SUBPICOSECOND EXCITED-STATE DYNAMICS OF A CAROTENOID (SPIRILLOXANTHIN) IN THE LIGHT-HARVESTING SYSTEMS OF CHROMATIUM-VINOSUM - RELAXATION PROCESS FROM THE OPTICALLY ALLOWED S-2 STATE

The excited-state dynamics of a carotenoid (spirilloxanthin (Spx)) in benzene solution and in chromatophores from Chromatium vinosum was stu died by sub-picosecond time-resolved absorption spectroscopy and picos econd time-resolved anti-Stokes Raman spectroscopy. From the time-reso lved absorption measurements, the lifetime of the S-1-state Spx was fo und to be 1.5-2 ps both in benzene and in the chromatophores. The reco very time of the S-1-state Spx was the same as the S-1-state lifetime in benzene, indicating that all the photoexcited S-2 molecules relax t o the S-0 state via S-1. On the other hand, the recovery time in the c hromatophores was faster than the S-1-state lifetime. This observation may be interpreted as a very rapid energy transfer from the S-2-state Spx to bacteriochlorophyll (BChl) in the chromatophores. Although the energy of the S-2-state Spx should be most probably transferred to th e 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 St state of Spx to the Q(x) state of BChl may also occur in the system treated in this study. (C) 1998 Elsevier Science B.V. All rights reserved.