Ultrafast excited-state intramolecular double proton transfer dynamics of [2,2′-bipyridyl]-3,3′-diamine

A femtosecond fluorescence upconversion study of [2,2′-bipyridyl]-3,3′-diamine (BP(NH2)2), in liquid solution, is reported. It is concluded that photo-excited BP(NH2)2 undergoes a branched intramolecular double proton transfer reaction comprising two trajectories: (a) ultrafast double proton transfer (<100 fs) followed by twisting (∼250 fs); (b) a combined process of double proton transfer and twisting, with an overall reaction time of ∼250 fs. Picosecond transient fluorescence is attributed to vibrational cooling in the excited product state. The lifetime of ∼10 ps of the tautomer product state is indicative of conical intersection of the product- and ground-state potential energy surfaces.