SYNTHESIS AND ELECTROCHROMIC PROPERTIES OF ASYMMETRIC VIOLOGEN DERIVATIVES

Abstract

Viologens, based on the 4,4' bipyridine scaffold, are promising functional materials due to their ability to reversibly change color under applied electrical potential. This electrochromic property makes them suitable candidates for applications in devices such as smart windows, electronic paper, and display technologies. The synthesis of asymmetric viologens provides opportunities to tailor their physicochemical properties, introduce additional functionalities, and enhance the long-term stability of both the compounds and the devices in which they are incorporated. However, the synthesis of asymmetric viologens remains a significant challenge, largely due to the electronic nature of the bipyridine core. In this study, four viologen derivatives, designated as V-COOH, V-OH, Me-V-COOMe, and Bn-V-COOH, were synthesized and employed as the active components in electrochromic devices. The compounds and resulting devices were characterized by using proton nuclear magnetic resonance spectroscopy, cyclic voltammetry, and ultraviolet-visible spectroscopy to investigate their optical behavior in both bleached and colored states. Key parameters such as maximum transmittance, optical contrast, and contrast ratio were evaluated. The results indicated that viologens with a second substituent exhibited superior electrochromic performance. Among these, Bn-V-COOH showed the lowest maximum transmittance in the colored state at 26.22 % and the highest contrast ratio of 2.26, while Me-V-COOMe demonstrated the highest optical contrast at 41.44 %. In contrast, the viologens lacking a second substituent showed markedly lower values across all optical parameters. These findings demonstrate that the presence of a second substituent is critical foroptimizing the electrochromic properties of viologen-based materials.