Modification of the photochemical behaviour of organic molecules by cyclodextrin: geometric isomerization of stilbenes and alkyl cinnamates

The photochemical behaviour of stilbenes and alkyl cinnamates complexed to cyclodextrin has been investigated in aqueous media. In benzene, on direct excitation, the photostationary state of stilbene, p-methylstilbene, p-methoxystilbene and p-fluorostilbene contains predominantly the cis isomer (greater than 85%); excitation of cyclodextrin complexes of either cis- or trans-stilbene in aqueous media resulted in a photostationary state enriched in trans isomers. Furthermore, the cyclization product of cis-stilbene, i.e. phenanthrene, which is formed in detectable amounts in benzene solution was found to be absent during aqueous β-cyclodextrin irradiation. However, for cinnamate esters the behaviours in solution and in cyclodextrin are identical. Irradiation in organic solvents such as benzene and methanol and irradiation in aqueous β-cyclodextrin solution result in the same 1:1 mixture of cis and trans isomers. The above results have been rationalized on the basis of the influence of the cyclodextrin cavity on the decay ratio of the twisted olefins