Does Marcus-Hush theory really work? Optical studies of intervalence transfer in acetylene-bridged biferrocene monocation at infinite dilution and at finite ionic strengths

Intervalence charge-transfer data for acetylene-bridged biferrocene monocation (Bf⁺) have been collected in five solvents in the presence and absence of excess electrolyte and in the limit of infinite chromophore dilution. The study was motivated by earlier work which demonstrated that the intervalence absorption maximum for Bf⁺ in methylene chloride could vary substantially with both chromophore concentration and added electrolyte concentration. In the present study similar (but smaller) variations are found in other solvents. The variations are ascribable to both ion pairing and higher order ionic association. For the available solvents the logarithms of the Bf⁺X^- pairing constant (X^- = PF₆ ^-, BF₄ ^-, or ClO₄ ^-) is found to vary inversely with the solvent's static dielectric constant, D_s. When the ion-pairing effects are eliminated by dilution, a genuine test of the Marcus-Hush prediction of the solvent dependence of the intervalence charge-transfer energy (E_op ^MMCT) is possible. For highly polar solvents (D_s ≥ 20) a good fit of E_op ^MMCT to 1/D_op - 1/D_s is found (where D_op is the optical dielectric constant and 1/D_op is the main variant in the two-term dielectric parameter). For methylene chloride, however, E_op ^MMCT falls well below the best-fit line for five other solvents. The deviation is tentatively attributed to dielectric saturation and is expected, therefore, to be manifest in other media possessing limited dielectric strength. In light of these findings, a speculative interpretation of pressure-induced solvent freezing effects is offered. It is suggested that, in frozen CD₃CN, E_op ^MMCT is simultaneously subjected to ion-pairing perturbations and to dielectric saturation effects and that the effects act in an offsetting, largely compensatory fashion. The speculation is supported by semiquantitative extrapolations from known liquid solvent behavior and by an analysis of intervalence bandwidths.