Azopyridine Research Articles

Photoresponse of new azo pyridine functionalized poly(2-hydroxyethyl methacrylate-co-methyl methacrylate)

A new azo polymer containing photoisomerizable azo pyridine functionalities was synthesized via Mitsunobu reaction of 4-(4-hydroxyphenylazo)pyridine with poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (p(HEMA-co-MMA)) for creating new photochromic materials. The resulting polymer with azo pyridine side groups was characterized for structural, thermal, and optical properties. UV–vis, 1H NMR and IR spectroscopies confirmed that all hydroxyl groups in p(HEMA-co-MMA) were substituted with azo dye. The obtained azo copolymer exhibited high thermal stability (around 240 °C) and a glass transition temperature (113 °C), promising for applications. The trans-to-cis isomerization upon UV irradiation and the thermal back reaction of the azo chromophore in the copolymer in the solid state was studied. A photostationary state with 50% content of cis-isomers upon 6 min of UV irradiation was reached, and during 48 h dark relaxation at ambient temperature, all cis-isomers converted to the trans form. Additionally, the possibility of efficient photogeneration of surface relief gratings with high amplitude of azo copolymer surface modulation was demonstrated.

Comprehensive investigations of trans-cis-trans isomerization in the solid state for azo polyimides

This work describes the thermal cis-trans isomerization in the solid-state for poly(ether imide)s and poly(ester imide)s with azobenzene or azo pyridine moieties. The cis-trans relaxation was monitored for covalently functionalized polyimides (main-chain, side-chain, „T-type”) and “guest-host” azo systems. The obtained results from the investigations allow us to formulate some general relationships between material aspects such as polyimide structure, its molar mass, the content of azo dye, the value of d-spacing, the thickness of polymer film, and thermal cis-trans isomerization. The reversibility of the trans-cis-trans reaction was studied in 10 subsequent cycles and 100 cycles for the selected polymers. Studies of the reversibility of the trans-cis-trans reaction were realized by irradiation using two wavelengths of radiation λ = 365 nm (P = 2.9 W) and λ = 460 nm (P = 850 mW). To the best of our knowledge, this is the first article that comprehensively presents solid-state cis-trans isomerization for azo polyimides.

Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System.

Finding stable analogues of three-dimensional (3D) lead halide perovskites has motivated the exploration of an ever-expanding repertoire of two-dimensional (2D) counterparts. However, the bandgap and exciton binding energy in these 2D systems are generally considerably higher than those in 3D analogues due to size and dielectric confinement. Such quantum confinements are most prominently manifested in the extreme 2D realization in (A)mPbI4 (m = 1 or 2) series of compounds with a single inorganic layer repeat unit. Here, we explore a new A-site cation, 4,4'-azopyridine (APD), whose size and hydrogen bonding properties endow the corresponding (APD)PbI4 2D compound with the lowest bandgap and exciton binding energy of all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI4 presents the first example of the ideal Pb-I-Pb bond angle of 180°, maximizing the valence and conduction bandwidths and minimizing the electron and hole effective masses. These effects coupled with a significant increase in the dielectric constant provide an explanation for the unique bandgap and exciton binding energies in this system. Our theoretical results further reveal that the requirement of optimizing the hydrogen bonding interactions between the organic and the inorganic units provides the driving force for achieving the structural uniqueness and the associated optoelectronic properties in this system. Our preliminary investigations in characterizing photovoltaic solar cells in the presence of APD show encouraging improvements in performances and stability.

Kinetics of the dark cis–trans isomerization of azobenzene and azo pyridine derivatives in ethanol and chloroform solutions

The paper presents a series of azobenzene and azo pyridine compounds containing hydroxyl or 6-hydroxyhexyloxyl substituents. The nitrogen atom is located at the ortho-, meta- or para-position in the structure of azo pyridine derivatives. The impact of the structural features of azo chromophores on their thermal (DSC, TGA) and optical (UV–Vis) properties was investigated. The dark cis–trans isomerization kinetics was studied in ethanol and chloroform at 298 K. The formation of noncovalent hydrogen bonds between azo molecules and/or chromophore-solvent was monitored using the 1H NMR spectroscopy and estimated by density functional theory (DFT) calculations. Based on experimental and computational studies we proposed the mechanism of the dark cis–trans isomerization for hydroxy-substituted azo compounds.

