Reversible Cis Research Articles

Installing Dynamic Molecular Photomechanics in Mesopores: A Multifunctional Controlled‐Release Nanosystem

Kicking out cholesterol: The release of guest molecules (cholesterol) included in the pores of mesoporous silica is promoted by simultaneous irradiation with UV and visible light. The reversible cis–trans photoisomerization of azobenzene substituents on the pore surface causes a stirring action, which accelerates the diffusion of the guest from the pores (see picture), thus providing an intelligent controlled-release method.

A three-dimensional model of the human transglutaminase 1: insights into the understanding of lamellar ichthyosis

The stratum corneum, the outer layer of the epidermis, serves as a protective barrier to isolate the skin from the external environment. Keratinocyte transglutaminase 1 (TGase 1) catalyzes amide crosslinking between glutamine and lysine residues on precursor proteins forming the impermeable layers of the epidermal cell envelopes (CE), the highly insoluble membranous structures of the stratum corneum. Patients with the autosomal recessive skin disorder lamellar ichthyosis (LI) appear to have deficient cross-linking of the cell envelope due to mutations identified in TGase 1, linking this enzyme to LI. In the absence of a crystal structure, molecular modeling was used to generate the structure of TGase 1. We have mapped the known mutations of TGase 1 from our survey obtained from a search of PubMed and successfully predicted the impact of these mutations on LI. Furthermore, we have identified Ca(2+) binding sites and propose that Ca(2+) induces a cis to trans isomerization in residues near the active site as part of the enzyme transamidation activation. Docking experiments suggest that substrate binding subsequently induces the reverse cis to trans isomerization, which may be a significant part of the catalytic process. These results give an interpretation at the molecular level of previously reported mutations and lead to further insights into the structural model of TGase 1, providing a new basis for understanding LI.

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-( ω-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 °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.

Photoswitchable luminescence of rhenium(i) tricarbonyl diimines.

The synthesis, characterization, and X-ray crystal structures of [Re(diimine)(CO)(3)(dpe)](PF(6)) (dpe = 1,2-di(4-pyridyl)ethylene) compounds are reported. The cis-dpe complexes exhibit yellow luminescence after UV excitation, whereas the trans-dpe counterparts are nonluminescent. The luminescence quantum yields of the cis-dpe complexes are strongly dependent on the identity of the diimine ligand. Irradiation (350 nm) of the trans-dpe complexes induces trans --> cis dpe-ligand isomerization with quantum yields on the order of 0.2, and this process leads to an on-switching of yellow luminescence. After long 350-nm irradiation times, a steady state composed of roughly 70% cis- and 30% trans-dpe complexes is reached. The reverse cis --> trans photoisomerization reaction is induced by irradiating the cis-dpe complexes at 250 nm, switching off the yellow luminescence. For 250-nm excitation, photodecomposition of the [Re(diimine)(CO)(3)(dpe)](+) complexes competes efficiently with photoisomerization.

Syntheses, crystal structures and isomerization kinetics of a series of [Co(dtc)2{P(OMe)3 − nPhn}2]+(n = 0–2) complexes (dtc−= N,N-dimethyldithiocarbamate): role of σ-donicity, π-acidity, and cone angle of the P-ligands in the trans influence and trans effect

