CNR-IFN, Dipartimento di Fisica, Politecnico di Milano, P.za L. da Vinci 32, Milan, Italy.
Phys Chem Chem Phys. 2012 May 14;14(18):6312-9. doi: 10.1039/c2cp23585d. Epub 2012 Feb 14.
In carotenoids internal conversion between the allowed (S(2)) and forbidden (S(1)) excited states occurs on a sub-picosecond timescale; the involvement of an intermediate excited state(s) (S(x)) mediating the process is controversial. Here we use high time resolution (sub-20 fs) broadband (1.2-2.5 eV) pump-probe spectroscopy to study the solvent dependence of excited state dynamics of spheroidene, a naturally-occurring carotenoid with ten conjugated double bonds. In the high polarizability solvent, CS(2), we find no evidence of an intermediate state, and the traditional three-level (S(0), S(1), S(2)) model fully accounts for the S(2)→ S(1) process. On the other hand, in the low polarizability solvent, cyclohexane, we find that rapid (~30 fs) relaxation to an intermediate state, S(x), lying between S(1) and S(2) is required to account for the data. We interpret these results as due to a shift of the S(2) energy, which positions the state above or below the energy of S(x) in response to changes in solvent polarizability.
类胡萝卜素中允许的（S(2)）和禁止的（S(1)）激发态之间的内部转换发生在亚皮秒时间尺度内；涉及中间激发态（S(x)）来介导该过程存在争议。在这里，我们使用高时间分辨率（亚 20 飞秒）宽带（1.2-2.5 eV）泵浦-探测光谱法来研究具有十个共轭双键的天然类胡萝卜素球烯的溶剂依赖性激发态动力学。在高极化率溶剂 CS(2)中，我们没有发现中间态的证据，传统的三能级（S(0)、S(1)、S(2)）模型完全解释了 S(2)→S(1)过程。另一方面，在低极化率溶剂环己烷中，我们发现需要快速（~30 飞秒）弛豫到中间态 S(x)，才能解释数据。我们将这些结果解释为 S(2)能量的移动，该能量根据溶剂极化率的变化而在 S(1)和 S(2)之间定位。
