Chapter 12:Viologens-based Electrochromic Materials and Devices

Viologens, or 1,1′-disubstituted-4,4′-bipyridiniums, are electrochromic materials with dramatic color change when reduced from transparent dication to colored radical cation. Many applications based on viologens have been developed, including anti-glare mirrors or displays in which viologens could be used to offer optical contrast. Additionally, by substituting different groups on bipyridiniums, the optical characteristics of viologens upon reduction could be tuned, such as alkyl for blue or aryl for green, which expands the versatility of viologens for displays. In an electrochromic device, viologens could be classified into three types of electrochromes depending on the environment or mechanism: type-1 for viologens in non-aqueous solvents, type-2 for precipitated viologens at radical cation state, and type-3 for immobilized viologens. Among them, type-3 viologen electrochromes are most known for their stability improved by immobilizing viologens, and therefore they were widely adopted in commercial electrochromic devices. The immobilization of viologens could be achieved by either anchoring viologens on substrates by direct bonding, polymerizing viologens, or introducing gel or solid-state electrolytes.