Electrochromic Viologen-Based 2D Cationic Porous Organic Polymer with Tunable Redox Potential

Viologen is a well-studied electrochromic material owing to its different colored states between the three reversible redox states: V²⁺ (dication) ↔V^+× (radical cation)↔V⁰ (neutral). As a small soluble organic molecule, viologen is usually incorporated directly into a liquid or gel electrolyte for the ease of processability. However, such electrochromic devices suffer from instability resulting from radical-radical dimerization, unwanted oxidation and slow switching speed. Immobilizing the viologen as a polymer thin film is a potential viable solution which has not been explored extensively. Additionally, viologen-based porous organic polymers (V-POP) with higher dimensionality is promising for their various tunable characteristics (porosity, redox potential, and color) owing to the programmability of their composition. In this work, 3 different viologen-based 2D cationic porous organic polymer thin films (V-POP1, V-POP2, V-POP3) 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). By using different linkers, tuning of the two redox potentials were investigated and compared with theoretical DFT calculations. Due to the cationic and porous nature of the polymer thin film that promote fast ionic diffusion, they overall showed superior performance in an aqueous condition. This was reflected from the rapid switching speed (<5s), high coloration efficiency (>100 cm²/C), low potential bias and various colour changes with significant DT%. This work has showcased the versatile properties of V-POP thin films, adding a new prospect in designing viologen thin film-based electrochromic devices.