A porous organic polymer nanosphere-based fluorescent biosensing platform for simultaneous detection of multiplexed DNA via electrostatic attraction and π–π stacking interactions

One key challenge in oligonucleotide sequence sensing is to achieve multiplexed DNA detection in one sensor. Herein, a simple and efficient fluorescent biosensing platform is constructed to simultaneously detect multiplexed DNA depending on porous organic polymer (POP) nanospheres. The developed sensor is based on the concept that the POP nanospheres can efficiently quench the fluorescence emission of dye-labeled single-stranded DNA (ssDNA). Fluorescence quenching is achieved by the non-covalent assembly of multiple probes on the surface of POP nanospheres through electrostatic attraction and π–π stacking interactions, in which the electrostatic attraction plays a more critical role than π–π stacking. The formed dsDNA could be released off the surface of POP via hybridizing with the target DNA. Consequently, the target DNA can be quickly detected by fluorescence recovery. The biosensor could sensitively and specifically identify three target DNAs in the range of 0.1 to 36 nM, and the lowest detection limits are 50 pM, 100 pM, and 50 pM, respectively. It is noteworthy that the proposed platform is successfully applied to detect DNA in human serum. We perceive that the proposed sensing system represents a simple and sensitive strategy towards simultaneous and multiplexed assays for DNA monitoring and early clinical diagnosis.

This communication reports a simple and efficient fluorescent biosensing platform to simultaneously detect multiplexed DNA depending on porous organic polymer (POP) nanospheres by electrostatic attraction and π–π stacking interaction.     

N-Aryl iminochromenes inhibit cyclooxygenase enzymes via π–π stacking interactions and present a novel class of anti-inflammatory drugs

Cyclooxygenase enzymes (COX1/2) have been widely studied and noted for their role in the biosynthesis of inflammation-induced proteins, prostaglandins and thromboxane. Multiple anti-inflammatory drugs have been developed to target these two enzymes, but most of them appeared to have notable adverse effects, especially on the cardiovascular system and lower gastrointestinal tract, suggesting an urgent need for new potent anti-inflammatory drugs. In this study, we screened twenty-two previously synthesized N-aryl iminochromenes (NAIs) for their anti-inflammatory activity by performing COX-1/2 inhibitory assays. Five compounds (1, 10, 14, 15, and 20) that gave the best in vitro anti-inflammatory results were subjected to an in vivo anti-inflammatory assay using the formalin-induced hind rat paw oedema method, followed by in silico studies using indomethacin and celecoxib as standard drugs. Among them, compound 10 stood out as the best candidate, and the percentage reduction in paw oedema at the dose of 20 mg kg⁻¹ body weight was found to be substantially higher with compound 10 than that with indomethacin. This is mostly due to the excellent suitability of the chromene-phenyl scaffold with a highly concentrated area of aromatic residues, which produced good π–π stacking interactions. Taken together, this study strongly suggests compound 10 as a potential candidate for anti-inflammatory drug research.

Screening of N-aryl iminochromenes for their anti-inflammatory activities by performing in vitro, in vivo, and in silico studies.     

Interface conjugation enhances the interface adhesion performance of carbon fiber-reinforced phthalonitrile composites by π–π stacking

There has been little research focus on the interface problems of phthalonitrile (PN) resin and carbon fiber. However, interface performance is related to the overall mechanical properties of composites and is very important. This study focused on the interfacial performance and adhesion mechanism of a carbon fiber C_f/PN composite. Micro-composites of C_f/PN and C_f/epoxy resins were prepared, and their interfacial shear strengths (IFSS) were tested by micro-droplet testing. The result showed that the IFSS of C_f/PN was higher than that of C_f/epoxy resin, indicating that the interfacial adhesion of the PN matrix composite must be more effective. To explain the obtained results, a number of tests, including SEM, SEM-EDS, FTIR, and TGA, were carried out. From the SEM analyses, cured PN polymer films were found on the surface of de-bonded carbon fibers. With the aid of SEM-EDS, the elements on the de-bonded carbon fiber surface of the C_f/PN composite were detected in situ. An interesting synchronous relationship was observed in the IFSS and SEM-EDS results. Through the FTIR spectra, the chemical structures of the PN polymers were identified. From the detailed analyses and discussion in this work, the effective interfacial bond function in the C_f/PN composite appears to be a complex result for all relative functions. The functional advantage of the PN composite may be the interface conjugation between the PN polymers and the graphene layer on the surface of the carbon fiber.

Cross-linking products and graphene can interact with each other through π–π stacking and promote the formation of interface conjugation.     

Theoretical study of D–A′–π–A/D–π–A′–π–A triphenylamine and quinoline derivatives as sensitizers for dye-sensitized solar cells

We have designed four dyes based on D–A′–π–A/D–π–A′–π–A triphenylamine and quinoline derivatives for dye-sensitized solar cells (DSSCs) and studied their optoelectronic properties as well as the effects of the introduction of alkoxy groups and thiophene group on these properties. The geometries, single point energy, charge population, electrostatic potential (ESP) distribution, dipole moments, frontier molecular orbitals (FMOs) and HOMO–LUMO energy gaps of the dyes were discussed to study the electronic properties of dyes based on density functional theory (DFT). And the absorption spectra, light harvesting efficiency (LHE), hole–electron distribution, charge transfer amount from HOMO to LUMO (Q_CT), D index, H_CT index, S_m index and exciton binding energy (E_coul) were discussed to investigate the optical and charge-transfer properties of dyes by time-dependent density functional theory (TD-DFT). The calculated results show that all the dyes follow the energy level matching principle and have broadened absorption bands at visible region. Besides, the introduction of alkoxy groups into triarylamine donors and thiophene groups into conjugated bridges can obviously improve the stability and optoelectronic properties of dyes. It is shown that the dye D4, which has had alkoxy groups as well as thiophene groups introduced and possesses a D–π–A′–π–A configuration, has the optimal optoelectronic properties and can be used as an ideal dye sensitizer.

