The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.

The recombination dynamics of ion pairs generated upon electron transfer quenching of perylene in the first singlet excited state by tetracyanoethylene in acetonitrile is quantitatively described by the extended unified theory of photoionization/recombination. The extension incorporates the hot recombination of the ion pair passing through the level-crossing point during its diffusive motion along the reaction coordinate down to the equilibrium state. The ultrafast hot recombination vastly reduces the yield of equilibrated ion pairs subjected to subsequent thermal charge recombination and separation into free ions. The relatively successful fit of the theory to the experimentally measured kinetics of ion accumulation/recombination and free ion yield represents a firm justification of hot recombination of about 90% of primary generated ion pairs.

The dynamics of the ion pairs produced upon fluorescence quenching of the electron donor 9,10-dimethylanthracene (DMeA) by phthalonitrile have been investigated in acetonitrile and tetrahydrofuran using transient absorption spectroscopy. Charge recombination to both the neutral ground state and the triplet excited state of DMeA is observed in both solvents. The relative efficiency of the triplet recombination pathway decreases substantially in the presence of an external magnetic field. These results were analyzed theoretically within the differential encounter theory, with the spin conversion of the geminate ion pairs described as a coherent process driven by the hyperfine interaction. The early temporal evolution of ion pair and triplet state populations with and without magnetic field could be well reproduced in acetonitrile, but not in tetrahydrofuran where fluorescence quenching involves the formation of an exciplex. A description of the spin conversion in terms of rates, i.e., incoherent spin transitions, leads to an overestimation of the magnetic field effect.