Dual-use synthesis of an asymmetric anthraquinone heptyl viologen (AQHV) for solution and gel-polymer electrolyte-based electrochromic devices

Abstract

Viologens, renowned for their exceptional color-switching properties, are widely utilized as electrochromic materials. However, their practical application is often limited by dimerization and aggregation of the reduced radical cations, which deteriorate redox reversibility and long-term device stability. In this work, we report the design and synthesis of a novel asymmetric viologen derivative, 1-((9,10-dioxo-9,10-dihydroanthracen-2-yl) methyl)-1′-heptyl-4,4′-bipyridinium di-tetrafluoroborate (denoted as AQHV(BF₄)₂), aimed at addressing these challenges. This viologen derivative incorporates a redox-active anthraquinone unit, enabling the formation of an intramolecular zwitterionic radical species (AQ⁻˙HV⁺˙) upon two-electron reduction. Electrochemical analyses, including cyclic voltammetry (CV) and differential pulse voltammetry (DPV), provide compelling evidence for this zwitterionic radical formation, which effectively suppresses the dimerization of viologen radical cations. The asymmetric architecture of AQHV introduces electronic and steric hindrance, minimizing π–π stacking and intermolecular interactions typically responsible for aggregation in symmetric viologens. Electrochromic devices (ECDs) were fabricated in both solution-type (s-ECD) and polymer gel-type (g-ECD) configurations, employing AQHV(BF₄)₂ as the cathodically colouring material and ferrocene (Fc) as the anodic redox mediator. The gel polymer electrolyte was prepared by in situ thermal polymerization of methyl methacrylate (MMA) with ethylene glycol dimethacrylate (EGDMA) as the cross-linker. The ECDs were characterized by in situ UV-visible absorption spectra and evaluated by dynamic transmittance curves. The s-ECD and g-ECD exhibited at 605 nm initial optical-transmittance changes (ΔT) of 60.2 and 62.2%, respectively, under the applied potentials of 0.0 (bleaching process) and 1.2 V (coloration process). The coloration efficiencies of s-ECD and g-ECD were calculated to be 49.0 cm² C⁻¹ and 60.9 cm² C⁻¹ at 605 nm, respectively. The s-ECD retained 96.4% of its initial ΔT after 1000 cycles of operation at 560 nm, while it was 98.8% in the case of g-ECD. The g-ECD showed a ΔT of 56.0% even after 5000 cycles (90.0% of its original ΔT was retained), when switched between 0 V and 1.2 V at 605 nm.