Abstract
In order to reveal the photo-protective functions and the excited-state dynamics of chlorosomes from Chlorobium tepidum was examined by pump-and-probe time-resolved absorption spectroscopy in subpicosecond-to-millisecond time region. Upon excitation using a ∼400 nm, 0.12 ps pulse, the singlet-singlet annihilation reaction as well as a pair of the singlet homofission and the reverse triplet-triplet annihilation reactions were seen as bi-molecular reactions. Upon excitation using a 355 nm, 12 ns pulse, a slow dissociation of chlorosomes into the piggy-back dimers was seen after the excited-state reactions; the chlorosome structure was reorganized within ∼20 ms. Thus, the cylindrical aggregate structure of chlorosomes facilitates efficient radiative energy dissipation either through the initial Qy stimulated emission or through the delayed Qy stimulated emission. The excess thermal energy that is produced by the internal-conversion processes seems to trigger an instantaneous shut down of the efficient light-harvesting function of the particular aggregate structure.
