Relative Electron Donor Strengths of Tetrathiafulvene Derivatives: Effects of Chemical Substitutions and the Molecular Environment from a Combined Photoelectron and Electrochemical Study

Interest in organic metals and superconductors has prompted studies of the effects of chemical substituents on the organic electron donor tetrathiafulvalene (TTF). Electron-donating substituents on TTF should lead to reduced ionization potentials and generally greater electron transfer in organic donor/acceptor compounds. However, the relative electron donor abilities may also be influenced by their molecular environment and intermolecular interactions. In order to address these questions, the valence ionization potentials of TTF and two derivatives, bis(ethylenedioxo)tetrathiafulvalene (BEDO-TTF) and bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), have been measured in the gas phase by photoelectron spectroscopy and compared with oxidation potentials from solution electrochemical measurements in a variety of solvents. The order of decreasing first ionization potentials is BEDT-TTF ≈ TTF (6.7 eV) > BEDO-TTF (6.46 eV). However, the order of solution oxidation potentials is BEDT-TTF > BEDO-TTF > TTF. The solvent dependence of these oxidation potentials scales linearly with the cation solvation power of the solvent, expressed as the difference between the first and second oxidation potentials of BEDO-TTF. Extrapolation to the zero-solvation limit (i.e., the gas phase) reproduces the order of ionization potentials determined by photoelectron spectroscopy. When predicting whether a derivative is a better electron donor than TTF, it is therefore necessary to consider the molecular environment of the donor molecule. The nature of solvation and/or interaction with the acceptor molecules may result in trends that are opposite to those based purely on ionization potentials or theoretical calculations on single isolated molecules.