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 T1 state appeared with the intersystem crossing time of ∼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-trans retinal. The frequency shifts of the all-trans T1 bands were observed in the early picosecond time region (τ ∼ 16 ps), which manifests the vibrational cooling process in the excited state. In the case of photoexcitation of 9-cis retinal, the all-trans T1 state slowly appeared with a time constant of ∼1 ns (1000 ± 150 ps), which corresponds to the 9-cis → all-trans structural change occurring in the T1 state. In addition, Raman signals due to the 9-cis T1 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 T1 state. The data obtained clearly showed that the 9-cis → all-trans photoisomerization predominantly takes place in the T1 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 T1 state and the 13-cis T1 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 T1 state and the 13-cis T1 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 S1 and 13-cis S1 states are relaxed to the corresponding T1 states separately with time constants inherent to each isomer. The singlet isomerization quantum yield was estimated approximately at ∼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.