We have designed four dyes based on D–A′–π–A/D–π–A′–π–A triphenylamine and quinoline derivatives for DSSCs and studied their optoelectronic properties as well as the effects of the introduction of alkoxy groups and thiophene group on the properties.     

Ultrahigh permeance of a chemical cross-linked graphene oxide nanofiltration membrane enhanced by cation–π interaction

Cross-linking with large flexible molecules is a common method to improve the stability and control the interlayer spacing of graphene oxide (GO) membranes, but it still suffers from the limitation of low water flux. Herein, a novel high flux GO membrane was fabricated using a pressure-assisted filtration method, which involved a synergistic chemical cross-linking of divalent magnesium ions and 1,6-hexanediamine (HDA) on a polyethersulfone (PES) support. The membrane cross-linked with magnesium ions and HDA (GO_{HDA–Mg²⁺}) exhibited a high water flux up to 144 L m⁻² h⁻¹ bar⁻¹, about 7 times more than that of cross-linked GO membranes without adding magnesium ions (GO_HDA), while keeping excellent rejection performance. The GO_{HDA–Mg²⁺} membrane also showed an outstanding stability in water for a long time. The effects of magnesium ions on the GO_{HDA–Mg²⁺} membrane were analyzed using several characterization methods, including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results indicated that magnesium ions not only promoted reasonable cross-linking, but also improved the stacking of GO sheets to give lower mass transfer resistance channels for water transport in the membranes, resulting in the ultrahigh permeance of the GO membranes.

Chemical cross-linking together with magnesium ions, potentially promoting reasonable cross-linking and improving the water channels of membrane in terms of flatness and surface with low mass transfer resistance.     

Effect of fluorine substituents on benzothiadiazole-based D–π–A′–π–A photosensitizers for dye-sensitized solar cells

Two D–π–A′–π–A organic dyes with triazatruxene (TAT) as the electron donor, thiophene as the π-spacer, benzoic acid as the anchor group, and benzothiadiazole (BT) or difluorobenzo[c][1,2,5]thiadiazole (DFBT) as the additional acceptor, namely LS101 and LS102, respectively, were applied to dye-sensitized solar cells (DSSCs). As fluorine substituents are usually strong electron-withdrawing groups, introducing two fluorine atoms into BT was expected to strengthen the electron-withdrawing ability of the auxiliary acceptor, resulting in DSSCs with a broader light capture region and further improved power conversion efficiency (PCE). Fluorine is the smallest electron-withdrawing group with an induction effect, but can also act as an electron-donating group owing to its conjugation effect. When the conjugation effect is dominant, the electron-withdrawing ability of additional acceptor DFBT decreases instead. Accordingly, the band gap of LS102 was broadened and the UV-vis absorption spectrum was blue-shifted. In the end, DSSCs based on LS101 achieved a champion PCE of 10.2% (J_sc = 15.1 mA cm⁻², V_oc = 966 mV, FF = 70.1%) while that based on LS102 gave a PCE of only 8.6% (J_sc = 13.4 mA cm⁻², V_oc = 934 mV, FF = 69.1%) under standard AM 1.5G solar irradiation (100 mW cm⁻²) with Co²⁺/Co³⁺ as the electrolyte.

The results and interpretations can clearly explain the reasons for the poor photovoltaic performance of DFBT in DSSCs.     

Elucidating π–π interaction-induced extension effect in sandwich phthalocyaninato compounds

Electrochemical and theoretical investigations over triple-quadruple-, quintuple-, and sextuple-decker sandwich-type compounds {[(Pc*)Sm][(Pc*)Cd_n(Pc*)_n][Sm(Pc*)]} (n = 0–3) elucidate successive π–π interaction-linked extension in the perpendicular direction of the phthalocyanine plane along with increasing the stacked tetrapyrrole number, significantly improving the nonlinear optical properties including effective imaginary third order molecular hyperpolarizability and optical limiting threshold.

π–π interaction-linked extension in the perpendicular direction to the monomers and corresponding effect on nonlinear optic properties have been clearly disclosed over the multiple-decker sandwich-type phthalocyaninato metal compounds.     

In situ generated hydrophobic micro ripples via π–π stacked pop-up reduced graphene oxide nanoflakes for extended critical heat flux and thermal conductivities

We report the synthesis of thermally heated pop-up reduced graphene oxide (Pop-rGO) and its nanofluid (Pop-rGO-Nf) in DI water for extended critical heat flux (CHF) in a nucleate pool boiling experiment. When Pop-rGO-Nf is boiled over a nichrome (NiCr) wire heater the CHF values were increased up to 132%, 156%, and 175% with increasing concentrations of 0.0005 vol%, 0.001 vol%, and 0.005 vol% at heat fluxes of q′′ = 264 333 kW m⁻², 339 202 kW m⁻², and 327 895 kW m⁻², respectively, because of the higher surface area of 430 m² g⁻¹. We also found a decrease in the CHF value from 0.05 vol% (175%) to 0.01 vol% (153%) for Pop-rGO-Nf due to the nanofluid concentration reaching the saturation point. After nucleate pool boiling, the developed Pop-rGO-Nf built-up layer on the NiCr wire surface showed regular π–π stacking with novel micro-rippled structures having uniform nanocavities and nanochannels. The nanocavities strongly helped vapor bubbles to escape from the NiCr wire surface. In addition, the nanochannels were formed by hydrogen bonding of adjacent carboxyl groups of each Pop-rGO nanosheet. The surface hydrophobicity of the built-up layers increased with the increase of the concentration of the Pop-rGO-Nfs, and the surface morphology, roughness average (R_a) and hydrophobicity were determined using FE-SEM, AFM and contact angle (CA) analysis. In our present investigation, during and after the nucleate CHF experiments with Pop-rGO-Nfs, for the first time, we obtained a higher CHF value of 175% at 0.01 vol% and a higher CA of 118° obtained at 0.05 vol%, due to the increase in surface hydrophobicity and the novel micro-rippled structures. We anticipate that the present results suggest that pool boiling employing Pop-rGO-Nf can dissipate the critical heat flux of electronic chips to a greater extent, allowing the enhancement of the cooling performance in existing two-phase heat transfer devices.

