Ultrafast Structural Response of Hydrogen Bonded Complexes to Electronic Excitation in the Liquid Phase

The response of hydrogen bonded complexes to electronic excitation is studied by site-specific vibrational spectroscopy in the femtosecond domain, providing insight into local changes of hydrogen bonding geometries. For the prototypical organic complex coumarin 102−(phenol) 1,2 in solution, electronic excitation of the coumarin chromophore initiates a response of intermolecular hydrogen bonds on two distinct time scales: the hydrogen donor (phenol) 1,2 is released from the acceptor coumarin within 200 fs, followed by a reorganization of the (phenol) 2 moiety with an 800 fs time constant. This structural response results in strong changes of vibrational dipole moments which are monitored for the first time.