Femtosecond laser selective intramolecular double-proton transfer in [2,2′-bipyridyl]-3,3′-diol

We report an excitation energy dependence of the dynamics of the excited-state intramolecular double-proton transfer of [2,2′-bipyridyl]-3,3′-diol in liquid solution. By means of femtosecond fluorescence upconversion experiments it is shown that an increase of the vibrational energy in the excited electronic S1 state results in an increase of the ratio of the reaction yields of the monoketo and diketo tautomeric products. The dominance of the concerted double-proton transfer process at the lower excitation energies is evidence for an energy barrier in the dienol–monoketo reaction pathway.