Ultrafast time-resolved spectroscopy of xanthophylls at low temperature
Ultrafast time-resolved spectroscopy of xanthophylls at low temperature
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DOI：
10.1021/jp0763681
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发表时间：
2008-03-20
影响因子：
3.3
通讯作者：
Frank, Harry A.
中科院分区：
文献类型：
--
作者：
Cong, Hong;Niedzwiedzki, Dariusz M.;Frank, Harry A.
叶黄素的许多光谱特征和物理性质以及它们在能量传递到叶绿素中的作用可以用三态模型来解释。叶黄素的特征性强可见吸收与从基态So（1（1）Ag（-））到S（2）（1（1）Bu（+））激发态的跃迁有关。表示为S（1）（2（1）Ag（-））的最低单重态是从基态吸收对称禁止的状态。超快光谱研究和量子计算表明，在S（1）（2（1）Ag（-））和S（2）（1（1）Bu（+））附近存在额外的激发单重态。其中之一表示为S*，并且在先前的工作中已经提出与S（1）（2（1）Ag（-））状态下分子的扭曲分子构象有关。在这项工作中，我们提出了三个主要的叶黄素从高等植物：紫黄质，叶黄素，玉米黄质的光谱调查的结果。这些分子具有从9个至11个共辄碳-碳双键的系统性增加的π电子共辄程度。分子的全反式异构体通过高效液相色谱（HPLC）纯化，并在77 K下通过稳态和超快时间分辨光谱进行研究。使用全局拟合技术的数据分析揭示了每个分子的激发单重态的固有光谱性质和超快动力学。测试了五种不同的全局拟合模型，发现使用动力学模型可以最好地解释数据，其中光激发导致分子进入S（2）（1（1）Bu（+））状态，随后经历衰变到振动热S（1）（1（1）Ag（-））状态，并且除了紫黄质之外也到S* 状态。然后，振动热的S（1）（1（1）Ag（-））态在小于1皮秒的时间内冷却到振动弛豫的S（1）（2（1）Ag（-））态。还发现，在失活过程中，一部分S* 群体转化为S（1）（2（1）Ag（-）），但发现该过程和S* 的相对产率取决于温度，这与其与叶黄素的扭曲构象相关联一致。全局拟合的结果表明，扭曲的构象的叶黄素亚群已经存在于光激发之前的基态。
Many of the spectroscopic features and photophysical properties of xanthophylls and their role in energy transfer to chlorophyll can be accounted for on the basis of a three-state model. The characteristically strong visible absorption of xanthophylls is associated with a transition from the ground state So (1(1)Ag(-)) to the S(2) (1(1)Bu(+)) excited state. The lowest lying singlet state denoted S(1) (2(1)Ag(-)), is a state into which absorption from the ground state is symmetry forbidden. Ultrafast optical spectroscopic studies and quantum computations have suggested the presence of additional excited singlet states in the vicinity of S(1) (2(1)Ag(-)) and S(2) (1(1)Bu(+)). One of these is denoted S* and has been suggested in previous work to be associated with a twisted molecular conformation of the molecule in the S(1) (2(1)Ag(-)) state. In this work, we present the results of a spectroscopic investigation of three major xanthophylls from higher plants: violaxanthin, lutein, and zeaxanthin. These molecules have systematically increasing extents of pi-electron conjugation from nine to eleven conjugated carbon-carbon double bonds. All-trans isomers of the molecules were purified by high-performance liquid chromatography (HPLC) and studied by steady-state and ultrafast time-resolved optical spectroscopy at 77 K. Analysis of the data using global fitting techniques has revealed the inherent spectral properties and ultrafast dynamics of the excited singlet states of each of the molecules. Five different global fitting models were tested, and it was found that the data are best explained using a kinetic model whereby photoexcitation results in the promotion of the molecule into the S(2) (1(1) Bu(+)) state that subsequently undergoes decay to a vibrationally hot S(1) (1(1)Ag(-)) state and with the exception of violaxanthin also to the S* state. The vibrationally hot S(1) (1(1)Ag(-)) state then cools to a vibrationally relaxed S(1) (2(1)Ag(-)) state in less than a picosecond. It was also found that a portion of the S* population is converted into S(1) (2(1)Ag(-)) during deactivation, but this process and the relative yield of S* was found to depend on temperature, consistent with it being associated with a twisted conformation of the xanthophyll. The results of the global fitting suggest that subpopulations of twisted conformers of xanthophylls already exist in the ground state prior to photoexcitation.