Excited-State Processes in Phycocyanobilin Studied by Femtosecond Spectroscopy

Abstract

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 (τ1 ∼ 3 ps), some tens of picoseconds (τ2 ∼ 30−50 ps) and some hundreds of picoseconds (τ3 ∼ 350−600 ps) with the exact values depending on the solvent used for the dissolution of phycocyanobilin. The two fast components (τ1 and τ2) can be attributed to the relaxation of two different excited states populated with excitation, while the longer-lived component (τ3) 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.