Steady-state and ultrafast transient absorption spectra were obtained for a series of conformationally-constrained, isomerically pure polyenes with 5–23 conjugated double bonds (N). These data and fluorescence spectra of the shorter polyenes reveal the N dependence of the energies of six 1Bu+ and two 1Ag− excited states. The 1Bu+ states converge to a common infinite polyene limit of 15,900 ± 100 cm−1. The two excited 1Ag− states, on the other hand, exhibit a large (~9000 cm−1) energy difference in the infinite polyene limit, in contrast to the common value previously predicted by theory. EOM-CCSD ab-initio and MNDO-PSDCI semi-empirical MO theories account for the experimental transition energies and intensities. The complex, multistep dynamics of the 11Bu+ → 21Ag− → 11Ag− excited state decay pathways as a function of N are compared with kinetic data from several natural and synthetic carotenoids. Distinctive transient absorption signals in the visible region, previously identified with S* states in carotenoids, also are observed for the longer polyenes. Analysis of the lifetimes of the 21Ag− states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 21Ag− decay kinetics of the carotenoid and polyene systems. These findings are important for understanding the mechanisms by which carotenoids carry out their roles as light-harvesting molecules and photoprotective agents in biological systems.
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机译:获得了一系列具有5-23个共轭双键(N)的构象约束,异构纯的多烯的稳态和超快瞬态吸收光谱。这些较短的多烯的数据和荧光光谱揭示了六个 1 sup> Bu + sup>和两个 1 sup> Ag - sup>激发态。 1 sup> Bu + sup>状态收敛到常见的无限多烯极限15,900±100 cm -1 sup>。另一方面,两个激发的 1 sup> Ag - sup>状态在无限多烯中表现出较大的能量差(〜9000 cm -1 sup>)极限,与先前通过理论预测的共同值相反。 EOM-CCSD从头算起和MNDO-PSDCI半经验MO理论考虑了实验的跃迁能和强度。 1 1 sup> Bu + sup>→2 1 sup> Ag - sup>→1 1的复杂的多步动力学将 sup> Ag - sup>作为N的函数的激发态衰变途径与来自几种天然和合成类胡萝卜素的动力学数据进行了比较。对于较长的多烯,还观察到了可见区域的独特瞬态吸收信号,该信号先前在类胡萝卜素中以S *状态标识。使用能隙定律对非辐射衰变的2 1 sup> Ag - sup>状态的寿命进行分析,发现2 1 < / sup> Ag - sup>的类胡萝卜素和多烯系统的衰减动力学。这些发现对于理解类胡萝卜素在生物系统中发挥其作为集光分子和光保护剂作用的机制非常重要。
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