Photoisomerization Behavior Research Articles

Photoisomerization in Photoaligned Azo Dyes Exhibiting Photostability

Here in this investigation, the molecules exhibiting photoisomerization in solutions is also photostable is observed. On the other hand molecules not showing photoisomerization behavior in solutions is not shown such kind of stability. Surprising behavior in which after writing laser is turned off, there is going to be increase in phase retardation (or order parameter) which is under deep consideration. Detail investigation is in progress to reveal the behavior of chemical structures when light is shined on them. Proposed investigation is crucial for selecting suitable azo dyes for photoalignment studies and also for many commercial applications which depends on many factors like stability, order parameter etc.

Synthesis of azobenzene‐functionalized two‐arm, three‐arm and four‐arm telomers using polyfunctional chain transfer agents

AbstractBACKGROUND: Star‐shaped polymers are very attractive because of their interesting properties such as reduced viscosity, good solubility, low glass transition temperature and fast response to external stimuli. The incorporation of azobenzene moieties in star‐shaped polymers could significantly widen their potential applications in various optical devices. One of the most important properties of the azobenzene chromophore is its reversible trans–cis photoisomerization induced by UV or visible light. Photoisomerization induces conformational changes in azopolymer chains, which in turn lead to macroscopic variations in chemical and physical properties of the surroundings and media.RESULTS: This study reports the synthesis of azobenzene‐functionalized two‐, three‐ and four‐arm telomers via free radical telomerization using the di‐, tri‐ and tetrafunctional chain transfer agents 1,2‐ and 1,4‐benzenedimethanethiol, trimethylolpropane‐tris(2‐mercaptoacetate) and pentaerythritol‐tetrakis(3‐mercaptopropionate), respectively, in the presence of azobisisobutyronitrile. Azotelomers were characterized using gel permeation chromatography and 1H NMR and Fourier transform infrared spectroscopy. Thermal phase transition behaviors were investigated using differential scanning calorimetry and polarized optical microscopy. Azotelomers synthesized in this study showed reversible photoisomerization and a fast generation of birefringence.CONCLUSION: Considering the photoisomerization behavior and birefringence of the two‐, three‐ and four‐arm azotelomers, it can be concluded that they could be potential candidates for use in various optical devices. Copyright © 2009 Society of Chemical Industry

Solvent dependent photo-isomerization of 4-dimethylaminoazobenzene carboxylic acid

We have observed reversible trans–cis photo-isomerization behaviors of 4-dimethylaminoazobenzene carboxylic acid (4DAzC) in ethyl acetate by alternating irradiations at 370 and 430nm. The photo-isomerization from trans- to cis-isomer was found to be strongly solvent dependent and not to occur efficiently in water and ethanol. Also photo-isomerization from cis- to trans-isomer was occurred upon 370nm illumination or via thermal relaxation. The activation energy for the thermal isomerization from cis to trans-isomer was estimated to be 49.2kJ/mol in ethyl acetate from the temperature-dependent kinetic absorption measurements.

Trans–cis Photoisomerization of Azobenzene‐Conjugated Dithiolato‐Bipyridine Platinum(II) Complexes: Extension of Photoresponse to Longer Wavelengths and Photocontrollable Tristability

