4-Ferrocenylphenyl-Substituted Tritylium Dyes with Open and Interlinked C+Ar2 Entities: Redox Behavior, Electrochromism, and a Quantitative Study of the Dimerization of Their Neutral Radicals

We report on seven 4-ferrocenylphenyl-substituted tritylium dyes Fc-C6H4-C+Ar2 with either unlinked or interlinked aryl residues Ar, including congeners with six-membered (thio)­xanthylium and seven-membered (dihydro)­dibenzo­[a,d]­cycloheptatrienylium motifs. All complexes are intensely colored and show more or less intense absorption bands owing to charge transfer from the 4-ferrocenylphenyl donor to the C+Ar2 acceptor unit as well as reversible electrochromism upon reduction and oxidation. The spectral profiles and redox potentials depend on whether or not the methylium center is incorporated into a 14-π-electron arene system. T-dependent EPR spectroscopy indicates that their one-electron-reduced neutral radicals dimerize. The ensuing monomer–dimer equilibria were studied by quantitative spin-counting methods, which revealed an unexpectedly large extent of 85.0–99.6% of dimerization.