A series of geometrical isomers of the new cobalt(III) complexes containing P(OMe)3 − nPhn (n = 0, 1 or 2), trans- and cis-[Co(dtc)2{P(OMe)3 − nPhn}2]+ (dtc− = N,N-dimethyldithiocarbamate), have been synthesized and the crystal structures and spectrometric properties were obtained. By comparison with reported structures of related [Co(dtc)2(P-ligand)2]+ complexes, it was shown that the Co–P bond lengths in [Co(dtc)2(P-ligand)2]+ were linearly dependent on the Tolman's cone angle, θT, of the P-ligands. On the other hand, the Co–P bond lengths as well as the transition energies of the splitting component (1A1g → a1Eg: D4h) of the first d–d band of trans-[Co(dtc)2(P-ligand)2]+ were not linearly dependent on the Tolman's electronic parameter (χ) of the P-ligands. In the trans-isomers, it was found that the mutual electronic trans influence was not significant in the [Co(dtc)2{P(OMe)3 − nPhn}2]+ species while it was far more evident in the complexes with strong σ-donating phosphines such as PMe3 and PMe2Ph. A reversible cis–trans inter-conversion was observed for the [Co(dtc)2{P(OMe)3 − nPhn}2]+ complexes: cis to trans isomerization was induced by irradiation of light while slow thermal trans to cis isomerization was observed as a dark reaction. Kinetic measurements of thermal trans to cis isomerization reactions revealed that the absorption change with time followed multi-exponential kinetics when no free P-ligand was added to the reaction mixture: by addition of excess amounts of free P-ligand, first-order kinetic traces were observed for all reactions. The apparent first-order rate constants were independent of the concentration of added free ligands for the isomerization reactions of [Co(dtc)2{P(OMe)2Ph}2]+ and [Co(dtc)2{P(OMe)Ph2}2]+, while saturation kinetics were observed for the reaction of [Co(dtc)2{P(OMe)3}2]+: all isomerizations involved pre-dissociation of one of the coordinated P-ligands. It was indicated that a significant amount of square pyramidal [Co(dtc)2{P(OMe)3}]+ exists in equilibrium with [Co(dtc)2{P(OMe)3}2]+, while merely a trace amount of such a 5-coordinate species was involved in the reactions of [Co(dtc)2{P(OMe)2Ph}2]+ and [Co(dtc)2{P(OMe)Ph2}2]+. A contradictory phenomenon observed in these complexes, a small static trans influence with a significant kinetic trans effect, was explained by the combination of (1) a lack of strong σ-donicity of P(OMe)3 − nPhn and (2) stabilization of the 5-coordinate square-pyramidal geometry through the electron sponge effect by the dtc− ligand.

Kinetics of photochromic reactions in a 10-membered dibenzoazo crown ether

The paper reports on results of measurements of the kinetics of the photochromic reaction in a member of the series of azobenzene-containing crown ethers. The 10-member crown (referred to as O2N2) undergoes a reversible photo- and thermally driven isomerisation, similarly as parent azobenzene. It is expected that constraints imposed by the crown on conformations of azobenzene-containing crown ethers may modify the energetics of the isomerisation process. The experiments were performed in pMMA matrices. The isomerisation was monitored spectrophotometrically. The trans – cis isomerisation was driven photochemically ( λ =365 or 500 nm) and the reverse cis – trans isomerisation was driven by the excitation in the visible (413 nm) or occurred spontaneously in the dark. The pre-exponential factor of the latter process amounts to ca. 6×10 11 s −1 and the activation energy is equal to ca. 118 kJ/mol, being higher than in other azobenzene-containing crown ethers and in parent azobenzene. The stability of the cis isomer at room temperature is much higher than that in other crown ethers, the extrapolated ambient-temperature time constant being of the order of several years. The stability of the cis form of O2N2 in the darkness, combined with a good reversibility is a promising feature for future applications of the material in optical data storage.

Heteroaromatic azo-sulfonamide monolayers: isomerisation and photocurrent generation

Abstract Trans–cis isomerisation of the azo-sulfonamide derivatives: 4-(N-methyl-N-n-dodecyl)amino-4′-(N-thiazol-2-yl)sulfonamidoazobenzene, 4-(N-methyl-N-n-dodecyl)amino-4′-(N-pyrimidin-2-yl)sulfonamidoazobenzene, and 4-(N-methyl-N-n-dodecyl)amino-4′-[N-(2-methyl)isoxazol-2-yl]sulfonamidoazobenzene and photocurrent generation in Langmuir–Blodgett (LB) films was investigated. The isomerisation took place on illumination with a light beam corresponding to the absorption band. The reverse cis–trans isomerisation proceeded as a thermal relaxation. It was observed that the LB films deposited during illumination of the trough were more effective in this photoreaction. The LB structures deposited onto conductive ITO glass were able to generate photocurrent in a standard three-electrode system. The photocurrent intensity was in linear relationship with bias voltage in the range from 0 to −0.35 V and in best conditions it reached 160 nA cm−2.

