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

ABSTRACT 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 3 2 . 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 3 2 / 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 3 2 . 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.