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

Abstract

In carotenoids internal conversion between the allowed (S₂) and forbidden (S₁) 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 (S₀, S₁, S₂) model fully accounts for the S₂ → S₁ process. On the other hand, in the low polarizability solvent, cyclohexane, we find that rapid (∼30 fs) relaxation to an intermediate state, S_x, lying between S₁ and S₂ is required to account for the data. We interpret these results as due to a shift of the S₂ energy, which positions the state above or below the energy of S_x in response to changes in solvent polarizability.