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 b1g 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 11B1u (π xπx*) and 11Au (π xπy* and/or πyπ 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 1Au ← S0 was detected in the fluorescence excitation spectra of the long DPY with N ≥ 3. The low-lying excited state with Au 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 21B1u (πyπ y*) states, which is consistent with the reduction of the nonlinear optical response of polyyne.
ASJC Scopus subject areas
- Colloid and Surface Chemistry