Electrochromism Through Redox Regulation of a Nearly Colorless Dynamic Covalent Radical Equilibrium

Radical-based dynamic covalent systems are attractive platforms for stimuli-responsive materials because reversible radical dimerization-cleavage reactions are often accompanied by pronounced changes in color and magnetic properties. In this study, we demonstrate reversible electrochromism based on redox control of the monomer-dimer equilibrium between a phenoxyl radical conjugated with an indandione skeleton (3•) and its σ-dimer 3₂. In contrast to most π-conjugated radicals, 3• is nearly colorless in solution. TD-DFT calculations revealed that the weak visible absorption of 3• originates from forbidden or weakly allowed low-energy transitions, whereas its reduced form 3^- exhibits intense visible absorption arising from an allowed HOMO-LUMO transition. The present system uses the nearly colorless 3₂/3• equilibrium as an OFF state and the strongly colored monoanion 3^- as an ON state. Electrochemical measurements revealed that reduction of the weakly absorbing radical 3• generates 3^-, thereby shifting the equilibrium toward dissociation of the σ-dimer 3₂. Spectroelectrochemical measurements showed pronounced and reversible color changes from nearly colorless to deep red upon electrochemical reduction and oxidation. Variable-temperature electrochemical measurements further demonstrated that the electrochemical behavior is strongly governed by the dynamic monomer-dimer equilibrium. These findings establish a rare electrochromic system driven by redox modulation of a radical-based dynamic covalent equilibrium.