Isomerization Of Stilbene Research Articles

Trans-to- cis isomerization of stilbene following an ultrafast laser pulse

Dynamical simulations are reported for trans-to- cis photoisomerization of stilbene induced by a femtosecond-scale laser pulse. The results imply that the basic mechanism is the simplest imaginable: Electronic excitation by the laser pulse weakens the central C–C bond, and also excites vibrations of this bond. Vibrational energy redistribution then leads to rotation about the bond, resulting in isomerization. Finally, electronic de-excitation near a conical intersection restores the molecule to the electronic ground state. On the other hand, these dynamical simulations demonstrate complexity in the details and reveal supporting roles for other degrees of freedom, such as torsional motion about the vinyl-phenyl side bonds.

Novel bile acid conjugates with aryl/alkenyl linker: Synthesis and characterization

Eight potential precursors for the design of bile acid derived receptors viz. 2,6-bis[dimethyl(3α-hydroxy-5β-cholan-24-amidoethyl)ammonio-methyl]naphthalene dibromide ( 1), 3,3′-bis[dimethyl(3α-hydroxy-5β-cholan-24-amidoethyl)ammoniomethyl]biphenyl dibromide ( 2), 3,3′-bis[dimethyl(3α,7α,12α-trihydroxy-5β-cholan-24-amidoethyl)ammoniomethyl]biphenyl dibromide ( 3), 4,4′-bis[dimethyl(3α-hydroxy-5β-cholan-24-amidoethyl)ammoniomethyl]stilbene dibromide ( 4), 4,4′-bis[dimethyl(3α,7α,12α-trihydroxy-5β-cholan-24-amidoethyl)ammoniomethyl]stilbene dibromide ( 5), allyl-dimethyl(3α,7α,12α-trihydroxy-5β-cholan-24-amidoethyl)ammonium bromide ( 6), 1,4′-bis[dimethyl(3α-hydroxy-5β-cholan-24-amidoethyl)ammonio]but-2-ene dibromide ( 7), 1,4′-bis[dimethyl(3α,7α,12α-trihydroxy-5β-cholan-24-amidoethyl)ammonio]but-2-ene dibromide ( 8) have been synthesized in moderate to almost quantitative yields, and their structures have been characterized by 1H, 13C, 13C DEPT-135, PFG 1H, 13C HMQC, and PFG 1H, 13C HMBC NMR spectra. Their molecular weights and elemental compositions have been determined by ESI-TOF mass spectrometry and elemental analyses. trans to cis isomerization of stilbene ( 4, 5) and 2-butenylene ( 7, 8) derivatives have also been studied.

Studies on thermotropic liquid crystalline polyphosphates containing photoreactive dual mesogens

A series of thermotropic liquid crystalline polyphosphates, containing photochromic molecules of stilbene in the backbone and substituted azobenzene in the side chain, were synthesized respectively. The inherent viscosity measurements were determined for all the polymers. The stability and char yield, Tg, Tm and Ti were investigated by thermal analysis. Polarizing optical microscopy of all the polymers exhibited birefringent melts with liquid crystalline behaviour. Variable temperature powder X‐ray diffraction techniques were performed for confirmation of textures. UV‐visible photolysis studies investigated the simultaneous behaviour of reactivity rates of crosslinking of stilbene units and isomerization, caused by azobenzene units in the main chain/side chain LCPs. Photoisomerization kinetics demonstrated the switching time rates for the trans‐cis conversion of the azobenzene unit. A model polymer was synthesized and compared for the effect of stilbene in the main chain. Dipole moment values were calculated for the simulated low molecular mass of the pendant substituents to predict the polarity using MOPAC 3D Pro.

Direct and sensitized (energy and electron transfer) geometric isomerization of stilbene within zeolites: a comparison between solution and zeolite as reaction media

The trans- and cis-stilbenes upon inclusion in NaY zeolite are thermally stable. Direct excitation and triplet sensitization results in geometric isomerization and the excited state behavior under these conditions are similar to that in solution. Upon direct excitation, a photostationary state consisting of 65% cis and 35% trans isomers is established. Triplet sensitization with 2-acetonaphthone gave a photostationary state consisting of 63% cis and 37% trans isomers. These numbers are similar to the ones obtained in solution. Thus, the presence of cations and the confined space within the zeolite have very little influence on the overall chemistry during direct and triplet sensitization. However, upon electron transfer sensitization with N-methylacridinium (NMA) as the sensitizer within NaY, isomerization from cis-stilbene radical cation to trans-stilbene occurs and the recombination of radical ions results in triplet stilbene. Prolonged irradiation gave a photostationary state (65% cis and 35% trans) similar to triplet sensitization. This behavior is unique to the zeolite and does not take place in solution. Steady state fluorescence measurements showed that the majority of stilbene molecules are close to the N-methylacridinium sensitizer. Diffuse reflectance flash photolysis studies established that independent of the isomer being sensitized only trans radical cation is formed. Triplet stilbene is believed to be generated via recombination of stilbene radical cation and sensitizer radical anion. One should be careful in using acidic HY zeolite as a medium for photoisomerization of stilbenes. In our hands, in these acidic zeolites isomerization dominated the photoisomerization.

