The Singlet–Triplet Fission of Carotenoid Excitation in Light-Harvesting Complexes from Thermochromatium tepidum

Abstract—Illumination of purple phototrophic bacteria in the carotenoid absorption band of light harvesting complexes often leads to low energy efficiency of absorbed light. This is due to singlet–triplet fission of carotenoid excitation. In the present study, the nature of this process was explored with a phototrophic bacterium Thermochromatium tepidum as an example. It was demonstrated by time-resolved EPR spectroscopy and magnetic field modulation of the fluorescence yield that the concept of intramolecular excitation fission that was developed in several publications was not confirmed experimentally. Evidence of the intermolecular character of excitation fission that involves two carotenoid molecules of the light-harvesting pigment–protein complexes, LH1-RC and LH2, has been obtained. The advantages of intermolecular excitation fission for application in photovoltaic solar energy converters are discussed.