Construction of a bioactive copper-based metal organic framework-embedded dual-crosslinked alginate hydrogel for antimicrobial applications

Metal-organic frameworks (MOFs) containing bioactive metals have the potential to exhibit antimicrobial activity by releasing metal ions or ligands through the cleavage of metal-ligand bonds. Recently, copper-based MOFs (Cu-MOFs) with sustained release capability, porosity, and structural flexibility have shown promising antimicrobial properties. However, for clinical use, the controlled release of Cu2+ over an extended time period is crucial to prevent toxicity. In this study, we developed an alginate-based antimicrobial scaffold and encapsulated MOFs within a dual-crosslinked alginate polymer network. We synthesized Cu-MOFs containing glutarate (Glu) and 4,4′-azopyridine (AZPY) (Cu(AZPY)-MOF) and encapsulated them in an alginate-based hydrogel through a combination of visible light-induced photo and calcium ion-induced chemical crosslinking processes. We confirmed Cu(AZPY)-MOF synthesis using scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, and thermogravimetric analysis. This antimicrobial hydrogel demonstrated excellent antibacterial and antifungal properties against two bacterial strains (MRSA and S. mutans, with >99.9 % antibacterial rate) and one fungal strain (C. albicans, with >78.7 % antifungal rate) as well as negligible cytotoxicity towards mouse embryonic fibroblasts, making it a promising candidate for various tissue engineering applications in biomedical fields.

Copper(II)-MOF Containing Glutarate and 4,4′-Azopyridine and Its Antifungal Activity

Antifungal activities of MOFs (metal organic frameworks) have been demonstrated in studies, and improvement in efficiency of fungal inactivation is a critical issue in the application of MOFs. In this study, we employed 4,4′-azopyridine (AZPY) in the construction of MOF to improve its antifungal activity. Three-dimensional (3D) copper metal organic framework containing glutarate (Glu) and AZPY (Cu(AZPY)-MOF) was synthesized by a solvothermal reaction. Glutarates bridge Cu2 dinuclear units to form two-dimensional (2D) layers, and these layers are connected by AZPY to form a 3D framework. When spores of two fungi, Candida albicans and Aspergillus niger, were treated with Cu(AZPY)-MOF for one day, number of CFU (colony forming unit) was continuously reduced over treated MOF concentrations, and maximum 2.3 and 2.5 log10CFU reductions (approximately 99% reduction) were observed in C. albicans and A. niger, respectively. Small amounts of CuII ions and AZPY released from Cu(AZPY)-MOF were not critical for fungal inactivation. Our results indicate that the level of antifungal activity of Cu(AZPY)-MOF is greater than that of Cu-MOF without AZPY constructed in our previous study, and intercalation of AZPY is able to improve the antifungal activity of Cu(AZPY)-MOF.

Tweaking of supramolecular hydrogel property of single and two-component gel systems by a bifunctional molecule

The properties of the supramolecular hydrogel built from the self-assembly of low molecular weight (LMW) compound play a crucial role in selecting the hydrogel for specific or targeted applications. Instead of developing new hydrogels, tweaking the properties of existing, well-known hydrogels using guest compounds may provide a way to design new hydrogel materials with desired properties. In this context, the influence of a bifunctional molecule (guest), namely 4,4′-Azopyridine (APy), on the thermal stability and viscoelastic properties of the supramolecular hydrogels built from a single (Fluorenylmethoxycarbonyl-L-Phenylalanine, FmF / Fluorenylmethoxycarbonyl-L-Tyrosine, FmY) and two-component (FmF+FmY) systems were analyzed. APy can connect the gelators through hydrogen bonding, and thereby it can tweak the property of the hydrogel. Various biophysical studies indicated that APy increased the gelation time of single and two-component systems while significantly improving the viscoelastic nature of both systems. Unlike FmF hydrogel, the regelation (thixotropy) efficiency of a two-component FmF+FmY gel system with APy showed complete recovery, suggesting that tweaking of thixotropic efficiency depends on the type of component gel systems. The Tgel of FmF gel was unaltered, while the two-component gel system had decreased stability in the presence of APy, suggesting that guest molecule can modulate the thermal stability of the two-component gel system. Gel characterization using various spectroscopic and microscopic techniques suggested that supramolecular self-assembly of gelators in a single and two-component systems with APy leads to fibrillar morphology, which is stabilized through intermolecular aromatic π-π stacking and hydrogen bonding interactions. NMR and FTIR analysis confirmed the interaction of APy with gelators in the self-assembled structure. Thus, the contemporary study demonstrated that the mechanical and thermal stability of the existing and/or new supramolecular gel systems could be modulated by co-assembly with guest molecules.

