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

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 Qy state (after reaction I) or through the delayed stimulated emission from the Qy 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).