Density functional studies of the ground- and excited-state potential-energy curves of stilbene cis-trans isomerization.

Using spin-unrestricted density functional theory (the VWN Becke-Perdew potential), including broken-symmetry and spin-projection methods, we have obtained the potential-energy curves as a function of the central torsional angle of stilbene in the ground (S0), the first excited triplet (T1), the first excited singlet (S1), and the doubly excited singlet (S2) states. The thermal trans-->cis isomerization of stilbene passes through a diradical broken-symmetry electronic structure around the twisted conformation (90 degrees central torsional angle) in the ground state. Our calculations support the proposed triplet mechanism for sensitized cis [symbol: see text] trans photoisomerization and the nonadiabatic singlet mechanism proposed by Orlandi and Siebrand. On the T1 potential-energy curve, the rotation of the C=C bond for both trans- and cis-stilbene will lead stilbene to the twisted conformation, from which the twisted stilbene will decay to the ground-state surface that is nearly isoenergetic with the T1 surface and has diradical electronic structure in the twisted region. On the S1 potential-energy curve, the energy increases in the direction from trans- to the twisted stilbene, and crosses with the neutral doubly excited S2 potential-energy curve, which has a minimum at the twisted structure and is lower in energy than the zwitterionic doubly excited state. The twisted stilbene around the energy minimum of the neutral doubly excited S2-state will decay onto the ground-state surface from where the rotation of the C=C bond leads the twisted stilbene to either the trans or cis configuration and the isomerization of stilbene is then completed. Similar studies have also been performed on a stilbene derivative with a substituent group, NHCOCH3.

Styrene hydration and stilbene isomerization in strong acid media. An excess acidity analysis

Rate constants obtained for the hydration of some ring-substituted styrenes, YC6H4CR=CH2 (R = H, CH3, CF3) (1-3), and for the isomerization of some cis-stilbenes substituted in both rings, YC6H4CH=CHC6H4Z (4), in aqueous H2SO4, D2SO4, and HClO4 media at 25°C and, in some cases, at other temperatures, have been subjected to an excess acidity analysis. Data obtained by different groups and in different media for the same substrates agree very well. The m‡ values obtained show that all the substrates react via the A-SE2 mechanism, rate-determining proton transfer to carbon; the transition state is late, with the proton transfer being about three-quarters complete. The reactivities of 1-3 in the aqueous standard state are obtained by extrapolation; they are 1:103:10-7, respectively. The stilbenes are relatively unreactive; log k0 values for compounds with substituents in the ring adjacent to the developing positive charge show a good correlation with σ+ with a large negative ρ+ value, but those for compounds with substituents in the other ring correlate with σ with a small negative ρ. Extrapolated log k0 values for 1 and 3 show a good correlation with σ+, but those for 2 correlate with neither σ+ nor σ, because the α-CH3 group twists the molecule and reduces the resonance interaction of ring substituents with the developing positive charge. On the other hand, this effect is not seen in 3, when the strongly deactivating α-CF3 group is present, meaning that the transition state is forced to be planar in this case. The activation parameters, solvent isotope effects, and excess acidity slope parameters m‡m* obtained in the analysis are tabulated and discussed. The parameters obtained for the parent styrenes 1 are very similar to those previously obtained for the phenylacetylenes YC6H4Ctriple bondCH, meaning that reactions with vinyl cation intermediates and reactions with benzyl cation intermediates can be quite similar. Key words: styrenes, stilbenes, hydration, excess acidity, LFER, mechanism.

