Electrochromic (EC) materials and their correspondent EC devices (ECD) that change colors on application of voltages have attracted great research interests in the past several decades since they have commercial applications as smart windows, auto dimming rearview mirrors and static displays.
Viologen has been widely investigated as an organic EC material since it has excellent redox properties accompanied with distinct color changes. However, most viologens only show color change from transparent or translucent yellow to blue. In order to make viologens more colorful, we designed and synthesized novel materials by inserting thiophene moiety into bipyridine. These chromophores are expected to have abundant color changes. Further, these new viologen-conjugates possess phosphoric acid groups, which could help them anchor on the titanium dioxide (TiO2) more firmly.
A layer of nanostructured TiO2 film (4.0 μm thick, TiO2 particles are about 20 nm large) was coated on conducting F-doped tin oxide (FTO) glass, and the new viologen-conjugate was adsorbed onto TiO2 particles by chemisorption. The films of new viologen-conjugate/TiO2/FTO were characterized by using cyclic voltammetry. Color changes from transparent at 0 V to purple at -0.8 V and rust red at -1.1 V were observed. Next, Prussian blue (PB) was electrodeposited onto FTO glass and used as counter electrode. And then ECD was assembled by combining the working electrode of new viologen-conjugate/TiO2/FTO, counter electrode of PB/FTO, and gel electrolyte. The ECD was characterized by using UV-vis spectrophotometer and electrochemical analysis system. The ECD showed a broad transmittance contrast over 60% at +2.5 V and -2.5 V.
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