Dosimetric Chromogenic Probe for Selective Detection of Sulfide via Sol-Gel Methodology.

Dinitrobenzenesulfonyl-protected naphthyl azo pyridine conjugate 1 has been designed and synthesized. Compound 1 acts as a nongelator in dimethyl sulfoxide (DMSO)–H2O (1:1, v/v) while its hydroxy counterpart 2 can form a nice gel in the same solvent. In the presence of sulfide, compound 1 undergoes rapid sulfonate ester hydrolysis and results in the formation of azo-naphthol 2 that responds in instant gelation. Such deprotection was extremely selective to sulfide; other analytes did not show measurable response. The sensing mechanism has been established by various spectroscopic techniques. Compound 1 in solution (DMSO–H2O) also shows a selective response toward sulfide over a series of other anions with a color change. Preparation of test kit with compound 1 allows detection of sulfide in solution and vapor states. Such kind of dosimetric sensing of chemical analytes by improvising the protection/deprotection of functional groups in gelator structure is rare in the literature, and to the best of our knowledge, this is the first example of a stimuli-responsive low-molecular-weight gelator which dosimetrically senses sulfide over other nucleophilic substrates.

Azopyridine molecular conductor: A superior device for molecular switch technology

The electron transport through a single molecular conductor consisting of an azopyridine (AZP) sandwiched between Au electrodes has been investigated using the nonequilibrium Green’s function formalism combined with the DFT method. We show that when AZP converts between its trans and cis conformations, this simple molecular device switches between a strongly and a weakly conducting state. We also found that the trans isomer can exhibit novel NDR behavior which can be used as the basis of memory, switching and logic functionality. The switching behavior and NDR characteristic are rationalized by analyzing the device transmission coefficients and electronic structures.

Methylated Azopyridine as a New Electron Transfer Mediator for the Electrocatalytic Oxidation of NADH

AbstractCarbon ionic liquid electrode (CILE) has been modified with a new synthesized mediator i.e. N,N′‐dimethyl‐4,4′‐azopyridinium hexafluorophosphate (MAZPHP) via sol process and the electron transfer mediating characteristics of this mediator has been evaluated. 4,4′‐Azopyridine (AZP) did not show any electrocatalytic activity toward the selected probe, NADH, while its synthesized methylated derivative, MAZPHP, is a very efficient mediator for the electrocatalytic NADH oxidation. Cyclic voltammetry of MAZPHP/Sol/CILE exhibited a pair of reversible peaks corresponding to incorporated mediator with a formal potential of about 221 mV vs. Ag/AgCl. MAZPHP/Sol/CILE is free from fouling effects by the oxidation products of NADH which generally give hindrance to amperometric detection of NADH. Using amperometric technique, NADH can be determined in the range of 1.0×10−5 M to 1.4×10−3 M with a detection limit of 2.0×10−6 M.

Density functional theory study on surface-enhanced Raman scattering of 4,4′-azopyridine on silver

Surface-enhanced Raman scattering (SERS) of 4,4′-azopyridine (AZPY) on silver foil substrate was measured under 1064 nm excitation lines. Density-functional theory (DFT) methods were used to calculate the structure and vibrational spectra of models such as Ag–AZPY, Ag 4–AZPY and Ag 6–AZPY complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of AZPY were identified on the ground of analog computation of potential energy distribution. The calculated spectra of Ag 4–AZPY and Ag 6–AZPY models were much approximated to the experimental results than that of Ag–AZPY model. The DFT results showed that the angles between two pyridyl rings keep 0° from AZPY to Ag–AZPY, Ag 4–AZPY and Ag 6–AZPY model. The energy gaps between the HOMO and LUMO changed from 363 to 1140 nm for AZPY-Ag complexes according to the DFT results. An conclusion was conceived that chemical enhancement mechanism may play an important role in the SERS of AZPY on silver substrate.

