the UV-vis range, extending even to the near infrared. This broad absorption is red-shifted in comparison with the one observed for NIP3T (Fig. 3), which reflects the formation of the new polymer network with an extended conjugation through the alpha positions of the terthiophenes.Using the corresponding cut-off wavelengths, the optical band gaps E_g found for NIP3T and NIP3T-ANW are 1.59 and 1.42 eV respectively. This optical result suggests that the incorporation of the NIP3T core into an extended conjugated system efficiently harvests photons from the visible range, even extending into the near IR region.To shed some light into the degree of crystallinity of the synthesized NIP3T-ANW network, we have carried out a battery of density functional theory (DFT)-based calculations with the QUANTUM ESPRESSO plane-wave DFT simulation code⁴¹ (see details in ESI^†). We have considered periodic boundary conditions to obtain a fully-relaxed ground-state crystal structure. Optimization of the cell-shape and size, simultaneously to the relaxation of the structure, reveals a hexagonal 2D lattice with an optimized parameter of 48.46 Å. Different interlayer stacking fashions have been tested, with only one yielding a good agreement with the experimental diffractogram from 2θ values >3°. The most favourable stacking predicted by theory consists in an intermediate configuration between the perfectly eclipsed and staggered configurations, with an interlayer distance of 3.42 Å, and permits an adequate accommodation of the layers profiting adjacent pores. Details on the structure can be found in the ESI.^†Additionally, we have computed the electronic band diagram of the obtained crystal structure along the high-symmetry k-path Γ → K → M → Γ, revealing a wide-gap (1.91 eV) semiconducting character, with rather dispersive valence and conduction bands, mainly resembling the molecular HOMO and LUMO of the molecular building blocks (see Fig. S7^†). Besides, computed time-dependent DFT (TDDFT) UV-vis spectrum manifests an excellent agreement with the experimental UV-vis spectra (Fig. S8^†). A broad and pronounced peak-feature is obtained between 600 and 800 nm, centred at around 720 nm (1.7 eV), which agrees with the optical gap of 1.6 eV found for NIP3T from Fig. 3a. This feature corresponds to electronic transitions between the valence and conduction bands, with an energy difference of around 0.2 eV between the optical and the electronic gap, which indicates that charge relaxation in excited states is not much significative. The good agreement between theoretical predictions on the canonically periodic computed system and the experimental evidences seems to justify the presence of some high-crystallinity regions from the synthesis.The band gap of a semiconductor material and the reduction and oxidation potentials are key parameters which determine its light-harvesting properties and types of reaction that can be conducted and therefore the overall photo-catalytic activity. A shift in the adsorption edge of a semiconductor towards longer wavelengths implies a narrower band gap and the efficient harvesting of a wider photons range.⁴²NIP3T-ANW seems to be an appealing material to be utilized as photocatalyst given (i) the optimal light harvesting properties as shown by the optical characterization, (ii) the efficient generation of electron–hole pairs owing to the insertion of terthiophene moieties, and (iii) the right energy band positions for the material.⁷ We therefore evaluated the photocatalytic activity of NIP3T-ANW under white light for the degradation of a model organic pollutant (Rhodamine B dye, RhB) in aqueous solution.⁴³ In the absence of catalyst, RhB remains stable in solution under illumination (Fig. 4a and S9^†). However, in presence of NIP3T-ANW nearly 90% of RhB in an aqueous solution is degraded after 120 min, showing the enhanced catalytic activity of the material (Fig. 4a and S10^†). Furthermore, a good stability is shown upon 4 straight catalytic cycles (Fig. 4b, S11 and S12^†). In contrast, in the presence of the NIP3T moiety, only a 55% degradation of RhB is observed in the same timeframe (Fig. 4a and S11^†).Open in a separate windowFig. 4RhB degradation curves. (a) Comparison between the degradation effect of NIP3T, NIP3T-ANW and without catalyst. (b) NIP3T-ANW stability after four recycling cycles.In the photocatalytic degradation of organic pollutants, they are typically broken down through the attack of superoxide and hydroxyl species, formed when atmospheric oxygen reacts with photogenerated electrons or when water or OH ions are oxidized by holes, respectively.⁴⁴ Additionally, electron–hole pairs (or excitons) can directly reduce or oxidize organic pollutants in aqueous environments. Consequently, the evaluation of the photodegradation mechanism of organic pollutants, despite challenging, can provide meaningful insights about the nature of a semiconductor photocatalyst.⁴⁵ With the aim of evaluating the photodegradation mechanism we performed the measurements in the presence of different scavengers, namely an aqueous solution of AgNO₃ (100 mg L⁻¹), which captures photogenerated electrons, or triethanolamine (TEOA), which traps photogenerated holes (Fig. S13^†).^46,47 In the presence of Ag⁺ we could observe that the photocatalytic efficiency is enhanced, while the addition of TEOA quenched the performance, therefore suggesting that holes are the active specie in the photodegradation mechanism (Fig. S14^†).In summary, we have presented an approach towards the incorporation of D–A π-conjugated monomeric assemblies into poly(azomethine) networks to yield a purely organic semiconductor for the photocatalytic degradation of organic pollutants in aqueous media. The poly(azomethine) network benefits from a straightforward poly(condensation) approach which favourably competes with the elaborate high-temperature protocols applied for the preparation of inorganic materials. This work enriches the family of donor–acceptor organic semiconductor networks and, given its modular nature, paves the way for the development of a promising family of materials for photocatalytic applications.     

