Femtochemistry of Hydrogen Bonded Complexes after Electronic Excitation in the Liquid Phase:  The Case of Coumarin 102

We review our results on hydrogen bonding dynamics in the electronically excited state of coumarin 102 (C102), where we deduced that a hydrogen bond cleavage occurs within 200 fs. We compare the electronic absorption and emission properties of C102 hydrogen bonded to CHCl 3, phenol or 2,2-dimethyl-3-ethyl-3-pentanol. We introduce the technique of two-pulse photon echo to show that an optical coherence between the optically coupled electronic ground and excited states survives until 180−200 fs, in the case of C102−(phenol) 1,2 complexes, and argue that this is an indication that an electronic state hopping occurs around this time. By comparison with the previous pump−probe and grating scattering results we deduce that this state hopping is accompanied by a hydrogen bond cleavage.