PICOSECOND TIME-RESOLVED RESONANCE RAMAN-SCATTERING AND ABSORBENCY CHANGES OF CAROTENOIDS IN LIGHT-HARVESTING SYSTEMS OF PHOTOSYNTHETIC BACTERIUM CHROMATIUM-VINOSUM

The excited electronic states of carotenoids in light-harvesting systems of photosynthetic bacteria are examined by picosecond laser spectroscopy. Picosecond changes in absorbance and in the intensity of resonance Raman bands are observed for carotenoids (mainly rhodopin and spirilloxanthin) in Chromatium vinosum. These data are obtained with picosecond transient absorption and picosecond time-resolved resonance Raman spectroscopies utilizing a two laser (8 ps pulses), pump-probe instrumental configuration. Following 560 nm pump laser excitation, sample absorbance between 573 and 650 nm initially increases and subsequently decays with a fast (6 ± 0.5 ps) and two slow (∼80 ps and >200 ns) components. The initial rise and the fastest decay component reflect, respectively, the formation and relaxation of the excited 21Ag state of the carotenoids. The slow decay constants both reflect the decay of excited triplet state carotenoids although the 80-ps component remains to be fully characterized. The intensities of the RR bands assignable to the ground 11Ag state of the carotenoids decrease by more than 30% during the laser excitation and then increase with a rate of 6 ps, reflecting the depletion of the 11Ag state followed by its repopulation due to the relaxation of the 21Ag state. The transient populations of excited triplet states in carotenoids are observed through picosecond time-resolved resonance Raman spectra containing several new resonance Raman bands, at 1475, 1260, 1171, 1124, and 1007 cm-1. © 1990, American Chemical Society. All rights reserved.