We report the synthesis of thermally heated pop-up reduced graphene oxide (Pop-rGO) and its nanofluid (Pop-rGO-Nf) in DI water for extended critical heat flux (CHF) in a nucleate pool boiling experiment.     

Mechanofluorochromism of (D–π–)2A-type azine-based fluorescent dyes

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC or h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes OUY-2, OUK-2, and OUJ-2 possessing intramolecular charge-transfer (ICT) characteristics from two (diphenylamino)carbazole–thiophene units as D (electron-donating group)–π (π-conjugated bridge) moieties to a pyridine, pyrazine, or triazine ring as A (electron-withdrawing group): grinding of the recrystallized dyes induced red or blue shifts of the fluorescent colors, that is, bathochromic or hypsochromic shifts of the fluorescence maximum wavelengths (λ^fl-solid_max). The degrees of MFC evaluated by the absolute value of differences (Δλ^fl-solid_max) in λ^fl-solid_max before and after grinding of the recrystallized dyes increased in the order of OUY-2 (+7 nm) < OUK-2 (−17 nm) < OUJ-2 (+45 nm), so that OUJ-2 exhibits obvious b-MFC, but OUK-2 exhibits h-MFC. X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) demonstrated that the recrystallized dyes were in the crystalline state but the ground dyes were in the amorphous state. When the ground solids were heated above their crystallization temperatures (T_c), the colors and fluorescent colors recovered to the original ones before grinding or converted to other ones, that is, heating the ground solids in the amorphous state induced the recrystallization to recover the original microcrystals or to form other microcrystals due to polymorph transformation. However, (D–π–)₂Ph-type fluorescent dye OTK-2 having a phenyl group as a substitute for the azine rings exhibited non-obvious MFC. Molecular orbital (MO) calculations indicated that the values of the dipole moments (μ_g) in the ground state were 4.0 debye, 1.4 debye, 3.2 debye, and 2.9 debye for OTK-2, OUY-2, OUK-2, and OUJ-2, respectively. Consequently, on the basis of experimental results and MO calculations, we have demonstrated that the MFC of the (D–π–)₂A-type azine-based fluorescent dyes is attributed to reversible switching between the crystalline state of the recrystallized dyes and the amorphous state of the ground dyes with changes in the intermolecular dipole–dipole and π–π interactions before and after grinding. Moreover, this work reveals that (D–π–)₂A fluorescent dyes possessing dipole moments of ca. 3 debye as well as moderate or intense ICT characteristics make it possible to activate the MFC.

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC of h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes: grinding of the recrystallized dyes induced bathochromic or hypsochromic shifts of the fluorescence bands.     

Impact of various heterocyclic π-linkers and their substitution position on the opto-electronic attributes of the A–π–D–π–A type IECIO-4F molecule: a comparative analysis

To investigate the consequence of different substitution positions of various π-linkers on the photovoltaic properties of an organic solar cell molecule, we have introduced two series of six three-donor molecules, by the substitution of some effective π-linkers on the A–π–D–π–A type reference molecule IECIO-4F (taken as IOR). In series “a” the thienyl or furyl bridge is directly linked between the donor and acceptor moieties, while in series “b” the phenyl ring of the same bridge is working as the direct point of attachment. The frontier molecular orbitals, density of states, transition density matrix, molecular electrostatic potential surfaces, exciton binding energy, excitation energy, wavelength of maximum absorption, open-circuit voltage, fill factor, and some other photovoltaic attributes of the proposed molecules were analyzed through density functional theory (DFT) and its time-dependent (TD) approach; the TD-DFT method. Though both series of newly derived molecules were a step up from the reference molecule in almost all of the studied characteristics, the “a” series (IO1_a to IO3_a) seemed to be better due to their desirable properties such as the highest maximum absorption wavelength (λ_max), open-circuit voltage, and fill factor, along with the lowest excitation and exciton dissociation energy, etc. of its molecules. Also, the studied morphology, optical characteristics, and electronic attributes of this series of proposed molecules signified the fact that the molecules with thienyl or furyl ring working as the direct link between the acceptor and donor molecules showed enhanced charge transfer abilities, and could provide a maximum quantum yield of the solar energy supplied.

We have introduced two series of six three-donor molecules, by the substitution of some effective π-linkers on the A–π–D–π–A type reference molecule IECIO-4F (taken as IOR) for efficient organic solar cells.     

