Kinetics and Energetics of Electron Transfer to Dimer Radical Cations

Spectra of the dimer cations naphthalene (Nap₂^•+) and ethene (Ethene₂^•+) were measured in liquid dichloromethane (DCM). The spectra peak at very different energies, 1.2 and 3.3 eV. In DCM dimerization stabilizes Nap₂^•+ by ΔG_d°(Nap₂^•+) = -218 meV relative to the monomer Nap^•+ as determined from the dimerization equilibrium constant. Both dimers can transfer a positive charge to hole acceptor molecules, but for both the rate constants rise more gradually with reaction energetics than do many charge transfer reactions previously studied. A striking observation finds that the rate constant for hole transfer from the Nap₂^•+ dimer to phenanthrene is smaller by two decades than that from biphenyl^•+ monomer to Nap, although both reactions have the same -ΔG° = 0.05 eV. A plausible interpretation for these observations is the presence of an energy of reorganization, λ(M₂), for the dimer that involves movement apart of the two partners in the dimer. While the dimerization equilibrium cannot be measured for Ethene₂^•+, the charge transfer data imply that both ΔG_d°(Ethene₂^•+) and λ(Ethene₂^•+) are considerably larger, perhaps by factors of 2-4 than for Nap₂^•+.