Thermal Cis-trans Isomerization Research Articles

A theoretical study on the thermal cis–trans isomerization of azoheteroarene photoswitches

Azoheteroarene photoswitches exhibit superior properties and the thermal cis–trans isomerization processes are sensitive to the different substitution patterns.

The Kinetics of the Cis-to-Trans Thermal Isomerization of 4-Anilino-4’- Nitroazobenzene are Highly Influenced by Solvent Polarity

The thermally stable trans isomers of donor-acceptor substituted azobenzene dyes can be easily transformed into unstable cis-form by light. Although azobenzene and its derivatives have been extensively investigated over the years, many aspects of the isomerization reaction kinetics are still unclear or controversial at best. Here, we investigate the kinetics of the thermal cis-trans isomerization of 4-anilino-4'-nitroazobenzene (4A4NAB) in six solvents of different polarities by means of flash photolysis. Using the transition state theory, we were able to determine a number of thermodynamic quantities including the standard enthalpy of activation Δ‡H0, the standard entropy of activation Δ‡S0, the Gibbs free energy of activation Δ‡G0, and the quasi-equilibrium constant K‡ of the isomerization reaction. Higher rates of isomerization and lower activation energies were observed in polar media, consistent with the formation of a charged transition state complex and the presence of solute−solvent interactions such as intermolecular hydrogen bonding. The data suggest that the cis-trans thermal isomerization of 4A4NAB is highly influenced by solvent polarity and the isomerization mechanism is likely to proceed via rotation about the azo double bond.

The role of the second order Jahn–Teller effect in the thermal reactions of ethylene

The UV absorption spectrum of ethylene during the pulse heating of an ethylene–argon mixture to prepyrolysis temperatures in the range 950 < Т < 1285 K was studied by kinetic spectroscopy in a free-piston adiabatic compression unit. New intense light absorption bands were found at 210 < λ < 260 and 440 < λ < 490 nm. Ab initio quantum-chemical calculations of the mechanism of the thermal cis-trans isomerization of ethylene were performed. When this mixture is heated, the point symmetry group of the ethylene molecule in the ground state S0 reduces to С1, which is characteristic for ethylene at the minimum of the S1 state, due to the second order Jahn–Teller effect.

Differing Isomerization Kinetics of Azobenzene-Functionalized Self-Assembled Monolayers in Ambient Air and in Vacuum.

Azobenzenealkanethiols in self-assembled monolayers (SAMs) on Au(111) exhibit reversible trans-cis photoisomerization when diluted with alkanethiol spacers. Using these mixed SAMs, we show switching of the linear optical and second-harmonic response. The effective switching of these surface optical properties relies on a reasonably large cross section and a high photoisomerization yield as well as a long lifetime of the metastable cis isomer. We quantified the switching process by X-ray absorption spectroscopy. The cross sections for the trans-cis and cis-trans photoisomerization with 365 and 455 nm light, respectively, are 1 order of magnitude smaller than in solution. In vacuum, the 365 nm photostationary state comprises 50-74% of the molecules in the cis form, limited by their rapid thermal isomerization back to the trans state. In contrast, the 455 nm photostationary state contains nearly 100% trans-azobenzene. We determined time constants for the thermal cis-trans isomerization of only a few minutes in vacuum and in a dry nitrogen atmosphere but of more than 1 day in ambient air. Our results suggest that adventitious water adsorbed on the surface of the SAM stabilizes the polar cis configuration of azobenzene under ambient conditions. The back reaction rate constants differing by 2 orders of magnitude underline the huge influence of the environment and, accordingly, its importance when comparing various experiments.

Function of Tyr185 in Stabilizing the Isomerization Equilibrium of the Retinal Chromophore in the Bacteriorhodopsin Ground State

