Reaction Path Specific Wavepacket Dynamics in Double Proton Transfer Molecules

Reaction Path Specific Wavepacket Dynamics in Double Proton Transfer Molecules 

For the concerted double proton transfer both donor-acceptor distances have to be reduced simultaneously, leading to a symmetric contraction of the molecule and to the coherent excitation ofthe symmetric stretch vibration. Forthe single proton transferthe donor-acceptor distance in only one of the two H-chelate rings is compressed by an antisymmetric bending motion. 

Besides the electronic degrees of freedom several vibrational modes contribute and the essential features of the dynamics can only be understood by a multidimensional model. The multidimensional character causes an irreversible course of the ESIPT even though the transfer itself takes only 50 fs and vibrational dephasing occurs on a picosecond time scale. Nevertheless, most of the energetically accessible vibrations do not play a significant role and a realistic description of the ESIPT has to consider only a restricted number of vibrational degrees of freedom. As discussed in Ref [22] we think that these are general features for many ultrafast molecular processes. The observation of vibrational wavepacket dynamics in a number of systems [6, 75, 76], which exhibit other ultrafast processes, supports this conclusion. 

In the case of intramolecular double proton transfer a wavepacket motion is found which depends, via the excess energy, on the branching ratio between concerted double and single proton transfer. It demonstrates that the coherent wave-packet dynamics in ESIPT molecules is driven by the ESIPT itself and is specific for the reaction path. 

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