Configurational Changes of Retinal in the Triplet State: Picosecond Time-Resolved Absorption Spectroscopy on the 7-Cis, 11-Cis, and 13-Cis Isomers and High-Performance Liquid Chromatography Analysis of Photoisomerization

Article abstract of DOI:10.1021/j100327a018

The triplet state of retinal was produced from the 7-cis, 11-cis, and 13-cis isomers by direct excitation with 355-nm ca. 10-ps pulses in deoxygenated n-hexane solution at room temperature.A set of transient absorption spectra was recorded for each isomer, in the time domain within 5 ns after excitation.The results indicated the following: (1) The 7-cis and 11-cis isomers initially produce their own short-lived, primary "7-cis" and "11-cis" triplet species which relax into the common, stationary "all-trans" triplet species. (2) The 13-cis isomer produces two different stationary triplet species, i.e., one its own "13-cis" and the other the above "all-trans" triplet species. (3) The above cis-isomers produce also "all-trans" triplet species immediately after excitation.The presence of two different long-lived, stationary triplet species, which were revealed by a previous study by resonance Raman spectroscopy, is confirmed, and short-lived, primary triplet species produced from the 7-cis and 11-cis isomers have been identified, in addition to the one from the 9-cis isomer, which was identified in a previous transient absorption study.The products of isomerization by benzil-sensitized triplet excitation of the all-trans, 7-cis, 9-cis, 11-cis, and 13-cis isomers were analyzed by means of high-performance liquid chromatography.The major product of isomerization from the cis isomers was the all-trans isomer.On the basis of the above results, the mechanisms of isomerization via the triplet state are discussed.