Multi-Color Electrochromic Devices Based on Phenyl and Heptyl Viologens Immobilized By an UV-Cured Polymer Electrolyte

Over the last few decades, viologens have been applied for electrochromic devices (ECDs). However, viologens are soluble in water or polar organic solvent and solution-type viologens suffer from leakage problem. To tackle this issue, many strategies have been proposed for preparing immobile viologens. In this study, an UV-crosslinked ethoxylated trimethylolpropane triacrylate (ETPTA) polymer network and succinonitrile (SN) were employed to prepare a solid polymer composite electrolyte (PCE) matrix [1] at room temperature to immobilize heptyl viologen (HV) and phenyl viologen (PV) at the electrode surface. PV exhibits good EC property, while aggregation of PV occurs immediately utilizing conventional liquid or gel electrolyte due to strong π-π stacking. Herein, an all-solid-state ECD is proposed to solve aggregation issue of PV. Moreover, a multi-color ECD is fabricated by incorporating HV, PV, and PCE. By changing the operating potential, the ECD exhibits green, blue, gray color at 0.6 V, 0.8 V, and 1.0 V, respectively (Fig. 1). The first set of redox peaks in Fig. 1 is due to redox reaction of PV, and the second set of redox peaks is the overlap of first reduced state of HV and second reduced state of PV. Fig. 2 shows the dynamic transmittance change of the proposed ECD between 0 and 1 V. The increment in transmittance at 1 V is due to the second reduced state of PV and HV at 438 nm and 610 nm, respectively. The color-tuning of the proposed ECD is controlled by the ratio of HV and PV molecules immobilized on the electrode surface, which can be  explained by the coloration efficiency and cyclic voltammogram of the ECD.