Photoswitchable architectural polymer: Toward azo‐based polyamidoamine side‐chain dendritic polyester

AbstractA versatile approach to the synthesis of novel polyamidoamine (PAMAM) side‐chain dendritic polyester (SCDPE) possessing azobenzene motifs in the polymeric core is described and displayed reversible cis–trans (E/Z) isomerization upon exposure to UV light. A polymerization reaction was conducted in solution using ester‐terminated PAMAM dendritic diol (1a, G 3.5) and azobenzene dicarboxylic acid chloride in the presence of triethylamine. PAMAM dendritic diol 1a as well as SCDPE (1) were thoroughly characterized by means of IR and NMR (1H and 13C) spectroscopies. The intrinsic viscosity of 1 at 36 °C in CHCl3 was found to be 0.38 dl/g. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4182–4188, 2001

Synthesis, characterization and reactivity of ReOMe 2(bipy)X complexes

The complexes cis- and trans-ReOMe 2(bipy)Cl, where cis or trans refers to the Me–Re–Me regiochemistry, were observed to react as follows: (1) with the alkylating agents RMgCl (R=CH 2SiMe 3 and CH 2Ph) and RLi (R=Me) to give ReOMe 2(bipy)R, and (2) with the alkoxides NaOR′ (R′=Me and Et) to give trans-ReOMe 2(bipy)(OR′). The complex ReOMe 2(bipy)CH 2SiMe 3 was observed by 1H NMR spectroscopy to undergo trans to cis isomerization, which proceeded via reversible first-order kinetics, with Δ H°=1.60(4) kcal mol −1 and Δ S°=3.7(1) eu. At room temperature, Δ G°=0.5(1) kcal mol −1 and K eq=0.42(2), indicating a weak thermodynamic preference for the trans isomer. The activation parameters for the trans to cis isomerization were Δ H ‡ 1=26(2) kcal mol− 1 and Δ S ‡ 1=10(7) eu, and those for the reverse cis to trans isomerization were Δ H ‡ −1=27(2) kcal mol −1 and Δ S ‡ −1=14(7) eu. Treatment of a mixture of cis- and trans-ReOMe 2(bipy)CH 2SiMe 3 with anhydrous HCl gave ReOMe(bipy)(CH 2SiMe 3)Cl; similar treatment of ReOMe 3(bipy) and ReOMe 2(bipy)CH 2Ph gave cis-ReOMe 2(bipy)Cl. Exposure of ReOMe(bipy)(CH 2SiMe 3)Cl to AgPF 6 in acetonitrile-d 3 gave the cationic nitrile adduct [ReOMe(bipy)(CH 2SiMe 3)(CD 3CN)][PF 6].

Hybrid Langmuir–Blodgett films of APT and cyanine with binary output modes

Hybrid Langmuir–Blodgett (LB) films were fabricated by transferring pure monolayers of an azobenzene derivative having a TCNQ (7,7,8,8-tetracyanoquinodimethane) moiety (APT) and then 1:1 mixed monolayers of cyanine/APT on solid substrates. The hybrid LB films have binary output modes. The alternate irradiation of UV and visible light yielded reversible cis- trans isomerization of APT, which was accompanied by the electrical conductivity switching associated with APT molecules and the change in the optical properties of the films due to the development of the J-band of the cyanine dye (CY). The order of the sequential deposition of the APT pure monolayers and the 1:1 mixed monolayers of CY/APT plays an important role in realizing the hybrid LB films with binary output modes.

