Electron Transfer in a Flexible, Tethered Donor-Acceptor Pair: The Influence of Solute Conformation on Solvent-Dependent Free Energies

ADMA (1-(9-anthryl)-3-(4-dimethylanilino)propane) undergoes charge transfer following excitation of the anthryl moiety and forms an exciplex. Two mechanisms of charge transfer have been identified in previous work, and the operative mechanism depends on the polarity of the solvent. These are referred to as the nonpolar and the polar mechanisms. In polar solvents the charge transfer is rapid and occurs in an extended conformation followed by folding to form the exciplex. In hydrocarbons the exciplex formation rate is much slower and requires the donor and acceptor moieties to attain the correct geometry prior to charge transfer. Prior to the present work, it has been assumed that the charge transfer mechanism in hydrocarbons is diffusion controlled. In this work we demonstrate that charge transfer in nonpolar solvents is an activated process. We measure the rate of charge transfer in ethers (solvents of modest polarity with ε = 3-4.3) and compare these to charge transfer rates in alkanes having similar viscosity. Whereas alkane charge transfer rates are well correlated to a viscosity power law, the rates in ethers are accelerated. We calculate the solvent-dependent driving force for the reaction using two different models, and the results also allow us to calculate the reaction reorganization energies. These are then used to estimate the activation barrier for the reaction, which demonstrate that the reaction is not encounter controlled. Analysis of the results demonstrates that solvent stabilization of the product state accelerates the charge transfer rate, in accord with Marcus theory, for solvents with dielectric constants between 2 and ∼4. Solvents with dielectric constants between 4 and 5 exhibit additional acceleration of the charge transfer reaction due to solvent dependence of the distance at which charge transfer occurs. This reflects a transition between the nonpolar and polar mechanisms.