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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Singlet excited-state dynamics of nitropolycyclic aromatic hydrocarbons: Direct measurements by femtosecond fluorescence up-conversion
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Singlet excited-state dynamics of nitropolycyclic aromatic hydrocarbons: Direct measurements by femtosecond fluorescence up-conversion

机译：硝基多环芳烃的单重态激发态动力学：飞秒荧光上转换直接测量

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摘要

Understanding the dynamics of the electronically excited states of nitrated polycyclic aromatic hydrocarbons (NPAHs) is of great importance since photochemical reactions determine the atmospheric stability of these toxic pollutants. From previous studies, it is known that electronically excited NPAHs evolve through two parallel pathways: The formation of the first triplet state and the dissociation of nitrogen (II) oxide. In this contribution, we present the first time-resolved emission measurements of the singlet excited states which are the precursors in the aforementioned photoprocesses. We analyzed 1-nitronaphthalene, 9-nitroanthracene, 1-nitropyrene, 6-nitrochrysene, and 3-nitrofluoranthene in solution samples. Although these compounds are considered nonfluorescent, with the frequency up-conversion method it was possible to detect the emission from the S-1 states despite their femtosecond and picosecond lifetimes. Except for 1-nitronapthalene, where a single exponential is observed, for the rest of the compounds, the emission shows double-exponential decays indicating ultrafast structural changes in the excited states. From anisotropy measurements, we conclude that no significant internal conversion occurs in the singlet manifold after excitation in the first absorption band. In accord with El-Sayed rules and with previous calculations, the highly efficient intersystem crossing implied by the large triplet yields and the ultrafast S-1 decays is accounted by the pi-pi* nature of the S-1 and T-1 states together with the existence of higher triplet configurations which act as receiver states. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule.
机译：由于光化学反应决定了这些有毒污染物的大气稳定性，因此了解硝化多环芳烃（NPAH）的电子激发态的动力学非常重要。从以前的研究中可以知道，电子激发的NPAHs通过两个平行的途径进化：第一个三重态的形成和氮（II）氧化物的离解。在此贡献中，我们提出了单重态激发态的第一时间分辨发射测量值，所述激发态是上述光处理中的前体。我们分析了溶液样品中的1-硝基萘，9-硝基蒽，1-硝基py，6-硝基ch和3-硝基荧蒽。尽管这些化合物被认为是非荧光的，但通过频率上转换方法，尽管它们具有飞秒和皮秒的寿命，仍可以检测到S-1状态的发射。除1-硝基萘以外，其余化合物均观察到单一指数，其发射显示出双指数衰减，表明激发态的结构快速变化。从各向异性测量，我们得出结论，在第一个吸收带中激发后，在单重态歧管中没有发生明显的内部转化。根据El-Sayed规则和先前的计算，由大三元组产量和超快S-1衰减所暗示的高效系统间交叉是由S-1和T-1状态的pi-pi *性质共同解释的存在更高的三重态构型，这些构型充当接收器状态。我们的测量结果表明，NPAH具有迄今为止在有机分子中观察到的最大的系统间交叉速率。

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