Synthesis of Poly(p-phenylene)-Based Photoresponsive Conjugated Polymers Having Azobenzene Units in the Main Chain

Poly(p-phenylene)-based novel conjugated polymers having photoisomerizable azobenzene units in the main chain were prepared by the palladium-catalyzed Suzuki coupling of 1,3-propanediol diesters of 1,4-phenylenediboronic acids with 4,4‘-diiodoazobenzenes, and the reversible cis−trans isomerizations of azobenzene units were achieved. Despite their stiff structures, the polymers are soluble in common organic solvents such as chloroform, toluene, and tetrahydrofuran if either or both of the monomers possess n-hexyl side chains on the aromatic rings. The resulting polymers possess high molecular weights (Mn > 7 × 103) and are thermally stable (T0 > 350 °C) and fluorescent. The azobenzene units in the conjugated polymer backbone underwent the photochemical cis−trans reversible isomerizations in both tetrahydrofuran and toluene upon irradiation with UV light of appropriate wavelengths at 20 °C. The thermal cis-to-trans isomerization was also readily accomplished in toluene at 80 °C. Moreover, the solid-state ph...

Tetrathiafulvalenenaphthalenophanes: planar chirality and cis/trans photoisomerization

A cyclophane incorporating one 1,5-dioxynaphthalene ring system and one tetrathiafulvalene (TTF) unit bridged by [SCH(2)CH(2)O] linkages has been synthesized. In this cyclophane, the TTF unit can adopt either cis or trans configurations. In addition, the 1, 5-dioxynaphthalene ring system imposes one element of planar chirality on this cyclophane. A second element of planar chirality is introduced by the trans form of the TTF unit. Thus, the cyclophane exists in diastereoisomeric forms as three pairs of enantiomers. The enantiomeric pairs associated with the cis form of the TTF unit, as well as one of those associated with the trans form, have been isolated by crystallization, and their structures assigned in the solid state by single-crystal X-ray analyses. In solution, cis/trans isomerization occurs when either the cis or the trans form of the cyclophane is exposed to light. The photoisomerization reaction can be followed by (1)H NMR and UV-vis spectroscopies, as well as by HPLC. The photoisomerization quantum yield has been measured at two different excitation wavelengths (406 and 313 nm). In both cases, the trans --> cis process (Phi = 0.20 at 406 nm) is much more efficient than the reverse cis --> trans process (Phi = 0.030 at 406 nm). Since the absorption spectra of the trans and cis isomers are different and the quantum yield of the trans --> cis photoisomerization reaction depends on the excitation wavelength, the mole fraction of the two diastereoisomers present at the photostationary state depends on the wavelength of the exciting light. No isomerization occurs when the solutions, regardless of the mole fraction of the two diastereoisomers, are stored in the dark.

Stimuli-responsive polymers. 4. Photo- and thermo-regulated chiroptical behavior in azobenzene-modified polymers fitted with main chain spirobiindane turns and chiral binaphthyl bends

A series of azobenzene-modified polyamides fitted with main chain spirobiindane turns and chiral binaphthyl bends was prepared from the solution polycondensation of trans-azobenzene-4,4′-dicarbonyl chloride with appropriate diamine monomers. When evaluated in their all trans-azobenzene configurations, these materials exhibited a good mix of physical properties suitable for high performance applications. Photoinduced trans→ cis isomerization reactions were effected by irradiating polymer solutions with near UV light. Reverse cis→ trans isomerization of the backbone azobenzene segments was triggered by either photochemical or thermal means and was monitored by optical absorbance spectroscopy. Thermally induced cis→ trans reorganization within each polymer followed the first-order rate law. Activation energies calculated for this process in DMAC all fell near 21–23 kcal mol −1 and were not strongly correlated to backbone content. Polymers containing axially asymmetric S-(−)- or R-(+)-2,2′-binaphthyl main chain linkages exhibited thermo- and photo-responsive chiroptical behavior when evaluated in dilute THF solutions. Specifically, the trans-azobenzene-modified materials were all characterized by CD spectra showing intense molar ellipticities within the 300–400 nm spectral window. Specific rotation magnitudes determined for the trans-polymers at the sodium D-line ranged into the hundreds of degrees and were dependent on the extent of binaphthyl loading along the polyamide backbone. The irradiation of the polymer samples to drive the trans→ cis isomerization reaction resulted in an immediate chiroptical response, with CD band intensities and optical rotations significantly diminished. These effects were fully reversible and were attributed to the presence of putative one-handed helical conformations in the trans-azobenzene-modified polymers that were severely disrupted following the trans→ cis isomerization reaction.