Bacteriorhodopsin (bR), a member of the microbial rhodopsin family's seven trans-membrane receptors, functions as a light-driven proton pump for light energy capture and conversion in Halobacterium Salinarum. In the dark-adapted (DA) state, bR contains two thermally interconvertible retinal isomers: 13-cis, 15-syn retinal (bRcis), and all-trans, 15-anti retinal (bRtrans) with a molar ratio of 6:4 in the purple membrane. Absorption of a photon causes photoisomerization of the chromophore from the all-trans to the 13-cis, 15-anti configuration and triggers a series of structural rearrangements of the protein that initiates a vectorial translocation of a proton out of the cell.The function of Tyrosine 185 (Tyr185) in bR photoreaction mechanism has long been a topic of discussion, and been suggested to play an important role in stabilizing the pentagonal hydrogen-bonded network on the extracellular side of the protonated Schiff base during the photoreaction. However, little is to know on coupling of Tyr185 with the protonated Schiff base with respect to the thermal isomerization of the retinal chromophore at the bR ground state. Heteronuclear single molecular manipulation, combined with homonuclear correlation experiments, UV spectroscopy, light-induced transient absorption change measurements, molecular dynamic simulations and mutagenesis, has been established as a powerful tool for providing insight into the conformation and coupling of Tyr185 with the retinal chromophore at the bR ground state in the purple membrane. Our results show that the retinal chromophore in 13-cis and 15-syn configurations has more accumulated charge and, is more twisted and rigid than that in the all-trans configuration. Moreover, the cis-trans thermal isomerization equilibrium with the molar ratio of 6:4 at the ground state is stabilized by coupling with Tyr185.

Supercritical transesterification of beef tallow for biodiesel production in a batch reactor

Supercritical transesterification experiments of beef tallow with ethanol were carried out in a batch reactor in order to assess the effect of temperature (350 - 400°C), reactant molar ratio (9:1 - 15:1) and reaction time (8 - 40 min). Composition of the produced biofuel was assessed in terms of the percentage of peak area of the long chain Fatty Acid Ethyl Esters (FAEEs) identified, including also the decomposition products generated as a result of thermal cracking or cis-trans isomerization reactions. Thermal decomposition of FAEEs produced from beef tallow included short chain ethyl esters (C8 - C13), ethyl pentadecanoate (C15:0), heptadecanoate (C17:0), not initially present or prevalent in triglycerides sources, as well as ethyl stearate (C18:0) and n-alkanes. Glycerol decomposition reactions were also verified by the presence of glycerol ethers and water content in the produced biodiesel. The highest conversion of triglycerides was verified at 400°C, 15:1 triglycerides to ethanol molar ratio and 40-minute reaction time. At these conditions, ethyl oleate (C18:1) decomposed completely to form cis-trans isomers as well as ethyl octadecanoate (C18:0), which was attributed to double bond hydrogenation reactions. Short chain ethyl esters and glycerol decomposition products have the potential to benefit certain properties of the fuel such as viscosity and cold flow.

Effect of cucurbituril on the primary photoprocesses in indocarbocyanine dyes in water

The effect of cucurbit[7,8]urils (CB7, CB8) on photoprocesses in indocarbocyanine dyes has been studied. Complexation with CB7 and CB8 reveals bathochromic shifts of the maxima in the absorption and fluorescence spectra. Fluorescence enhancement and decreasing have been observed for the complexes with CB7 and CB8, respectively. The dyes and their complexes are capable of trans–cis photoisomerization and hardly undergo intersystem crossing to the triplet state. The rate of the thermal cis–trans isomerization reaction is decreased due to complexation.

Fast thermal cis–trans isomerization depending on pH and metal ions of water-soluble azobenzene derivatives containing a phosphate group

We observed unique thermal cis–trans isomerization properties of water-soluble azobenzene derivatives in which a phosphate group was substituted at ortho position of an azobenzene via an amide linkage. Here, we report the effect of the structure of the phosphate groups (the length and the position), pH, and metal ions on thermal cis–trans isomerization. We revealed that the water-soluble azobenzene derivatives, in which di- or tri-phosphate group was substituted at ortho position of the azobenzene via the amide linkage, showed the fast thermal isomerization in low pH condition. In addition, we showed that the effects of the metal ions on the thermal isomerization rate.

Quantum Monte Carlo Treatment of the Charge Transfer and Diradical Electronic Character in a Retinal Chromophore Minimal Model.

The penta-2,4-dieniminium cation (PSB3) displays similar ground state and first excited state potential energy features as those of the retinal protonated Schiff base (RPSB) chromophore in rhodopsin. Recently, PSB3 has been used to benchmark several electronic structure methods, including highly correlated multireference wave function approaches, highlighting the necessity to accurately describe the electronic correlation in order to obtain reliable properties even along the ground state (thermal) isomerization paths. In this work, we apply two quantum Monte Carlo approaches, the variational Monte Carlo and the lattice regularized diffusion Monte Carlo, to study the energetics and electronic properties of PSB3 along representative minimum energy paths and scans related to its thermal cis–trans isomerization. Quantum Monte Carlo is used in combination with the Jastrow antisymmetrized geminal power ansatz, which guarantees an accurate and balanced description of the static electronic correlation thanks to the multiconfigurational nature of the antisymmetrized geminal power term, and of the dynamical correlation, due to the presence of the Jastrow factor explicitly depending on electron–electron distances. Along the two ground state isomerization minimum energy paths of PSB3, CASSCF calculations yield wave functions having either charge transfer or diradical character in proximity of the two transition state configurations. Here, we observe that at the quantum Monte Carlo level of theory, only the transition state with charge transfer character can be located. The conical intersection, which becomes highly sloped, is observed only if the path connecting the two original CASSCF transition states is extended beyond the diradical one, namely by increasing the bond-length-alternation (BLA). These findings are in good agreement with the results obtained by MRCISD+Q calculations, and they demonstrate the importance of having an accurate description of the static and dynamical correlation when studying isomerization and transition states of conjugated systems.

