Viologen-functionalized poly(ionic liquids): Spectroelectrochemical and QCM-D studies

Abstract

Ionic Liquids (ILs) and poly(Ionic Liquids) (PoILs) have fully tunable properties due to their different ion combinations as well as cation or anion functionalization, which results in smart materials with endless possibilities. In this study, a solid electrochromic PoIL was synthesized using a novel functionalizable and polymerizable IL (1-propyl-3-vinylimidazolium chloride bromide) and a viologen (4,4′-bipyridine) that reacted to form a monomer. Its subsequent polymerization resulted in a mechanically stable, highly ionically and electronically conductive electrochromic polyelectrolyte with a thermal decomposition temperature higher than 309°C and a glass transition temperature of 13°C. Spectroelectrochemical studies of the polymer deposited as a thin film over a transparent conductive substrate demonstrated a reversible redox process with reduction and oxidation potentials of −0.45 and −0.3V, respectively, which was accompanied by a reversible color change from colorless (oxidized state) to purple (reduced state) with an optical contrast of 19% at 525nm, a switching time of nearly 20s and a coloration efficiency of 60cm2C−1. Additionally, the film was repetitively switched from colorless to purple and began to lose electroactivity after approximately 4h and 400cycles. However, electroactivity was still observed after 16h or 1600cycles. Moreover, quartz crystal microbalance with dissipation monitoring indicated that the polymer charge compensation process was accompanied by a huge viscoelastic change in the film, as demonstrated by the spread of the harmonics and important changes in dissipation.