Explaining the Temperature Dependence of Spirilloxanthin's S* Signal by an Inhomogeneous Ground State Model

We investigate the nature of the S* excited state in carotenoids by performing a series of pump-probe experiments with sub-20 fs time resolution on spirilloxanthin in a polymethylmethacrylate matrix varying the sample temperature. Following photoexcitation, we observe sub-200 Is internal conversion of the bright S-2 state into the lower-lying S-1 and S* states, which in turn relax to the ground state on a picosecond time scale. Upon cooling down the sample to 77 K, we observe a systematic decrease of the S*/S-1 ratio. This result can be explained by assuming two thermally populated ground state isomers. The higher lying one generates the S* state, which can then be effectively frozen out by cooling. These findings are supported by quantum chemical modeling and provide strong evidence for the existence and importance of ground state isomers in the photophysics of carotenoids.