Fluorescence properties of protonated and unprotonated Schiff bases of retinal at room temperature

The fluorescence spectra, quantum yields and kinetics of protonated (PSBR) and unprotonated (SBR) Schiff bases of retinal in different solvents were investigated at room temperature. The emission quantum yields and lifetimes show that the S-1 --> S-0 transition is strongly forbidden in SBR and allowed in PSBR. The allowed SBR S-2 --> S-0 emission is observed and the S-2 state lifetime is estimated to be about 30 fs for hexane solution. The SBR S-1 state lifetime in non-alcoholic solvents is in the 35-50 ps range and depends strongly on the type of H-bonding solvent (alcohols). The PSBR S-1 state lifetime in different solvents is less than 5-7 ps. The radiative lifetime of the SBR S-1 state (tau(f)(0)) is more than two orders of magnitude longer than the radiative lifetime (tau(a)(0)) calculated from the absorption spectra. The discrepancy between tau(f)(0) and tau(a)(0) for PSBR is less than 3-10 in different solvents. The fluorescence anisotropy of SBR in a polymer matrix and PSBR in ethanol is close to 0.4. These data, together with corresponding results for other solvents, indicate that the dipole oscillators of the absorption and fluorescence transitions for SBR and PSBR are directed along the long molecular axes.