Thermal Cis-trans Isomerization Research Articles

Responsive behavior of 4-(N-maleimido)azobenzene in polymers with aromatic main chain and side chain linked units

Polymer matrices with aromatic main chain and side chain linked units (polystyrene, polysulfone) were employed to construct thin films containing maleimidoazobenzene molecules in order to develop new materials with responsive properties. The photochromic behavior of the active compound was studied both in the solid and solution state for comparison. The thermal cis-trans isomerization of azobenzene chromophores in polymer films follows two different kinetic pathways. A double-exponential relaxation process (fast and slow) took place in polystyrene matrix under 70 oC while the thermal isomerization at 70 oC and higher follows a first-order kinetics. The cis isomer content corresponding to the photostationary state was found to be lower in polymer films than in solution.

The selective thermal cis-trans isomerization of 1,2-diphenylsulfonyl ethylene in the presence of carbon dioxide

The selective isomerization of cis-1,2-diphenylsulfonyl ethylene to trans-1,2-diphenylsulfonyl ethylene proceeded in supercritical carbon dioxide above 393 K without an addition of catalyst. The product yield increased with increasing temperature and was almost constant against carbon dioxide density. The 73% of product yield was obtained at 493 K and 10 MPa of carbon dioxide pressure for 60 min. The product yield obtained in supercritical carbon dioxide was larger than those obtained in organic solvents. The supercritical carbon dioxide was a dilute solvent compared with organic solvent, which probably reduced the suppression of the reaction by dense solvent. Further, only cis-sulfonyl ethylenes isomerized among several cis-ethylene derivatives. The sulfonyl group played an important role for the isomerization.

Synthesis and Photoresponsive Behavior of a Liquid-Crystalline Polymer with a 4-Ring Azotolane Moiety with Huge Birefringence

A novel liquid-crystalline polymer (LCP) containing an azobenzene group directly connected to a tolane moiety (azotolane moiety) was synthesized. The LCP showed a nematic phase in a broad temperature range (> 200°C). Trans-cis photoisomerization and cis-trans thermal isomerization occurred even though the LCP contains a long azotolane moiety. The homogeneously aligned film of the LCP exhibited an extremely large value of Δn (0.70 at 633 nm). Upon irradiation with a pumping beam in the LC phase, a large change in Δn was induced due to destruction of the homogeneous alignment of the azotolane moieties.

Photochromic reaction-induced changes of ordering in liquid crystalline films

Kinetics of photochemically and thermally driven isomerization was studied in a photochromic azobenzene derivative dissolved in three liquid crystalline matrices. Deviations from the Arrhenius law, observed for the thermal cis-trans isomerization reaction in the vicinity of nematic–isotropic (N–I) phase transitions, have been explained by a reduction of the intensity of light measured in the experiments, due to scattering. The isotropic phase was found to appear in all studied systems upon UV irradiation, at temperatures substantially lower than the thermodynamic temperatures of the respective N–I phase transitions. The nematic phase reappears due to the cis-trans isomerization reaction that can be thermally driven or can occur upon irradiation with the visible light. The occurrence of ‘photo-induced phase transitions’ reported for photochromic dye-doped liquid crystals can be rationalised taking into account different compatibilities (and hence different solubilities) of both forms of photochromic systems. The effects observed should be common to many other photochromic systems dissolved in liquid crystals, in the vicinity of phase transitions of the matrices.

Isomerization Behavior of an Azopolymer in Terms of the Langmuir−Blodgett Film Thickness and the Transference Surface Pressure

The photoinduced trans-cis and thermal cis-trans isomerizations of an azopolymer were investigated in terms of LB film thickness and the transference surface pressure by means of UV-vis spectroscopy and AFM microscopy. Three different photochemical behaviors have been observed depending on the number of layers: (i) for 1 layer, the percentage of cis isomers in the photostationary state is around 25%, (ii) in films of 5-20 layers, this percentage decreases to ca. 15%, and (iii) for 35 layers, the percentage of isomers cis formed after irradiation is more than 50%. Furthermore, the thermal cis-trans isomerization varies from a first-order kinetics to an expression with two exponential terms when the LB films change from 1 layer to multilayers. Morphologic changes in the surface of the films during the photoisomerization process were observed by means of AFM, and these changes also depend on the number of layers. In the cis state some hills were observed in the films of 1 and 35 layers, whereas practically no changes occurred in the films of 10 layers. These hills can be interpreted as the rupture of bidimensional structure of the film during the photoisomerization process. Three different schemes have been proposed to explain the observed behaviors.

