Relaxation dynamics of high-energy excited states of carotenoids studied by UV excitation and pump–repump–probe transient absorption spectroscopy

Abstract

The excited states of carotenoids have been a subject of numerous studies. While a majority of these reports target the excited state dynamics initiated by the excitation of the S₂ state, the upper excited state(s) absorbing in the UV spectral region (denoted as S_UV) has been only scarcely studied. Moreover, the relation between the S_UV and S_n, the final state of the well-known S₁–S_n transition of carotenoids, remains unknown. To address this yet-unresolved issue, we compared the excited state dynamics of two carotenoids, namely, β-carotene and astaxanthin, after excitation of either the S_UV or S_n state. The S_UV state was excited directly by UV light, and the excitation of the S_n state was achieved via re-pumping the S₁–S_n transition. The results indicated that direct S_UV excitation produces an S₁–S_n band that is significantly broader than that obtained after S₂ excitation, most probably due to the generation of multiple S₁ conformations produced by excess energy. No such broadening is observed if the S_n state is excited by the re-pump pulse. This shows that the S_n and S_UV states are different, each initializing a specific relaxation pathway. We propose that the S_n state retains the coupled triplet pair character of the S₁ state, while the S_UV state is the higher state of Bu⁺ symmetry accessible by one-photon transition.