Femtosecond ultraviolet-visible absorption study of all-trans→13-cis•9-cis photoisomerization of retinal

The all-trans --> 13-cis.9-cis photoisomerization reaction of retinal in aerated nonpolar solvents has been studied by femtosecond time-resolved ultraviolet-visible (UV-VIS) absorption spectroscopy. The excited-state absorption spectra in the wavelength region 400-800 nm indicate that there is no all-trans --> 13-cis.9-cis isomerization reaction pathway that is complete in the electronic excited singlet manifold of S-1, S-2, and S-3. The ground-state bleaching recovery of all-trans retinal monitored in the near UV (ultraviolet) wavelength region 310-390 nm shows that a perpendicular excited singlet state (p*) takes part in the all-trans --> 13-cis.9-cis isomerization reaction. The lifetime of p* is about 7 ps, and the precursor of p* is most probably the S-2 state. The isomerization quantum yield derived from the femtosecond UV absorption data agrees well with those determined by the HPLC analysis of the photoproduct. The temperature dependence of the isomerization quantum yield indicates the existence of a potential-energy barrier as high as (1.2 +/- 0.6) x 10(3) cm(-1) on the reaction pathway from the S-2 state to the p* state. (C) 1998 American Institute of Physics.