Carotenoid Single-Molecular Singlet Fission and the Photoprotection of a Bacteriochlorophyll b -Type Core Light-Harvesting Antenna.

Carotenoid (Car) in photosynthesis plays the major roles of accessary light harvesting and photoprotection, and the underlying structure-function relationship attracts continuing research interests. We have attempted to explore the dynamics of Car triplet excitation ( ³ Car*) in the bacteriochlorophyll b (BChl b )-type light harvesting reaction center complex (LH1-RC) of photosynthetic bacterium Halorhodospira halochloris . We show that the LH1 antenna binds a single Car that was identified as a lycopene derivative. Although the Car is hardly visible in the LH1-RC stationary absorption, it shows up conspicuously in the triplet excitation profile with distinct vibronic features. This and the ultrafast formation of ³ Car* on direct photoexcitation of Car unequivocally manifest the unimolecular singlet fission reaction of the Car. Moreover, the Car with even one molecule per complex is found to be rather effective in quenching ³ BChl b *. The implications of different ³ Car* formation mechanisms are discussed, and the self-photoprotection role of BChl b are proposed for this extremophilic species.