Stilbene photoisomerization

Optically induced cis-trans isomerization is a key structural dynamic element for many types of photochromic switches as stilbenes and azobenzene derivatives and for photosensor proteins as bacteriorhodopsin, rhodopsin, and photoactive yellow protein. 

The photoisomerization of stilbenes is found to be a simple and convenient model for a detailed study of factors affecting the unimolecular photoreaction dynamics. The steady-state equilibrium constant of the photoisomerization process resulting from monochromatic excitation is given as 

That is why slowing down the photoisomerization process will cause an increase in fluorescence. The observed rate constant of the steady-state photoisomerization is given as 

Stilbenes. Applications in Chemistry, Life Sciences and Materials Science. Gertz Likhtenshtein Copyright 2010 WILEY-VCH Verkg GmbH Co. KGaA, Weinheim ISBN 978-3-527-32388-3 

Saltiel and coworkers proposed and developed first classical mechanisms for the light-induced trans-cis photoisomerization of trans-stilbene that have formed the basis for subsequent works [2-10]. According to pioneering works of the Saltiel group [2, 3], the trans-cis photoisomerization of stilbenes can proceed by two in-principle mechanisms (i) the direct process involving exited single state 

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Figure A3.6.8. Isothemis of k ) for tran.s-stilbene photoisomerization in n-hexane at temperatures between...

H. Hamaguchi I would like to comment on the stilbene photoisomerization in solution. We recently found an interesting linear relationship between the dephasing time of the central double-bond stretch vibration of Si franj-stilbene, which was measured by time-resolved Raman spectroscopy, and the rate of isomerization in various solutions. Although the linear relationship has not been established in an extensive range of the isomerization rate, I can point out that the vibrational dephasing time measured by Raman spectroscopy is an important source of information on the solvent-induced vibrational dynamics relevant to the reaction dynamics in solution. 

Saltiel, J., Waller, A.S., and Sears, D. F., Jr. (1992) Dynamics of cis-stilbene photoisomerization the adiabatic pathway to excited trans-stilbene, J. Photochem. Photobiol., A 65, 29-40. 

Saltiel, J., Sun, Y.-P. (1989), Application of the Kramers Equation to Stilbene Photoisomerization in n-Alkanes Using Translational Diffusion Coefficients to Define Microviscosity," J. Phys. Chem. 93, 8310. 

J. Frederick, Y. Fujiwara, J. H. Penn, Y. Yoshihara, and H. Petek, Models for stilbene photoisomerization Experimental and theoretical studies ofthe excited-state dynamics of 1,2-diphenycycloalk-enes, J. Phys. Chem. 95, 2845-2858 (1991). 

Recently, we have carried out an ab initio study of the ground- and excited-state PESs of stilbene that showed that pyramidalization is likely to be an important coordinate in stilbene photoisomerization.indeed, the similarities... 

Sun Y-P and Saltiel J 1989 Application of the Kramers equation to stilbene photoisomerization in n-alkanes using translational diffusion coefficients to define microviscosity J. Phys. Chem. 93 8310-16... 

Figure 5. Azulene effect on the benzophenone-sensitized stilbene photoisomerization in benzene line 1, degassed line 2, O2 atmosphere line 3, degassed with 2.90 x 10 M p-C. From Ref. 41, with permission of North-Holland Publishing.

The rate constants measured for trans-cis stilbene photoisomerization were found to be similar for both the free and the anti-DNP-bound StDNP tracer. Computer modeling suggested that the dinitrophenyl segment of StDNP is squeezed between two tryptophans. 

The role of the triplet state in the cis-trans isomerization of stilbenes effected by photosensitizers, such as acetophenone, benzophenone, or anthraquinone, which have large So Ti excitation energies, was first revealed in Ref. [65]. Theoretical considerations and experimental data on intermolecular triplet-triplet energy transfer leading to the sensitized stilbene photoisomerization are described in Section 4.2.2. It was shown that data on positional dependence of the heavy-atom effect on the cis-trans photoisomerization of bromostilbenes were consistent with the fact that, in contrast to the para position, the meta position is near a node in the highest occupied and the lowest unoccupied MO of stilbene [66]. According to [67], internal and external heavy-atom effects induce phosphorescence in frans-stilbene... 

In the first step of the direct stilbene photoisomerization, the stUbene molecule is excited to the singlet or triplet state when it absorbs a quantum of energy. After that, solvent molecules undergo reorganization, thus reducing the energy of the system. Next step is the exdted-state decay by two competitive processes radiative (fluorescence or phosphorescene emission) or nonradiative (photochemical) deactivation. 

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