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 alpha,omega-diphenylpolyynes (DPY) having 1-6 triple bonds. The a(g) vibrational modes of the CdropC 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 big modes of the phenyl ring twisting (similar to400 cm(-1)) and of the C-H bending (similar to900 cm(-1)) were detected. The observed fluorescent states of DPY with N less than or equal to 2 and N greater than or equal to 3 are assigned to the 1(1)B(1u) (pi(x)pi(x)(*)) and 1(1)A (pi(x)pi(y)(*) and/or pi(y)pi(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 1(1)B(1u) <-- So but also the forbidden transition of 1(1)A(u) <-- S-0 was detected in the fluorescence excitation spectra of the long DPY with N greater than or equal to 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-alpha,omega-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction between the 1(1)B(1u) and 2(1)B(1u) (pi(y)pi(y)(*)) states, which is consistent with the reduction of the nonlinear optical response of polyyne.