Ultrafast single and double proton transfer in photo-excited [2,2′-bipyridyl]-3,3′-diol

We report on a subpicosecond fluorescence study of [2,2′-bipyridyl]-3,3′-diol using the fluorescent up-conversion technique with a time resolution of ≈300 fs. A new short-living fluorescence band, peaking at 568 nm, is observed, which is attributed to the emissive keto-enol tautomer. This tautomer is discussed to be formed in its excited singlet state in a stepwise proton transfer mechanism; eventually the keto-enol tautomer decays into the double-protonated di-keto product, which alternatively could also have been formed in a direct double proton transfer reaction.