Abstract
Within various classifications of electrochromic compounds, viologen compounds have been identified as leading candidates due to their advantageous properties, such as fast switching kinetics, rapid and prominent color changes, and low operating voltage parameters, all of which contribute to improved energy efficiency and device longevity [1]. However, traditional symmetric viologens suffer significant problems such as limited color palettes, environmental sensitivity, and poor cycling endurance, limiting their broad use in advanced applications [2]. In order to overcome these drawbacks, asymmetric viologens (AVs), which include two different substituents on the bipyridinium scaffold (Benzyl-4,4′-dipyridyl-R (Bn-V-R) architecture), were successfully found as a transformative subclass with enhanced electrochromic properties. Their structural asymmetry allows for fine-tuning of electrical configurations, and wider optical absorption profiles, resulting in more brilliant and varied color modulation [3]. Importantly, rational design strategies—particularly the use of electron-drawing substituents on aromatic or benzyl moieties—improve redox ability, optical visibility, and long-term reliability during operation. Microwave irradiation was used for the synthesis of asymmetric viologens, providing an efficient and fast approach for both viologens and polyviologens. The approach involved three steps: initial aryl substitution using the recognized two-step Zincke reaction, subsequent alkylation with the appropriate alkyl halide, and optional first-step alkyl attachment in a nonpolar solvent to produce monoalkyl intermediates. Ion chromatography and ¹H/¹³C NMR were used to validate structural integrity. Two methodologies were used in device fabrication: pre-anchoring and electrochromic gel creation. UV-Vis spectroscopy was used to assess both their bleached and colored states. Bn-V-COOH had the lowest transmittance in the colored state (Tmax = 26%) and a higher contrast ratio (CR = 2.26), while Me-V-COOMe had the largest optical modulation (ΔT = 41%). This comprehensive study focuses on the significant structure-property interactions that regulate the performance of asymmetric viologens, highlighting their promise as next-generation candidates for robust, energy-efficient, and diverse electrochromic systems. Acknowledgements This research was funded by the project AP23489476 "Design and development of multi-colored electrochromic films for energy storage applications" from the Ministry of Science and Higher Education of the Republic of Kazakhstan. References [1] Nuroldayeva, G.; Balanay, M.P. Flexing the spectrum: Advancements and prospects of flexible electrochromic materials. Polymers 2023, 15 , 2924. [2] Yin, T.; Duanmu, J.; Liu, L. Viologen-based aqueous organic redox flow batteries: Materials synthesis, properties, and cell performance. J. Mater. Chem. A 2024, 12 , 15519–15540. [3] Balamurugan, G.; Park, J.S. Metallo-terpyridine modified asymmetric viologen exhibiting remarkable optical memory effect in single-layered electrochromic devices. Electrochim. Acta 2021, 382 , 138308.