Remote electron transfer in donor–acceptor complexes: the influence of photoexcitation pulse carrier frequency

Spectral effect in ultrafast photochemistry reveals itself as dependence of kinetics and quantum yield of chemical reaction on the carrier frequency of excitation laser pulse. Such dependence is repeatedly observed experimentally in donor–acceptor complexes (DACs)—simple molecular compounds involving a pair of molecules acting as electron donor and electron acceptor upon excitation of the charge transfer (CT) absorption band.
Two general approaches exist so far for interpretation of this phenomenon. The first approach elucidates the spectral effect in terms of nonequilibrium wavepackets formed on the excited state electronic term as a result of DAC optical pumping. Within this approach, the differences in ultrafast recombination kinetics in DACs upon excitation at various frequencies are the result of different trajectories of their motion along the solvent relaxation coordinates. Numerical simulations show that the spectral effect in this case can be either positive (acceleration of charge recombination with the rise of excitation carrier frequency) or negative. Another interpretation of the spectral effect is tied up with excitation of intramolecular vibrational degrees of freedom in donor and acceptor compounds upon DAC absorption of a photon. Distinctions in recombination kinetics after the DAC excitation at the red or blue ends of the CT absorption band are then explained by population of different vibrational sublevel of the excited (ionic) state.
However the origin of the spectral effect in DACs can be associated not only with nonequilibrium solvent polarization or intramolecular high-frequency vibrations, but with any other energetic coordinates as well. The most obvious candidate for this role is the distance $r$ between the donor and acceptor molecules. It is well-known that the solvent reorganization energy $E_{rm}$ and the free energy of charge recombination $\Delta G_{CR}$ depend on r significantly. This implies that excitation of DACs at different wavelengths may result in predominant formation of ion pairs either at contact or noncontact distances and, thus, leads to different rates of charge recombination.
In this study we explore this mechanism of the spectral effect within a simple two-level model involving the ground (neutral) and excited (ionic) electronic states of the DAC, and a set of vibrational sublevels. Polar solvent around the reactants is taken into consideration through the Marcus energetic coordinate $q$. Using the resonant approximation for the excitation rate, analytic expression for the density of ion pairs in the $\{r, q \}$ space after optical pumping is derived. Analysis of this expression shows the principal possibility of control for the kinetics and quantum yield of ultrafast recombination in DAC by variation of carrier frequency of the laser pulse.