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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >On the nature of the 'dark S*' excited state of β-carotene
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On the nature of the 'dark S*' excited state of β-carotene

机译：关于β-胡萝卜素“暗S *”激发态的性质

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Femtosecond transient absorption spectroscopy has been applied to the isolated carotenoid β-carotene under a large variety of experimental conditions regarding solvent, temperature, excitation wavelength, and intensity to study the excited state relaxation dynamics in order to elucidate the origin of the so-called "dark S* state", which has been discussed very controversially in the literature. The results are analyzed in terms of lifetime density maps, and various kinetic models are tested on the data. The sample purification was found to be critical. The appearance of a component with a lifetime longer than that of the relaxed S1 state (i.e., π > 10 ps), which has been associated previously with the S* (or S ~±) state is due to the presence of an impurity. For pure samples, four lifetimes are typically observed (all a‰?10 ps at room temperature). Consideration of the large body of experimental data leads us to exclude relaxation schemes implying a separate "dark S* state" in β-carotene formed in parallel to the normal S_2 → S_1 relaxation scheme. Vibrational cooling in the S1 state can explain fully all the features of the transient spectra on the picosecond time scale within a S_2 → S_(1v) → S _(1v) → S_1 → S_0 relaxation scheme without invoking any additional electronic or distinctly different conformational states. Thus, we exclude assignments of the previously reported "S* state" signals in β-carotene (i) to require the postulate of a separate electronic state, (ii) to require the postulate of a large conformational change and/or a partial cis configuration formed in the relaxation pathway, or (iii) to require a vibrationally excited ground state (GS) species. High intensity excitation leads in part to a two-photon excitation to the S_(2N) state which upon relaxation gives rise to a different vibrational excitation pattern in the initially created hot S_1 state(s). The spectral changes in the S_(1v) state observed upon both short wave excitation as well as high intensity excitation can be explained well by such a modified vibrational excitation pattern. In contrast, the variations in the difference spectra of the partially (S_(1v')) and fully vibrationally relaxed S_1 states (S_1) are minor. The data do not provide any evidence that would require one to postulate the existence of a separate "S* state".
机译：飞秒瞬态吸收光谱法已在溶剂，温度，激发波长和强度等各种实验条件下应用于分离的类胡萝卜素β-胡萝卜素，以研究激发态弛豫动力学，从而阐明所谓的“暗S *状态”，这在文献中已引起很大争议。根据寿命密度图分析结果，并在数据上测试各种动力学模型。发现样品纯化很关键。寿命长于先前与S *（或S〜±）状态相关联的松弛S1状态（即π> 10 ps）寿命更长的组件的出现是由于杂质的存在。对于纯样品，通常观察到四个寿命（室温下均为a？10 ps）。对大量实验数据的考虑导致我们排除了弛豫方案，这意味着平行于正常S_2→S_1弛豫方案形成的β-胡萝卜素中单独的“暗S *状态”。在S1状态下的振动冷却可以在S_2→S_（1v）→S _（1v）→S_1→S_0弛豫方案中完全解释皮秒时间尺度上的瞬态谱的所有特征，而无需调用任何其他电子或明显不同的构象状态。因此，我们排除了β-胡萝卜素中先前报告的“ S *状态”信号的分配（i）要求独立的电子状态的假设，（ii）要求大的构象变化和/或部分顺式的假设在松弛路径中形成的构型，或（iii）需要振动激发的基态（GS）物种。高强度激发部分地导致双光子激发到S_（2N）状态，这在松弛时会在最初创建的热S_1状态下产生不同的振动激发模式。通过这种修改的振动激发模式，可以很好地解释在短波激发和高强度激发下观察到的S_（1v）状态的光谱变化。相反，部分（S_（1v'））和完全振动松弛的S_1状态（S_1）的差异谱的变化较小。该数据没有提供任何证据来要求假设一个单独的“ S *状态”的存在。

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