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 2 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 S 1 -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 S 1 -S n transition. The results indicated that direct S UV excitation produces an S 1 -S n band that is significantly broader than that obtained after S 2 excitation, most probably due to the generation of multiple S 1 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 S 1 state, while the S UV state is the higher state of Bu + symmetry accessible by one-photon transition.