The excited triplet (T 1) state structure and vibrational properties of 2,2′-bipyridine

The geometry of the lowest lying excited triplet state (T 1) of 2,2′-bipyridine (22BPY) was optimized by using the time-dependent density functional theory (TD-DFT) with the B3LYP functional and the SVP basis set. The T 1 state is of 3B u symmetry and results from a nearly one-electron ππ ∗ transition from the 3b g HOMO to the 4a u LUMO. Its geometry is trans-planar and essentially characterized by a reinforcement of the interring CC bond and a quinoidal distortion of the rings. This calculated triplet structure is firmly validated by an unequivocal agreement, for four 22BPY isotopomers, between the derived theoretical vibrational frequencies and previously reported experimental time-resolved resonance Raman (TR3) spectra. Moreover, vertical transitions to the 10 lowest energy triplet states T n were calculated and the corresponding T 1 state resonance Raman intensities estimated, in the short-time dynamics approximation of the Franck-Condon scattering mechanism, from the gradient of the T n potential surfaces at the T 1 geometry along the totally symmetric modes. Excellent agreement with the experimental resonance Raman intensities was observed for a 4a u (LUMO) → 5b g (LUMO + 3) T 1 → T n transition. This analysis provides a further support of the TD-DFT optimized T 1 state structure of 22BPY.