Research in the area of electron transfer reactions focuses on the fundamentals of the elementary process of activating the transfer of electron between the donor and acceptor molecules in soft condensed solvents. The various problems we are addressing can broadly be separated into the mechanistic aspects of electron transfer reactions and microscopic models of the activation barrier.

Mechanistic aspects

This research addresses the question of how to generally describe the activated event of electron transfer in terms of parameters characteristic of the donor and acceptor molecules and the solvent they are dissolved in. We are particularly interested in understanding electron transfer in systems characterized by extensive delocalization of the electronic density between the donor and acceptor and in systems with a substantial variation of the polarizability with the electronic transition.

Electronically delocalized systems present a substantial challenge both experimentally and theoretically since the change in the electronic population is sensitive to many parameters including the fluctuating field of the solvent. The fact that the electronic states of the donor and acceptor are coupled results in dramatic effects on both optical spectroscopy and redox kinetics. Thi is illustrated by the dependence of the absorption and emission width in the optical dye coumarin 153 on the Stokes shift. Without electronic delocalization, both the emission and absorption widths should be linear functions of the Stokes shift with the unit slope. The large transition dipole between the ground and excited singlet states of coumarin results in substantial narrowing of the emission band, particularly in strongly polar solvents. The goal of this project is to develop a theoretical procedure allowing to predict and analyze the experimental optical spectra with strong electronic coupling between the electronic states (see J. Phys. Chem. A 105 (2001) 8516).

Electron transfer in systems where the polarizability of the reactants changes with electron transfer is an exciting problem because of strong deviations of the electron transfer free energy surfaces from parabolas of the classical Marcus-Hush theory. The model developed to account for such effects, the Q-model, predicts several important effects of the variation of the solute polarizability on the reorganization energy of electron transfer and the reaction activation barrier (see J. Chem. Phys. 113 (2000) 5413; J. Chem. Phys. 115 (2001) 8933; J. Am. Chem. Soc. 125 (2003) 7470; J. Phys. Chem. A 108 (2004) 2087).

Molecular models of the activation barrier

The goal of this research is to develop a computational algorithm for calculating the activation barrier of electron transfer from first principles based on the interaction potentials between the reactants and the liquid solvent and the molecular properties of the solvent. We have recently developed a formalism allowing to calculate reorganization energies of electron transfer for an arbitrary molecular shape and charge distribution of the donor-acceptor complex dissolved in an arbitrary molecular dielectric. The theory incorporates partial atomic charges and atomic coordinates along with the polarization structure factors of the microscopic solvent into the calculation of the solvent reorganization energy and free energy of solvation (see J. Phys. Chem B 107 (2003) 14509; J. Chem. Phys. 120 (2004) 7532).

Electron transfer in Nematics.

Electron transfer in liquids undergoing transition from the isotropic to nematic liquid crystalline phase is a part of a bigger problem of the influence of polarization anisotropy of the solvent on the activation barrier of electron transfer. The main effect arise from the dependence of the polarization response of the nematic phase on the angle between the direction of the external electric field and the direction of the nematic director. This anisotropy is reflected by the polarization structure factors obtained by Monte Carlo simulations shown in the Figure. The resultant solvent reorganization energy and the activation barrier both show a pronounced dependence on the angle between the orientation of the donor-acceptor complex in the nematic solvent (see J. Phys. Chem. B 107 (2003) 1937; J. Chem. Phys. 119 (2003) 1559.)