Azobenzene derivatives modified with dithiolato-bipyridine platinum(II) complexes were synthesized, revealing their highly extended photoresponses to the long wavelength region as well as unique photocontrollable tristability. The absorptions of trans-1 and trans-2 with one azobenzene group on the dithiolene and bipyridine ligands, respectively, cover the range from 300 to 700 nm. These absorptions are ascribed, by means of time-dependent (TD)DFT calculations, to transitions from dithiolene(pi) to bipyridine(pi*), namely, interligand charge transfer (CT), pi-pi*, and n-pi* transitions of the azobenzene unit, and pi-pi* transitions of the bipyridine ligand. In addition, only trans-1 shows distinctive electronic bands, assignable to transitions from the dithiolene(pi) to azobenzene(pi*), defined as intraligand CT. Complex 1 shows photoisomerization behavior opposite to that of azobenzene: trans-to-cis and cis-to-trans conversions proceed with 405 and 312 nm irradiation, which correspond to excitation with the intraligand CT, and pi-pi* bands of the azobenzene and bipyridine units, respectively. In contrast, complex 2 shows photoisomerization similar to that of azobenzene: trans-to-cis and cis-to-trans transformations occur with 365 and 405 nm irradiation, respectively. Irradiation at 578 nm, corresponding to excitation of the interligand CT transitions, results in cis-to-trans conversion of both 1 and 2, which is the longest wavelength ever reported to effect the photoisomerization of the azobenzene group. The absorption and photochromism of 4, which has azobenzene groups on both the dithiolato and bipyridine ligands, have characteristics quite similar to those of 1 and 2, which furnishes 4 with photocontrollable tristability in a single molecule using light at 365, 405, and 578 nm. We also clarified that 1 and 2 have high photoisomerization efficiencies, and good thermal stability of the cis forms. Complexes 3 and 5 have almost the identical photoresponse to those of their positional isomers, complexes 2 and 4.

A Novel Molecular Architecture of a Liquid-crystalline Polymer showing Nanophase-separated Surfaces

Novel azotolane derivatives containing an oligooxyethylene moiety have been synthesized via palladium-catalyzed cross coupling reactions as the key step. The thermotoropic liquid crystalline behavior of the compound was investigated by polarizing optical microscopy and differential scatering calorimetry. Furthermore, we observed that the compound shows trans-cis photoisomerization behavior upon irradiation of ultraviolet light, as well as a conventional azobenzene moiety.

A Novel Ferroceneylazobenzene Self-Assembled Monolayer on an ITO Electrode: Photochemical and Electrochemical Behaviors

A novel ferroceneylazobenzene self-assembled monolayer (SAM) has been constructed on an indium-tin oxide (ITO) electrode via the covalent attachment of 4-(4'-11-ferrocenyl-undecanoxyphenylazo)benzoic acid ( FcAzCOOH) onto a silanized ITO substrate surface and verified by reflectance infrared spectroscopy and water contact angle. Atomic force microscopy (AFM) and cyclic voltammogram (CV) indicated that the FcAzCOOH formed a uniform and reproducible SAM on the ITO electrode with a surface coverage of ca. 1.9 x 10 (-10) mol/cm (2) (87 A (2)/molecule). The reversible photoisomerization behavior of the SAM was characterized by UV-vis spectra. The azo pi-pi* transition band intensity of the SAM gradually decreased with UV (365 nm) irradiation and was almost recovered again when subsequent exposure to ambient room light (400-800 nm). The increased tilt angle of the molecules on the ITO substrate after UV irradiation further confirmed the trans-to- cis isomerization of azobenzene moieties. The CV of the trans- FcAzCOOH modified ITO electrode showed a pair of waves due to redox of the ferrocene groups in the potential range of 0 to +800 mV (vs SCE), and the peak separation of the redox wave became larger after UV irradiation and almost returned to its original value after subsequent exposure to the visible light. Rate-dependent CV curves indicated that the charge transfer rate between the ferrocene species in the SAM and the ITO electrode was slowed down after UV irradiation due to the smaller porosity of the monolayer film and the more compact barrier layer between the redox species and the ITO electrode. It is the first time to directly observe the influence of photoisomerization of the azobenzene moiety on the redox behavior of redox species in the ferroceneylazobenzene-functionalized SAM. The present results provide profound insight into the role of redox microenvironment on electron transfer kinetics and also provide a simple and facile approach to the preparation of photocontrollable electrodes.

