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

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Abstract

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.

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    The fact that the fluorescence decays become single-exponential at 9 mM α-CD, which is a concentration at which 1:2 complexes predominate (Fig. 3), indicates a total suppression of photoisomerization in the very rigid microenvironment provided by these complexes. It may be noted that such differences in the effect of α- and β-cyclodextrins on photoisomerization processed have been reported for other compounds [11–15]. Various explanations have been offered for such behavior.

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