Unsymmetrical naphthalene diimides as electrochromic materials: tailoring structure for controlled optical and redox properties.

Naphthalene diimides (NDIs) are attractive electron-deficient building blocks for electrochromic materials owing to their excellent electrochemical stability and reversible redox behavior. Herein, we report three unsymmetrical NDI derivatives bearing aromatic N-substituents with progressively increasing electron-donating ability, enabling systematic tuning of their electrochemical and optical properties. All compounds exhibit two reversible reduction processes, whereas donor-functionalized derivatives additionally undergo reversible oxidation, resulting in multistate electrochromism. Spectroelectrochemical studies reveal three, four, and five distinct optical states for NDI-1, NDI-2, and NDI-3, respectively, accompanied by pronounced absorption changes across the visible and near-infrared regions. Owing to its broad electrochemical window and rich optical response, NDI-3 was employed as the active material in a solid-state electrochromic device, which demonstrated reversible multicolor switching, optical modulation in the visible and near-infrared regions, switching times of 1.4-8.7 s, with particularly good stability observed for the radical anion/dication transition. These findings demonstrate that molecular engineering of unsymmetrical NDI derivatives provides an effective strategy for developing multistate electrochromic materials with tunable optical properties for advanced electrochromic devices and energy-efficient smart window applications.