Ionomeric poly(urethane semicarbazides) with azobenzene groups in the main chain – studies on photoswitching behaviour and mechanical properties

Poly(urethane semicarbazide) anionomers containing azobenzene chromophores in the main chain were prepared by chain extending the isocyanate terminated prepolymers with chromophoric dihydrazide. Variations were done with respect to the hard segment content and the nature of the diisocyanate and the effect of the variations on the thermal, mechanical properties and wettability were studied. The polymers in solution showed reversible photoisomerization behaviour evinced by UV/vis spectroscopy in which irradiation of polymer solutions with UV light was observed as a decrease in intensity of absorbance corresponding to trans-form and increase in the absorbance of cis-form. The back relaxation took place in the presence of visible light. Photoswitching or the wettability of the films induced by the reversible cis– trans isomerization of the azobenzene chromophores was experimentally shown from water contact angle measurements. Irradiation of the films with 365 nm light effected a decrease in the water contact angle. X-ray diffraction results indicated dense arrangement of crystallized hard segments in compositions containing higher hard segment content. Thermal stability up to 300 °C was exhibited by the polymers as shown by thermogravimetric analysis and the phase separated morphology was confirmed by dynamic mechanical analysis. Tensile strength measurements showed that the films with increasing hard segment content exhibited increasing tensile strength and modulus but decreasing values of elongation.

Synthesis of organoboron compounds bearing an azo group and substituent effects on their structures and photoisomerization

2-(Phenylazo)phenylboranes bearing several substituents were synthesized and substituent effects on their structures and photoisomerization behaviors were investigated to reveal the scope of the photoswitching of the coordination number of the boron by using an azobenzene-based photoresponsive ligand, 2-(phenylazo)phenyl group. 11B NMR, X-ray crystallographic analysis, and UV–vis spectra revealed that electron-donating ability of the substituents at both the boron atom and the azobenzene moiety determined the strength of the interaction between the boron and the nitrogen of the azo group. Photoisomerization behaviors of 2-(phenylazo)phenylboranes are largely affected by the B–N interaction.

Synthesis and photoisomerization behavior of a novel thiazolylazo polyimide

AbstractPolyimide (PI) with thiazolylazo side chain was prepared via a post azocoupling reaction scheme. The PI film could easily be obtained by spin coating and consequent imidization from the precursor polymer, which was soluble in several polar organic solvents such as N‐methyl‐2‐ketopyrrolidine and N,N‐dimethylformarride. The polymer showed a reverse trans–cis photoisomerization when irradiated by the lights of 365 nm and 440 nm. The glass‐transition temperature (Tg) and temperature at 10% of weight loss under N2 of the polymer are 138.9°C and 510.6°C, respectively, which confirm that the thermal stability of the polymer is quite good. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Synthesis and Photoisomerization of Fullerene− and Oligo(phenylene ethynylene)−Azobenzene Derivatives

The presence of fullerenes and oligo(phenylene ethynylene)s (OPEs) in azobenzene derivatives have a large effect on the photoisomerization behavior of the molecules. Fullerenes reduce the photoisomerization yield for cis isomers, and the OPEs, when directly attached to the azobenzenes, have a similar yet smaller effect when compared with the fullerenes. While these effects have not been previously considered for fullerene--and OPE-azobenzene derivatives, they were clearly detected in our work using NMR and UV-vis spectroscopy methods. The intramolecular electronic energy transfer between the fullerene and azobenzene moiety was examined in two cases in which separation of the two functional groups was small, as in 1, or large, as in 2. Almost no photoisomerization was observed for 1, while significant photoisomerization was observed for 2, apparently due to the effective isolation and blocking of electronic communication between the two functional groups.

Preparation of side‐on bisazobenzene‐containing homopolymers and block copolymers via ATRP and studies on their photoisomerization and photoalignment behaviors

AbstractThe preparation of a series of novel homopolymers and copolymers containing bisazobenzene chromophores with side‐on structure in the side chains via atom transfer radical polymerization (ATRP) were presented. UV–vis spectra of the thin films of these polymers under irradiation of 488 nm Ar+ laser suggested that the photoisomerization of the bisazobenzene chromophores happened mainly on one of the two azo groups in the bisazobenzene chromophores with similar probability due to their side‐on structure. Good photoalignment behaviors of these polymers were proved by photoinduced birefringence measurements because side‐on structure permitted the two azo groups in the bisazobenzene chromophores both participated in the trans–cis–trans photoisomerization cycles equally to induce the whole chromophore reorientation. Furthermore, the reorientation axis located at the middle of chromophore decreased the sweep volume during photoalignment. The impetus for this study was to evaluate the photoisomerization and photoalignment process of side‐on bisazobenzene‐containing polymers and to find possible applications in the photosensitive devices. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3460–3472, 2007

