Structural studies and photochromism of mercury(II)-iodo complexes of (arylazo)imidazoles

Neat reaction between HgI2 and 1-methyl-2-(phenylazo)imidazole (Pai-Me) under microwave irradiation has isolated a novel compound whose structure shows intercalated HgI2 in the layers of Pai-Me. They exist independently in interpenetrated arrays. In a solution phase study, the same reaction has synthesized an iodo-bridged azoimidazole-Hg(II) complex, [Hg(RaaiR')(mu-I)(I)]2 (RaaiR' = 1-alkyl-2-(arylazo)imidazole). The structures have been characterized by X-ray diffraction studies. Chloro-bridged Hg(II) complexes of azoimidazoles, [Hg(RaaiR')(mu-Cl)(Cl)]2, are also known. These complexes upon irradiation with UV light show trans-to-cis isomerization. The reverse transformation, cis-to-trans isomerization, is very slow with visible light irradiation. Quantum yields (phi t-->c) of trans-to-cis isomerization are calculated, and the free ligand shows higher phi than their Hg(II) complexes. The cis-to-trans isomerization is a thermally induced process. The activation energy (Ea) of cis-to-trans isomerization is calculated by controlled temperature reaction. The Ea's of free ligands are much higher than that of halo-bridged Hg(II)-azoimidazole complexes. Chloro-bridged Hg(II) complexes show lower Ea's than those of iodo-bridged complexes. DFT calculation has been adopted to rationalize the experimental results.     

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.     

Helical conformational dynamics and photoisomerism of alternating pyridinedicarboxamide/m-(phenylazo)azobenzene oligomers

Alternating sequences of pyridine-2,6-dicarboxamides and meta-(phenylazo)azobenzenes have been assembled into oligomers composed of four (8) and eight (9) azobenzene linkages. X-ray crystallography confirmed that oligomer 8 adopts a two-turn helical conformation with a helical pitch of approximately 3.4 Angstroms in the solid state. The presence of a two- and four-turn helical conformation of 8 and 9, respectively, in polar and nonpolar solvents was elucidated by the anisotropic upfield shifting of protons located within the helices, NOE enhancements between protons oriented toward the helix interior, and the diastereotopicity of the terminal benzyloxycarbonyl (CBz) methylene protons. (1)H NMR line shape analysis of the CBz methylene hydrogens at the chain ends revealed a dynamic equilibria interconverting M and P helical conformations with energetic barriers (DeltaG) of 11.1 (DeltaS = -19.4 +/- 1.6 cal mol(-1) K(-1); DeltaH = 6.5 +/- 0.4 kcal/mol) for 8 and 13.8 kcal/mol (DeltaS = -6.6 +/- 6.2 cal mol(-1) K(-1); DeltaH = 11.8 +/- 1.8 kcal/mol) for 9. Irradiation of the oligomers with 350 nm light induces an E --> Z isomerization of the azo linkages that decreases in efficacy at longer helix lengths. The suppression of E --> Z isomerization is a consequence of the contrasting behavior of the azo linkages located at the helix termini, which afford Z/E ratios similar to those of model compound 7d, and the internal azo groups, which undergo significantly lower Z/E conversion ratios compared with 7e.     

Cadmium(II) complexes of (arylazo)imidazoles: synthesis, structure, photochromism, and density functional theory calculation

Reaction between CdX2 and 1-alkyl-2-(phenylazo)imidazole (RaaiR') has isolated complexes of composition Cd(RaaiR')2X2 in MeOH or MeCN. Crystallization of Cd(RaaiR')2I2 from N,N-dimethylformamide (DMF) has separated [Cd(RaaiR')I2.DMF], while Cd(RaaiR')2X2 (X = Cl and Br) remains unchanged in its composition upon crystallization under identical conditions. The structure has been established by spectral (UV-vis and 1H NMR) data and confirmation in the latter case by a single-crystal X-ray diffraction study of [Cd(TaiMe)I2.DMF] [where TaiMe = 1-methyl-2-(p-tolylazo)imidazole]. UV-light irradiation in a MeCN solution of Cd(RaaiR')2I2 and [Cd(RaaiR')I2.DMF] shows trans-to-cis isomerization of coordinated azoimidazole. The reverse transformation, cis-to-trans, is very slow with visible light irradiation. Quantum yields (phit-->c) of trans-to-cis isomerization are calculated, and the free ligand shows higher phi values than their cadmium(II) iodo complexes. The cis-to-trans isomerization is a thermally induced process. The activation energy (Ea) of cis-to-trans isomerization is calculated by a controlled-temperature experiment. The effects of the anions (Cl-, Br-, I-, and ClO4-) and the number of coordinated azoimidazoles (RaaiR') [Cd(RaaiR') or Cd(RaaiR')2] on the rate and quantum yields of photochromism are established in this work. A slow rate of photoisomerization of [Cd(RaaiR')4](ClO4)2 compared to Cd(RaaiR')I2 or Cd(RaaiR')2X2 may be associated with the increased mass and rotor volume of the complexes. The rate of isomerization is also dependent on the nature of X and follows the sequence Cd(RaaiR')2Cl2 < Cd(RaaiR')2Br2 < Cd(RaaiR')2I2. It may be related to the size and electronegativity of halide, which reduces the effective molar association in the order of I < Br < Cl and hence the rate. Gaussian 03 calculations of representative complexes and free ligands are used to explain the difference in the rates and quantum yields of photoisomerization.     

