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 •+.