Trans-cis photoisomerization of stilbene

Sundstrdm V and Gillbro T 1985 Dynamics of trans-cis photoisomerization of stilbene in hydrocarbon solutions Ber. Bunsenges Phys. Chem. 89 222-6... 

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... 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

The trans-to-cis photoisomerization of stilbene-like organic species can be carried out upon their coordination to the Re1 centre to form the fac-[Re(CO)3(NN)( rans-L)]+ complexes [26]. Another spectacular example of photoisomerization is generation of several different isomers by photochemical rearrangement of norbornadiene in the presence of different metal catalysts, such as Cu1 complexes or carbonyls of Fe, Ni, and Cr [27, 28]. 

The trans-cis photoisomerization of the stilbene-type ligand in the Re(I) diimine complexes has been reported (55). 

Interest is shown still in the isomerization of stilbene and its derivatives. A study of the pathways for the cis -trans isomerization of 4-nitro, 4,4-dinitro, and 4-nitro-4 -methoxy stilbenes has been reported. The dynamics of the photoisomerization of stilbenes in hydrocarbon solution has been studied and a comparison of the trans -cis isomerization of stilbene in low viscosity liquid alkanes and in the gas phase has been carried out. The photoisomerization of stilbene in straight chain alcohols has provided evidence for the existence of rotational relaxation in the excited and the ground states. An analysis of the isomerization rates of photoexcited stilbene has been made and the photobehaviour of poly deuteriated stilbene has been studied. ... 

The photoswitching properties associated with azobenzene and stilbene derivatives (Figure 1) are attributed to the reversible trans-cis photoisomerization of N=N and C=C double bonds, respectively. As a result of the steric constraints in the cis isomer, which reduce the n electron delocalization, the absorption and triplet-state energies for the trans-isomer are generally lower than those of the cis-counterpart, thus leading to the photochromic behavior. ... 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

Photochemistry offers many examples of large-scale movement at the molecular level. The cis-trans isomerization127-33- of an azo group is a very attractive process for modification of the geometry and the properties of compounds, although several other photochemical reactions can be utilized,134-371 including the photoisomerization of stilbene-like compounds. 

The photoisomerization of stilbene is one of the most extensively studied photoreactions (25). Solvent effects have been thoroughly investigated for both the direct and photosensitized isomerizations, and a model has been developed which attributes these effects to solvent viscosity (26). Increased viscosity inhibits direct photoisomerization of the cis isomer, but facilitates that of trans-stilbene. As a result, the cis/trans ratio of the photostationary state increases with increasing solvent viscosity. The wide range of viscosities which are attainable by pressure manipulation of supercritical carbon dioxide provides an excellent opportunity to probe the effect of viscosity on stilbene photochemistry in the same solvent. 

The E Z photoisomerization of stilbene has been studied in considerable detail. We use the customary labels T (for trans) to designate structures near that of ( )-stilbene, c (for cis) for those near (Z)-stilbene and p (for perpendicular) for those near the transition-state geometry for E Zisomerization in the ground state. Figure 5.20 shows schematic potential energy surfaces (PESs) for S0, Si and Ti that are based largely on the experimental data discussed below. 

Almost all modern textbooks on photochemistry and many review articles discuss the photoisomerization of stilbenes sensitized by triplet donors. It is therefore adequate to give only a very brief summary of this field here. The photostationary state depends on the transfer mechanism and the properties of both the sensitizer and the olefin [139, 206 236]. A plot of photostationary cis/trans ratios (([c]/[t])sens) versus the excitation triplet energy (Er) [25, 208, 209] has been used to estimate unknown ET values of sensitizers [210]. 

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