Unsymmetrically Substituted 1,1′-Biferrocenylenes Maintain Class III Mixed-Valence Character

UV–vis–near-IR electronic absorption spectra of unsymmetrically substituted derivatives of 1,1′-biferrocenylene (bis­(fulvalene)­diiron = BFD) have been systematically evaluated upon oxidation using spectroelectrochemical techniques. Upon reversible electrochemical oxidation to the BFD⁺ monocation and BFD²⁺ dication state, the characteristic intervalence charge-resonance (IVCR) transitions of a class III mixed-valence system in solution state was confirmed. The class III mixed-valence character of the parent BFD⁺ system was not altered upon unsymmetric substitution. Peak-fit analysis and TD-DFT calculations were utilized to assign the observed IVCR transitions.