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 CHCl3, 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.

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We study the reaction dynamics of OClO in cyclohexane, acetonitrile, and water by femtosecond pump–probe spectroscopy. In all solvents we observe a quantum beat in a 403 nm one-color pump–probe experiment with 55 fs temporal resolution, that decays with a 1.3–1.5 ps time constant. From this we conclude that, in contrast to previous reports, not all OClO molecules dissociate after excitation with 403 nm light. In both cyclohexane and water we observe in the 403 nm experiment an increase in stimulated emission between 0.5 and 2 ps that appears to be connected to the quantum beat decay. We explain these results as the consequence of vibrational relaxation of the bending mode of OClO. Relaxation from (ν1,1,0) to (ν1,0,0) leads to population of a state with a two times higher transition dipole moment, which accounts for the increased stimulated emission. Further proof that not all OClO molecules dissociate immediately after excitation is found in the identification of a stimulated emission contribution in femtosecond 400 nm pump/800 nm probe experiments, which also decays with about a 1.5 ps time constant. Femtosecond 400 nm pump/267 nm probe measurements indicate that a fraction of the OClO molecules dissociate very rapidly, with dissociation times of ⩽60, 80, and 140 fs, in acetonitrile, water, and cyclohexane, respectively. An anisotropy decay is resolved at 267 nm of the formed ClO in water and cyclohexane, with anisotropy decay times of 0.17 and 0.27 ps, respectively. In all solvents a fraction of the ClO+O fragments recombine, with time constants of 1.2 and 4.1 ps in water, 6.0 ps in acetonitrile, and 8.9 ps in cyclohexane. In acetonitrile a secondary dissociation pathway is identified with a 2.1 ps time constant. This pathway might also be responsible for the biexponentiality of the recombination process in water. In particular, in acetonitrile and cyclohexane the data indicate cage escape of a significant amount of fragments.

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