Symmetry breaking wavepacket dynamics in double proton transfer

In this contribution we show unambiguously that a coherent wavepacket motion results from an ultrafast reaction and the associated electronic configuration change and does not reflect mainly the direct optical excitation of the contributing vibronic levels. [2,2´-Bipyridyl]-3,3´-diol (BP(OH)₂) is an ideal candidate to study this long standing question because of its inversion symmetry. It exhibits in aprotic solvents both single and double proton transfer when it is promoted to the electronically excited state. Thereby the hydrogen atom of the hydroxyl group switches to the nitrogen. The simultaneous double proton transfer conserves the inversion symmetry of the molecule, whereas the single proton transfer at one side of the molecule leads to a spontaneous symmetry breaking. The optically active vibrational modes are symmetric and thus of minor importance for the single proton transfer. Here we report on experimental results on BP(OH)₂ obtained by UV-visible pump probe spectroscopy with an unprecedented time resolution of 30 fs. This allows us for the first time to observe both transfer processes and the associated coherent wavepacket dynamics in real time.