Fluorescence of retinal Schiff base in alcohols

Spectra, quantum yields, and kinetics of the unprotonated n-butyl Schiff base of retinal (SBR) fluorescence are investigated in n-alcohols at room temperature. While the absorption spectra in H-bonding solvents are only slightly shifted to longer wavelengths compared to the absorption in hexane, the emission spectra are strongly influenced by H-bond formation. The radiative S-1 state lifetimes are found to be in the 100-200 ns range, indicating that the S-1 --> S-0 transition is strongly forbidden. With decreasing polarity the rate of the S-1 state decay decreases and in pure alcohols the rate exhibits an exponential dependence on the dielectric constant. Semiempirical calculations indicate a larger probability of proton transfer from alcohol to SBR in the excited state than in the ground state, which may result in protonation of the SBR upon excitation. The theoretical model also shows that the polar environment increases the probability of such a proton transfer from alcohol to SBR. The observed experimental results can be explained by an increase of the S-1 state lifetime by H-bonding. Within the model an increase of solvent polarity makes the nonradiative decay faster in the H-bonded complex due to proton transfer from the alcohol to SBR.