Low-temperature energy transfer in FMO trimers from the green photosynthetic bacterium Chlorobium tepidum.

The pump-probe kinetics of the slowest spectral equilibrations between inequivalent BChl a Qy states in FMO trimers from Chlorobium tepidum are decelerated by nearly two orders of magnitude when the temperature is lowered from 300 K to 19 K. The pump-probe anisotropy decays are also markedly slower at 19 K than at 300 K. Singlet-singlet annihilation in FMO trimers is negligible at the laser powers used here. However, reduced temperatures greatly accentuate the probability of singlet-triplet annihilation, due to accumulation of metastable BChl a states under high laser repetition rates.