Kuznetsova Valentyna, Fuciman Marcel, Polívka Tomáš
Department of Physics, Faculty of Science, University of South Bohemia, Branišovská 1760, 370 05 České Budějovice, Czech Republic.
Phys Chem Chem Phys. 2023 Aug 23;25(33):22336-22344. doi: 10.1039/d3cp02485g.
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.
类胡萝卜素的激发态一直是众多研究的主题。虽然这些报告中的大多数针对的是由S态激发引发的激发态动力学，但在紫外光谱区域吸收的较高激发态（表示为S）却很少被研究。此外，S态与类胡萝卜素著名的S-S跃迁的终态S之间的关系仍然未知。为了解决这个尚未解决的问题，我们比较了两种类胡萝卜素，即β-胡萝卜素和虾青素，在S态或S态激发后的激发态动力学。S态通过紫外光直接激发，而S态的激发是通过重新泵浦S-S跃迁实现的。结果表明，直接的S态激发产生的S-S带比S态激发后获得的带明显更宽，这很可能是由于多余能量产生了多种S构象。如果通过重新泵浦脉冲激发S态，则不会观察到这种展宽。这表明S态和S态是不同的，各自启动特定的弛豫途径。我们提出，S态保留了S态的耦合三重态对特征，而S态是通过单光子跃迁可及的具有Bu对称性的较高态。
