A semi-solid hydrogel state electrochromic device based on an asymmetric disubstituted viologen containing a quaternary ammonium group with enhanced electrochromic stability

This work aims to investigate the electrochromic behavior of an asymmetric disubstituted viologen containing a quaternary ammonium group in semi-solid hydrogel state electrochromic devices. The asymmetric disubstituted viologen BTMAPP³⁺3Br⁻ was synthesized as the electrochromic material, and two symmetric disubstituted viologens (DTMAPP⁴⁺4Br⁻ and DBP²⁺2Br⁻) were synthesized as the control electrochromic materials. Polyacrylamide hydrogel state ECDs based on three electrochromic materials were assembled via in situ photoinitiated polymerization of acrylamide under UV light, and their electrochemical and electrochromic performances were investigated. They showed similar electrochemical performance. They also required the same working voltage and achieved a similar color change from a colorless bleached state to a blue-purple colored state. It is noteworthy that the ECD based on BTMAPP³⁺3Br⁻ showed better cycling stability (retaining of the initial optical contrast after 16 000 s monitored: 95.6% versus 70.7% and 78.5%) and a higher coloration efficiency (115 cm² C⁻¹versus 96 cm² C⁻¹ and 103 cm² C⁻¹) compared to the PAM ECDs based on two symmetric disubstituted viologens. The results indicate that an asymmetric disubstituted viologen containing a quaternary ammonium group can enhance the cycling stability of the gel-state ECD through suppressing the formation of radical cation dimers. The all-in-one semi-solid hydrogel state ECDs based on BTMAPP³⁺3Br⁻ with advantages of easy-to-make process, good cycling stability, low operating voltage, high coloration efficiency and low power consumption offer a new choice for electronic labels, solar cell powered displays, smart windows, etc.