Effects of Ion Aggregation on the Intervalence Transfer Band of the Mixed-Valence Biferrocenium Cation in Solution

The concentration dependence of the energy (E_op) of the intervalence transfer (IT) electronic absorption band of mixed-valence biferrocenium triiodide dissolved in either nitrobenzene or dichloromethane is examined. In nitrobenzene, E_opincreases rapidly from ∼5.57 to ∼6.05 X 10³cm^-1in the 0.24-0.4 mM range, whereupon further increases in the concentration of biferrocenium triiodide lead to E_opincreasing more gradually to a value ∼6.3 X 10³cm^-1at 24 mM. On the other hand, the full width at half maximum(Δv_1/2) and the transition moment of the IT band decrease with increasing concentration for biferrocenium triiodide in nitrobenzene. E_opincreases, Δv_1/2remains constant, and the transition moment of the IT band increases with increasing concentration for biferrocenium triiodide in dichloromethane in the more limited range of 0.30-0.95 mM. These concentration dependencies of the IT band for biferrocenium triiodide are attributable to variable degrees of ion aggregation in solution. The concentration dependencies of the electrical conductivity of solutions of biferrocenium triiodide in either nitrobenzene or dichloromethane substantiate this proposal. IT band contours obtained at different concentrations are fit to the PKS vibronic model to yield parameters that characterize the electronic and vibronic coupling within the mixed-valence biferrocenium cation. It is concluded that in the 0.24-24 mM range the IT band contour of biferrocenium triiodide in nitrobenzene is likely a superposition of variable amounts of two IT bands, one for an ion-paired cation and the other for a non-ion-paired cation. The increase in E_opwith increasing concentration reflects an increasing percentage of ion-paired mixed-valence cations that have a higher energy IT band than the non-ion-paired cation because the ion pairing probably introduces a zeropoint energy separation between the two vibronic states of the cation.