Creation of azobenzene-based photochromic amorphous molecular materials—synthesis, glass-forming properties, and photochromic response

A novel family of azobenzene-based photochromic amorphous molecular materials has been created. They were found to readily form amorphous glasses with well-defined glass-transition temperatures and to exhibit photochromism as amorphous films as well as in solution. It was found that their quantum yields of trans–cis photoisomerization were smaller as amorphous films than in solution and that the backward cis–trans thermal isomerization reactions as amorphous films were either accelerated or retarded relative to those in solution, depending upon their molecular structures. In addition, the rate acceleration for the cis–trans thermal isomerization as amorphous films relative to solution was found to be more prominent as the irradiation time for generating the cis-isomer became shorter.

Thermal cis–trans isomerization of triazo-benzene

Abstract 1,3,5-Hydroxy-triazo-benzene (H-TAB) was synthesized through a coupling-oxidation protocol. Temperature-controlled UV, IR, and ab initio calculation were carried out to investigate the cis–trans thermal isomerization of H-TAB. In temperature-controlled UV experiments, λ max of the π–π ∗ band and for the trans conformation at 335 nm and that for the cis form at 282 nm are shifted by increased temperature; band intensities of the π–π ∗ transition decrease and λ max of the π–π ∗ band is shifted toward the high-energy region. The maximum peak at 2922 cm −1 is shifted to 2926 cm −1 , and that at 2852 cm −1 is shifted to 2856 cm −1 at increased temperature in the temperature-controlled IR experiment. Ab initio calculation reveals that the cis conformation of H-TAB is more stable than the trans form because the cis form has less spatial repulsion. Therefore, the ground-state energy difference induced by steric repulsion of the benzene unit is the driving force of the blue shift in the thermal IR and UV spectra for the triazo-benzene.

Relationship between the absorption spectra and spatial hindrance in starch graft copolymers

AbstractThe photochemical trans–cis isomerization of graft copolymers of azo dyes onto starch was studied in liquids spectrophotometrically. The relationship between the absorption spectra and spatial hindrance was investigated. The experimental results showed that the structure of the graft copolymers had a marked influence on photoisomerization. The smaller the spatial hindrance was, the easier the photoisomerization was. The copolymers showed better photoisomerization than their corresponding monomers, and this showed that the backbone of starch in the main chain influenced not the maximum‐wavelength position of the copolymers but the retroactive structure of the copolymers and favored their photoisomerization. In addition, the thermal cis–trans isomerization rate of the graft copolymers was studied in various polar solvents. The results suggested that the solvent polarity had a large effect on the thermal back‐isomerization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2123–2126, 2006

Linear Tetrapyrroles as Functional Pigments in Chemistry and Biology

AbstractFor Abstract see ChemInform Abstract in Full Text.

Photochromism of Crown Ethers with Incorporated Azobenzene Moiety

The kinetics of the thermal cis-trans isomerization was determined in a series of crown ethers containing the azobenzene moiety incorporated into the crowns of various sizes (10- to 19-membered crowns), dissolved in liquid isooctane and in polymer matrixes (PMMA and polystyrene). The kinetic parameters (activation energies and pre-exponential factors) were determined from temporal evolution of the absorbance measured at one of the maxima due to the pi-pi* transition of the trans forms of the molecules, using both isothermal and nonisothermal procedures. Molecular structures and electronic spectra of both trans and cis forms were calculated employing an ab initio method with 3-21G* basis set and semiempirical ZINDO-S, respectively. A comparison of the quantum-chemical calculations with the experimental results shows that one may exclude the splitting of the UV absorption bands of the trans forms of the molecules under study that resulted from a coexistence of several conformations of these molecules. A linear relationship was found between the activation energy and logarithm of the pre-exponential factor of the thermally driven cis-trans isomerization: all molecules follow a common dependence, except for the smallest (10-membered) crown ether.