Photoisomerization and Thermal Isomerization of Arylazoimidazoles

Photoisomerization and thermal isomerization behaviors of an extensive series of arylazoimidazoles are investigated. Absorption spectra are characterized by a structured pipi* absorption band around 330-400 nm with a tail on the lower energy side extending to 500 nm corresponding to an npi* transition. The trans-to-cis photoisomerization occurs on excitation into these absorption bands. The quantum yields are dependent on the excitation wavelength, as observed for azobenzene derivatives, but are generally larger than those of azobenzene. The thermal cis-to-trans isomerization rates are also generally larger than that of azobenzene and are comparable to those of 4-N,N-dimethylaminoazobenzene and 4-nitroazobenzene. Arylazoimidazoles with no substituent on the imidazole nitrogen are unique in that the quantum yield for the trans-to-cis photoisomerization and the rate of thermal cis-to-trans isomerization are particularly large. It is proposed that the fast thermal isomerization is due to an involvement of self-catalyzed and protic molecule-assisted tautomerization to a hydrazone form.

Colloidal Sphere Formation, H-Aggregation, and Photoresponsive Properties of an Amphiphilic Random Copolymer Bearing Branched Azo Side Chains

This work studied the colloidal sphere formation, H-aggregation, and photoresponsive properties of an amphiphilic random copolymer functionalized with branched azo side chains (POAPB6P-AC). The colloidal spheres were prepared through gradual hydrophobic association of the polymer chains in THF−H2O media, which was induced by a continuous increase in the water content. The forming process and morphology of the colloidal spheres were characterized by DLS, SLS, and TEM. UV−vis spectroscopy was used to study the azo chromophore H-aggregation and structure variation in the colloid formation process by measuring the parameters such as λmax, the trans-to-cis photoisomerization rate, and the isomerization degree at the photostationary state. The results indicate that the colloidal sphere formation undergoes several different stages as the water content increases. The polymeric chains start to associate at the critical water content (CWC). CWC is related with the initial concentration of the polymer in THF and is estimated to be 23 vol % when the initial concentration is 1.0 mg/mL. When the water content increases beyond the CWC, more and more polymer chains change from “isolated” single chains to clusters of associated chains. When the water content reaches 37%, almost all polymer chains are involved in the clusters. As the water content further increases, the clusters experience a collapse caused by the strong hydrophobic interaction, which is indicated by a dramatic size contraction detected from the DLS measurements. When the water content reaches about 60 vol %, the azo chromophores of POAPB6P-AC start to form H-aggregates, which is indicated by a significant blue shift (from 360 to 339 nm) of the UV−vis maximum absorption band. The structure variation can also be detected from the photoisomerization behavior of the systems. When the water content changes from 50 to 60 vol %, both the trans-to-cis photoisomerization rate and the isomerization degree at the photostationary state decrease dramatically. Finally, when the water content increases from 95 to 100 vol %, the isomerization rate and degree significantly decrease again. In this stage, THF in the microphase is almost completely replaced by H2O, and colloidal structure is “frozen” because of the free volume decrease and chain entanglement. The above observations give insight into the occurrence of H-aggregation in colloids of amphiphilic azo polymers for the first time. The understanding can lead to further development in expanding functionalities of the photoresponsive colloidal spheres through manipulation of the chromophore aggregation.

