Carotenoid Single-Molecular Singlet Fission and thePhotoprotection of a Bacteriochlorophyllb-Type Core Light-Harvesting Antenna

Carotenoid (Car) in photosynthesis plays the major roles of accessarylight harvesting and photoprotection, and the underlying structure-functionrelationship attracts continuing research interests. We have attempted to explorethe dynamics of Car triplet excitation (3Car*) in the bacteriochlorophyllb(BChlb)-type light harvesting reaction center complex (LH1-RC) of photosynthetic bacteriumHalorhodospira halochloris. We show that the LH1 antenna binds a single Car that wasidentified as a lycopene derivative. Although the Car is hardly visible in the LH1-RCstationary absorption, it shows up conspicuously in the triplet excitation profile withdistinct vibronic features. This and the ultrafast formation of3Car*on directphotoexcitation of Car unequivocally manifest the unimolecular singletfission reactionof the Car. Moreover, the Car with even one molecule per complex is found to berather effective in quenching3BChlb*. The implications of different3Car*formationmechanisms are discussed, and the self-photoprotection role of BChlbare proposedfor this extremophilic species.