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

Abstract

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 S3, S2, S1, and T1 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 S3, S2, and T1 states, were identified in the mixed solvents. The band assigned to the S2 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 S3 and T1 states, but that all of the retinal molecules are hydrogen-bonded in the S2 state. These observations indicate that an ultrafast hydrogen-bond-formation reaction takes place during or just after the S3 → S2 internal conversion and is complete within a time scale much shorter than the S2 lifetime. Dissociation of the hydrogen bonding is most likely to take place during or after the S1 → T1 intersystem crossing and is complete within a time scale much shorter than the T1 lifetime. The observed longer lifetime of the hydrogen-bonded S2 state is consistent with the higher isomerization quantum yield in protic solvents than in aprotic nonpolar solvents.