Cation–π interactions drive hydrophobic self-assembly and aggregation of niclosamide in water

The beneficial medicinal effects of niclosamide have been reported to be hampered by poor aqueous solubility and so a higher concentration dosage is required. In this work, we have studied the aggregation properties of niclosamide in water by varying the number of monomers. We have employed all-atom classical molecular dynamics simulation in order to explore such properties. The equilibrium structure exists in an aggregated state with structural rearrangements of the stacking units. Niclosamide monomers tend to form clusters in an orderly manner and tend to aggregate in parallel and antiparallel orientations of the phenyl rings as the monomers are increased in number from 4 to 9. Upon increasing the size from 9 to 14, and from 49 to 150, a considerable dominance of the metastable parallel arrangement is observed, resulting in the formation of a closely packed cluster with hydrophobic contacts. The metastable conformation self-arranges to a T-shape before forming a stable planar antiparallel displaced conformation. The aggregated π–π parallel and cation–π antiparallel clusters in water exist in a β-conformer. We further observed that formation of a stable cluster aggregate entails the formation of an intermediate metastable cluster that disperses in solution forming a large stable cluster. We also discovered that movement of the water is faster in less aggregated clusters and as the cluster size increases, the mobility rate becomes much slower.

In this work, we have studied the aggregation properties of niclosamide in water by varying the number of monomers.     

Probing the supramolecular features via π–π interaction of a di-iminopyrene-di-benzo-18-crown-6-ether compound: experimental and theoretical study

A novel DPyDB-C N-18C6 compound was synthesised by linking a pyrene moiety to each phenyl group of dibenzo-18-crown-6-ether, the crown ether, through –HC N– bonds and characterized by FTIR, ¹H-NMR, ¹³C-NMR, TGA, and DSC techniques. The quantitative ¹³C-NMR analysis revealed the presence of two position isomers. The electronic structure of the DPyDB-C N-18C6 molecule was characterized by UV-vis and fluorescence spectroscopies in four solvents with different polarities to observe particular behavior of isomers, as well as to demonstrate a possible non-bonding chemical association (such as ground- and excited-state associations, namely, to probe if there were forming dimers/excimers). The interpretation of the electronic structure was realized through QM calculations. The TD-CAM-B3LYP functional, at the 6-311+G(d,p) basis set, indicated the presence of predominant π → π* and mixed π → π* + n → π* transitions, in line with the UV-vis experimental data. Even though DPyDB-C N-18C6 computational studies revealed a π-extended conjugation effect with predominantly π → π* transitions, thorough fluorescence analysis was observed a weak emission, as an effect of PET and ACQ. In particular, the WAXD analysis of powder and thin films obtained from n-hexane, 1,2-dichloroethane, and ethanol indicated an amorphous organization, whereas from toluene a smectic ordering was obtained. These results were correlated with MD simulation, and it was observed that the molecular geometry of DPyDB-C N-18C6 molecule played a defining role in the pyrene stacking arrangement.

Herein, we report the formation of a potential supramolecular arrangement mediated by inter- and intra-molecular interactions between di-iminopyrene-dibenzo-18-crown-6-ether molecules.     

First principles study of electronic and nonlinear optical properties of A–D–π–A and D–A–D–π–A configured compounds containing novel quinoline–carbazole derivatives

Materials with nonlinear optical (NLO) properties have significant applications in different fields, including nuclear science, biophysics, medicine, chemical dynamics, solid physics, materials science and surface interface applications. Quinoline and carbazole, owing to their electron-deficient and electron-rich character respectively, play a role in charge transfer applications in optoelectronics. Therefore, an attempt has been made herein to explore quinoline–carbazole based novel materials with highly nonlinear optical properties. Structural tailoring has been made at the donor and acceptor units of two recently synthesized quinoline–carbazole molecules (Q1, Q2) and acceptor–donor–π–acceptor (A–D–π–A) and donor–acceptor–donor–π–acceptor (D–A–D–π–A) type novel molecules Q1D1–Q1D3 and Q2D2–Q2D3 have been quantum chemically designed, respectively. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations are performed to process the impact of acceptor and donor units on photophysical, electronic and NLO properties of selected molecules. The λ_max values (321 and 319 nm) for Q1 and Q2 in DSMO were in good agreement with the experimental values (326 and 323 nm). The largest shift in absorption maximum is displayed by Q1D2 (436 nm). The designed compounds (Q1D3–Q2D3) express absorption spectra with an increased border and with a reduced band gap compared to the parent compounds (Q1 and Q2). Natural bond orbital (NBO) investigations showed that the extended hyper conjugation and strong intramolecular interaction play significant roles in stabilising these systems. All molecules expressed significant NLO responses. A large value of β_tot was elevated in Q1D2 (23 885.90 a.u.). This theoretical framework reveals the NLO response properties of novel quinoline–carbazole derivatives that can be significant for their use in advanced applications.

Materials with nonlinear optical properties have significant applications in nuclear science, biophysics, medicine, chemical dynamics, solid physics & materials science. We show how π bridges, donors & acceptors can be reconfigured to improve optical properties.     

Synthesis,solid state self-assembly driven by antiparallel π⋯π stacking and {⋯H–C–C–F}2 dimer synthons,and in vitro acetyl cholinesterase inhibition activity of phenoxy pendant isatins

A series of novel phenoxy pendant isatins PI1–12 have been synthesized in excellent yields by a simple nucleophilic substitution reaction involving isatins and 1-(2-bromoethoxy)-4-substituted benzenes, and characterized by their FT-IR, ¹H NMR, ¹³C NMR and GC-MS data, and in the case of PI4 by its single crystal X-ray analysis. The solid-state structure of PI4 showed an intriguing and unique 1D-supramolecular chain-based self-assembled structure, the driving force of which is mainly the strong antiparallel π⋯π stacking and {⋯H–C–C–F}₂ dimer synthons. This compound not only highlights the potential of the isatin moiety in forming strong antiparallel π⋯π stacking interactions but also provides a platform to have considerable insight into the nature, strength and directionality of much debated π–π and C–H⋯F–C interactions. The in vitro biological studies revealed that three phenoxy pendant isatins PI1, PI2 and PI4 are highly potent inhibitors of acetylcholinesterase enzyme with IC₅₀ values of 0.52 ± 0.073 μg ml⁻¹, 0.72 ± 0.012 μg ml⁻¹ and 0.68 ± 0.011 μg ml⁻¹, respectively, showing comparable activity to the standard drug, donepezil (IC₅₀ = 0.73 ± 0.015 μg ml⁻¹). A simple and efficient synthesis of phenoxy pendant isatins PI1–12 from inexpensive and commercially available starting materials, and their high potential of acetyl cholinesterase inhibition provide an attractive opportunity to find more effective medication for Alzheimer''s disease (AD).

