Femtosecond fluorescence anisotropy studies of excited-state intramolecular double-proton transfer in [2,2′-bipyridyl]-3,3-diol in solution

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 similar to350 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 "diketo" excited states is indicative of a (quasi-)barrierless reaction. To explain that the rapid (similar to350 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 (similar to10 ps) and the rotational diffusion motions of the solute in the liquid (20-40 ps).