Cis-to-trans Isomerization of Spheroidene in the Triplet State as Detected by Time-Resolved Absorption Spectroscopy

Cis-to-trans isomerization upon excitation of the all-trans, 9‘-cis, 13‘-cis, 9-cis, 13-cis, and 15-cis isomers of spheroidene to the triplet state (by the use of anthracene as a sensitizer) was traced by submicrosecond time-resolved absorption spectroscopy. The time-resolved spectra were analyzed by means of singular-value decomposition and subsequent global fitting using a scheme having potential minima at the cis and trans positions in both the T1 and S0 states. The quantum yields of triplet-sensitized isomerization were also determined. The process of cis-to-trans isomerization in the T1 state was clearly seen for the 13‘-cis and 9-cis isomers. The quantum yield and the rate of cis-to-trans isomerization increased systematically in the order 13‘-cis, 9-cis, 13-cis, and 15-cis T1 species, whereas their rates of T1 → S0 intersystem crossing were similar to one another and 6 times larger than that of the all-trans T1 species. The T n ← T1 absorption maximum shifted to the blue in the above order, and reached to that of the all-trans T1 species. Those observations were explained in terms of “the triplet-excited region” localized in the central part of the conjugated chain. A possible reason for the selection of the 15-cis configuration by a bacterial photoreaction center containing this carotenoid was discussed.