Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive MC 0314, La Jolla, California 92093, United States.
J Phys Chem B. 2012 Sep 6;116(35):10617-30. doi: 10.1021/jp3069514. Epub 2012 Aug 9.
The carotenoids have low-lying triplet excited states and can self-assemble in some solvents to form weakly or strongly coupled aggregates. These qualities make carotenoid aggregates useful for studies of singlet fission, where an outstanding goal is the correlation of interchromophoric coupling to the dynamics and yield of triplet excited states from a parent singlet excited state. Three aggregates of zeaxanthin, two weakly coupled and one strongly coupled, are characterized by steady-state spectroscopic methods including temperature-dependent absorption, fluorescence, and resonance Raman spectroscopy. The absorption spectra for each type of aggregate are distinct; however, an analysis of band positions reveals some important shared characteristics and suggests that the strongly coupled H-aggregate contains a subpopulation of weakly coupled constituents. Temperature-dependent absorption spectroscopy indicates that one of the weakly coupled aggregates can be converted to the other upon heating. The emission spectra of the three aggregates have similar profiles that are overall red-shifted by more than 1000 cm(-1) relative to the monomer. The emission quantum yields of the aggregates are 5 to 30 times less than that of the monomer, with the lowest yield for the strongly coupled aggregate. The vibrational spectra of the chromophores support only slight perturbations from the structure of solvated monomers. Our findings support the conclusion that all three aggregates are best characterized as H-aggregates, in agreement with a prior theoretical study of lutein aggregates.
类胡萝卜素具有低位三重激发态，并可在某些溶剂中自组装形成弱或强耦合的聚集体。这些特性使得类胡萝卜素聚集体在研究单重态裂变方面很有用，单重态裂变的一个突出目标是将发色团间耦合与母体单重激发态的三重激发态的动力学和产率相关联。叶黄素的三种聚集体，两种弱耦合和一种强耦合，通过稳态光谱方法进行了表征，包括温度依赖的吸收、荧光和共振拉曼光谱。每种类型的聚集体的吸收光谱都不同；然而，对波段位置的分析揭示了一些重要的共同特征，并表明强耦合 H-聚集体包含弱耦合成分的亚群。温度依赖的吸收光谱表明，一种弱耦合聚集体可以在加热时转化为另一种聚集体。三种聚集体的发射光谱具有相似的轮廓，与单体相比整体红移超过 1000 cm(-1)。聚集体的发射量子产率比单体低 5 到 30 倍，其中强耦合聚集体的量子产率最低。发色团的振动光谱仅支持从溶剂化单体结构的轻微扰动。我们的发现支持以下结论，即所有三种聚集体都最好被描述为 H-聚集体，这与先前对叶黄素聚集体的理论研究一致。
