The excited triplet (T1) state structure and vibrational properties of 2,2′-bipyridine

The geometry of the lowest lying excited triplet state (T1) 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 T1 state is of 3Bu symmetry and results from a nearly one-electron ππ∗ transition from the 3bg HOMO to the 4au 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 Tn were calculated and the corresponding T1 state resonance Raman intensities estimated, in the short-time dynamics approximation of the Franck-Condon scattering mechanism, from the gradient of the Tn potential surfaces at the T1 geometry along the totally symmetric modes. Excellent agreement with the experimental resonance Raman intensities was observed for a 4au (LUMO) →  5bg (LUMO + 3) T1 → Tn transition. This analysis provides a further support of the TD-DFT optimized T1 state structure of 22BPY.