Design of viologen-based 2D cationic covalent organic polymer for multi-colored electrochromic devices with tuneable redox potential

Viologen-based ionic covalent organic polymers (V-iCOP) with higher dimensionality is promising for their various characteristics (porosity, redox potential, and color) owing to the tunability of their composition. In this work, 3 different viologen-based 2D cationic covalent organic polymer thin films (V-iCOP1, V-iCOP2, V-iCOP3) were synthesized by Zincke reaction with linkers of varying electron affinity: tris(4-aminophenyl)amine (TAPA), 1,3,5-tris(4-aminophenyl)benzene (TAPB), 1,3,5-tris-(4-aminophenyl)triazine (TAPT). The structural and electronic effects of the different linkers on the electrochromic device performances were investigated and compared with theoretical DFT calculations. Due to the cationic nature of the polymer thin film that promotes fast ionic diffusion, they overall show superior electrochromic performance in aqueous conditions. This is reflected from the rapid switching speed (<10 s), high coloration efficiency (maximum CE of 836.08 cm2/C), low potential bias and multichromic properties with significant transmittance difference (ΔT%). The constructed V-iCOP electrochromic devices with quasi-solid hydrogel electrolyte exhibit long term cycling stability (92.5% at 2000 cycles). The acceptor-acceptor moiety of V-iCOP3 with the triazine linker exhibited the largest color modulation and highest coloration efficiency. This work showcases the versatile properties and promising molecular design prospectus of V-iCOP thin films for electrochromic devices. Viologen, known for its electrochromic properties, has been extensively studied for applications in electrochromic devices (ECDs) due to its tunable color changes and high transmittance contrast. This study explores the synthesis of viologen-based two-dimensional ionic covalent organic polymers (V-iCOPs) to address stability issues in ECDs. Researchers synthesized three V-iCOPs using the Zincke reaction, incorporating different electron-donating, neutral, and withdrawing ligands to investigate their effects on optical and electrochemical properties. The V-iCOP films demonstrated stable cycling and distinct color changes across redox states, with V-iCOP3 showing the best performance due to its acceptor-acceptor moiety. The study highlights the importance of ligand selection in enhancing ECD performance, supported by density functional theory calculations. These findings suggest that V-iCOPs hold promise for next-generation electrochromic applications, offering a pathway for improved stability and efficiency in ECDs. This summary was initially drafted using artificial intelligence, then revised and fact-checked by the author. Viologen-based 2D ionic covalent organic polymer (V-iCOP) thin films are synthesized via the Zincke reaction using three different linkers with varying electron affinities. The structural and electronic effects of the different linkers on the electrochromic device (ECD) performances are investigated and compared with theoretical DFT calculations. Due to the cationic viologen moiety, the V-iCOP films promote fast ionic diffusion with photocurable hydrogel electrolyte, with the ECD resulting in rapid switching (<10 s), high coloration efficiency (up to 836.08 cm² C⁻¹), low operating bias, multichromic behavior, and long-term cycling stability (92.5% retention after 2000 cycles).