Department of Physics, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Osaka 558-8585, Japan.
Phys Chem Chem Phys. 2011 Jun 14;13(22):10762-70. doi: 10.1039/c0cp02568b. Epub 2011 May 6.
Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.
含有与多烯主链共轭的羰基的类胡萝卜素是海洋生物中天然存在的色素，对水生藻类的光合作用光捕获功能至关重要。这些类胡萝卜素表现出归因于分子内电荷转移 (ICT) 态的光谱特征，由于羰基的存在，这种 ICT 态在极性溶剂中出现。在这里，我们通过稳态吸收和飞秒泵浦探测测量报告了在极性（甲醇）和非极性（环己烷）溶剂中羰基类胡萝卜素岩藻黄素的光谱性质。在甲醇中，通过单光子激发到光学允许的 S(2) (1(1)B) 态，观察到与光学禁阻 S(1) (2(1)A) 态和/或 ICT 态相关的瞬态吸收。在甲醇中进行的瞬态吸收测量表明，随着激发能量的降低，ICT 态与 S(1) 态形成的比例增加。我们还表明，基于 Franck-Condon 分析，在甲醇中的稳态吸收中可以清楚地看到 ICT 特征。结果表明，在极性环境中存在两种岩藻黄素的光谱形式，蓝色和红色。
