Tailoring Valence Tautomerism by Using Redox Potentials: Studies on Ferrocene‐Based Triarylmethylium Dyes with Electron‐Poor Fluorenylium and Thioxanthylium Acceptors

Three new electrochromic ferrocenyl triarylmethylium dyes with fluorenylium (1 a (+), 1 b (+)) or thioxanthylium (1 c (+)) residues were selected in order to keep the intrinsic differences of redox potentials for ferrocene oxidation and triarylmethylium reduction small and to trigger valence tautomerism (VT). UV/Vis/NIR and quantitative EPR spectroscopy identified paramagnetic diradical isomers 1 a (..+)–1 c (..+) alongside diamagnetic forms 1 a (+)–1 c (+), which renders these complexes magnetochemical switches. The diradical forms 1 a (..+)–1 c (..+) as well as the one‐electron‐reduced triarylmethyl forms of the complexes were found to dimerize in solution. For radical 1 a (.), dimerization occurs on the timescale of cyclic voltammetry; this allowed us to determine the kinetics and equilibrium constant for this process by digital simulation. Mößbauer spectroscopy indicated that 1 a (+) and 1 b (+) retain VT even in the solid state. UV/Vis/NIR spectro‐electrochemistry revealed the poly‐electrochromic behaviour of these complexes by establishing the distinctly different electronic absorption profiles of the corresponding oxidized and reduced forms.