Triplet States of Carotenoids Bound to Reaction Centers of Photosynthetic Bacteria: Time‐resolved Resonance Raman Spectroscopy

Time‐resolved, low‐temperature resonance Raman spectra of triplet states of the carotenoids specifically present in bacterial reaction centers in a strained cis conformation have been obtained, thus demonstrating the possibility of studying intermediate transient states of these structures using resonance Raman spectroscopy. Resonance Raman spectra of triplet cis spheroidene and cis methoxyneurosporene present in reaction centers of Rhodopseudomonas spheroides, (strains 2.4.1. and Ga, respectively) exhibit marked differences with those of triplet, all‐ trans carotenoids previously studied in vitro. These differences, together with the frequency shifts measured for the v 1 modes, indicate that triplet carotenoids bound to reaction centers retain a cis conformation, and that probably no isomerization occurs to all‐ trans carotenoids upon T ← S 0 excitation. P i electron distributions along the polyene backbone are probably less regular in the triplet state than in the singlet ground state, although probably not to the extent suggested by previous theoretical calculations. The apparently anomalous behaviour of the v 2 bands of all‐ trans carotenoids upon T ← S 0 excitation is shown to result largely from the actual complexity of this region of the Raman spectra, together with a weak participation of the v c—–c internal coordinate in the corresponding modes. Finally, the Raman scattering efficiency of triplet spheroidene bound to reaction centers is lower than that of the singlet, ground state form, under equivalent excitation conditions.