Nonlinear Effects during Inscription of Azobenzene Surface Relief Gratings

Surface relief gratings were inscribed on azobenzene polymer films using a pulselike exposure of an Ar + laser. The inscription process was initiated by a sequence of short pulses followed by much longer relaxation pauses. The development of the surface relief grating was probed by a He-Ne laser measuring the scattering intensity of the first-order grating peak. The growth time of the surface relief grating was found to be larger than the length of the pulses used. This unusual behavior can be considered as a nonlinear material response associated with the trans-cis isomerization of azobenzene moieties. In this study the polymer stress was assumed to be proportional to the number of cis-isomers. One-dimensional viscoelastic analysis was used to derive the polymer deformation. The rate of trans-cis isomerization increases with the intensity of the inscribing light; in the dark it is equal to the rate of thermal cis-trans isomerization. The respective relaxation times were estimated by fitting theoretical deformation curves to experimental data.

Convenient synthesis of fully and partially deuterated stereoregular poly(phenylacetylene)s bearing a carboxy pendant and helicity induction on the polymers with chiral amines and its memory

AbstractPoly[(4‐carboxyphenyl)acetylene] and its derivatives deuterated in the main chain and/or on the phenyl group (poly‐1's) were conveniently synthesized by the polymerization of 4‐ethynylbenzoic acid or its derivative deuterated on the phenyl group in the presence of bases with a water‐soluble rhodium complex in water or deuterium oxide. The obtained polymers possesses highly cis–transoidal stereoregular structures according to NMR and Raman spectroscopy. Possible deuterium isotope effects of these partially and fully deuterated poly‐1's during the helicity induction with chiral amines and its memory assisted by achiral amines were investigated. The thermal cis–trans isomerization of the methyl esters of poly‐1's was also studied with 1H NMR spectroscopy. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4711–4722, 2004

Linear tetrapyrroles as functional pigments in chemistry and biology

1,19,21,24-tetrahydro-1,19-bilindione is the framework of pigments frequently found in nature, which includes biliverdin IX α, phytochromobilin and phycocyanobilin. 1,19-bilindiones have unique features such as (1) photochemical and thermal cis-trans isomerization, (2) excited energy transfer, (3) chiroptical properties due to the cyclic helical conformation, (4) redox activity, (5) coordination to various metals, and (6) reconstitution to proteins. 1,19-bilindione can adopt a number of conformations since it has exocyclic three double bonds and three single bonds that are rotatable thermally and photochemically. In solution, biliverdin and phycocyanobilin adopt a cyclic helical ZZZ, syn, syn, syn conformation, but other conformations are stabilized depending on the experimental conditions and substituents on the bilin framework. The conformational changes in 1,19-bilindiones are related to the biological functions of a photoreceptor protein, phytochrome. Structural and conformational studies of bilindiones are summarized both in solution and in protein. The conformational changes of bilins can be used for other functions such as a chirality sensor. The bilindiones and the zinc complexes of bilindiones can be employed as a chirality sensor due to the helically chiral structure and the dynamics of racemization of enantiomers. In this paper, we discuss the conformational equilibria and dynamics of bilindiones and its implications in photobiology and materials science.

