Violation of fluctuation-dissipation relations for electron transfer in nonpolar solvents

Abstract

Nonpolar materials are increasingly employed as media for electron transfer, particularly in applications related to solar energy conversion. What should be the mechanism of activation for electron tunneling in the absence of rotating permanent dipoles considered in standard theories is not clear. We suggest that compression and decompression (density) fluctuations shifting positions of polarizable molecules of the medium is the mechanism for radiationless transitions. These fluctuations affect the induction interactions between the medium induced dipoles and the localized electron (induction forces). Solvent reorganization energy must be a signature of such fluctuations, but it has never been directly measured for electron transfer in nonpolar media. Here, absorption and emission spectra of a charge-transfer complex are analyzed as functions of temperature in cyclohexane. Significant reorganization energies, 0.2–0.5 eV, are found. They strongly differ between the ground and photoexcited charge-transfer states in violation of fluctuation-dissipation relations establishing the basis for modern theories of electron transfer. The reorganization energies are decaying functions of temperature, also in violation of the macroscopic fluctuation-dissipation relations.