Ultrafast formation of a carotenoid radical in LH2 antenna complexes of purple bacteria

The SI-mediated carotenoid to BChl a energy-transfer pathway and carotenoid radical formation have been studied using femtosecond time-resolved transient absorption spectroscopy. A series of LH2 complexes (LH2 = light-harvesting complex 2) from Rhodobacter sphaeroides incorporating carotenoids neurosporene, spheroidene, and spheroidenone were used to explore the effect of conjugation length. The S, lifetimes of carotenoids in the LH2 complex are significantly shorter than in solution, resulting in efficiencies of the S-1-mediated energy transfer of 94%, 82%, and 76%, respectively. In addition to the S-1 decay, a low-amplitude component of similar to9 ps was revealed for all carotenoids in LH2 and assigned to the S* state. Besides energy transfer, excitation of the carotenoid S-2 state in LH2 complexes also leads to the formation of a carotenoid radical, and this pathway is also conjugation length dependent. The carotenoid radical is formed as a result of electron transfer between the carotenoid and B800 BChl a molecules. The yield of the radical formation is highest for neurosporene (10-15%), while a lower efficiency was observed for spheroidene (5-8%). For spheroidenone, no distinct radical signal was found, The precursor of the carotenoid radical is a charge-transfer state that is populated directly from the S-2 state. This charge-transfer state, which represents an excited state of the carotenoid-B800 complex, decays in 300 fs (neurosporene) or 400 fs (spheroidene) to form ground-state carotenoid cation and BChl anion. The charge-separated state has a lifetime of 2 ps (neurosporene) and 8 ps (spheroidene).