Triplet (T1) state time-resolved resonance Raman investigation of 2,2′-bipyridine

Time-resolved resonance Raman spectra of the lowest triplet state T1 (ππ*) of 2,2′-bipyridine (22BPY) have been obtained for different isotopomers in solution and have been interpreted in detail on the basis of previously reported analyses of the vibrational properties of the ground state S0 and anion radical R− of 22BPY. A notable analogy is found between the T1 and R− species that are characterized by similar spectral changes relative to the S0 state. It is concluded that the T1 state is characterized by a trans-planar structure of C2h symmetry, showing a distortion with notable quinoid character, comparable to the distortion evidenced in R−. The electronic distribution is symmetrically delocalized in the two pyridyl rings as in S0 and R−. It differs in this regard from the asymmetric S1 (nπ*) state configuration studied earlier, which is characterized by the confinement of the excitation in one of the pyridyl rings.