The phenoxy pendant isatins were observed to be highly potent inhibitors of acetylcholinesterase. In addition, the solid-state structure of a phenoxy pendant isatin showed an intriguing 1D-supramolecular self-assembled structure.     

Photophysical properties and fluorosolvatochromism of D–π–A thiophene based derivatives

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models. Intramolecular charge transfer characteristics of the structurally similar 2-fluoro-4-(5-(4-methoxyphenyl)thiophen-2-yl)benzonitrile (MOT) and 4-(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-2-fluorobenzonitrile (DMAT) were investigated. Significant enhancement of intramolecular charge transfer strength has been observed through molecular structure modification of the electron donating group from a methoxy to dimethylamine group. Ground state absorption spectra show a small red shift of about 10 nm and 18 nm while the fluorescence emission spectra show a large red shift of about 66 nm and 162 nm on changing from the nonpolar cyclohexane to the aprotic polar DMSO for MOT and DMAT, respectively. Dipole moment change from the ground state to the charge transfer excited state is calculated to be 6.6 D in MOT and 9.0 D in DMAT. The fluorescence quantum yield, fluorescence lifetime and the derived radiative and non-radiative rate constants were found to be better correlated to the emission energy rather than any of the solvent properties. Three multi-parametric relationships were used in the interpretation of the specific versus non-specific solute–solvent interactions, namely, Kamlet–Taft, Catalán and Laurence et al. models. The findings of these approaches are used to extract useful information about different aspects of solvent effects on the photophysical properties of the two studied compounds. Kamlet–Taft solvatochromic model indicates that non-specific interactions are dominant in controlling the photophysical properties. Catalán''s solvent dipolarity/polarizability parameter is found to play a significant role in solvatochromic behaviour which is also designated by the Laurence model.

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models.     

Phenoxy pendant isatins as potent α-glucosidase inhibitors: reciprocal carbonyl⋯carbonyl interactions,antiparallel π⋯π stacking driven solid state self-assembly and biological evaluation

Carbonyl–carbonyl (CO⋯CO) interactions are recently explored noncovalent interactions of significant interest owing to their role in the stability of biomacromolecules. Currently, substantial efforts are being made to understand the nature of these interactions. In this study, twelve phenoxy pendant isatins 1–12 have been evaluated for their α-glucosidase inhibitory potential in addition to the analysis of X-ray single crystals of 4 and 9. Both compounds 4 and 9 showed intriguing and unique self-assembled structures. The CO⋯CO and antiparallel displaced π⋯π stacking interactions are mainly involved in the formation of 1D-stair like supramolecular chains of 4 whereas antiparallel π⋯π stacking interactions drive the formation of 1D-columnar stacks of 9. These compounds not only highlight the potential of the isatin moiety in forming strong CO⋯CO and antiparallel π⋯π stacking interactions but also are interesting models to provide considerable insight into the nature of these interactions. The in vitro biological studies revealed that all twelve phenoxy pendant isatins 1–12 are highly potent inhibitors of α-glucosidase enzyme with IC₅₀ values ranging from 5.32 ± 0.17 to 150.13 ± 0.62 μM, showing many fold more potent activity than the standard drug, acarbose (IC₅₀ = 873.34 ± 1.67). Easy access and high α-glucosidase inhibition potential of these phenoxy pendant isatins 1–12 provide an attractive platform for finding more effective medication for controlling postprandial hyperglycemia.

Carbonyl–carbonyl (CO⋯CO) interactions are recently explored noncovalent interactions of significant interest owing to their role in the stability of biomacromolecules.     

A novel indole-based conjugated microporous polymer for highly effective removal of heavy metals from aqueous solution via double cation–π interactions

A novel indole-based conjugated microporous polymer (PTIA) with three coplanar indole units, designed and synthesized by an oxidative coupling reaction, was utilized as a platform for removing heavy metals. Owing to the conjugation of the three coplanar indoles, the highly electron-rich large π planes can simultaneously attract six heavy metal atoms via double cation–π interactions, endowing this microporous material with remarkable heavy metal adsorption capacity and efficiency.

A novel indole-based conjugated microporous polymer (PTIA) with three coplanar indole units, designed and synthesized by an oxidative coupling reaction, was utilized as a platform for removing heavy metals.     

