Proton-transferring systems studied by vibrational spectroscopy and theoretical ab initio calculations. The S₀ and T₁ states of [2,2′-bipyridine]-3,3′-diol

Tautomerization and phototautomerization of [2,2′-bipyridine]-3,3′-diol is investigated by means of infrared, Raman, and time-resolved resonance Raman spectroscopies and ab initio theoretical calculations. Full ab initio SCF (self-consistent field method) geometry optimization followed by MBPT (2)/6-31G** (second-order many-body perturbation theory) energy calculations of both tautomeric forms in the ground state and the lowest excited triplet state is reported and discussed. Two tautomeric forms. A (dihydroxy) and B (dioxo) are found to be minima on the S₀ and T₁ potential energy surfaces. While form A is the most stable one in the ground state, the energetic ordering is reversed when going to the triplet state. A good agreement between the experimental and calculated vibrational spectra allows a reliable assignment of most vibrational bands corresponding to tautomer A in the ground state. To assign the observed triplet-triplet optical transition and estimate intensities of resonance Raman bands in the triplet state, excitation energies and oscillator strengths of higher triplet states relative to T₁ are calculated. Tentative assignments for the resonance Raman bands observed in the lowest excited triplet state are given. All observed vibrational bands are assigned to totally symmetric modes of the most stable B form. From a comparison of the observed and calculated T₁ resonance Raman spectra, it is concluded that two transitions, namely, T₁ → T₅ and T₁ → T₈, contribute to the resonance Raman scattering and to the triplet-triplet absorption band observed around 430 nm. These states are both of A_g symmetry and are calculated at 3.15 (T₅) and 4.80 eV (T₈) above T₁ (B_u). In view of the fact that the exciting wavelength (around 430 nm, 2.88 eV) is substantially out of resonance with the calculated T₁ → T₈ transition, vibronic coupling between T₅ and T₈ is considered as a possibility.