Photocrosslinked polymer and interpenetrating polymer network for nonlinear optics

Two photocrosslinkable NLO polymers of poly(glycidyl methacrylate) substituted with 4-nitro-4′-hydroxyl stilbene (PGMAS) and acryloyl-functionalized epoxy-based polymer (PENAA) carrying 4-nitroaniline moieties were synthesized and characterized. By using the sulfonium salt cationic photoinitiator BDS · 2PF6 which can induce cationic or/and radical polymerization, the photocrosslinking of PGMAS and the interpenetrating polymer network (IPN) formed by the photocrosslinking of PGMAS and PENAA simultaneously were reported. The poled and photocrosslinked polymer films and IPN films exhibit relatively stable second-order nonlinear optical activity. The influence of stilbene isomerization in PGMAS films with different crosslink densities on the SHG stability was also investigated. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1081–1087, 1999

Styrene hydration and stilbene isomerization in strong acid media. An excess acidity analysis

Styrene hydration and stilbene isomerization in strong acid media. An excess acidity analysis

Distance-dependent photoinduced electron transfer in synthetic single-strand and hairpin DNA

The singlet state of stilbene-4,4′-dicarboxamide can serve as a fluorescent probe of both DNA conformation and electron transfer. Covalent incorporation of the stilbene-dicarboxamide into DNA structures with restricted conformational mobility results in inhibition of stilbene isomerization and an increase in its fluorescence quantum yield and lifetime. The fluorescence of stilbenedicarboxamide is selectively quenched by proximate guanine, but not by the three other DNA nucleobases. Selective quenching occurs via an electron transfer mechanism in which stilbene serves as the electron acceptor and guanine as the electron donor. Kinetic analysis of the distance dependence of electron transfer in stilbene-bridged hairpins suggests that duplex DNA is more effective than proteins as a medium for electron transfer, but that it does not function as a molecular wire.

Quantum harmonic transition state theory—Application to isomerization of stilbene in liquid ethane

A harmonic quantum transition state theory, suggested recently by Pollak and Gershinsky [in Lectures on Stochastic Dynamics, edited by W. Lutz and T. Poeschel, Lecture Notes in Physics (Springer Verlag, New York, 1997)], is applied for the first time to a realistic reacting system. The isomerization of trans-stilbene in the gas phase and in the presence of dense liquid ethane solvent is investigated. We find that the overall quantum effect at room temperature is rather small. The quantum correction to the classical reaction rate estimate is approximately 23% for gas phase stilbene at room temperature. The addition of the dense solvent lowers the correction down to 13%, thus making the reacting system even more “classical,” justifying the extensive use of classical molecular dynamics in investigating this reaction.

ChemInform Abstract: Mechanism of cis‐to‐trans One‐Way Isomerization of Stilbene and Formation of Its Stable Dimer Cation Radicals in Zeolite Cavities.

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Isomerization of stilbene in the gas phase: Theoretical study of isotopic and clustering effects

This paper presents a continuation of our previous theoretical studies on the rate of isomerization of trans-stilbene from the first excited electronic state based on the potential energy surface of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. Harmonic RRKM computations and molecular dynamics and Monte Carlo based classical rates are presented for deuterated isotopes of stilbene as well as hexane clusters of stilbene of varying size. Good agreement with experiment is found for energy dependent rates of d12 vs h12 stilbene. However, we find that the rate for d2 stilbene is greater than for d10 stilbene in contradiction to the experimental observations. For the hexane clusters we find that addition of hexane molecules causes a systematic decrease in the rate, in agreement with experiment.

Mechanism of Cis-to-Trans One Way Isomerization of Stilbene and Formation of Its Stable Dimer Cation Radicals in Zeolite Cavities

Abstract Cis-to-trans one way isomerization of stilbene in the first case catalyzed by proton, and then, secondly through electron transfer, occurred in acidic zaolites without light, suggesting that dimer cation radicals with charge transfer interaction between trans-stillbene and its cation radical may be formed in H-ZSM-5, whereas 1,4-dimer cation radicals of stilbene may be produced in H-mordenite.

Unimolecular reactions in the gas and liquid phases: A possible resolution to the puzzles of the trans-stilbene isomerization

Previous theoretical and experimental investigations of the trans-stilbene isomerization reaction in the excited S1 state indicated that the gas phase thermal rate at room temperature is much smaller than the thermal rate in the liquid phase. This was based on the observations that: (a) A combination of measured energy-dependent rates and RRKM calculations led to an isolated molecule thermal rate at T=300 K of 2×109 s−1; (b) An experiment of Balk and Fleming [J. Phys. Chem. 90, 3975 (1986)] in which stilbene vapor at 300 K excited at the S0 to S1 zero point to zero point electronic transition energy (000), gave a lifetime in the excited state of ∼780 ps. The liquid state lifetime in ethane is ∼30 ps. In this paper we present theoretical computations of the rate in the gas and liquid phases, based on a new potential model of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. We find that: (a) RRKM rates are in agreement with measured energy-dependent rates; (b) The thermal rate derived from the new RRKM rates is the same as the thermal rate in liquid ethane; (c) The laser excitation experiment of Balk and Fleming leads to laser cooling of the excited state suggesting that their measured lifetime is longer than the lifetime in the liquid. The surrounding liquid heats up the molecule on a time scale which is faster than the isomerization lifetime. Experiments are suggested to verify this interpretation.

