Excited-State Dynamics of a Protonated Retinal Schiff Base in Solution

The dynamics of all-trans and 13-cis retinal protonated Schiff base (RPSB) were studied in different solvents by means of picosecond transient spectroscopy. The decay time of the excited state absorption was found to be wavelength dependent due to the contribution of the faster decay of stimulated emission. The stimulated emission has a lifetime of a 2.5−4 ps while the excited state absorption decay is biexponential with lifetimes of 2.5−4 and 10−12 ps. The fluorescence quantum yield is strongly temperature dependent, but viscosity has a small effect on both excited-state lifetime and fluorescence quantum yield. This leads to the conclusion that there is a ∼600 cm-1 barrier in the excited-state which results from intramolecular electronic factors and not from the solvent viscosity. The comparison of these results with those for the retinal in rhodopsin and bacteriorhodopsin is discussed in terms of the protein catalysis for the retinal photoisomerization.