The energies and kinetics of triplet carotenoids in the LH2 antenna complexes as determined by phosphorescence spectroscopy

The triplet (T1) states of carotenoids (Cars) and bacteriochlorophyll a (BChl) in the LH2 antenna complexes from Rhodobacter sphaeroides G1C, Rba. sphaeroides 2.4.1 and Rhodospirillum molischianum, containing neurosporene, spheroidene and lycopene, respectively, were examined by stationary-state and time-resolved phosphorescence spectroscopy. The T1 energies of Cars were determined, irrespective of the Car or BChl excitation, to be 7030 cm−1 (neurosporene), 6920 cm−1 (spheroidene) and 6870 cm−1 (lycopene), respectively, whereas that of BChl to be 7590 cm−1. In the Rba. sphaeroides G1C, the Car and BChl triplet states decayed in similar time constant as the BChl Qy state, a fact which indicates that the pair of triplet states decays through the triplet–triplet annihilation mechanism.