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-Bu+) state were precisely determined by the time-resolved fluorescence. The kinetics of the optically forbidden S1(21Ag-) 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 Sx 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 → Sx→ 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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry