Picosecond Kinetics of Excited State Relaxation in Biliverdin Dimethyl Ester

The relaxation processes of biliverdin IXα dimethyl ester have been studied by time‐resolved absorption and fluorescence detection, using pump‐probe and single‐photon timing techniques, respectively. The results complement and extend those of earlier investigations.1,3–5In 2‐methyltetrahydrofuran, chloroform, ethanol, and ethyl lactate, four emitting species, two short‐lived (picosecond) and two long‐lived ones (nanosecond), have been detected. In methanol, the number of emitters is possibly reduced to three. In the cases of four emitters, these can be attributed to either three or four ground state forms. We conclude that the radiationless relaxation of the short‐lived excited states is simultaneously induced both by proton transfer processes and by conformational changes, as previously discussed.3The spectroscopic properties of the long‐lived species point to the presence of relatively rigid conformations of the bilatriene in liquid solutions at room temperature. The possible relevance of this observation with regard to the influence of protein interactions on the conformational equilibrium of tetrapyrroles in phycobiliproteins is discussed.