Singlet Excited-State Dynamics of Nitropolycyclic Aromatic Hydrocarbons: Direct Measurements by Femtosecond Fluorescence Up-Conversion

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• 作者：Rodrigo Morales-Cueto ; Mariana Esquivelzeta-Rabell ; Jimena Saucedo-Zugazagoitia ; Jorge Peon
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：February 1, 2007
• 年：2007
• 卷：111
• 期：4
• 页码：552 - 557
• 全文大小：152K
• 年卷期：v.111,no.4(February 1, 2007)
• ISSN：1520-5215

文摘

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 thesetoxic pollutants. From previous studies, it is known that electronically excited NPAHs evolve through twoparallel pathways: The formation of the first triplet state and the dissociation of nitrogen (II) oxide. In thiscontribution, we present the first time-resolved emission measurements of the singlet excited states which arethe precursors in the aforementioned photoprocesses. We analyzed 1-nitronaphthalene, 9-nitroanthracene,1-nitropyrene, 6-nitrochrysene, and 3-nitrofluoranthene in solution samples. Although these compounds areconsidered nonfluorescent, with the frequency up-conversion method it was possible to detect the emissionfrom the S₁ states despite their femtosecond and picosecond lifetimes. Except for 1-nitronapthalene, where asingle exponential is observed, for the rest of the compounds, the emission shows double-exponential decaysindicating ultrafast structural changes in the excited states. From anisotropy measurements, we conclude thatno 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 impliedby the large triplet yields and the ultrafast S₁ decays is accounted by the -* nature of the S₁ and T₁ statestogether with the existence of higher triplet configurations which act as receiver states. Our measurementsshow that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule.