Electrochromic Property of Adsorbed Carboxylated Viologen on Zinc Oxide
Authors: Yamaguchi Hiroto , Saito Erika , Uno Koya , Yamakado Ryohei , Okada Shuji , Yoshida Tsukasa 
Published: December 2020

Electrochromic displays (ECD) are non-emissive, reflective, non-volatile and energy-saving, as they are based on reversible color change by electrochemical redox reactions. Unlike the electrophoretic e-ink displays already used in book readers, which can only switch between two tones (black and white), ECD can produce gray tones. While the established inorganic WO3 EC device used as automatic antiglare mirror is monotonous, full color ECD can be envisioned by employing reversible color change of organic chromophore. However, most of solid thin films of organic dye molecules cannot achieve electrical conduction and associated ion exchange needed for near-complete redox.
A promising strategy is to use mesoporous inorganic electrode with adsorbed monolayer of organic dyes, just like that used in dye-sensitized solar cells. While sufficient color density can be assured by the virtue of high surface area, electrical conduction and ionic transport are also facilitated. The challenge is to find combination of the organic chromophore which undergo reversible color change, preferably in W/Y, W/M, W/C to achieve full color, and suitable electrolyte to prevent dye desorption from the electrode surface.
In this study, we have synthesized carboxylated viologen derivatives (N,N’-bis(2-carboxyethyl)-4,4’-bipyridinium dichloride (CE2V) and 3-[4-(N-methyl-4-pyridino)-1-pyridinio]carboxyethyl diiodide (CEMV)) (Fig. 1a), which are adsorbed onto mesoporous ZnO nanoparticulate electrode to test their electrochromism. Viologens are known to exhibit a sharp color change from colorless to deep blue upon its reduction to its cation radical that is stable and reversibly oxidized. In situ spectroelctrochemical measurements were performed to evaluate the capabilities of the newly developed systems.
CE2V was synthesized by carboxylation of 4,4’-bipyridine with acrylic acid and subsequent conversion to chloride with hydrochloric acid (yield 73%). CEMV was prepared by monomethylation of 4,4’-bipyridine with methyl iodide and subsequent carboxylation with acrylic acid (yield 46%). Mesoporous ZnO film (10 μm thick) prepared by doctor blading was soaked in ethanolic solution 500 µM of CE2V or CEMV for 3 hours. Transmission absorption spectra were measured in situ by Otsuka Electronics MDPC-7000 diode array spectrophotometer during cyclic voltammetry on the viologen modified ZnO electrodes in a 0.1 M tetrabuthylammonium hexafluorophosphate (TPAPF6) in acetonitrile under N2.
The amounts of CE2V and CEMV adsorbed on ZnO were 1.31×10-7, 7.28×10-8 mol cm-2, respectively, indicating superior adsorption stability of double anchor molecule. Both of them exhibited stable reversible redox in two-steps (Fig. 1b). Associated color changes were observed from colorless to purplish blue and yellow to purple for CE2V and CEMV, respectively, which were then bleached in the second reduction. Singly reduced cation radicals intensely absorb in the visible, but doubly reduced states are not colored. The oxidation back to the singly reduced state regenerated the color, which was again bleached on its oxidation back to the original dication state. The in situ spectra monitored for the reversible color change are shown for ZnO/CE2V electrode (Figs. 1c,d). The charges consumed in the primary reduction were calculated from the integral of the peak areas to determine the redox active fractions as 6.83×10-8 and 4.48×10-8 mol cm-2 for CE2V and CEMV, respectively, which correspond to 52 and 62% of the totally adsorbed amounts.
Although the spectra of cation radicals of viologens developed in the present study are bit too broad to serve as white to “cyan” chromic device, the relatively fast and stable reversible electrochromism has been achieved by combination of porous inorganic electrode and adsorbed monolayer of organic chromophore.

Figure 1