Au nanoparticle scaffolds modulating intermolecular interactions among the conjugated azobenzenes chemisorbed on curved surfaces: tuning the kinetics of cis-trans isomerisation.

π-π Intermolecular interactions among adjacent conjugated azobenzenes chemisorbed on (non-)flat Au surfaces can be tuned by varying the curvature of the Au nanoparticles. Here we show that such interactions rule the thermal cis-trans isomerization kinetics, towards a better control on the azobenzene bistability for its optimal integration as a responsive material.

Kinetics of thermal activation of an ultraviolet cone pigment.

Visual pigments can be thermally activated via isomerization of the retinyl chromophore and hydrolysis of the Schiff base (SB) through which the retinyl chromophore is bound to the opsin protein. Here, we present the first combined experimental and theoretical study of the thermal activation of a Siberian hamster ultraviolet (SHUV) pigment. We measured the rates of thermal isomerization and hydrolysis in the SHUV pigment and bovine rhodopsin. We found that these rates were significantly faster in the UV pigment than in rhodopsin due to the difference in the structural and electrostatic effects surrounding the unprotonated Schiff base (USB) retinyl chromophore in the UV pigment. Theoretical (DFT-QM/MM) calculations of the cis-trans thermal isomerization revealed a barrier of ∼23 kcal/mol for the USB retinyl chromophore in SHUV compared to ∼40 kcal/mol for protonated Schiff base (PSB) chromophore in rhodopsin. The lower barrier for thermal isomerization in the SHUV pigment is attributed to the (i) lessening of the steric restraints near the β-ionone ring and SB ends of the chromophore, (ii) displacement of the transmembrane helix 6 (TM6) away from the binding pocket toward TM5 due to absence of the salt bridge between the USB and the protonated E113 residue, and (iii) change in orientation of the hydrogen-bonding networks (HBNs) in the extracellular loop 2 (EII). The results in comparing thermal stability of UV cone pigment and rhodopsin provide insight into molecular evolution of vertebrate visual pigments in achieving low discrete dark noise and high photosensitivity in rod pigments for dim-light vision.

Theoretical study of solvent effects on the ground and low-lying excited free energy surfaces of a push-pull substituted azobenzene.

The ground and low-lying excited free energy surfaces of 4-amino-4'-cyano azobenzene, a molecule that has been proposed as building block for chiroptical switches, are studied in gas phase and a variety of solvents (benzene, chloroform, acetone, and water). Solvent effects on the absorption and emission spectra and on the cis-trans thermal and photo isomerizations are analyzed using two levels of calculation: TD-DFT and CASPT2/CASSCF. The solvent effects are introduced using a polarizable continuum model and a QM/MM method, which permits one to highlight the role played by specific interactions. We found that, in gas phase and in agreement with the results found for other azobenzenes, the thermal cis-trans isomerization follows a rotation-assisted inversion mechanism where the inversion angle must reach values close to 180° but where the rotation angle can take almost any value. On the contrary, in polar solvents the mechanism is controlled by the rotation of the CN═NC angle. The change in the mechanism is mainly related to a better solvation of the nitrogen atoms of the azo group in the rotational transition state. The photoisomerization follows a rotational pathway both in gas phase and in polar and nonpolar solvents. The solvent introduces only small modifications in the nπ* free energy surface (S1), but it has a larger effect on the ππ* surface (S2) that, in polar solvents, gets closer to S1. In fact, the S2 band of the absorption spectrum is red-shifted 0.27 eV for the trans isomer and 0.17 eV for the cis. In the emission spectrum the trend is similar: only S2 is appreciably affected by the solvent, but in this case a blue shift is found.