Synthesis and optical storage properties of a novel polymethacrylate with benzothiazole azo chromophore in the side chain

A new amorphous high-Tg azo polymer with benzothiazole group in the side chain, poly{2-{4-[[N-(2-methacryloyl)ethyl]-N-ethyl]phenylazo}-6-nitro-benzothiazole} (pBTAMC), was prepared and its optical storage properties were investigated in thin films. Comparison is made with three push-pull azobenzene polymers: poly{4-[[2-(methacryloyloxy)ethyl]ethylamino]-4′-(methylsulfonyl)azobenzene}(pDRSAM), poly{4-[2-(methacryloyloxy)ethylsulfonyl]-4′-(diethylamino)azobenzene}(pDRASM} and poly{4-[[2-(methacryloyloxy)ethyl]ethylamino]-4′-nitroazobenzene} (pDR1M) in order to study the effect of chromophore structure on optical storage properties. The thermal cis-trans isomerization behavior of azo groups in polymer films, measured by means of flash photolysis, revealed that the rate constant of thermal isomerization increases with the increase in dipole moment of azo chromophores. In these systems, the relaxation of the cis-isomer was analyzed as the sum of two first-order reactions, a fast and a slow one. The azo chromophore orientation induced by irradiation of polymer films with linearly polarized light was measured as birefringence. Both the level of photoinduced birefringence and the rate of achieving it follow the order pDR1M > pDRASM > pDRSAM > pBTAMC. pBTAMC, which contains the strongest pair of donor-acceptor substituents in the azo chromophore side chain, exhibits the highest stability of the induced order (93% of photoinduced birefringence is maintained after relaxation). In pBTAMC films, the chromophore orientation is accompanied by some irreversible chromophore bleaching.

Preparation and photoinduced isomerization study of a homopolymer and a copolymer based on 4′‐(6‐acryloxy)hexyloxy‐4‐methoxyazobenzene and acrylic acid

AbstractThe synthesis and characterization of a copolymer based on 4′‐(6‐acryloxy)hexyloxy‐4‐methoxyazobenzene (MAB6) and acrylic acid (AA) are reported. A reaction was carried out by free‐radical polymerization, yielding an MAB6 homopolymer and an AA–MAB6 copolymer with an MAB6 concentration of 16–80%. A nematic phase was observed in the copolymer when the MAB6 content was 44% or higher. Both nematic and smectic phases were observed in the MAB6 homopolymer. All of the polymers were investigated for trans–cis–trans isomerization in a solid film. The samples were irradiated with nonpolarized ultraviolet light (385 nm) before absorption measurements were taken with an ultraviolet–visible spectrometer. The copolymer and homopolymer exhibited a thermal cis–trans isomerization, which could be described by a double‐exponential relaxation process (fast and slow). The relaxation experiment suggested that the hydrogen bonding may have hindered the slow process but had no effect on the fast process. A film of a copolymer sample with a high MAB6 content could be optically aligned by the exposure of the sample to polarized light (385 nm). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 4017–4024, 2003

Synthesis of bipyridine‐linked dimers of azanonaborane: electronic absorption and molecular‐orbital calculations

AbstractA new class of azanonaborane–bipyridine derivatives was synthesized from the reaction of bidentate nitrogen ligands with the azanonaborane cluster [(RNH2)B8H11NHR] in benzene under reflux. The exchange of the exo‐amino ligand by bipyridine derivatives is a convenient route to yield a dimer of the azanonaborane cluster in good yield. Thus, 4,4′‐bipyridine (PyPy) and trans‐1,2‐di‐(4‐pyridyl)‐ethene (PyPy) replace the exo‐NH2R unit to give [(PyPy)B8H11NHR] or [RHNH11B8PyPyB8H11NHR] and [(PyPy)B8H11NHR] or [RHNH11B8PyPyB8H11NHR] respectively as colored products due to electronic interaction between the {B8N} unit and the bonded bipyridine units. This interaction has been investigated by UV–vis spectroscopy and by AM‐1 molecular‐orbital calculations. In the case of [(PyPy)B8H11NHR], the 1H and 13C NMR analysis of the products revealed two isomeric forms (cis and trans). This is attributed to thermal cis–trans isomerization. Copyright © 2003 John Wiley & Sons, Ltd.

