Doctor John T.M. Kennis
Department of Chemistry, University of California and Physical Biosciences Division
Lawrence Berkeley National Laboratory
Berkeley, California

Informal

Thursday, May 4, 2000
2:00 pm (CT)
3169 Beckman Institute

Abstract

Excited state dynamics upon excitation of the carotenoid spirilloxanthin in the photosynthetic bacteria Rhodospirillum rubrum and Rs. marinum were studied by means of femtosecond spectroscopy. Transient absorption spectroscopy indicated the formation of multiple spectrally distinguishable species upon spirilloxanthin excitation. Remarkably, a species earlier identified as a spirilloxanthin triplet state was formed on a timescale of 100 fs with an estimated pairwise yield of 26%, which subsequently decayed by about half with a time constant of 3 ps, after which it persists on the timescale of the experiment. We explain these results by asserting that fission of a spirilloxanthin singlet state into a pair of triplet states on individual spirilloxanthin molecules takes place. Subsequently, triplet-triplet annihilation occurs on a 3 ps timescale in a fraction of the triplet pairs. None of these phenomena were observed in extracted spirilloxanthin in an organic solvent, where upon optical excitation, internal conversion to the S1 state in 180 fs is followed by internal conversion to the ground state in 1.3 ps. This is the first ever direct observation of a singlet to triplet state conversion process on the femtosecond timescale in a carotenoid or a photosynthetic antenna, and it constitutes a new pathway of excited energy deactivation in these systems.