Preparation and characterization of an edible metal–organic framework/rice wine residue composite

In this communication, using rice wine residue (RWR) as the support, an edible γ-cyclodextrin-metal–organic framework/RWR (γ-CD-MOF/RWR) composite with a macroscopic morphology was synthesized. The obtained edible composite is promising for applications in drug delivery, adsorption, food processing, and others.

An edible metal–organic framework/rice wine residue composite was made with large surface area for potential applications in drug delivery, adsorption, food processing, and others.

As a typical class of porous materials, metal–organic frameworks (MOFs) have attracted increasing attention since being first proposed by Yaghi and co-workers.¹ Over the past two decades, owing to their large surface area, ultrahigh porosity and tunable pore size,² MOFs have exhibited great prospects for gas storage and separation,^3,4 catalysis,^5–8 sensors,⁹ drug delivery,^10–12etc. Among numerous reported MOFs, γ-cyclodextrin-MOF (γ-CD-MOF), which is connected by the (γ-CD)₆ units of alkaline earth metal ions, was initially synthesized and reported by Stoddart et al.^13,14 in the 2010s. Owing to the –OCCO– groups derived from γ-CD, this kind of MOF is edible and therefore opens a new path for preparing green, biocompatible and edible MOF materials.^13,15,16 For example, Stoddart et al.¹¹ reported a co-crystallization approach to trap ibuprofen and lansoprazole inside γ-CD-MOF, and the resultant composite microspheres can be used for sustained drug delivery. Zhang et al.¹⁷ proposed a strategy to graft cholesterol over the surface of γ-CD-MOF to form a protective hydrophobic layer to improve its water stability. Many researchers succeeded in preparing oral delivery medicine with high drug loading and an enhanced therapeutic effect by combining the drug molecules with γ-CD-MOF.^16,18–20 These works present the excellent application prospects of γ-CD-MOF in the medical field.Since MOFs possess so many attractive advantages, extensive studies have focused on combining MOFs with many other functional materials (metal nanoparticles, quantum dots, carbon matrices and polyoxometalates, etc.) by means of the synergistic effect, leading to the formation of novel composites designed for targeted applications.^21–28 However, these reported composites were still presented as loose powders, which may not be convenient for the applications. Therefore, the question of how to prepare MOFs-based composites for larger particles at low cost is of great significance. On the other hand, as a traditional alcoholic beverage, rice wine has been popular in southern China and some other Asian nations for thousands of years.²⁹ The rice wine lees or rice wine residue (RWR) is a by-product of the fermentation process of rice wine. It is a mixture of proteins, amino acids and polysaccharides. It is traditionally a health food in some Asian nations.³⁰ The edibility, extensive source, low cost and specific macroscopic shape make RWR a potential functional material for further use of MOFs.Herein, a facile and environmental-friendly strategy has been developed to realize the growth of γ-CD-MOF on rice wine residue, resulting in the formation of an edible MOF/RWR composite in the shape of rice grains. The material characterization confirmed the obtained composite possesses the characteristics of MOF. Except for the edible γ-CD-MOF/RWR, other MOF/RWR composites (HKUST-1, ZIF-67 and MIL-100(Fe)/RWR composites; shown in Fig. S1^†) were prepared to demonstrate the universality of this synthesis strategy.The synthesis procedure of the γ-CD-MOF/RWR composite is schematically illustrated in Fig. 1. The rice wine residue was soaked in deionized water for 12 h and then washed with deionized water three times before vacuum freeze-drying. Similar to the synthesis of γ-CD-MOF powder,¹⁵ KOH was dissolved into water. Then certain amounts of the aforementioned dry rice wine residue were soaked into the K⁺-containing solution for 2 h in order to absorb the sufficient potassium ions. K⁺ was then linked by the coordination of –OCCO– units in γ-CD and RWR with the three-dimensional interconnected network. After vapor diffusion of MeOH and some other procedures described in the synthesis of γ-CD-MOF powder (seen in ESI), the γ-CD-MOF/RWR composite (Fig. 2) was obtained. This method is convenient as no extra binders are needed during the whole process. The same procedure was employed to prepare the RWR composites with other MOFs (HKUST-1, ZIF-67 and MIL-100(Fe)). And the syntheses are briefly described in the ESI. The images of the obtained composites are shown in Fig. S1.