Synthesis of azo-conjugated metalladithiolenes and their photo- and proton-responsive isomerization reactions

A versatile synthetic method of azo-conjugated metalladithiolenes was developed, and new complexes with various central metals and substituent groups were synthesized. Molecular structures of the azo-conjugated metalladithiolenes of Ni(II), Pd(II), and Pt(II) with diphenylphosphinoethane as a co-ligand were determined by X-ray crystallography. While the energy of the reversible trans-to-cis photoisomerization is considerably lower than that of azobenzene, the thermal stability of the cis form is much higher than that of the organic azobenzene derivatives showing similar low-energy trans-to-cis photoisomerization. A novel proton response of the azo group occurs, and the combination of photoisomerization and protonation leads to a novel proton-catalyzed cis-to-trans isomerization, the rate of which correlates with the redox potential of the metalladithiolene moiety. The study including other azo-conjugated metalladithiolenes has indicated that the protonation is a common feature for the azo-conjugated metalladithiolenes, but trans-to-cis photoisomerization is strongly dependent on the electronic structure of the trans form or a steric effect in the cis form.     

Increasing the isomerisation kinetics of azo dyes by chemical bonding to liquid-crystalline polymers

It is well known that the proper substitution of the azobenzene core allows tuning the thermal cis-to-trans isomerisation kinetics of azo dyes. The thermal isomerisation process of nitro-substituted azobenzenes is accelerated up to 13 times with respect to that in the common isotropic solvents when they are doped in nematic low molar mass liquid crystals. This kinetic acceleration is even stronger when these azo dyes are covalently linked to a nematic siloxane polymer. In this environment, the isomerisation process is accelerated more than 10(3) times. This effect is presented herein for the first time. The possible application of the networks obtained as possible photo-actuators has been also considered.     

Effect of packing on orientation and cis-trans isomerization of azobenzene chromophore in Langmuir-Blodgett film

Photoactive film material of long-chain azobenzene derivative, p-(omega-trimethylammoniodecyloxy)-p'-octyloxyazobenzene bromide (TAOAB), was fabricated into a Langmuir-Blodgett (LB) film by a polyion-complex technique using poly(sodium 4-styrenesulfonate) as a polyanion. To investigate the effect of the packing state of the azobenzene chromophore on its orientation and cis-trans isomerization, TAOAB was mixed with methyl stearate in the LB film matrix at various mole fractions (X(TAOAB)), and structural characterizations were subsequently carried out by means of Fourier transform infrared and ultraviolet-visible spectroscopies, X-ray diffraction analysis, and atomic force microscopy. The results obtained show that as the degree of packing increases, both the azobenzene chromophores and the hydrocarbon chains orient more perpendicularly to the surface of the LB film. In addition, reversible cis-trans photoisomerization of TAOAB took place upon alternate irradiation with UV and visible light even in a mixed LB film with the chromophores in a dense lateral packing state. In the process of thermal cis-to-trans isomerization, we found that the reaction rate is strongly affected by the packing state of TAOAB at 20 degrees C, reflecting the differences in steric hindrance among LB films of various X(TAOAB). In addition, higher activation energy was obtained for thermal cis-to-trans isomerization when the free volume around the chromophores became smaller.     

