The dependence of the ultrafast relaxation kinetics of the S₂ and S₁ states in β -carotene homologs and lycopene on conjugation length studied by femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies

The ultrafast relaxation kinetics of all-trans- β -carotene homologs with varying numbers of conjugated double bonds n (n=7-15) and lycopene (n=11) has been investigated using femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies, both carried out under identical excitation conditions. The nonradiative relaxation rates of the optically allowed S ₂(1^-B_u⁺) state were precisely determined by the time-resolved fluorescence. The kinetics of the optically forbidden S₁(2¹A_g^-) state were observed by the time-resolved absorption measurements. The dependence of the S₁ relaxation rates upon the conjugation length is adequately described by application of the energy gap law. In contrast to this, the nonradiative relaxation rates of S₂ have a minimum at n=9 and show a reverse energy gap law dependence for values of n above 11. This anomalous behavior of the S₂ relaxation rates can be explained by the presence of an intermediate state (here called the S_x state) located between the S₂ and S₁ states at large values of n (such as n=11). The presence of such an intermediate state would then result in the following sequential relaxation pathway S₂ → S_x→ S ₁ → S₀. A model based on conical intersections between the potential energy curves of these excited singlet states can readily explain the measured relationships between the decay rates and the energy gaps.