Reaction path dependent coherent wavepacket dynamics in excited state intramolecular double proton transfer

The primary steps and the wavepacket dynamics of the ultrafast photoinduced double proton transfer in (2,2′-bipyridyl)-3,3′-diol are observed. A two-color pump probe spectrometer with 30 fs time resolution allows the variation of the vibrational excess energy. We find that the first step of the sequential proton transfer and the concerted double proton transfer both proceed in about 50 fs. We observe two types of coherently excited vibrational modes: (i) two symmetric stretching modes are closely associated with the concerted double proton transfer indicating that a simultaneous and symmetric reduction of the donor acceptor distances in both chelate rings takes place during the transfer; (ii) a nonsymmetric in-plane-bending mode is associated with the sequential proton transfer and provides evidence that the transient reduction of the donor acceptor distance occurs only in one chelate ring. This mode is exclusively excited by the reaction and cannot be excited optically according to symmetry selection rules. This constitutes a direct proof that a vibrational wavepacket can be excited by an ultrafast reaction itself.