Synthesis, structure, photochromism and DFT calculations of copper(I)-triphenylphosphine halide complexes of thioalkylazoimidazoles

[Cu(SRaaiNR′)(PPh₃)X] complexes are synthesized by the reaction of CuX (X = Cl, Br, I), triphenylphosphine and 1-alkyl-2-[(o-thioalkyl)phenylazo]imidazole (SRaaiNR′). The single crystal X-ray structure of [Cu(SEtaaiNH)(PPh₃)I] (SEtaaiNH = 2-[(o-thioethyl)phenylazo]imidazole) shows a distorted tetrahedral geometry of the copper center with bidentate, N(azo), N(imidazole) chelation of SEtaaiNH and coordination from PPh₃ and iodine. These complexes show a trans-to-cis isomerization upon irradiation with UV light. The reverse transformation, cis-to-trans isomerization, is very slow with visible light irradiation and is thermally accessible. The quantum yields (?_t→c) of the trans-to-cis isomerization of [Cu(SRaaiNR′)(PPh₃)X] are lower than the free ligand values. This is due to the increased mass and rotor volume of the complexes compared to the free ligand data. The rate of isomerization follows the order: [Cu(SRaaiNR′)(PPh₃)Cl] < [Cu(SRaaiNR′)(PPh₃)Br] < [Cu(SRaaiNR′)(PPh₃)I]. The activation energy (E_a) of the cis-to-trans isomerization is calculated by a controlled temperature reaction. DFT computation of representative complexes has been used to determine the composition and energy of the molecular levels.     

Isomerization dynamics of photochromic spiropyran molecular switches in phospholipid bilayers

Transient absorption spectroscopy was used to investigate the dynamics of the photochromic indolinobenzospiropyran reaction in toluene solution and in phosphatidylcholine bilayers (1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)). After excitation with UV light, colorless (R/S)-2-(3',3'-dimethyl-6-nitro-3'H-spiro[chromene-2,2'-indol]-1'-yl)ethanol derivatives are converted to colored merocyanine products in high yield; Phi = 0.45 in DMPC liposomes. We find that the reaction occurs in the bilayer aliphatic region in the gel (P(beta)(')) and liquid (L(alpha)) phases. The Arrhenius activation energy for the isomerization in DMPC bilayers was approximately 3.5 times larger in the liquid phase (L(alpha), E(a) = 26.0 +/- 1.0 kJ mol(-1)) than that in the gel phase (P(beta)('), E(a) = 7.3 +/- 1.6 kJ mol(-1)). Analysis of the isomerization rate constant temperature dependence allows an estimation of the bilayer viscosity and free volume properties in the L(alpha) phase.     

Theoretical studies on the butene double bond isomerization catalyzed by 5-H of 1-ethyl-3-methyl-imidazolium fluoride

The isomerization of 1-butene to trans-2-butene catalyzed by 5-H proton of 1-ethyl-3-methyl-imidazolium fluoride (EMImF) has been studied with density functional theory of quantum chemistry. The equilibrium states geometries and transition state geometry are optimized at the levels of B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p), respectively. The apparent activation barrier of isomerization is about 208 kJ/mol theoretically. It indicates that the 5-H proton on the imidazole ring of EMImF has certain catalytic activity to the butene double bond isomerization, which is similar to that of the 4-H proton. According to the data of intrinsic coordinate path, it can be determined that the isomerization is an elementary course and the hydrogen exchange of butene with EMImF is synergetic.     

The rate enhancement for prolyl cis-to-trans isomerization of cyclic CPFC peptide is caused by an increase in the vibrational entropy of the transition state

The conformational preferences and prolyl cis-trans isomerization of oxidized and reduced Ac-Cys-Pro-Phe-Cys-NH2 (CPFC peptides) have been carried out using the ab initio HF/6-31+G(d) and hybrid density functional B3LYP/6-311++G(d,p) levels of theory. The most preferred conformations of oxidized and reduced CPFC peptides with the trans prolyl peptide bond have a type-I beta-turn for the Pro-Phe sequence in common. In particular, the transition states for both forms are stabilized by the intramolecular hydrogen bonds between the prolyl nitrogen and the N-H group of the Phe3 residue. The rotational barrier DeltaGct to the cis-to-trans isomerization for the oxidized CPFC peptide is calculated to be 19.37 kcal/mol at the B3LYP/6-311++G(d,p)//HF/6-31+G(d) level of theory, which is lower by 0.88 kcal/mol than that of the reduced CPFC peptide. This may indicate that the rate constant kc-->t of the prolyl cis-to-trans isomerization for the oxidized form is about 4 times larger than that of the reduced form, which is reasonably consistent with the value deduced from NMR experiments. In particular, the increase in vibrational entropy for the transition state of the oxidized form over that of the reduced form contributes to enhance the rate constant for the prolyl cis-to-trans isomerization of the oxidized form.     

