Kinetics and Energetics of Electron Transfer to Dimer Radical Cations

Abstract

Spectra of the dimer cations naphthalene (Nap 2 •+ ) and ethene (Ethene 2•+ ) were measured in liquid dichloromethane (DCM). The spectra peak at very different energies, 1.2 and 3.3 eV. In DCM dimerization stabilizes Nap 2•+ by ΔG d °(Nap 2 •+ ) = −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 Nap 2•+ 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(M 2 ), for the dimer that involves movement apart of the two partners in the dimer. While the dimerization equilibrium cannot be measured for Ethene 2•+ , the charge transfer data imply that both ΔG d °(Ethene 2 •+ ) and λ(Ethene 2•+) are considerably larger, perhaps by factors of 2−4 than for Nap 2•+ .