Singlet photosensitization of simple alkenes. Part 1. cis–trans-Photoisomerization of cyclo-octene sensitized by aromatic esters

Abstract

The use of aromatic carboxylic esters, e.g. methyl benzoate, as sensitizers gave an unusual cis–trans-photoisomerization of cyclo-octene, in which the trans : cis-ratio in the photostationary state was anomalously high, provided the usual triplet sensitization is operating (Table 1). From Stern–Volmer quenching studies and the consequent comparison of the decay ratios for three different sensitizers in two different solvents with those for direct or triplet-sensitized photoisomerizations (Table 4), it is concluded that the present photoisomerization takes place via a singlet mechanism which involves non-vertical singlet energy transfer from an excited aromatic ester to the cyclo-octene moiety in an exciplex resulting in 90° twisted cyclo-octene (1p), which in turn decays to ground state cis- or trans-cyclo-octene in the ratio of 0.56 : 0.44. The asymmetric cis–trans-photoisomerization induced by chiral aromatic esters also supports the involvement of an exciplex.