A DFT study on the double bond migration of butene catalyzed by ionic pair of 1-ethyl-3-methyl-imidazolium fluoride

The double bond migration of butene catalyzed by 1-ethyl-3-methyl-imidazolium fluoride (EmimF) has been studied using quantum chemical method. The geometries of reactant, transition state and product for the isomerization have been optimized by density functional theory (DFT) at the B3PW91/6-31G(d,p), 6-311++G(d,p) and aug-cc-PVDZ levels. The computed results show that the 4-H atom on imidazole ring of EmimF has a good catalytic activity to the double bond migration of butene and the catalytic reaction of 1-butene to 2-butene is a synergetic and elementary process. The apparent activation energy of isomerization is about 197 kJ/mol.     

Photoisomerization of disperse red 1 studied with transient absorption spectroscopy and quantum chemical calculations

The photoisomerization of the push-pull substituted azo dye Disperse Red 1 is studied using femtosecond time-resolved absorption spectroscopy and other spectroscopic and computational techniques. In comparison with azobenzene, the pipi* state is more stabilized by the effects of push-pull substitution than the npi* state, but the latter is probably still the lowest in energy. This conclusion is based on the kinetics, anisotropy of the excited state absorption spectrum, the spectra of the ground states, and quantum chemical calculations. The S(1)(npi*) state is formed from the initially excited pipi* state in <0.2 ps, and decays to the ground state with time constants of 0.9 ps in toluene, 0.5 ps in acetonitrile, and 1.4 ps in ethylene glycol. Thermal isomerization transforms the Z isomer produced to the more stable E isomer with time constants of 29 s (toluene), 28 ms (acetonitrile), and 2.7 ms (ethylene glycol). The pathway of photoisomerization is likely to be rotation about the N=N bond. Quantum chemical calculations indicate that along the inversion pathway ground and excited state energy surfaces remain well separated, whereas rotation leads to a region where conical intersections can occur. For the ground-state Z to E isomerization, conclusive evidence is lacking, but inversion is more probably the favored pathway in the push-pull substituted systems than in the parent azobenzene.     

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.     

Lipid-mediated light activation of a mechanosensitive channel of large conductance

This paper describes the reversible activation of a mechanosensitive channel via a light-sensitive lipid mimic. For these experiments, the mechanosensitive channel of large conductance (MscL) protein from Lactococcus lactis and Escherichia coli was reconstituted in lipid bilayers composed of 80 mol % 1,2-dioleoyl-sn-glycero-3-phosphocholine and 20 mol % di-(5-[[4-(4-butylphenyl)azo]phenoxy]pentyl)phosphate (4-Azo-5P). Light-induced isomerization of the azobenzene moiety of 4-Azo-5P from trans to cis was used to activate MscL.     

Cyano- and isocyanotris(trifluoromethyl)borates: syntheses, spectroscopic properties, and solid state structures of K[(CF3)3BCN] and K[(CF3)3BNC

A two step synthesis to the isocyanotris(trifluoromethyl)borate anion, [(CF3)3BNC]-, and its isomerization to the cyanotris(trifluoromethyl)borate anion, [(CF3)3BCN]-, at temperatures above 150 degrees C are presented. In the first step (CF3)3BNCH was obtained by reacting (CF3)3BCO with hydrogen cyanide followed by deprotonation of the HCN adduct with Li[N(SiMe3)2] in toluene. The thermal behavior of K[(CF3)3BNC] and K[(CF3)3BCN] were investigated by differential scanning calorimetry (DSC), and K[BF4] was identified as a major solid decomposition product. The enthalpy of the isocyanide-cyanide rearrangement, deltaH(iso) = -35 +/- 4 kJ mol(-1), was obtained from DSC measurements, and the activation energy, E(a) = 180 +/- 20 kJ mol(-1), from kinetic measurements. The isomerization was modeled as an intramolecular reaction employing DFT calculations at the B3LYP/6-311+G(d) level of theory yielding a reaction enthalpy of deltaH(iso) = -36.1 kJ mol(-1) and an activation energy of E(a) = 155.7 kJ mol(-1). The solid-state structures of K[(CF3)3BNC] and K[(CF3)3BCN] were determined by single-crystal X-ray diffraction. Both salts are isostructural and crystallize in the orthorhombic space group Pnma (no. 62). In the crystals the borate anions possess C(s) symmetry, while for the energetic minimum C3 symmetry is predicted by DFT calculations. The borate anions have been characterized by IR and Raman spectroscopy as well as by NMR spectroscopy. The assignment of the IR and Raman bands is supported by their calculated wavenumbers and intensities. The spectroscopic and structural properties of both borate anions are compared to the properties of the isoelectronic borane carbonyl (CF3)3BCO and the [B(CF3)4]- anion as well as to those of other related species.     