Theoretical study of the trans-stilbene isomerization reaction in ethane

A theoretical investigation of the experimental measurements of the isomerization rate of trans-stilbene in liquids is presented. Monte Carlo and molecular dynamics simulations of the reaction indicate that the predominant solvent effect is in raising the isomerization barrier in the potential of mean force as the solvent density is increased. Dynamic friction effects are small. Good agreement is obtained between the numerical and experimental rates.

Femtosecond Real-Time Probing of Reactions. 20. Dynamics of Twisting, Alignment, and IVR in the trans-Stilbene Isomerization Reaction

The femtosecond real-time dynamics of the isomerization reaction of trans-stilbene under collisionless conditions are studied using (2+1) resonance-enhanced multiphoton ionization (REMPI) and femtosecond depletion spectroscopy (FDS) in a pump−probe scheme. The observed transients reflect the macroscopic sample anisotropy decay (rotational coherence) and intramolecular vibrational energy redistribution (IVR) as well as the ethylenic twisting isomerization reaction. Polarization-resolved measurements are performed to isolate the influence of rotational dynamics, and the measured anisotropy decay is compared with theoretical calculations of rotational coherence at room temperature. The IVR and nonradiative (isomerization) processes of trans-stilbene are studied as a function of S1 excess vibrational energy up to ∼6500 cm-1. These results are compared with previous measurements of trans-stilbene under collisionless conditions and with predictions of RRK and RRKM theories.

Nonradiative relaxation of 2-phenylindene in solution and its implications for isomerization of stilbenes

The solvent dependence of the excited state photophysics of 2-phenylindene (PI) is explored here. The results show that the nonradiative process is activated with a barrier in excess of 6.8 kcal/mol. This finding is consistent with recent observations on PI in a supersonic jet. The implications of these findings for stilbene isomerization are discussed.

Microscopic friction and solvation in barrier crossing: isomerization of stilbene in size-selected hexane clusters

In order to understand the roles of bulk friction and solvation in barrier crossing reactions, here we report our first study of the isomerization rates for a beam of t-stilbene-hexane n clusters, n = 1–5, at excess energies covering the range E ex = 0–4000 cm −1. We observe that rates measured at excess energies well above the t-stilbene isomerization barrier decrease monotonically with increasing number of solvent molecules. The influence of microscopic solvation on the rates is systematically studied and related to changes in barrier height and internal friction, both key to the understanding of behavior in the bulk.

Vibrational coherence in the solution phase photoisomerization reaction of cis-stilbene

Coherent low frequency (100–125 cm −1) nuclear motion was detected in the cis → trans condensed phase isomerization of stilbene. A mode, associated with phenyl torsional motion, has a vibrational dephasing time of 100 fs and exhibits solvent-dependent wavepacket dynamics. Selective probe wavelengths (600, 650 and 690 nm) exhibit phase shifts for the wavepacket dynamics that are also solvent sensitive.

Chromophore production in lignin model compounds — Anaerobic reactions

The anaerobic photochemistry of a number of plausible lignin model compounds (i.e. I: 3,4-Dimethoxy-α-(2-methoxyphenoxy)-β-hydroxypropiophenone;II: 1-(3,4-Dimethoxyphenyl)-2-(2-methoxyphenoxy)-propan-1,3-diol;Pol A: Poly(4-methoxyacrylophenone);Pol B: Poly(3,4-dimethoxyacrylophenone);St 1: 3,5-Dimethoxy-4-hydroxystilbene; andSt 2: 3,5,3′,5′-Tetramethoxy-4-hydroxystilbene) was studied, thin films of these materials being exposed to long-wave (λ≥300 nm) radiation under high vacuum conditions (10−6 torr). In all cases, the only low molecular weight products formed were methane and ethane, and quantum yields were estimated for these reactions. All materials underwent colouration (yellow) and a number of changes were also observed in both the absorbing and emitting characteristics. The colouration was attributed to the presence ofo-quinones which were formed (by further photolysis) from the phenoxy radicals, which were, in turn, produced by O−CH3 fission, the resulting methyl radicals being the precursors of methane and ethane. The stilbenes were in all cases much more reactive (by a factor of about 100); however, they also absorbed higher intensities of radiation in the 300<λ<350 nm region on account of the greater extent of red-shifting of the longest-wave π-π′ aromatic transitions. Gel permeation data indicate the formation of products of cyclization and isomerization of stilbenes and the dimerization of phenoxy radicals while new absorbances in the infrared and13C NMR confirm the presence ofo-quinones in all the models.