On spontaneous and photoinduced orientational mobility of dye molecules in polymers

A few years ago, a theoretical model has been introduced for the simulation of photoinduced reorientation of dye molecules in amorphous polymers (photoinduced anisotropy, photoassisted electrical poling and all-optical poling). In the present paper, approximations and validity conditions are discussed in details. Simulations of the transient behavior of photoinduced anisotropy and of photoassisted poling, have been computed for different sets of parameters. Here, simulations are compared with new experimental results on photoinduced dichroism and electrooptic properties of azo-dye-containing polymer films. Experiments show an inhomogeneity and a time variation of angular diffusion constants, but also of absorption spectra. These inhomogeneities cannot be introduced in the theoretical model, nevertheless, simulations describe satisfyingly the mechanisms of molecular reorientation induced by trans– cis photoisomerization. The influence of different parameters can be understood, for instance the reverse cis– trans photoisomerization and the anisotropy of the cis state. Some similar results with spiropyran are summarized.

Stimuli-responsive polymers. III. Poly(aryl ether ketone amide)s with reversibletrans- ?cis-4,4?-azobenzene and fixed 2,2?-binaphthyl kinking elements in the main chain

The low-temperature polycondensation of trans-azobenzene-4,4′-dicarbonyl chloride with (S)-(−)-1,1′-binaphthyl-2,2′-diamine and/or 1,4-bis(3-aminophenoxy-4′-benzoyl)benzene afforded a new series of poly(aryl ether ketone amide)s with both fixed and photoinducible kinking elements positioned randomly along the main chain. In their lower energy, trans-azobenzene configurations, the orange, film-forming materials were amorphous, highly tractable, and thermally stable under air or nitrogen up to about 420°C. Variants endowed with higher loadings of the bent binaphthyl monomer were soluble in a variety of organic solvent media including THF and acetone. The introduction of cis-azobenzene backbone kinks into these materials was carried out by irradiating the polymer solutions with near-UV light. Up to 70% of the azobenzene moieties in these polymers were capable of assuming the higher energy cis-configuration, thus greatly increasing the number of bent or kinked sites positioned along each polymer backbone. In solution, reverse cis → trans isomerization reactions were triggered thermally and were quantitatively tracked by both optical absorbance and 1H NMR spectroscopies. Activation parameters calculated for cis → trans reorganization of the polymer backbone were not dependent upon the chemical composition or molecular weight of the polymers but did exhibit a small dependence upon the nature of the solvent medium used to conduct the isomerization experiment. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2827–2837, 1998

The inner-hydrogen migration and ground-state structure of porphycene

Following on from previous work on free-base porphyrin, we present the results of a comprehensive study on the structure and inner-hydrogen migration in porphycene, a structural isomer of porphyrin. We used density functional theory with the hybrid B3-LYP exchange-correlation functional, and both the 6-31G(d) and a triple-zeta double-polarization (TZ2P) basis set (the latter containing 726 contracted basis functions). Full geometry optimizations were carried out and all stationary points were characterized by vibrational analysis. A scaled quantum mechanical (SQM) treatment of the theoretical force constants shows convincingly that the trans-isomer is the ground state, with trans–trans inner-hydrogen migration taking place—as is the case with porphyrin—in a two-step process via a (highly unstable) cis intermediate. With the TZ2P basis, excluding zero-point effects, the trans–cis barrier height is 4.9 kcal/mol, the cis–trans energy difference is 2.4 kcal/mol and the reverse cis–trans barrier height is only 2.5 kcal/mol. We also map out and fully characterize an alternative, high-energy migration path involving a second, nonplanar cis isomer.

Photoinduced Switching in Self-Assembled Multilayers of an Azobenzene Bolaamphiphile and Polyelectrolytes

The reversible cis–trans photoisomerization of azobenzene and its derivatives has potential applications in optical data storage systems. But can the photoisomerization occur also in multilayers? It is shown that differences in photochemical activity can be ascribed to the different packing of azobenzene derivatives in the layers, free volume, such as illustrated in the Figure, being necessary to enable the photoisomerization to take place.

