Picosecond time-resolved resonance Raman study of the photoisomerization of retinal

Picosecond time-resolved Raman spectroscopy was applied to the study of the photoisomerization dynamics of all-trans, 9-cis and 13-cis retinal in nonpolar solvents. It was found that picosecond time-resolved spontaneous Raman spectra are obtainable from retinal in solution despite a high fluorescence background when the probe wavelength is in rigorous resonance with the T-T absorption. In the case of photoexcitation of all-trans retinal, the transient Raman bands ascribed to the all-trans T-1 state appeared with the intersystem crossing time of similar to 30 ps. No Raman signal attributable to the product was recognized within the signal-to-noise ratio, reflecting the low isomerization quantum yield of all-traits retinal. The frequency shifts of the all-trans T-1 bands were observed in the early picosecond time region (tau similar to 16 ps), which manifests the vibrational cooling process in the excited state. In the case of photoexcitation of 9-cis retinal, the all-trans TI state slowly appeared with a time constant of similar to 1 ns (1000 +/- 150 ps), which corresponds to the 9-cis --> all-trans structural change occurring in the T-1 state, In addition, Raman signals due to the 9-cis T-1 state (e.g., 1400 cm(-1)) were recognized in the early delay time and they disappeared in accordance with the appearance of the all-trans T-1 state. The data obtained clearly showed that the 9-cis - all-trans photoisomerization predominantly takes place in the T-1 state with thermal activation to cross the potential barrier from the 9-cis configuration to the all-trans. In contrast, with photoexcitation of 13-cis retinal, the transient Raman signals attributable to the mixture of the all-trans T-1 state and the 13-cis T-1 state appeared in a few tens of picoseconds, and no spectral change was observed after 100 ps up to a few nanoseconds. The quantitative analysis indicated that the all-trans T-1 state and the 13-cis T-1 state appeared with different time constants, It suggests that the 13-cis - all-trans isomerization takes place in the excited singlet state before the intersystem crossing and that the resultant all-trans S-1 and 13-cis S-1 states are relaxed to the corresponding T-1 states separately with time constants inherent to each isomer. The singlet isomerization quantum yield was estimated approximately at similar to 0.2 from the obtained picosecond Raman data. These results indicated that the singlet mechanism is a major pathway (or one of major pathways) in photoisomerization of 13-cis retinal. The present time-resolved Raman study showed that the cis - trans photoisomerization mechanism and dynamics of retinal significantly depend on the position of the double bond to rotate.