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.     

Surface-relief-grating formation behavior of two hyperbranched azo polymers

In this work,surface-relief-grating formation behavior was studied by using two hyperbranched azo polymers.The hyperbranched polymers containing pseudo-stilbene type azo chromophores throughout the hyperbranched structure were synthesized by step-growth polycondensation of AB₂ type monomers.The AB₂ monomer,4-（4’-（bis（2-chloroethyl）amino）-phenylazo） benzoic acid（BAA）,was prepared through azo-coupling reaction between N,N’-bis（2-chloroethyl）aniline and 4- aminobenzenic acid.The another AB₂ monomer,4-（4’-（bis（2-chloroethyl）amino）phenylazo）-3-nitro-benzoic acid（BANA）, was prepared through azo-coupling reaction between N,N-bis（2-chloroethyl）aniline and 3-nitro-4-aminobenzcnic acid.The hyperbranched polymers（PBAA and PBANA） were prepared through nucleophilic substitution reaction of BAA and BANA, respectively.The polymers synthesized were characterized by using spectroscopic methods and thermal analysis.The photoinduced dichroism and photo-induced surface-relief-grating（SRG） formation of the hyperbranched polymers were investigated upon irradiation with Ar⁺ laser at 488 nm.PBAA shows typical photoinduced dichroism SRG formation behavior.On the contrary,PBANA does not show the photoresponsive properties.The results indicate that the nitro at the ortho position of azo group of PBANA shows the effect of inhibiting the photoinduced variations.The effect can be attributed to the blockage of the trans-cis isomerization of the azobenzene moieties by the steric hindrance.     

CO rebinding to protoheme: investigations of the proximal and distal contributions to the geminate rebinding barrier

The rebinding kinetics of CO to protoheme (FePPIX) in the presence and absence of a proximal imidazole ligand reveals the magnitude of the rebinding barrier associated with proximal histidine ligation. The ligation states of the heme under different solvent conditions are also investigated using both equilibrium and transient spectroscopy. In the absence of imidazole, a weak ligand (probably water) is bound on the proximal side of the FePPIX-CO adduct. When the heme is encapsulated in micelles of cetyltrimethylammonium bromide (CTAB), photolysis of FePPIX-CO induces a complicated set of proximal ligation changes. In contrast, the use of glycerol-water solutions leads to a simple two-state geminate kinetic response with rapid (10-100 ps) CO recombination and a geminate amplitude that can be controlled by adjusting the solvent viscosity. By comparing the rate of CO rebinding to protoheme in glycerol solution with and without a bound proximal imidazole ligand, we find the enthalpic contribution to the proximal rebinding barrier, H(p), to be 11 +/- 2 kJ/mol. Further comparison of the CO rebinding rate of the imidazole bound protoheme with the analogous rate in myoglobin (Mb) leads to a determination of the difference in their distal free energy barriers: DeltaG(D) approximately 12 +/- 1 kJ/mol. Estimates of the entropic contributions, due to the ligand accessible volumes in the distal pocket and the xenon-4 cavity of myoglobin ( approximately 3 kJ/mol), then lead to a distal pocket enthalpic barrier of H(D) approximately 9 +/- 2 kJ/mol. These results agree well with the predictions of a simple model and with previous independent room-temperature measurements of the enthalpic MbCO rebinding barrier (18 +/- 2 kJ/mol).     

Mechanistic study of trans⇆cis isomerization of the substituted azobenzene moiety bound on a liquid‐crystalline polymer

A mechanistic study of the trans?cis isomerization of the azobenzene moiety in a side‐chain liquid‐crystal polymer system was carried out with six liquid‐crystalline polymethacrylates in which different electron‐withdrawing substituents were attached to the para‐positions of the azobenzene chromophores. Compared to the non‐nitro‐substituted azo polymers, the nitro‐substituted azo polymers exhibited two quite different behaviors: an extraordinarily high reaction rate of the thermal cis–trans isomerization and an unexpected composition of cis–trans isomers obtained from the photochemical trans–cis isomerization process. A potential energy profile for the isomerization process was established on basis of the structures of the proposed transition states and was employed to elucidate the reaction mechanism. The results confirmed that the nitro‐substituted azo polymer system proceeded via a rotation mechanism in either direction of the trans?cis isomerization reaction, whereas the non‐nitro‐substituted species were more likely to follow an inversion mechanism. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2296–2307, 2001