Exploring the role of counterions in the electrochromic performance of redox-active viologen

Viologen, a dicationic organic family, has gained significant attention as an active electrochromic (EC) component that reveals bias-driven, reversible, and detectable color changes. Tuning the electrochromic properties of viologens through counterion selection represents a promising approach. However, a thorough study concerning the effects of various counterions remains nearly unexplored. The present work highlights the crucial roles played by three different counterions, namely, perchlorate (ClO₄^-, V₁), tetrafluoroborate (BF₄^-, V₂), and bis(trifluoromethanesulfonyl)imide (TFSI^-, V₃), by considering three ion-pair complexes formed via a post-synthesis counterion exchange in benzyl viologen. These three counterions differ significantly in size, polarity, and mobility, which in turn affects the performance of electrochromic devices. The EC efficacy of the viologens was investigated through integration with poly(3-hexylthiophene) (P3HT), where the synergy between viologen and P3HT is decisive in determining their performance. ECD₁, composed of V₁, showed a superior EC performance compared with those of ECD₂ and ECD₃, which contain V₂ and V₃, respectively. Notably, ECD₁ exhibited a significantly higher coloration efficiency (approximately 1038 ± 25.7 cm² C^-1) than ECD₂ (approximately 318 ± 19.9 cm² C^-1) and ECD₃ (approximately 421 ± 20 cm² C^-1). We infer that the bias-induced mobility of the counterions and electrostatic interactions between viologens and counterions are the influential factors controlling electronic communication, thereby affecting the EC performance.