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, & beta;-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.