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

Abstract

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.