Symmetry Switching of the Fluorescent Excited State in ,-Dipheny

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• 作者：Yasutomo Nagano ; Tadaaki Ikoma ; Kimio Akiyama ; and Shozo Tero-Kubota
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：November 19, 2003
• 年：2003
• 卷：125
• 期：46
• 页码：14103 - 14112
• 全文大小：206K
• 年卷期：v.125,no.46(November 19, 2003)
• ISSN：1520-5126

文摘

By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we havecomprehensively studied the low-lying excited singlet states of ,-diphenylpolyynes (DPY) having 1-6triple bonds. The a_g vibrational modes of the CC stretching and of the phenyl ring motion were observedin the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in thefluorescence spectra of the long DPY with the triple-bond number (N) more than two, the phenyl ring motionwith a_g symmetry disappeared and the b_1g modes of the phenyl ring twisting (~400 cm^-1) and of the C-Hbending (~900 cm^-1) were detected. The observed fluorescent states of DPY with N 2 and N 3 areassigned to the 1¹B_1u (_xx*) and 1¹A_u (_xy* and/or _yx*) states, respectively, based on the vibronicstructures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not onlythe allowed transition of 1¹B_1u S₀ but also the forbidden transition of 1¹A_u S₀ was detected in thefluorescence excitation spectra of the long DPY with N 3. The low-lying excited state with A_u symmetryis characteristic in polyyne, which does not exist in polyene. The oscillator strength (f) of the first absorptionband in DPY decreases with an increase in N, which is the opposite behavior of the all-trans-,-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction betweenthe 1¹B_1u and 2¹B_1u (_yy*) states, which is consistent with the reduction of the nonlinear optical responseof polyyne.