Contrasting electroabsorbance behavior of two borderline class II/class III mixed-valence systems

Intervalence Stark effect spectroscopy of a symmetrically substituted and an asymmetrically substituted derivative of the Creutz-Taube ion, (NH3)5Ru-pyrazine-Ru(NH3)55+, is reported. Like the formally mixed-valent parent ion, both appear to lie near the boundary between valence delocalization and valence localization. Remarkably, despite the close structural similarities of the two compounds investigated, the corresponding electroabsorbance spectra are grossly different. Furthermore, neither spectrum is well fit by a classic Liptay approach. An alternative, qualitative approach that allows for the possibility that the externally applied electric field itself affects the degree of valence delocalization and, therefore, the electroabsorbance spectrum, was employed. The alternative approach relies upon the availability of compounds closely analogous to the investigated compounds, but differing slightly in degree of redox asymmetry. A subtle method for creating such compounds is to partially encapsulate a parent compound with a crown ether, such that a lower symmetry assembly is obtained. Using this approach the unusual Stark responses, including the disparate responses for symmetric vs. asymmetric structures, are reasonably well reproduced. Intervalence-enhanced resonance Raman spectra provide additional support for the proposed interpretation.