Excited-State Processes in Phycocyanobilin Studied by Femtosecond Spectroscopy

The light-induced photoprocesses of phycocyanobilin were studied by femtosecond spectroscopy. After excitation with 100 fs pulses at 610 nm the transient absorption spectra were measured throughout the visible region as a function of time. Following excitation of phycocyanobilin bleaching of the initial ground-state absorption and broad excited-state absorptions located at the shortwave and longwave sides of the bleaching region were observed. The decay of both bleaching and the excited-state absorptions was accompanied by the build-up of a comparatively long-living transient absorption between 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 are well described by three exponential components with lifetimes in the range of a few picoseconds (τ 1 ∼ 3 ps), some tens of picoseconds (τ 2 ∼ 30−50 ps) and some hundreds of picoseconds (τ 3 ∼ 350−600 ps) with the exact values depending on the solvent used for the dissolution of phycocyanobilin. The two fast components (τ 1 and τ 2 ) can be attributed to the relaxation of two different excited states populated with excitation, while the longer-lived component (τ 3 ) is associated with the decay of the photoproduct. A possible kinetic model that explains the mechanism of the light-induced relaxation processes in phycocyanobilin is presented.