PHOTOISOMERIZATION OF SIXTEEN ISOMERS OF RETINAL. INITIAL PRODUCT DISTRIBUTION IN DIRECT AND SENSITIZED IRRADIATION. PHOTOCHEMISTRY OF POLYENES 31*

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Abstract
Abstract— Initial product distributions of all 16 geometric isomers of retinal in hexane solutions have been determined. With direct irradiation, the product ratios are characterized by a preference for isomerization at the 13,14‐bond. In particular, all isomers containing the 13‐cis geometry give the corresponding 13‐trans isomer as the major product. Preference for one‐photon‐one‐bond isomerization was also noted, although a substantial amount of the all‐trans isomer was detected for all poly‐cis, 13‐trans isomers. In sensitized irradiation, the initial mixture shows extensive one‐photon‐two(or multiple)‐bond isomerization to the corresponding unhindered isomers. In cases of hindered isomers, multiple‐bond isomerized products are dominant. The different results are accountable by the different shapes of the excited state potential curves for singlet and triplet states.