Ultrafast dynamics and excited state spectra of open-chain carotenoids at room and low temperatures
Ultrafast dynamics and excited state spectra of open-chain carotenoids at room and low temperatures
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DOI：
10.1021/jp070500f
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发表时间：
2007-05-31
影响因子：
3.3
通讯作者：
Frank, Harry A.
中科院分区：
文献类型：
--
作者：
Niedzwiedzki, Dariusz;Koscielecki, Jeremy F.;Frank, Harry A.
类胡萝卜素的许多光谱特征和光物理性质都可以使用三态模型来解释，其中分子的强可见光吸收与 S-0 (1(1)A(g)(-)) -> S-2 (1(1)B(u)(+)) 跃迁相关，而最低的单线态 S-1 (2(1)A(g)(-)) 是这样的状态： 对称性禁止从基态的吸收。然而，半经验和从头算量子计算表明，额外的激发单线态可能位于 S-1 (2(1)A(g)(-)) 和 S-2 (1(1)B(u)(+)) 之间或附近，并且一些超快光谱研究报告了这些状态的证据。一种这样的状态，表示为 S*，被认为是 S-2 (1(1)B(u)(+)) 减少的中间体，并且是光捕获复合物中类胡萝卜素三重态形成的途径。在这项工作中，我们提出了对一系列源自光合细菌的开链类胡萝卜素进行超快、时间分辨光谱研究的结果，并系统地增加了它们的π电子碳-碳双键（n）的数量。这些分子是神经孢子烯 (n = 9)、球状蛋白 (n = 10)、视紫红质葡糖苷 (n = 11)、紫光菌素 (n = 12) 和螺菌黄质 (n = 13)。在室温下，在丙酮和 CS2 溶剂中研究了这些分子。这些实验探讨了溶剂极性和极化率对分子光谱和动力学行为的影响。还在 77 K 的乙醚/异戊烷/乙醇 (EPA) 玻璃中研究了这些分子，其中光谱分辨率大大提高。使用全局拟合技术对数据进行分析揭示了激发态的超快动力学以及与其衰变相关的光谱变化，包括以前未报道的光谱特征。这些数据与 S* 被识别为具有扭曲的构象结构一致，其产率在具有较长π电子共轭的分子中增加。特别是，对于该系列中最长的分子螺菌黄素，实验和详细的量子计算分析揭示了与松弛 S-1 (2(1)A(g)(-)) 构象相关的两个 S* 态的存在，涉及近平面的 6-s-cis 和 6-s-trans 几何形状。我们提出，在极性溶剂中，螺菌黄质的基态呈现螺旋构象，产生净溶质偶极矩，同时降低空腔形成能。在激发和弛豫到 S-1 (2(1)A(g)(-)) 状态时，多烯解开并展平为具有可比较的 6-s-反式和 6-s-顺式构象群的更平面的几何形状。
Many of the spectroscopic features and photophysical properties of carotenoids are explained using a three-state model in which the strong visible absorption of the molecules is associated with an S-0 (1(1)A(g)(-)) -> S-2 (1(1)B(u)(+)) transition, and the lowest lying singlet state, S-1 (2(1)A(g)(-)), is a state into which absorption from the ground state is forbidden by symmetry. However, semiempirical and ab initio quantum calculations have suggested additional excited singlet states may lie either between or in the vicinity of S-1 (2(1)A(g)(-)) and S-2 (1(1)B(u)(+)), and some ultrafast spectroscopic studies have reported evidence for these states. One such state, denoted S*, has been implicated as an intermediate in the depopulation of S-2 (1(1)B(u)(+)) and as a pathway for the formation of carotenoid triplet states in light-harvesting complexes. In this work, we present the results of an ultrafast, time-resolved spectroscopic investigation of a series of open-chain carotenoids derived from photosynthetic bacteria and systematically increasing in their number of pi-electron carbon-carbon double bonds (n). The molecules are neurosporene (n = 9), spheroidene (n = 10), rhodopin glucoside (n = 11), rhodovibrin (n = 12), and spirilloxanthin (n = 13). The molecules were studied in acetone and CS2 solvents at room temperature. These experiments explore the effect of solvent polarity and polarizability on the spectroscopic and kinetic behavior of the molecules. The molecules were also studied in ether/isopentane/ethanol (EPA) glasses at 77 K, in which the spectral resolution is greatly enhanced. Analysis of the data using global fitting techniques has revealed the ultrafast dynamics of the excited states and spectral changes associated with their decay, including spectroscopic features not previously reported. The data are consistent with S* being identified with a twisted conformational structure, the yield of which is increased in molecules having longer pi-electron conjugations. In particular, for the longest molecule in the series, spirilloxanthin, the experiments and a detailed quantum computational analysis reveal the presence of two S* states associated with relaxed S-1 (2(1)A(g)(-)) conformations involving nearly planar 6-s-cis and 6-s-trans geometries. We propose that in polar solvents, the ground state of spirilloxanthin takes on a corkscrew conformation that generates a net solute dipole moment while decreasing the cavity formation energy. Upon excitation and relaxation into the S-1 (2(1)A(g)(-)) state, the polyene unravels and flattens into a more planar geometry with comparable populations of 6-s-trans and 6-s-cis conformations.