AIE and reversible mechanofluorochromism characteristics of new imidazole-based donor–π–acceptor dyes

Four new imidazole-based donor–π–acceptor 2a–2d dyes have been synthesized, and their solvatochromism, aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties were investigated. The new dyes 2a–2d were designed to have 1,4,5-triphenyl-1H-imidazole as an electron donor (D) and 1-indanone, 1,3-indandione, 2-phenylacetonitrile and 2-thiopheneacetonitrile as electron acceptors (A) linked through a phenyl bridge. The maximum absorption wavelength of 2a–2d dyes in DCM solution appeared at 376, 437, 368, and 375 nm, respectively. The dyes exhibit a high molar extinction coefficient (ε) and large Stokes shift, making them useful in optoelectronic applications. Solvatochromic properties of dyes 2a–2d have been studied and showed bathochromic changes in emission wavelengths, from 449 to 550 nm for 2a, 476 to 599 nm for 2b, 438 to 520 nm for 2c, and from 439 to 529 nm for 2d, as the solvent polarity increased from n-hexane to acetonitrile. Moreover, in dioxane/water mixture systems, AIE behaviors were observed, and the emission intensity of 2b–2d dyes increased by around 5, 3, and 3 times in the mixed solvent (dioxane : water = 10 : 90) in contrast to pure dioxane. In addition, the XRD data of the 2a–2d dyes in pristine, ground, and fumed states illustrate that the transition between the ordered crystalline and disordered amorphous phases is the primary cause of MFC behaviors mechanism. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) showed that the highest occupied molecular orbital (HOMO) of dyes is distributed on the donor unit. In contrast, the lowest unoccupied molecular orbital (LUMO) is mainly placed on the acceptor unit to reveal that the HOMO–LUMO transition has a great ICT character.

Four new imidazole-based donor–π–acceptor 2a–2d dyes have been synthesized, and their solvatochromism, aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties were investigated.     

First theoretical probe for efficient enhancement of optical nonlinearity via structural modifications into phenylene based D–π–A configured molecules

The design of nonlinear optical (NLO) materials using conjugated molecules via different techniques is reported in the literature to boost the use of these systems in NLO. Therefore, in the current study, designed phenylene based non-fullerene organic compounds with a D–π–A framework were selected for NLO investigation. The initial compound (PMD-1) was taken as a reference and its seven derivatives (PMDC2–PMDC8) were made by introducing different acceptor moieties into the chemical structure of PMD-1. To explain the NLO findings, frontier molecular orbital (FMO), transition density matrix (TDM), density of states (DOS), natural bond orbital (NBO) and UV-Vis study of the title compounds was executed by applying the PBE1PBE functional with the 6-311G(d,p) basis set. The descending order of band gaps (E_gap) was reported as PMDC7 (2.656) > PMDC8 (2.485) > PMD-1 (2.131) > PMDC3 (2.103) > PMDC2 (2.079) > PMDC4 (2.065) > PMDC5 (2.059) > PMDC6 (2.004), in eV. Global reactivity parameters (GRPs) were associated with E_gap values as PMDC6 with the lowest band gap showed less hardness (0.0368 E_h) and high softness (13.5785 E_h). The UV-Vis investigation revealed that the maximum λ_max (739.542 nm) was exhibited by PMDC6 in dichloromethane (DCM) as compared to other derivatives. Additionally, natural bond orbital (NBO) based findings revealed that PMDC6 exhibited the highest stability value (34.98 kcal mol⁻¹) because of prolonged hyper-conjugation. The dipole moment (μ), average linear polarizability 〈α〉, first hyperpolarizability (β_tot) and second hyperpolarizability (γ_tot) were evaluated for the reference and its derivatives. Consequently, among the designed compounds, the highest β_tot (4.469 × 10⁻²⁷ esu) and γ_tot (5.600 × 10⁻³² esu) values were shown by PMDC6. Hence, it''s concluded from said results that these structural modifications proved PMDC6 as the best second and third order NLO candidate for various applications like fiber optics, signal processing and data storage.

The design of nonlinear optical (NLO) materials using conjugated molecules via different techniques is reported in the literature to boost the use of these systems in NLO.     

Facile synthesis of aminated indole-based porous organic polymer for highly selective capture of CO2 by the coefficient effect of π–π-stacking and hydrogen bonding

A new aromatic aminated indole-based porous organic polymer, PIN-NH₂, has been successfully constructed, and it was demonstrated that the coefficient effect endows this porous material with outstanding CO₂ absorption capacity (27.7 wt%, 1.0 bar, 273 K) and high CO₂/N₂ (137 at 273 K and 1 bar) and CO₂/CH₄ (34 at 273 K and 1 bar) selectivity.

It was demonstrated that the coefficient effect endows POP PIN-NH₂ with outstanding CO₂ absorption capacity and high selectivity.

