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 (similar to 250 fs); (b) a combined process of double proton transfer and twisting, with an overall reaction time of similar to 250 fs. Picosecond transient fluorescence is attributed to vibrational cooling in the excited product state. The lifetime of similar to 10 ps of the tautomer product state is indicative of conical intersection of the product and ground-state potential energy surfaces. (c) 2005 Elsevier B.V. All rights reserved.