Supramolecular Hydrogen-Bonded Liquid Crystals Formed from 4-(4′-Pyridylazophenyl)-4″-alkoxy Benzoates and 4-Substituted Benzoic Acids

Four pyridine-based derivatives (I 10, I 12, I 14, and I 18) with molecular formula CnH2n+1O-C6H4COOC6H4-N = N-C5H4N, that differ from each other in the number of carbon atoms, n, were prepared and characterized for their mesophase behavior. The number of carbons in the alkoxy chain varies between 10, 12, 14, to 18 atoms, respectively. Liquid crystalline properties were studied for two series of hydrogen-bonded liquid crystal complexes, consisting of the central 4-pyridylazophenyl benzoate and substituted benzoic acids. In the first series of the supramolecular complexes (group A), the lengths of the alkoxy chains on both of the azo pyridine and the benzoic acid components have varied between 10, 12, 14, and 18 carbons. In the second series (group B), the same proton-acceptors, i.e., the pyridyl azo derivatives, were used but with benzoic acids para-substituted with small compact polar groups, X. All of the newly prepared pyridine-based derivatives and their hydrogen-bonded complexes were found to be mesomorphic. The newly prepared pyridine-based derivatives and their supramolecular complexes in both groups were characterized for their mesophase behavior by differential scanning calorimetry, DSC, and polarized light microscopy, PLM. Four 4-alkoxybenzoic acids (CmH2m+1OC6H4COOH, II 10, II 12, II 14, and II 18) were used in group A series; the number of carbons (m) of their alkoxy groups also varies between 10, 12, 14, and 18, respectively. Substituents on the other group of acids (III a−f) in group B series vary, respectively, between CH3O, CH3, H, Br, CN, and NO2. Smectic C mesophase is induced in most of the binary mixtures investigated.

Spectroscopy of 4,4′-azopyridine by density functional theory and surface-enhanced Raman scattering

The molecular geometry and vibrational frequencies of 4,4′-azopyridine (AZPY) were calculated using the density functional theory (DFT) methods. Fourier transform infrared (FTIR), fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of AZPY on the silver foil substrate have been recorded. On the basis of the computed results, FTIR, FT-Raman and NIR-SERS bands were assigned. It is shown that the calculated and observed frequencies are in good agreement with each other for FTIR and FT-Raman spectra. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of AZPY on the silver foil substrate surface.

Synthesis and photochemical behavior of metalloporphyrin complexes containing a photochromic axial ligand

Metalloporphyrin complexes with a photochromic axial ligand were synthesized and photoisomerization behavior of those complexes was investigated. In this study, 3,3′-azopyridine ( AZP) was used as the axial ligand. We found that the structure of the axial-coordinated complexes depended on the center metal of porphyrin. When magnesium was employed as the center metal, a 1 : 1 coordinated complex was obtained. On the other hand, zinc or rhodium porphyrins formed the sandwich-bonded complexes. The photoisomerization behavior also depended on the center metal of the porphyrins, and the cis-to- trans “one-way” photoisomerization of the AZP axial ligand took place in the complexes of zinc and rhodium porphyrins. The one-way isomerization may be interpreted as the results of the photoinduced electron transfer from porphyrin to AZP.

Hydrogen-bonded azopyridine and succinic acid co-crystal

In the 1:1 supramolecular adduct of azopyridine (AZP) and succinic acid (SA) [systematic name: di-4-pyridyldiazene-succinic acid (1/1)], C10H8N4.C4H6O4 or AZP.SA, both components lie on inversion centers. Alternating AZP and SA molecules are linked by O-H...N hydrogen bonds to form a linear chain extending in the [31-1] direction. Between chains there is a strong pyridyl-azo pi-pi interaction, with a 3.340 A separation between the inversion center at the mid-point of the azo bond and the centroid of the pyridine ring; this interaction results in the formation of sheets.

Two-dimensional correlation spectroscopic studies of hydrogen-bonded supramolecular assemblies between azaaromatic molecule with dicarboxylic acid

Infrared (IR) spectra have been measured for a supramolecular assembly based on intermolecular hydrogen bonding between sebacic acid (SEA) molecule as a proton donor and azopyridine (AZP) molecules as a proton acceptor ([SEA·AZP]) over a temperature range from 29 to 155 °C in order to explore the involved hydrogen-bonding species and heat-induced structural variations. The temperature-dependent IR spectra have shown that the intermolecular hydrogen bondings are formed and stable enough in the supramolecular assembly but they become slightly decoupled when temperature is higher than the clearing point of supramolecular assembly. Two-dimensional (2D) correlation spectra in 1780–1660 cm −1 region have provided information about the structural variations of intermolecular hydrogen bonding concerning with CO group. While, 2D correlation spectra in 880–820 cm −1 region have revealed hydrogen bonding changes concerning with proton in the adjacent position of pyridyl ring. Three different hydrogen-bonded species in the supramolecular assembly have been clarified by the 2D correlation analysis. The specific order of the temperature-induced structural changes in the involved intermolecular hydrogen-bonded species in supramolecular assembly has also been elucidated in the present study.

