Ultrafast Electronic Relaxation and Hydrogen-Bond-Formation/Dissociation Dynamics of Photoexcited All-trans Retinal in Protic Solvents

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The ultrafast electronic relaxation and the hydrogen-bond-formation/dissociation dynamics of photoexcited all- trans retinal in 1-butanol/cyclohexane mixed solvents have been studied by femtosecond time-resolved visible absorption spectroscopy. Four transient absorption bands, which can be assigned to the S 3, S 2, S 1, and T 1 states, were observed in neat cyclohexane. The shapes and the dynamics of these absorption bands agree very well with those reported previously for all- trans retinal in hexane. In contrast, only three transient absorption bands, which can be assigned to the S 3, S 2, and T 1 states, were identified in the mixed solvents. The band assigned to the S 2 state showed a time-dependent peak shift, which is attributed to solvent reorganization on a picosecond time scale. A kinetic analysis of the three transient absorption bands has led to the conclusion that no state-ordering change of the (n, π*) and (π, π*) states takes place in the excited singlet manifold upon hydrogen-bond formation. The 1-butanol concentration dependence of the absorption spectra shows that the free and hydrogen-bonded species coexist in the S 3 and T 1 states, but that all of the retinal molecules are hydrogen-bonded in the S 2 state. These observations indicate that an ultrafast hydrogen-bond-formation reaction takes place during or just after the S 3 → S 2 internal conversion and is complete within a time scale much shorter than the S 2 lifetime. Dissociation of the hydrogen bonding is most likely to take place during or after the S 1 → T 1 intersystem crossing and is complete within a time scale much shorter than the T 1 lifetime. The observed longer lifetime of the hydrogen-bonded S 2 state is consistent with the higher isomerization quantum yield in protic solvents than in aprotic nonpolar solvents.