Chemically Induced Dynamic Electron Polarization Study on the Mechanism of Exchange Interaction in Radical Ion Pairs Generated by Photoinduced Electron Transfer Reactions

Photoinduced electron transfer reactions were studied by using the continuous wave time-resolved electron paramagnetic resonance and Fourier-transformed electron paramagnetic resonance spectroscopy in polar solvents. The chemically induced dynamic electron polarization was investigated in both singlet and triplet precursor intermolecular electron transfer systems. The signs of the exchange interaction, which are defined by the energy differences between the singlet and triplet radical ion pairs, depended on the free energy changes for the charge recombination processes. The results are interpreted in terms of the mechanism that the spin selective stabilization and destabilization are caused by the perturbation through the electronic coupling from the ground state and the locally excited triplet state of the donor - acceptor pair at the equilibrium nuclear coordinate. In the singlet precursor electron transfer systems, the positive exchange interaction resulted from the selective triplet stabilization in the radical ion pair, when the ion pair state crossed with the locally excited triplet state at the normal region. In the triplet precursor electron transfer systems, the negative exchange interaction resulted from the selective singlet stabilization when the ion pair state crossed with the singlet ground state at the normal region. When the free energy change is larger than about 1.8 eV, the positive exchange interaction resulted from the spin-selective destabilization in the singlet ion pair, since the level crossing occurs at the inverted region.