Self-assembly of transition-metal-based macrocycles linked by photoisomerizable ligands: examples of photoinduced conversion of tetranuclear-dinuclear squares.

A series of hetero- and homometallic square complexes bridged by a photoactive 4,4'-azopyridine (AZP) or 1,2-bis(4-pyridyl)ethylene (BPE) ligand, cyclobis[[cis-(dppf)M](mu-L)(2)(fac-Re(CO)(3)Br)](OTf)(4) (M = Pd, L = trans-AZP (5); M = Pt, L = trans-AZP (7); M = Pd, L = trans-BPE (8); M = Pt, L = trans-BPE (10)), cyclo[[cis-(dppf)M](mu-L)(2)(fac-Re(CO)(3)Br)](OTf)(2) (M = Pd, L = cis-AZP (6); M = Pd, L = cis-BPE (9)), [cis-(dppf)Pd(mu-trans-AZP)](4)(OTf)(8) (11), and [cis-(dppf)Pd(mu-cis-AZP)](2)(OTf)(4) (12), where dppf is 1,1'-bis(diphenylphosphino)ferrocene and OTf is trifluoromethanesulfonate anion, were prepared by thermodynamically driven self-assembly processes. The photophysical and photochemical properties of these complexes have been investigated, and all of them show a lack of luminescence in room temperature solution. Upon irradiation at 313 or 366 nm, Pd(II)-Re(I)-containing tetranuclear squares 5, 8, and 11 undergo photoisomerization and convert to their corresponding dinuclear complexes 6, 9, and 12, whereas Pt(II)-Re(I)-based squares 7 and 10 show only slow square disassembling processes. The tetranuclear squares can be fully recovered by heating the photoisomerized solution for several hours.

Synthesis, photophysical, photochemical and electrochemical properties of rhenium(I) diimine complexes with photoisomerizable pyridyl-azo, -ethenyl or -ethyl ligands

A series of rhenium(I) diimine complexes with pyridyl-azo, -ethenyl or -ethyl type ligands, [Re(CO)3(N–N)L]ClO4 and [{Re(CO)3(N–N)}2L′][ClO4]2 {N–N = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen); L = 4-phenylazopyridine (PHAZO), 4-styrylpyridine (STYPY), [2-(4-pyridyl)ethyl]benzene (PEB), 4-(4-nitrostyryl)pyridine (NSP); L′ = 4,4′-azopyridine (AZO), 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPENB), 1,4-bis[2-(4-pyridyl)ethyl]benzene (BPEB)} have been synthesized and their photophysical properties studied. The quantum yields for the photoinduced isomerization of the complexes [Re(CO)3(bpy)(NSP)]+ and [Re(CO)3(phen)(NSP)]+ have been determined.

Synthesis and structural studies by infrared and Mössbauer spectroscopy of adducts of tin(IV) and organotin(IV) derivatives with 2,2′-azopyridine

A number of complexes have been prepared by the reaction between 2,2′-azopyridine(AZP) and tin(IV) halides and organotin(IV) halides, and characterized by elemental analysis and infrared and variable temperature 119Sn Mössbauer spectroscopies. All of the new compounds have 1:1 stoichiometry, with the AZP ligand occupying two coordination sites by bonding through one of the ring and one of the azo group nitrogen atoms, to give rise to distorted octahedral structures. In the diorganotin complexes the two organic groups occupy trans positions. The infrared and Mössbauer spectroscopic data suggest that these compounds are monomeric in the solid state.

The Electrochemical Behavior of 4,4'‐Azopyridine‐1,1' ‐Dioxide

The electroreduction of 4,4'‐azobispyridine‐1,1'‐dioxide (APDO) in dimethylformamide solutions was studied by polarography, cyclic voltammetry, controlled potential coulometry, and electron spin resonance, Voltammetric reduction showed two well‐defined, reversible one‐electron transfers. Coulometric reduction at a potential corresponding to the plateau of the first wave gave evidence of chemical reactions following the initial electron transfer producing intermediates which were also electroactive at the same potentials. Coulometry at the second wave gave an over‐all value of six Faradays per mole for the complete reduction of APDO to the dianion of azopyridine (AP), identified by electrochemical and electron spin resonance techniques. A mechanism for the reduction process consistent with the experimental evidence is presented.