Today, one of the most serious environmental problems is climate change, such as global warming and sea-level rises, which are caused by increased concentrations of carbon dioxide (CO₂) in the atmosphere.^1–3 As we all know, CO₂ mainly arises from fossil-fuel combustion in power plants, and the flue gas is always mixed with other gases including nitrogen (N₂), methane (CH₄) and so on. Therefore, it is necessary to design materials for selectively separating and adsorbing CO₂ from these industrial and energy-related sources to improve the environmental problems.^4–6 Aqueous amine solutions are the most common adsorbents for CO₂ separation and capture,⁷ however, not only do these adsorbents degrade over time and are corrosive, toxic, and volatile, but also the regeneration process is highly energy demanding for these systems due to the chemical capture mechanism. As alternatives, porous organic polymers (POPs)^8–10 relying on physical adsorption have become the research focus due to their low density, large specific surface area, good thermal stability, and narrow pore size distribution, but the low uptake capacity, and especially, the poor selectivity are two urgent issues that need to be addressed that seriously restrict the commercialization of POP adsorbents.¹¹ Hence, in the past few years, many methods have been developed to improve the POP performance including increasing the surface area and adjusting the pore size.^12,13Recently, based on the rapid development of supramolecular interactions^14,15 and the unique advantage of POP materials, i.e., the structure designability, researchers found that introducing special active sites into the framework, such as heteroatoms and diverse organic groups, is a simple and effective way to ameliorate the adsorption performance by the formation of some special non-covalent interactions and various functional groups have been explored.^16–18 Recently, Chang et al.¹⁹ have designed and prepared an novel aerogel (PINAA) that contains both amide and indole groups and they demonstrated that the CO₂ can be rapidly adsorbed on the heteroaromatic ring of indole because of its relatively large binding area via strong π–π-stacking interactions, and then, the desorbed CO₂ molecule can be captured by an adjacent amide group because of “electrostatic in-plane” interaction. This synergistic effect of electrostatic in-plane and dispersive π–π-stacking interactions of amide and indole with CO₂ endows the resulting aerogel enhanced CO₂ adsorption capacity and CO₂/CH₄ and CO₂/N₂ selectivity. Inspired by this fascinating study, we hypothesized that when the indole group is aminated, the CO₂ can be rapidly adsorbed on the heteroaromatic ring of indole because of its relatively large binding area via strong π–π-stacking interaction (Fig. 1a), after that, the hydrogen bonding interactions between the O of the CO₂ and –NH of the aniline group would make the CO₂ to further form a stable conformation with the aminated indole system (Fig. 1b), as a result, the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would ensure the high CO₂ adsorption capacity and further enhanced CO₂/CH₄ and CO₂/N₂ selectivity.Open in a separate windowFig. 1Schematic representation showing the heteroaromatic ring of indole adsorbing CO₂via π–π-stacking interactions (a) and the CO₂ molecule is further stabled via the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions (b). (c) Synthetic route of PIN-NH₂ aerogel.To verify our suppose, in this work, we tactfully designed and fabricated an aminated indole-based aerogel PIN-NH₂via Friedel–Crafts alkylation (Fig. 1c), and its CO₂ adsorption capacity and CO₂/CH₄ and CO₂/N₂ selectivity were immediately investigated. The successful preparation of PIN-NH₂ was confirmed by Fourier transform infrared spectroscopy (FT-IR) and ¹³C solid state cross-polarization magic-angle-spinning nuclear magnetic resonance (¹³C CP/MAS NMR) spectrometer, and the results are in good agreement with the proposed structures (Fig. S1 and S2, ESI^†). In the ¹³C CP/MAS NMR spectrum of PIN-NH₂, the peaks at 169–103 ppm are ascribed to the indole group carbons, and the signals located at 35–40 ppm are assigned to the methylene carbons (Fig. S1, ESI^†). For FT-IR spectrum (Fig. S2, ESI^†), the peak at 3438 cm⁻¹ is attributed to the stretching vibrations of N–H in amine unit and indole amine. The peaks at 2999 cm⁻¹ and 2927 cm⁻¹ are assigned to the stretching vibration of –CH₂– in the polymer network and the peaks at 1630 cm⁻¹ and 1480 cm⁻¹ are ascribed to the vibrations of the aromatic ring skeleton.The porosity of PIN-NH₂ was quantified by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM) and N₂ adsorption–desorption isotherms at 77 K. As shown in Fig. 2a, the SEM image displays that the PIN-NH₂ consists of aggregated particles with sub-micrometer sizes. And the microporous characteristic can be observed clearly from the TEM image as shown in Fig. 2b, the presence of porous structure provides the essential condition for CO₂ capture and separation. As shown in Fig. 2c, at a low pressure (0–0.1 bar), there is a rapid raise in the N₂ adsorption–desorption isotherm, indicating its microporous nature, and the increase in the N₂ sorption at a relatively high pressure (∼0.9 bar) shows the presence of meso- and macrostructures of the PIN-NH₂. The specific surface area calculated in the relative pressure (P/P₀) range from 0.01 to 0.1 shows that the Brunauer–Emmett–Teller (BET) specific surface area of PIN-NH₂ is up to 480 m² g⁻¹. Additionally, the pore-size distribution (PSD)²⁰ calculation result was shown in Fig. 2d and S3 ESI,^† which indicating the pore diameter is about 14 Å and further confirming the microporous feature of the PIN-NH₂. To gain further insight into the microstructural information, the powder wide-angle X-ray diffraction (PXRD) was further performed on the PIN-NH₂ polymer. As shown in Fig. S4, ESI,^† only a broad peak at 17.8° 2θ in the PXRD pattern is present, which clearly suggests that the polymer is mainly amorphous in nature. Additional, the thermogravimetric analysis (TGA) show that the microporous material is stable up to 370 °C indicating its potential in post combustion processes operated at high temperatures (Fig. S5, ESI^†).Open in a separate windowFig. 