Molecular engineering of viologens with strong electronegative groups and asymmetric substitution for electrochromic devices with high cycling stability

Viologen-based materials are among the most promising organic electrochromic systems due to their high optical contrast, fast redox response, and molecular tunability. However, their practical deployment is fundamentally constrained by the dimerization and irreversible side reactions of viologen radical cations (V +• ), which lead to rapid performance degradation and limited cycling stability. Here, we propose an asymmetric molecular design strategy that synergistically integrates electron-withdrawing inductive effects with moderate steric hindrance to fundamentally suppress radical cation dimerization. Using site-selective functionalization of 4,4′-bipyridine, a novel asymmetric viologen derivative, 4-( p -cyanobenzyl)-4′-( p -fluorobenzyl)-4,4′-bipyridinium (FCBV), is rationally designed and synthesized.The combined effects of dual electronegative substituents and benzyl-induced steric hindrance not only inhibit V +• aggregation but also promote directional electron transfer, resulting in simultaneously enhanced stability and kinetics. Electrochromic devices based on FCBV deliver a high optical contrast exceeding 60 % in the 500–700 nm range, an exceptional coloration efficiency of 155.8 cm 2 ·C -1 , and outstanding durability, retaining 98.9 % of their initial optical modulation after 20,000 switching cycles. The devices are further demonstrated in smart windows with effective light–thermal regulation and in adaptive anti-glare rearview mirrors, confirming their practical applicability in building and automotive optoelectronic systems.