Spectral and time-resolved properties of novel hetarylazo dyes containing hydrogenated quinolines and triazole moieties

The spectral characteristics of novel hetarylazo dyes containing triazole and hydrogenated quinoline moieties and the spectral and time-resolved parameters of photochemical processes occurring upon their photoexcitation were studied by stationary spectrophotometry and pulse photolysis. All compounds have an intense absorption maximum in the visible spectral range at 470–500 nm with ɛ = 17000–27000 L mol−1 cm−1. Upon excitation with the visible light, reversible trans-cis photoisomerization occurs, which is followed by thermal cis-trans isomerization. The temporal parameters of the isomerization depend on the azo dye structure. The introduction of bulky substituents in the triazole and hydroquinoline moieties results in the increase in characteristic times of the thermal transformation of the generated transient species from milliseconds to seconds and in the partial irreversibility of the process.

Enzyme immobilization in a photosensitive conducting polymer bearing azobenzene in the main chain

A new photosensitive and thermosensitive monomer, namely bis(4-(3-thienyl ethylene)-oxycarbonyl)diazobenzene (TDAZO), was synthesized. The photochemical and thermal cis–trans isomerization of the monomer has been investigated. The rate constants of the photoisomerization of TDAZO in ACN and DCM were 0.195 and 0.308 min−1, respectively. For spectroelectrochemical investigation and enzyme immobilization application, TDAZO copolymerized with thiophene and pyrrole. Electrochemical and spectroelectrochemical properties of P(TDAZO-co-Th) were investigated and invertase was immobilized in P(TDAZO-co-Py) copolymer. Immobilization of enzymes was carried out by the entrapment of the enzyme in conducting polymer matrices during electrochemical polymerization of pyrrole through thiophene moieties of the TDAZO. Optimum conditions for this electrode, such as pH, temperature, kinetic parameters (K m and V max) and operational stability were investigated. Kinetic parameters invertase-immobilized in copolymer were smaller than free enzyme. The optimum operational temperature was 10 °C higher for immobilized enzyme than that of the free enzyme. Due to strong interaction between enzyme and diazo group in the polymer main chain, thermal, pH and operational stability of enzyme has been enhanced.

옥타[8-{4-(4'-시아노페닐아조)펜옥시}]옥틸 그리고 옥타[8-{4-(4'-시아노페닐아조) 펜옥시카보닐}]헵타노화 이당류의 열방성 액정과 광화학적 상전이 거동

Octa[8-{4-(4'-cyanophenylazo)phenoxy}]octyl and octa[8-{4-(4'-cyanophenylazo)phenoxycarbonyl}]heptanoated disaccharide derivatives were synthesized by reacting cellobiose, maltose, and lactose with 1-{4-(4'-cyanophenylazo) phenoxy}octylbromide or 1-{4-(4'-cyanophenylazo)phenoxycarbonyl}]heptanoyl chloride, and their thermotropic liquid crystalline and photochemical phase transition behavior were investigated. All the {(cyanophenylazo)phenoxy} octyl disaccharide ethers (CADETs) formed monotropic nematic (N) phases, whereas all the {(cyanophenylazo) phenoxycarbonyl}heptanoated disaccharide esters (CADESs) exhibited enantiotropic N phases. Compared with CADETs, CADESs showed higher isotropic (I)-to-N phase transition temperatures. Photoirradiation of the disaccharide derivatives in a glass cell or in a cell with a rubbed polyimide (PI) alignment layer at a N phase resulted in disappearance of the N phase due to trans-cis photoisomerization of azobenzene, and the initial N phase was recovered when the irradiated sample was kept in the dark because of cis-trans thermal isomerization and reorientation of trans-azobenzenes. The rates of the photochemical N-I and the thermal I-N phase transition of disaccharide derivatives in a cell with a rubbed PI alignment layer were faster than those in a glass cell, and were significantly different from those observed for the monomesogenic compounds containing cyanoazobenzene and the 4-{4'-(cyanophenylazo)phenoxy}octyl glucose and cellulose ethers. The results were discussed in terms of difference in cooperative motion of azobenzene groups due to the flexibility of the main chain, the number of mesogenic units per repeating units, and the distance between the azobenzene groups.

Azobenzene Photoisomerization under High External Pressures: Testing the Strength of a Light-Activated Molecular Muscle

The photoinduced isomerization and thermal back relaxation of an azobenzene-functionalized polymer poly(disperse red 1 acrylate) were investigated at increasing external pressures up to 1.5 GPa inside a diamond-anvil spectroscopic cell. The thermal cis-trans isomerization was monitored by laser pump-probe spectroscopy, which demonstrated an increase in the half-life of the isomerization process with increasing pressure. Additionally, the cis content of the photostationary state gradually decreased as a function of pressure, with complete arrest of the trans-cis photoisomerization above 1.5 GPa. The fact that the photoswitching behavior however could still be observed beyond 1 GPa is remarkable and is effectively a measure of the strength of the azobenzene chromophore as an artificial muscle. The changes in the Raman shifts of both trans- and cis-azobenzene were also investigated from ambient pressure up to 4 GPa, and no discontinuities were observed in the pressure vs wavenumber plots indicating no change in phase. The cis-trans photoisomerization of azobenzene was shown however to still be inducible at all the pressures investigated, confirming the suitability of these molecules for high-efficiency light actuation.

