Optical transitions of symmetrical mixed-valence systems in the Class II–III transition regimeElectronic supplementary information (ESI) is available: derivation of eqn. (39c), table summarizing the relationships between band maxima and band widths predicted by the two-state model and table of spectral properties of mixed-valence ruthenium(II)/(III) bridged by pyrazine and dicyanamide. See http://www.rsc.org/suppdata/cs/b0/b008034i/

In the Robin and Day classification, mixed-valence systems are characterized as Class I, II or III depending on the strength of the electronic interaction between the oxidized and reduced sites, ranging from essentially zero (Class I), to moderate (Class II), to very strong electronic coupling (Class III). The properties of Class I systems are essentially those of the separate sites. Class II systems possess new optical and electronic properties in addition to those of the separate sites. However, the interaction between the sites is sufficiently weak that Class II systems are valence trapped or charge localized and can the be described by a double-well potential. In Class III systems the interaction of the…

• UD
  U.S. Department of Energy
• BE
  Basic Energy Sciences
• BN
  Brookhaven National Laboratory