Dual-Use Synthesis of an Asymmetric Anthraquinone Heptyl Viologen (AQHV) for Solution and Gel-Polymer Electrolyte-based Electrochromic Device

Abstract

Viologens, renowned for their exceptional color-switching properties, are widely utilized as electrochromic materials. However, a significant challenge arises from their propensity for dimerization and electrode aggregation in reduced states. These phenomena result in impaired redox switching, noticeably compromising the performance and long-term stability of electrochromic devices (ECDs). In this work, a novel asymmetric-functional derivative of viologen, 1-((9,10-dioxo-9,10-dihydroanthracen-2-yl) methyl)-1'-heptyl-4,4'-bipyridinium di-tetrafluoroborate (denoted as AQHV(BF4)2), was synthesized to confront the dimerization issue in viologen-based ECD. The redox-active anthraquinone group was conceived to mitigate the dimerization reactions by forming zwitterionic radical ions through a two-electron reduction process. The solution-type (s-ECD) and polymeric gel-type (g-ECD) devices were fabricated utilizing the viologen part of AQHV(BF4)2 as the cathodically coloring material and ferrocene (Fc) as the anodic material or mediator. The polymeric gel electrolyte was synthesized by an in-situ thermal polymerization method, using methyl methacrylate (MMA) as the monomer and 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 (T) 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 and 60.9 cm2/C 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 T of 56.0% even after 5000 cycles (90.0% of its original T was retained), when switched between 0 V and 1.2 V at 605 nm.