Photoinduced Switching in Self-Assembled Multilayers of an Azobenzene Bolaamphiphile and Polyelectrolytes

The reversible cis–trans photoisomerization of azobenzene and its derivatives has potential applications in optical data storage systems. But can the photoisomerization occur also in multilayers? It is shown that differences in photochemical activity can be ascribed to the different packing of azobenzene derivatives in the layers, free volume, such as illustrated in the Figure, being necessary to enable the photoisomerization to take place.

Azobenzene modified poly(aryl ether ketone amide)s. 2. Photo- and thermo-responsive behaviour in dilute solution

A number of azobenzene modified poly(aryl ether ketone amide)s with differing backbone geometries wereevaluated for their photo- and thermo-regulated behaviour in dilute solution. Photoinduced trans → cis isomerization reactions were carried out by irradiating the polymer samples with ultraviolet light at wavelengths between 370 and 400 nm. Photostationary state compositions achieved under these conditions typically consisted of about 70% of the higher energy cis isomer distributed along the polymer main chain. Reverse cis → trans isomerization of the backbone azobenzene moieties was triggered by either photochemical or thermal means and was monitored by optical absorbance and 1H n.m.r. spectroscopies. Thermally induced cis → trans return in each of the polymers obeyed the first-order rate law. Activation energies calculated for the ‘dark’ isomerization reaction fell near 21 kcal mol −1 for each of the polymer samples evaluated. These values were not dependent on the overall structure or molecular weight of the polymer backbone and were nearly identical to those determined for several lower molecular weight model compounds. Calculated half-lives for the isomerization of cis-azobenzene linkages buried in the polymer backbone ranged from 1 day near room temperature to about 1 h at the 60°C isotherm. Data gleaned from SEC experiments suggested that polymers endowed with conformationally restricted geometries underwent a two-fold reduction in hydrodynamic radius in response to ultraviolet light exposure. Photo-contractions in more flexible polymer samples appeared to be less dramatic, consistent with molecular modelling and dilute solution viscosity measurements.

Stationary Points on the Ground-State Potential Energy Surface of Dimethyldiazene. Isomerization and Decomposition in Competition

The singlet ground-state potential energy surface as related to the thermal unimolecular mechanisms of dimethyldiazene was investigated, in order to obtain activation energies for trans−cis isomerization, as well as for the isomerization leading to 1,1-dimethyldiazene and for the simple C−N bond cleavage in the formation of CH3N2• and CH3• radicals. We used approximate density functional theory (DFT) with B3LYP potential to locate minimum energies and transition-state structures, as well as to determine harmonic vibrational frequencies and resulting zero-point vibrational energies. To obtain more reliable results, some structures were reoptimized by the multiconfiguration SCF wave function. The energetics were determined using quadratic configuration interaction singles and doubles (QCISD), perturbation theory with UHF and CAS reference wave functions (MP2-MP4 and CASPT2), and the configuration interaction wave function, which includes single and double excitations from the selected set of reference configurations constructed with the CASSCF molecular orbitals (MR-CISD). In contrast to previous theoretical studies, our results show that the trans−cis isomerization of 1,2-dimethyldiazene, which proceeds by rotation around the NN bond, or semilinearization, requires an activation energy near to that of its decomposition. It was also confirmed that isomerization to 1,1-dimethyldiazene is less favored than the cis−trans isomerization. The following barrier heights have been estimated: 229 kJ/mol for decomposition, 207 kJ/mol for isomerization by rotation, 227 kJ/mol for isomerization by semilinearization, and 424 kJ/mol for rearrangement to 1,1-dimethyldiazene. Calculation of RRKM rate coefficients showed that in the temperature range from 450 to 600 K the trans−cis isomerization and a direct decomposition proceed in competition. However the more stable trans-1,2-dimethyldiazene preferentially decomposes directly, as a consequence of the fast reverse cis−trans isomerization rate. At the temperatures above 600 K a direct decomposition increasingly predominates over isomerization.