Ion Size-Dependent Electrochromism in Air-Stable Napthalenediimide-Based Conjugated Polymers

Conjugated polymers (CPs) that show stable and reversible cation insertion/deinsertion under ambient conditions hold great potential for optoelectronic and energy storage devices. However, n-doped CPs are prone to parasitic reactions upon exposure to moisture or oxygen. This study reports a new family of napthalenediimide (NDI) based conjugated polymers capable of undergoing electrochemical n-type doping in ambient air. By functionalizing the NDI-NDI repeating unit with alternating triethylene glycol and octadecyl side chains, the polymer backbone shows stable electrochemical doping at ambient conditions. We systematically investigate the extent of volumetric doping involving monovalent cations of varying size (Li⁺, Na⁺, tetraethylammonium (TEA⁺)) with electrochemical methods, including cyclic voltammetry, differential pulse voltammetry, spectroelectrochemistry, and electrochemical impedance spectroscopy. We observed that introducing hydrophilic side chains on the polymer backbone improves the local dielectric environment of the backbones and lowers the energetic barrier for ion insertion. Surprisingly, when using Na⁺ electrolyte, the polymer films exhibit higher volumetric doping efficiency, faster-switching kinetics, higher optical contrast, and selective multielectrochromism when compared to Li⁺ or TEA⁺ electrolytes. Using well-tempered metadynamics, we characterize the free energetics of side chain-ion interactions to find that Li⁺ binds more tightly to the glycolated NDI moieties than Na⁺, hindering Li⁺ ion transport, switching kinetics, and limiting the films' doping efficiency.