Solvent Effect on the Dimerization of Tetracyanoquinodimethane Anion Radical

The thermodynamic quantities of the dimerization of tetracyanoquinodimethane (TCNQ) anion radical in water–ethanol mixtures were determined. The free energy of dimerization −ΔG showed a monotone decrease with an increase in ethanol content, whereas the enthalpy and the entropy of dimerization, −ΔH and −ΔS, showed a maximum. This unusual behavior is explained by proposing a solvation model in which the water molecules are attracted more strongly around the TCNQ− dimer than are the ethanol molecules. The entropy was found to play an important role in the dimerization.