Heterogeneity and Relaxation Dynamics of the Photoexcited Retinal Schiff Base Cation in Solution

We present steady-state and broadband femtosecond fluorescence spectra of the protonated Schiff base of retinal in various protic and aprotic solvents, as a function of the excitation wavelength. A detailed spectral decomposition of the time-resolved fluorescence spectra allows us to isolate three spectral components: (i) the vibrationally relaxed S-1 fluorescence, (ii) a vibrationally hot S, fluorescence, and (iii) a higher-lying emission that undergoes spectral evolution on a time scale of 300-400 fs, which we assign to S-2 fluorescence. The vibrationally "cold" S-1 fluorescence exhibits three decay components upon 400 nm excitation (except in acetonitrile, which has two), but only two of them upon 570 nm excitation. These components clearly demonstrate the heterogeneity of the S, state in the sense that emission stems from several shallow potential surface minima. We discuss a connection between these decay channels and reactive and nonreactive excited-state paths on the basis of their solvent-dependent population and of previous high-performance liquid chromatography studies. There is no clear trend of the fluorescence decay times with solvent properties. Rather, a solvent effect manifests itself in acetonitrile, in that the number of fluorescence decay channels is smaller and that the quenching of the hot fluorescence seems more efficient. This effect has to do with the population pathways leading to the fluorescent states. These observations stress the heterogeneity of excited retinal Schiff base, influencing the decay but also the population channels. They also reinforce the claim that steric effects play an important role in the dynamics of the protein.