Institute of Physics and Biophysics, Faculty of Science , University of South Bohemia , Branišovská 1760 , CZ 370 05 České Budějovice , Czech Republic.
Department of Chemistry, Faculty of Science , Gebze Technical University , 41400 Gebze , Kocaeli , Turkey.
J Phys Chem B. 2018 Mar 22;122(11):2922-2930. doi: 10.1021/acs.jpcb.8b00743. Epub 2018 Mar 13.
We used ultrafast transient absorption spectroscopy to study excited-state dynamics of the keto-carotenoid fucoxanthin (Fx) and its two derivatives: 19'-butanoyloxyfucoxanthin (bFx) and 19'-hexanoyloxyfucoxanthin (hFx). These derivatives occur in some light-harvesting systems of photosynthetic microorganisms, and their presence is typically related to stress conditions. Even though the hexanoyl (butanoyl) moiety is not a part of the conjugated system of hFx (bFx), their absorption spectra in polar solvents exhibit more pronounced vibrational bands of the S state than for Fx. The effect of the nonconjugated acyloxy moiety is further observed in transient absorption spectra, which for Fx exhibit characteristic features of an intramolecular charge transfer (ICT) state in all polar solvents. For bFx and hFx, however, much weaker ICT features are detected in methanol, and the spectral markers of the ICT state disappear completely in polar, but aprotic acetonitrile. The presence of the acyloxy moiety also alters the lifetimes of the S/ICT state. For Fx, the lifetimes are 60, 30, and 20 ps in n-hexane, acetonitrile, and methanol, whereas for bFx and hFx, these lifetimes yield 60, 60, and 40 ps, respectively. Testing the S/ICT state lifetimes of hFx in other solvents revealed that some ICT features can be induced only in polar, protic solvents (methanol, ethanol, and ethylene glycol). Thus, bFx and hFx represent a rather rare example of a system in which a nonconjugated functional group significantly alters excited-state dynamics. By comparison with other carotenoids, we show that a keto group at the acyloxy tail, even though it is not in conjugation, affects the electron distribution along the conjugated backbone, resulting in the observed decrease of the ICT character of the S/ICT state of bFx and hFx.
我们使用超快瞬态吸收光谱研究了生色酮类胡萝卜素叶黄素（Fx）及其两种衍生物：19'-丁酰氧基叶黄素（bFx）和 19'-己酰氧基叶黄素（hFx）的激发态动力学。这些衍生物存在于一些光合微生物的光捕获系统中，其存在通常与应激条件有关。尽管己酰基（丁酰基）部分不是 hFx（bFx）的共轭体系的一部分，但它们在极性溶剂中的吸收光谱表现出比 Fx 更明显的 S 态振动带。非共轭酰氧基部分的影响在瞬态吸收光谱中进一步观察到，对于 Fx，在所有极性溶剂中都表现出分子内电荷转移（ICT）态的特征。然而，对于 bFx 和 hFx，在甲醇中检测到的 ICT 特征要弱得多，并且在极性但无质子的乙腈中，ICT 态的光谱标记完全消失。酰氧基部分的存在也改变了 S/ICT 态的寿命。对于 Fx，在正己烷、乙腈和甲醇中的寿命分别为 60、30 和 20 ps，而对于 bFx 和 hFx，这些寿命分别为 60、60 和 40 ps。在其他溶剂中测试 hFx 的 S/ICT 态寿命表明，一些 ICT 特征只能在极性质子溶剂（甲醇、乙醇和乙二醇）中诱导。因此，bFx 和 hFx 代表了一个相当罕见的系统，其中非共轭官能团显著改变了激发态动力学。通过与其他类胡萝卜素的比较，我们表明酰氧基尾部的酮基即使不在共轭中，也会影响沿共轭主链的电子分布，导致 bFx 和 hFx 的 S/ICT 态的 ICT 特征观察到的降低。
