Energy Dissipations in Chlorosomes: Emission from the Q_y State Following Singlet-Singlet and Triplet-Triplet Annihilation Reactions in the Cylindrical Aggregate and Its Reversible Dissociation into the Piggy-Back Dimers

摘要

The excited-state dynamics of chlorosomes from Chlorobium tepidum, upon excitation above the Soret state, was examined by pump-and-probe time-resolved absorption spectroscopy in the subpicosecond to millisecond time region. Upon excitation using a 397-nm, 0.10-ps pulse, the singlet-singlet annihilation (reaction I) as well as the singlet homofission followed by the triplet-triplet annihilation (reaction II) were seen as bimolecular reactions in the subpicosecond to picosecond time regime. Upon excitation using a 355-nm, 12-ns pulse, the dissociation of chlorosomes into the piggy-back dimers (reaction III) was seen in submicroseconds; the chlorosome structure was reorganized within ～20 ms. Thus, the cylindrical aggregate structure of chlorosomes facilitates efficient radiative-energy dissipation either through the initial stimulated emission from the Q_y state (after reaction I) or through the delayed stimulated emission from the Q_y state (after reaction II). The excess thermal energy that is produced by the internal-conversion and vibrational-relaxation processes seems to be dissipated during the instantaneous dissociation of the aggregate structure (after reaction III).