Sub-picosecond excited-state dynamics of a carotenoid (spirilloxanthin) in the light-harvesting systems of Chromatium vinosum.: Relaxation process from the optically allowed S2 state Original Research Article

The excited-state dynamics of a carotenoid (spirilloxanthin (Spx)) in benzene solution and in chromatophores from Chromatium vinosum was studied by sub-picosecond time-resolved absorption spectroscopy and picosecond time-resolved anti-Stokes Raman spectroscopy. From the time-resolved absorption measurements, the lifetime of the S1-state Spx was found to be 1.5–2 ps both in benzene and in the chromatophores. The recovery time of the S0-state Spx was the same as the S1-state lifetime in benzene, indicating that all the photoexcited S2 molecules relax to the S0 state via S1. On the other hand, the recovery time in the chromatophores was faster than the S1-state lifetime. This observation may be interpreted as a very rapid energy transfer from the S2-state Spx to bacteriochlorophyll (BChl) in the chromatophores. Although the energy of the S2-state Spx should be most probably transferred to the Qx state of BChl under the so far accepted energy-transfer theory, the present results of time-resolved Raman measurements suggest that the energy transfer from the S2 state of Spx to the Qy state of BChl may also occur in the system treated in this study.