Relaxation Dynamics of High-energy Excited States of Carotenoids Studied by UV Excitation and Pump-repump-probe Transient Absorption Spectroscopy

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 S state, the upper excited state(s) absorbing in the UV spectral region (denoted as S) has been only scarcely studied. Moreover, the relation between the S and S, the final state of the well-known S-S 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 or S state. The S state was excited directly by UV light, and the excitation of the S state was achieved re-pumping the S-S transition. The results indicated that direct S excitation produces an S-S band that is significantly broader than that obtained after S excitation, most probably due to the generation of multiple S conformations produced by excess energy. No such broadening is observed if the S state is excited by the re-pump pulse. This shows that the S and S states are different, each initializing a specific relaxation pathway. We propose that the S state retains the coupled triplet pair character of the S state, while the S state is the higher state of Bu symmetry accessible by one-photon transition.