Symmetry Switching of the Fluorescent Excited State in α,ω -Diphenylpolyynes

By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we have comprehensively studied the low-lying excited singlet states of α,ω-diphenylpolyynes (DPY) having 1-6 triple bonds. The ag vibrational modes of the C≡C stretching and of the phenyl ring motion were observed in the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in the fluorescence spectra of the long DPY with the triple-bond number (N) more than two, the phenyl ring motion with ag symmetry disappeared and the b_1g modes of the phenyl ring twisting (∼400 cm^-1) and of the C-H bending (∼900 cm ^-1) were detected. The observed fluorescent states of DPY with N ≤ 2 and N ≥ 3 are assigned to the 1¹B_1u (π _xπ_x*) and 1¹A_u (π _xπ_y* and/or π_yπ _x*) states, respectively, based on the vibronic structures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not only the allowed transition of 1¹B _1u ← S₀ but also the forbidden transition of 1 ¹A_u ← S₀ was detected in the fluorescence excitation spectra of the long DPY with N ≥ 3. The low-lying excited state with A_u symmetry is characteristic in polyyne, which does not exist in polyene. The oscillator strength (f) of the first absorption band in DPY decreases with an increase in N, which is the opposite behavior of the all-trans-α,ω-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction between the 1¹B _1u and 2¹B_1u (π_yπ _y*) states, which is consistent with the reduction of the nonlinear optical response of polyyne.