2The microstructures of PIN-NH₂ framework. (a) SEM, (b) TEM, (c) the nitrogen adsorption–desorption isotherms and (d) the pore size distribution of PIN-NH₂ framework.Owing to the artful structure design and particular preparation method, there is a reserved aniline group on the side of the indole group in the PIN-NH₂ network. It was expected that after the rapidly capture of the CO₂ molecule via the π–π-stacking interactions, the next aniline group would assist to further stabilize the CO₂ molecule via hydrogen bonding interactions, in other words, the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would make this porous organic polymer more efficiently attract CO₂ molecules, which inspires us to investigate the gas uptake capacity. Physisorption isotherms for CO₂ (at 273 K) measured with a pressure more than 1.0 bar indicated that the resulting PIN-NH₂ network exhibited a high carbon dioxide uptake of 27.7 wt% at 1.0 bar, as shown in Fig. 3a. Comparing with most of reported porous materials such as metal–organic frameworks,²¹ activated carbons,²² and microporous organic polymers,^23,24 the porous organic polymer PIN-NH₂ shows an enhanced CO₂ uptake (Table S1, ESI^†). The calculation of isosteric heat of adsorption of PIN-NH₂ shows that the heat of adsorption is 35.7 kJ mol⁻¹ (Fig. S6, ESI^†), which is higher than that of the reported azo-linked polymers (27.9–29.6 kJ mol⁻¹),²⁵ the acid-functionalized porous polymers (32.6 kJ mol⁻¹).^26,27 and the indole-based porous polymers.^28,29 The high value of the heat of adsorption indicated the strong physisorption effect owing to the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions.Open in a separate windowFig. 3Gas adsorption isotherms of PIN-NH₂ for CO₂ at 273 K (a), adsorption and desorption isotherms of PIN-NH₂ for different gases at 273 K (b), a CO₂ molecule is adsorbed on the heteroaromatic ring of indole via π–π-stacking interaction (c) and the CO₂ molecule is further stabled via the coefficient effect of π–π-stacking and hydrogen bonding interactions (d), adsorption isotherms of PIN-NH₂ for different gases with 3% RH of water at 273 K (e), reversibility of the PIN-NH₂ polymer in CO₂ capture measured by TGA at 273 K (f).The application in CO₂ separation and adsorption field of the traditional POPs is limited in a great degree by the poor gas selectivity as the flue gas and natural gas are both mixed gas. Here, we believed that the CO₂ can be easily attracted by the heteroaromatic ring via the π–π-stacking interactions and then stabled with the assist of hydrogen bonding interactions, which leading an enhanced gas selectivity. Therefore, we urgently evaluated the selective gas uptake of the PIN-NH₂ network for small gases (CO₂/CH₄, CO₂/N₂). In the calculation, the ratio of CO₂/N₂ is 15/85 and the ratio of CO₂/CH₄ is 5/95, which is the typical composition of flue gas and natural gas, respectively, the test results were shown in Fig. 3b. It can be found there is a rapid increase for the CO₂ uptake while there is a negligible increase for the CH₄ and N₂ uptake with the increase of the pressure, which maybe due to the unique local dipole–π interactions between the porous organic framework PIN-NH₂ and CO₂ molecule. The test results shows that the CO₂ uptake of PIN-NH₂ is up to 5.92 mmol g⁻¹ at a pressure of 1.0 bar and a temperature of 273 K while the CH₄ and N₂ uptake of PIN-NH₂ is only 0.18 and 0.04 mmol g⁻¹, respectively. The estimated ideal CO₂/CH₄ and CO₂/N₂ adsorption selectivities are up to 34 and 137, respectively. Additionally, the selectivities of PIN-NH₂ toward CO₂ over CH₄ and N₂ at 291 and 303 K were also investigated, respectively, and the results indicated that the resulting polymer PIN-NH₂ still exhibited good selectivity at higher temperatures (Fig. S7 and S8, ESI^†).The high gas selectivities of this microporous framework may attribute to the strong affinity for CO₂ compared with N₂ and CH₄ arising from the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions between the sorbent and CO₂ guest molecule, to further attest the above surmise, we used density functional theory (DFT)³⁰ at the M06-2X level with the aug-cc-pVDZ basis set to investigate the interaction of aminated indole system with CO₂ and the details of the calculation is shown in the ESI.^†Fig. 3c and d shows the snapshot for CO₂ capture by a model compound. The calculation result shows that owing to the electron-rich and large binding area, the CO₂ was very easily attracted by the indole plane at a distance of 3.706 Å, and the computational binding energy was 13.23 kJ mol⁻¹ (Fig. 3c). Soon, the balance structure was changed, the CO₂ molecular was moved towards the amino group till the distance between the amino group and CO₂ molecular was 3.037 Å, indicating a hydrogen bonding interaction was formed in this system. As a result, the distance between the indole plane and CO₂ molecular decreased to 3.257 Å from 3.706 Å, and the computational binding energy increased to 42.15 kJ mol⁻¹, which meaning a more steady system was formed (Fig. 3d). In the sense of computational chemistry, the expected strong coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would favor the uptake of CO₂ of the PIN-NH₂ network. Additional, The DFT result also indicated that the interaction energy between CO₂ and the imine group of indole is relatively weak with a correlation distance at 4.041 Å.As we all know that the CO₂ adsorption property will be affected in a great degree for porous polymers in the presence of water.³¹ In real industrial applications, the flue gas from a power plant is a mixture of CO₂, water vapor, and others. As a result, it has very important practical significance to study the CO₂ capture performance under humid condition. Here, the CO₂ capture property of PIN-NH₂ was studied at a relative humidity of 3% RH, as shown in Fig. 3e, the CO₂ adsorption capacity of PIN-NH₂ decreased from 5.92 to 4.88 mmol g⁻¹ (1.0 bar, 273 K), however, the uptake of CH₄ and N₂ does not affected by the water. These results indicate that adsorption of water diminishes the CO₂ capture. Although the selectivity (CO₂/N₂ = 104, CO₂/CH₄ = 21) is decreased under humid condition, PIN-NH₂, to the best of our knowledge, still has very good CO₂ selectivity over other CO₂ capture materials in similar conditions.³² Moreover, the CO₂ adsorption process is fully reversible (Fig. 3f). Herein, the new aminated indole-based aromatic porous organic polymer PIN-NH₂ synthesized from easily available starting materials demonstrated not only remarkable CO₂ capture capacity, but also prominent CO₂/N₂ and CO₂/CH₄ selectivities. Further, the dynamic breakthrough separation experiments of gas mixture at 298 K using a fixed-bed column packed with PIN-NH₂ was carried out to evaluate the performances of PIN-NH₂ aerogel in an actual adsorption-based separation process. The details of the experiment process were described in ESI.^† As shown in Fig. S9 and S10, ESI,^† the CH₄ and N₂ penetrated through the bed firstly with a retention time for only 6.5 and 3.4 min, respectively, while PIN-NH₂ column can retain CO₂ until above 23 min, which means the high CO₂ adsorption capacity and selectivity of the PIN-NH₂ adsorbent in actual application.