Anion and solvent dependency of the electronic coupling strength in mixed valent class II systems

The influence of the coordination and ion pairing properties of electrolyte anions on electronic coupling in cationic class II mixed valent species was studied. In order to cover a range of electronic coupling strengths within the class II regime, weakly coupled 2,5-diferrocenyl-3,4-thiadiazol, moderately coupled 2,5-diferrocenyl thiophene and strongly coupled N-(4-dimethylaminophenyl)-2,5-diferrocenyl-1H-pyrrole were chosen as analytes. The electrochemical properties of these compounds were determined by cyclic and square wave voltammetry using electrolytes with varying ion pairing capabilities, such as [NBu₄][Cl], [NBu₄][PF₆] and [NBu₄][BArF] ([NBu₄][B(C₆F₅)₄]), as well as solvents with increasing dielectric constants (dichloromethane (ε_r = 8.93), acetone (ε_r = 20.56), acetonitrile (ε_r = 35.94) and propylene carbonate (ε_r = 64.92)). It is shown that the choice of electrolyte has a considerable impact on the electrostatic and the electron transfer features of the mixed valent compounds when solvents of low polarity and low relative permittivity such as dichloromethane are used. For the use of more polar solvents such as propylene carbonate the electrochemical and spectroscopic properties are almost electrolyte independent. The solvatochromic and ion-related changes in the spectroscopic properties are most pronounced for weakly coupled systems and decrease with an increase in the electron transfer coupling strength.