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

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 Q_x 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 Q_y state of BChl may also occur in the system treated in this study.