Ultrafast Formation of a Carotenoid Radical in LH2 Antenna Complexes of Purple Bacteria

The S 1 -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 1 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 ∼9 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).