Photonic crystal fibre as an optofluidic reactor for the measurement of photochemical kinetics with sub-picomole sensitivity

Photonic crystal fibre constitutes an optofluidic system in which light can be efficiently coupled into a solution-phase sample, contained within the hollow core of the fibre, over long path-lengths. This provides an ideal arrangement for the highly sensitive monitoring of photochemical reactions by absorption spectroscopy. We report here the use of UV/vis spectroscopy to measure the kinetics of the photochemical and thermal cis-trans isomerisation of sub-picomole samples of two azo dyes within the 19-μm diameter core of a photonic crystal fibre, over a path length of 30 cm. Photoisomerisation quantum yields are the first reported for "push-pull" azobenzenes in solution at room temperature; such measurements are challenging because of the fast thermal isomerisation process. Rate constants obtained for thermal isomerisation are in excellent agreement with those established previously in conventional cuvette-based measurements. The high sensitivity afforded by this intra-fibre method enables measurements in solvents in which the dyes are too insoluble to permit conventional cuvette-based measurements. The results presented demonstrate the potential of photonic crystal fibres as optofluidic elements in lab-on-a-chip devices for photochemical applications.

Spontaneous nano pattern formation under pulsed exposure: relaxation kinetics study

Formation of spontaneous surface relief grating under pulsed exposure on the surface of an azo polymer film has been investigated. We did experiments in different repetition rates and pulse widths by use an electro mechanical chopper and a diode-pumped solid-state laser with wavelength of 532 nm as pump beam. Incidence of pump beam, has been set normal to the surface of azo-polymer film. Spontaneous surface relief grating with grating pitch of ~1,250 nm was formed. For low repetition rate, relaxation due to cis-trans isomerization becomes dominant and growth rate becomes slower. A threshold for spontaneous pattern formation was found. In addition to an increase in the diffraction intensity during illumination, anomalous variation of diffraction intensity by switching on and off the laser was observed. By switching off the recording beam sharp enhancement occurred. By switching on the recording beam, after a sharp decrease in the intensity of diffracted beam, it increased further under illumination. Anomalous variations, strongly depends on polarization of the probe beam and the irradiation time for the formation of spontaneous SRG. Sharp enhancement has been attributed to cis-trans thermal isomerization and sharp decrease has been attributed to isomeration in whole surface under illumination.

Noble Reaction Features of Bromoborane in Oxidative Addition of B–Br σ-Bond to [M(PMe3)2] (M = Pt or Pd): Theoretical Study

Through detailed calculations by density functional theory and second-order Møller-Plesset perturbation theory (MP2) to fourth-order Møller-Plesset perturbation theory including single, double, and quadruple excitations [MP4(SDQ)] methods, we investigated the oxidative addition of the B-Br bond of dibromo(trimethylsiloxy)borane [Br(2)B(OSiMe(3))] to Pt(0) and Pd(0) complexes [M(PMe(3))(2)] (M = Pt or Pd) directly yielding a trans bromoboryl complex trans-[MBr{BBr(OSiMe(3))}(PMe(3))(2)]. Two reaction pathways are found for this reaction: One is a nucleophilic attack pathway which directly leads to the trans product, and the other is a stepwise reaction pathway which occurs through successive cis oxidative addition of the B-Br bond to [M(PMe(3))(2)] and thermal cis-trans isomerization. In the Pt system, the former course occurs with a much smaller energy barrier (E(a) = 5.8 kcal/mol) than the latter one (E(a) = 20.7 kcal/mol), where the DFT-calculated E(a) value is presented hereafter. In the Pd system, only the latter course is found in which the rate-determining steps is the cis-trans isomerization with the E(a) of 15.1 kcal/mol. Interestingly, the thermal cis-trans isomerization occurs on the singlet potential energy surface against our expectation. This unexpected result is understood in terms of the strong donation ability of the boryl group. Detailed analyses of electronic processes in all these reaction steps as well as remarkable characteristic features of [Br(2)B(OSiMe(3))] are also provided.

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.