New thiophene-based azo ligands containing azo methine group in the main chain for the determination of copper(II) ions

New kind of azo chromophores containing thiophene moiety and salicyaldimine-based ligand as side chains have been synthesized by a sequential process for optical sensing of Cu 2+ metal ions in organic solution. Compounds have been characterized by IR and UV–vis spectrophotometer, 1H NMR, 13C NMR, MS, TGA and CV instruments. Spectral characteristics of the synthesized compounds have been investigated in five organic solvents of different polarity. Polarizibility effects of the solvents on the spectral characteristics and the dipole moments in the excited state are estimated. The fluorescence quantum yields of the synthesized compounds have been calculated to be in the range of 0.0035–0.0095 in solvents of different polarity. The fluorescence emission quenching experiments between the synthesized compounds and electron donor (pyrene, anthracene), electron acceptor (Co 2+) compounds give the bimolecular quenching rate constants of 10 11 and 10 14 M −1 s −1, respectively. The free energies of photo-electron transfer process (Δ G ET) between the azo dyes and the quenchers have been found to be about −19 kcal/mol. Cyclic voltammetry studies indicate that synthesized azo ligands undergo two- or three-reversible reduction potentials (versus ferrocene) and give LUMO energy value of −3.07 eV which is lower than that of TiO 2 conduction band and has a band gap value of ∼2.5 eV. These results may point that synthesized azo dyes could be used as hole conducting materials in solid DSSC (Dye Sensitized Solar Cell) devices. Thermal decomposition behavior of the azo dyes gives more information about the structure of the studied materials. The photoisomerization behavior of the synthesized compounds has been investigated in ethyl acetate under Xe lamp irradiation in the fluorescence spectrophotometer for 1 h. Photoisomerization rate constants of cis– trans orientation ( k c–t) have been found to be about 10 −5 s −1. The complexation process of synthesized thienylidene azo dyes gives subtle changes in their absorption spectra.

Influence of Wavelength of Light on Photoinduced Orientation of Azobenzene-Containing Polymethacrylate Film

Influence of the wavelength of linearly polarized (LP) light on the generation of photoinduced optical anisotropy of E- and Z-isomers of a polymethacrylate which comprises 4-methoxyazobenzene side groups was investigated using LP 313 nm, 365 nm, 405 nm, 436 nm, 532 nm, and 633 nm lights. Different photoisomerization and molecular reorientation behavior were observed depending on the wavelength of the light. Large in-plane orientation was achieved by the use of LP 313 and LP 532 nm lights. The ratio of the absorbance between E- and Z-isomers plays an important role in the photoinduced orientation behavior.

Stereoselective Synthesis of cis- and trans-Oligo(phenylenevinylene)s via Palladium-Catalyzed Cross-Coupling Reactions

cis-Oligo(phenylenevinylene)s (OPVs) are synthesized by Suzuki-Miyaura coupling of arylboronic acids with (Z)-bromoalkenes in over 96% geometrical purity. On the other hand, trans-oligo(phenylenevinylene)s can be synthesized by Hiyama coupling of aryl iodide with (E)-alkenylsilanes in almost perfect purities. Effect of pi-conjugation chain length on photoisomerization behavior of OPVs is described.

Photoisomerization behavior and photoinduced surface relief gratings on azopolymer film by a monochromatic light irradiation

Liquid crystalline azobenzene functionalized polymer, P3AB6Ac has been synthesized and its photoisomerization and holographic properties have been studied. The liquid crystalline properties of the polymer were confirmed by differential scanning calorimetry and optical polarizing microscopy. The polymer showed reversible photoisomerization in both solution and in the film. Surface relief gratings were induced on the polymer films prepared by spin-coating from its solution in THF by the irradiation of linearly polarized and circularly polarized Ar + laser individually through photomask. The surface relief structures were investigated using atomic force microscopy. A surface grating with a surface modulation depth of 400–500 nm was obtained by irradiation of circularly polarized Ar + laser for 30 min. The period of record of the polymer film was also found to be considerably high, more than one year.

