Sensitized photoisomerization of all-trans- and 11-cis-retinal

Article abstract of DOI:10.1021/ja00202a032

The photoisomerization of all-trans-retinal (all-trans) and 11-cis-retinal (11-cis) sensitized by a range of sensitizers (porphyrins, Zn-porphyrins, naphthacene and a Zn-phthalocyanine) with varying triplet energies above and below the vertical triplet energy of all-trans-retinal (E(T) = 149 kJ/mol) has been investigated by continuous illumination and HPLC analysis of the products. The composition of the photostationary states, the relative isomerization quantum yields of all-trans and 11-cis, and the primary product ratios are reported. One photon-one bond isomerizations are dominant. With Zn-TPP as sensitizer it is shown that 11-cis and possibly all-trans undergoes one photon-two bond isomerizations as well. The quantum yields of photoisomerization of 11-cis sensitized by biphenyl (E(T) = 275 kJ/mol) and anthracene (E(T) = 178 kJ/mol) were found to be identical. Absolute measurements allowed evaluation of the overall triplet isomerization quantum yield of 11-cis, Φ(iso)(T)(11-cis→) = 1.0 ± 0.2, from which the corresponding quantity for all-trans, Φ(iso)(T)(trans-→) = 0.15 ± 0.05, was obtained. The overall photoisomerization quantum yield of 11-cis with direct excitation at 254 nm was determined as Φ(iso)(d)(11-cis→) = 0.42 ± 0.05 assuming all-trans and 13-cis to be the primary products. Quenching rate constants of several of the sensitizer triplet states by retinal were measured by laser flash photolysis. Possible mechanisms that could account for the experimental results are discussed, and it is concluded that the photoisomerizations most likely involve the retinal triplet states as intermediates. Within this model the observed sensitizer triplet energy dependence of the primary product ratios suggests that the triplet states of the different retinal isomers are different mixtures of triplet excited species, and that the composition of these mixtures depends on the starting isomers as well as on the sensitizer triplet energy for energies below the vertical triplet energy of all-trans-retinal. In the light of a review of the existing literature on the direct and the sensitized photoisomerization of retinals these conclusions suggest substantial corrections to the accepted picture of retinal photochemistry.