Redox Isomerism in Ethynyl-Bridged Triazatruxene-Diarylmethylium Dyads

We present two triazatruxene-triarylmethylium TAT-Tr⁺ donor-acceptor dyads <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> with either 4-CF₃- (<b>1</b>^<b>+</b>) or 4-F-substituted phenyl rings (<b>2</b>^<b>+</b>) at the tritylium (Tr⁺) site. In spite of rather large differences between the redox potentials for TAT oxidation and Tr⁺ reduction, the diamagnetic TAT-Tr⁺ forms of these dyads coexist with their paramagnetic TAT^+•-Tr^• valence tautomers with one unpaired spin at every redox site. The major quantity of the diradical isomers is trapped as dimers with concomitant loss of the unpaired spin density of the Tr^• entity. The dimers were also observed by cyclic and square wave voltammetry and form readily by one-electron reduction of the dyads to the corresponding trityl radical. Dimerization of the neutral radicals occurs at a much faster rate than dissociation of their two-electron oxidized forms, indicating hysteretic behavior.