UV−Vis, NMR, and Time-Resolved Spectroscopy Analysis of Photoisomerization Behavior of Three- and Six-Azobenzene-Bound Tris(bipyridine)cobalt Complexes

The photoisomerization properties of tris(bipyridine)cobalt complexes containing six or three azobenzene moieties, namely, [Co(II)(dmAB)3](BF4)2 [dmAB = 4,4'-bis[3''-(4'''-tolylazo)phenyl]-2,2'-bipyridine], [Co(III)(dmAB)3](BF4)3, [Co(II)(mAB)3](BF4)2 [mAB = 4-[3' '-(4' ''-tolylazo)phenyl]-2,2'-bipyridine], and [Co(III)(dmAB)3](BF4)3, derived from the effect of gathering azobenzenes in one molecule and the effect of the cobalt(II) or cobalt(III) ion were investigated using UV-vis absorption spectroscopy, femtosecond transient spectroscopy, and 1H NMR spectroscopy. In the photostationary state of these four complexes, nearly 50% of the trans-azobenzene moieties of the Co(II) complexes were converted to the cis isomer, and nearly 10% of the trans-azobenzene moieties of the Co(III) complexes isomerized to the cis isomer, implying that the cis isomer ratio in the photostationary state upon irradiation at 365 nm is controlled not by the number of azobenzene moieties in one molecule but rather by the oxidation state of the cobalt ions. The femtosecond transient absorption spectra of the ligands and the complexes suggested that the photoexcited states of the azobenzene moieties in the Co(III) complexes were strongly deactivated by electron transfer from the azobenzene moiety to the cobalt center to form an azobenzene radical cation and a Co(II) center. The cooperation among the photochemical structural changes of six azobenzene moieties in [Co(II)(dmAB)3](BF4)2 was investigated with 1H NMR spectroscopy. The time-course change in the 1H NMR signals of the methyl protons indicated that each azobenzene moiety in [Co(II)(dmAB)3](BF4)2 isomerized to a cis isomer with a random probability of 50% and without interactions among the azobenzene moieties.

Langmuir and Grafted Monolayers of Photochromic Amphiphilic Monodendrons of Low Generations

Four generations of monodendrons with multiple dodecyl alkyl tails (AA-N, N representing number of alkyl tails from 1 to 8), an azobenzene spacer group, and a carboxylic acid polar head have been studied at the air-water and air-solid interface using AFM, GIXD, X-ray reflectivity, and UV-vis spectrometry. The one and two tail molecules formed orthorhombic lateral packing with long-range intramonolayer ordering. Good agreement between molecular models and thickness measurements indicated that the one and two tail molecules orient along the surface normal. The increase in the cross-sectional mismatch caused by the presence of the multiple chains for the higher generations disrupted the long-range ordering and forced the alkyl tails to adopt quasi-hexagonal structure. The higher generations (AA-4 and AA-8) formed a kinked structure with the alkyl tails oriented perpendicular to the surface with the azobenzene group tilted at a large degree toward the surface. The photoisomerization behavior in dilute solutions, at the air-water interface, and for grafted layers demonstrated that lower generation monodendrons maintained the photochromic behavior after chemical grafting to the silicon substrates, although the confinement of the molecules in monolayers significantly increased the reorganization time.

Photon-, Electron-, and Proton-Induced Isomerization Behavior of Ferrocenylazobenzenes

3-, 4-, and 2-ferrocenylazobenzenes, 1, 2, and 3, respectively, and several derivatives of 1 were synthesized, and their photoisomerization behaviors were examined. The molecular structures of 1 and its derivatives, 2-chloro-5-ferrocenylazobenzene (5) and 3-ferrocenyl-4'-hydroxylazobenzene (11), were determined by X-ray diffraction analysis. 3-Ferrocenyl compound 1 undergoes reversible trans-to-cis isomerization with a single green light source and the Fe(III)/Fe(II) redox change. 4- and 2-Ferrocenyl compounds, 2 and 3, also respond to green light in addition to UV light, exciting the pi-pi* transition, but the cis molar ratio in the photostationary state (PSS) is lower than that of 1. The response to green light in 2 and 3 is caused by the MLCT (from Fe d orbital to azo pi* orbital) band excitation, while the character of the MLCT band, as estimated by time-dependent density functional theory calculations, differs between 1 and 2. The oxidized form of 2 undergoes facile cis-to-trans thermal isomerization. Both 1 and 2 undergo facile protonation and show proton-catalyzed cis-to-trans isomerization. Among the derivatives of 1, 2-chloro-5-ferrocenylazobenzene (5) exhibits the highest cis molar ratio (47%) in the PSS of green light irradiation.