The role of the triplet state in depopulation of the electronically excited proton-transferring system [2,2′-bipyridyl]-3,3′-diol

Electronic excitation of [2,2′-bipyridyl]-3,3′-diol (BP(OH)2) generates the diketo-tautomeric form of the system. The long-lived transient absorption (TA) of BP(OH)2 was assigned as the T-T absorption of the diketo-tautomer. The effect of the stimulated emission pumping on TA excludes the essential contribution of the absorption from the ground-state diketo form. The rate constants of T-T annihilation and of oxygen quenching are kTTA = 2.4 × 1010 M−1 s−1 and kqO2 = (2–3) × 109 M−1 s−1. The contribution of the second-order process to the overall decay of the triplet BP (OH)2, and the origin of the delayed fluorescence of the diketo form, are discussed.