Excited-state processes in phycocyanobilin studied by femtosecond spectroscopy

The light-induced photoprocesses of phycocyanobilin were studied by femtose cond spectroscopy. After excitation with 100 fs pulses at 610 nm the transi ent absorption spectra were measured throughout the visible region as a fun ction of time. Following excitation of phycocyanobilin bleaching of the ini tial ground-state absorption and broad excited-state absorptions located at the shortwave and longwave sides of the bleaching region were observed. Th e decay of both bleaching and the excited-state absorptions was accompanied by the build-up of a comparatively long-living transient absorption betwee n 600 and 750 nm, which is ascribed to the formation of a photoproduct. On the basis of a global analysis the observed transient absorption changes ar e well described by three exponential components with lifetimes in the rang e of a few picoseconds (tau(1) similar to 3 ps), some tens of picoseconds ( tau(2) similar to 30-50 ps) and some hundreds of picoseconds (tau(3) simila r to 350-600 ps) with the exact values depending on the solvent used for th e dissolution of phycocyanobilin. The two fast components (tau(1) and tau(2 )) can be attributed to the relaxation of two different excited stares popu lated with excitation, while the longer-lived component (tau(3)) is associa ted with the decay of the photoproduct. A possible kinetic model that expla ins the mechanism of the light-induced relaxation processes in phycocyanobi lin is presented.