Femtosecond Fluorescence Anisotropy Studies of Excited-State Intramolecular Double-Proton Transfer in [2,2‘-bipyridyl]-3,3‘-diol in Solution

Abstract

For [2,2‘-bipyridyl]-3,3‘-diol (BP(OH)2), dissolved in aprotic solvents, the time dependence of the fluorescence anisotropy has been studied using the femtosecond fluorescence up-conversion technique. A fast fluorescence anisotropy decay, with a characteristic time of ∼350 fs, is observed when detection is at 460 nm, i.e., near the blue edge of the BP(OH)2 broad-band emission. It is discussed that the fast decay is typical of emission from the “di-enol” excited Franck−Condon state for which the lifetime is limited by the first steps of a branched excited-state double double-proton-transfer process. The branched reaction includes concerted and consecutive double-proton-transfer. The fast decay of the “di-enol” excited state into “mono-keto” and “di-keto” excited states is indicative of a (quasi-)barrierless reaction. To explain that the rapid (∼350 fs) initial decay is manifested in the fluorescence anisotropy, it is conjectured that the electronic wave function characteristic of the emissive state is a reaction-coordinate dependent admixture of diabatic wave functions, these functions being characteristic of the “enol”- and “keto”-tautomers. The progress of the double-proton-transfer reaction is accompanied by a change of the admixture of the excited-state tautomer wave functions and in this way gives rise to a rapid decay in the fluorescence anisotropy. The fluorescence anisotropy of BP(OH)2 furthermore includes two slower decay components. These components, with time constants of a few tens of picoseconds, are related to the second step of the consecutive double proton-transfer kinetics (∼10 ps) and the rotational diffusion motions of the solute in the liquid (20−40 ps).