On the role of the triplet state in the photoisomerization of retinal isomers

The photoisomerization of 11-cis- and all-trans-retinals is investigated in aerated and deaerated solutions using direct singlet excitation as well as via sensitization from the triplet states of biacetyl and biphenyl. Triplet quantum yields are established for all-trans- and 11-cis-retinal using N₂-laser photolysis and pulse radiolysis. It is concluded that triplet energy transfer to 11-cis-retinal leads to isomerization with an efficiency of 15-17%, while no detectable isomerization can be induced by triplet transfer to the all-trans isomer. An analysis of the oxygen effects indicates that photosensitized isomerization precedes the formation of the fully thermalized retinal triplet. A mechanism in which isomerization is taking place from nonrelaxed vibronic triplet levels in competition with thermal deactivation is proposed. The new data are compared with available theoretical calculations and their relevance to the mechanism of isomerization induced by direct excitation is discussed.