Novel Bipyridinium Ionic Liquids as Liquid Electrochromic Devices

Novel mono and dialkylbipyridinium (viologens) cations combined with iodide, bromide, or bis(trifluoromethanesulfonyl)imide NTf₂] as anions were developed. Selective alkylation synthetic methodologies were optimized in order to obtain the desired salts in moderate to high yields and higher purities. All prepared mono‐ and dialkylbipyridinium salts were completely characterized by ¹H, ¹³C, and ¹⁹F NMR spectroscopy, Fourier‐transform IR spectroscopy, and elemental analysis (in the case of NTf₂ salts). Melting points, glass transition temperatures by differential scanning calorimetry (DSC) studies, and decomposition temperatures were also checked for different prepared organic salts. Viscosities at specific temperatures and activation energies were determined by rheological studies (including viscosity dependence with temperature in heating and cooling processes). Electrochemical studies based on cyclic voltammetry (CV), differential pulsed voltammetry (DPV), and square‐wave voltammetry (SWV) were performed in order to determine the redox potential as well as evaluate reversibility behavior of the novel bipyridinium salts. As proof of concept, we developed a reversible liquid electrochromic device in the form of a U‐tube system, the most promising dialkylbipyridinium‐NTf₂ ionic liquid being used as the electrochromic material and the room‐temperature ionic liquid (RTIL), 1‐ethyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)‐imide EMIM]NTf₂], as a stable and efficient electrolyte.