Relaxation kinetics of photoinduced surface relief grating on azopolymer films

The relaxation kinetics of the photoinduced surface relief grating (SRG) formed on an azopolymer film after stopping laser beam writing has been systematically investigated. In addition to a photoinduced increase of the diffraction efficiency in the SRG, an anomalous further enhancement of the efficiency was observed even without light irradiation, after the recording light beam was switched off. This anomalous relaxation process consists of two components with short and long relaxation times, and strongly depends on the temperature and polarization of the probe beam used for the diffraction measurement. At lower temperatures the anomalous growth is more effective and the fast component is dominant. From the results of the polarization dependence of the diffraction efficiency and optical absorption, it has been manifested that the fast component of the relaxation originates from the cis-trans thermal isomerization of azobenzene in the side chain. The origin of the slow decay is also discussed in terms of the order parameter change of the azomolecules caused by the reorientational relaxation.

Dramatic decrease of the cis content and molecular weight of cis‐transoidal polyphenylacetylene at 23 °C in solutions prepared in air

AbstractThe thermal cis–trans isomerization of cis‐transoidal polyphenylacetylene (PPA) synthesized with Noyori's catalyst [Rh(CCPh)(norbornadiene)(PPh3)2] has been investigated under both ambient and inert atmospheres in solution and in bulk. In all cases, an intramolecular cyclization results in cis–trans isomerization, and subsequent chain cleavage produces 1,3,5‐triphenylbenzene. This reaction is accelerated by the presence of air, particularly when the reaction takes place in solution. Decreases in the cis content and molecular weight show that the intramolecular cyclization process takes place at 23 °C in solution. The mechanism of this reaction is identical to that reported previously for cis‐cisoidal and cis‐transoidal PPA synthesized with Ziegler–Natta catalysts. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3212–3220, 2002

Thermal decomposition and isomerization of cis-permethrin and beta-cypermethrin in the solid phase.

The stability to heart of cis-permethrin and beta-cypermethrin in the solid phase was studied and the decomposition products identified. Samples heated at 210 degrees C in an oven in the dark showed that, in the absence of potassium chlorate (the salt present in smoke-generating formulations of these pyrethroids), cis-permethrin was not isomerized, although in the presence of that salt, decomposition was greater and thermal isomerization occurred. Other salts of the type KXO3 or NaXO3, with X being halogen or nitrogen, also led to a considerable thermal isomerization. Heating the insecticides in solution in the presence of potassium chlorate did not produce isomerization in any of the solvents assayed. Salt-catalysed thermal cis-trans isomerization was also found for other pyrethroids derived from permethrinic or deltamethrinic acid but not for those derived from chrysanthemic acid. The main thermal degradation processes of cis-permethrin and beta-cypermethrin decomposition when potassium chlorate was present were cyclopropane isomerization, ester cleavage and subsequent oxidation of the resulting products. Permethrinic acid, 3-phenoxybenzyle chloride, alcohol, aldehyde and acid were identified in both cases, as well as 3-phenoxybenzyl cyanide from beta-cypermethrin. A similar decomposition pattern occurred after combustion of pyrethroid fumigant formulations.

Synthesis of a new polymer surface bearing grafted azo polymer chains

AbstractAzobenzene, a photosensitive chromophore that undergoes photoinduced and thermal cis–trans isomerization, can be applied in a nonlinear optical field. {4′‐[(Hydroxy)ethyl]amino}‐4‐nitroazobenzene (disperse red 1) corresponds to one of these azo compounds, which can be grafted to a polymer chain as a part of the main chain, as a dangling group, or onto the polymer surface. In the last case, disperse red 1 is transformed into an acrylic monomer and then grafted onto a polypropylene surface modified with a cold carbon dioxide‐plasma treatment. A method is proposed for quantifying the radicals formed during the plasma treatment and, consequently, for optimizing the grafting reaction. The best conditions (the nature of the solvent, temperature, monomer concentration, and duration) are given. Both IR and Raman spectroscopies were used as efficient techniques for grafting characterization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3052–3061, 2001