^†Open in a separate windowFig. 1Schematic illustration of the synthesis procedure of γ-CD-MOF/RWR composite.Open in a separate windowFig. 2Digital photo of the γ-CD-MOF/RWR composite.The rice wine residue, of which the elemental analysis is shown in Table S1,^† is mainly composed of polysaccharides and proteins. Thus, a broad peak at around 22.2° in the XRD patterns of rice wine residue can be observed (Fig. S2^†), which is due to its poor crystallinity.³¹ The XRD patterns of γ-CD-MOF and γ-CD-MOF/RWR composite samples are shown in Fig. 3a. The characteristic peaks at 5.6°, 6.9°, 13.3°, 16.6°, 20.6° and 23.2°, observed from the XRD patterns of γ-CD-MOF, agree with the previously reported works.^32,33 Meanwhile, compared with γ-CD-MOF, the γ-CD-MOF/RWR composite shows similar characteristic peaks with lower intensity, indicating a lower crystallinity of the MOF within the composite. Fig. 3b shows the FT-IR spectra of different samples. Compared with the rice wine residue, the peaks in regions 1 and 2 of γ-CD-MOF and γ-CD-MOF/RWR can be ascribed to the stretching vibration of –CH₂ and –C–O–C– of the MOF, respectively.^15,34 These results further confirm the formation of the γ-CD-MOF in the γ-CD-MOF/RWR composite.Open in a separate windowFig. 3XRD patterns (a) and FT-IR spectra (b) of γ-CD-MOF/RWR composite, γ-CD-MOF and RWR.The SEM images were collected to further investigate the micromorphology of the as-prepared samples. As shown in Fig. 4a, a three-dimensional layered network structure and rich macropores of the rice wine residue rough surface can be seen. γ-CD-MOF (Fig. 4b) exhibits a uniform body-centered cubic shape with an average size of 4.27 μm, which is in accordance with the reported works.^15,35,36 Meanwhile, the images of the γ-CD-MOF/RWR composite (Fig. 4c and d) show that the cubic γ-CD-MOF crystals are well dispersed on the surface of the rice wine residue and even partially integrated into the framework of the rice wine residue. Compared with the pristine γ-CD-MOF, some γ-CD-MOF in γ-CD-MOF/RWR is not an intact cubic structure, exhibiting a significantly different morphology. This suggests a synergistic effect between the MOF crystals and the rice wine residue during the growth of MOF crystals, rather than a simple physical mixture of the two materials. The thermal stability of the γ-CD-MOF/RWR composite was investigated via TGA analysis. As shown in Fig. S3,^† the decomposition temperature of γ-CD-MOF/RWR composite slightly increased compared with those of pristine γ-CD-MOF and rice wine residue. Moreover, the γ-CD-MOF/RWR composite was stable in water, methanol and ethanol (shown in Fig. S4^†) even under mild stirring. These results indicate an improved physiochemical stability of γ-CD-MOF after the incorporation of rice wine residue. This finding further confirms the synergistic effect between them.Open in a separate windowFig. 4SEM images of rice wine residue (a), γ-CD-MOF (b) and γ-CD-MOF/RWR composite (c and d). Fig. 5a shows the nitrogen sorption isotherms of the γ-CD-MOF and γ-CD-MOF/RWR composite. Both pristine γ-CD-MOF and γ-CD-MOF/RWR exhibit typical type-I isotherms, demonstrating their microporous structures. The pore size distributions of pure γ-CD-MOF and γ-CD-MOF/RWR (Fig. 5b) confirm the existence of micropores (between 1 and 2 nm). The calculated Brunauer–Emmett–Teller (BET) surface areas, micropore volume and total pore volume are listed in 35,37 The specific surface area of the γ-CD-MOF/RWR composite is 651 m² g⁻¹, which is significantly higher than that of the pure rice wine residue (10.8 m² g⁻¹). Thus, the increase in the specific surface area of γ-CD-MOF/RWR composite can be attributed to the growth of γ-CD-MOF on the RWR support. Therefore, γ-CD-MOF/RWR composite inherits both the high porosity of γ-CD-MOF and the macroscopic morphology of rice wine residue, which should contribute to its practical applications.Open in a separate windowFig. 5N₂ adsorption and desorption isotherms (a) and pore size distributions (b) of γ-CD-MOF/RWR composite and corresponding comparative samples.Summary of the BET areas (S_BET), micropore volume (V_micro) and total pore volume (V_tot) of γ-CD-MOF, γ-CD-MOF/RWR composite and pure rice wine residue