Solvent-dependent activation of intermediate excited states in the energy relaxation pathways of spheroidene

In carotenoids internal conversion between the allowed (S-2) and forbidden (S-1) excited states occurs on a sub-picosecond timescale; the involvement of an intermediate excited state(s) (S-x) mediating the process is controversial. Here we use high time resolution (sub-20 fs) broadband (1.2-2.5 eV) pump-probe spectroscopy to study the solvent dependence of excited state dynamics of spheroidene, a naturally-occurring carotenoid with ten conjugated double bonds. In the high polarizability solvent, CS2, we find no evidence of an intermediate state, and the traditional three-level (S-0, S-1, S-2) model fully accounts for the S-2 -> S-1 process. On the other hand, in the low polarizability solvent, cyclohexane, we find that rapid (similar to 30 fs) relaxation to an intermediate state, Sx, lying between S-1 and S-2 is required to account for the data. We interpret these results as due to a shift of the S-2 energy, which positions the state above or below the energy of Sx in response to changes in solvent polarizability.