Linear polyether-containing electrochemichromic solutions and related devices

#The present invention relates to electrochemichromic solutions and devices based thereon. Such solutions are well-known and are designed to either color or clear, depending on desired application, under the influence of applied voltage.

#Such devices have been suggested for use as rearview mirrors in automobiles such that in night driving conditions, application of a voltage would darken a solution contained in a cell incorporated into the mirror (U.S. Pat. No. 3,280,701, Oct. 25, 1966). Similarly, it has been suggested that windows incorporating such cells could be darkened to block out sunlight, and then allowed to lighten again at night. Electrochemichromic cells have been used as display devices and have been suggested for use as antidazzle and fog-penetrating devices in conjunction with motor vehicle headlamps (British Patent Specification 328017, May 15, 1930).

#U.S. Pat. No. 4,090,782 to Bredfeldt et al, U.S. Pat. No. 4,752,119 to Ueno et al (Jun. 1988), Chemical Abstract 86:196871c, 72-Electro. Chemistry, Vol. 86, 1977, I.V. Shelepin et al in Electrokhimya, 13(3), 404-408 (Mar. 1977), O. A. Ushakov et al, Electrokhimya, 14(2), 319-322 (Feb. 1978), U.S.S.R. Patent 566863 to Shelepin (Aug. 1977), U.S. Pat. No. 3,451,741 to Manos, European Patent Publication 240,226 published Oct. 7, 1987 to Byker, U.S. Pat. No. 4,902,108 to Byker, U.S. Pat. No. 3,806,229 to Schoot et al, U.S. Pat. No. 4,093,358 to Shattuck et al, European Patent Publication 0012419 published June 25, 1980 to Shattuck and U.S. Pat. No. 4,139,276 to Clecak et al all disclose electrochemichromic solutions of anodic and cathodic electrochromically coloring components which provide self-erasing, high color contrast, single compartment cells. Such anodic and cathodic coloring components comprise redox couples selected to exhibit the following reaction: ##STR1## The redox couple is selected such that the equilibrium position of the mixture thereof lies completely to the left of the equation. At rest potential, the anodically coloring reductant species RED.sub.1, and the cathodically coloring oxidant species OX.sub.2 are colorless. To cause a color change, voltage is applied and the normally colorless RED.sub.1 is anodically oxidized to its colored antipode OX.sub.1, while, simultaneously, OX.sub.2 is cathodically reduced to its colored antipode, RED.sub.2. These cathodic/anodic reactions occur preferentially at the electrodes which, in practical devices, are typically transparent conductive electrodes. Within the bulk of the solution, the redox potentials are such that when RED.sub.2 and OX.sub.1 come together, they revert to their lower energy form.

#FIG. 1 is a cross-sectional view of an electrochemichromic cell;

#FIG. 2 is a graph of leakage current (Amps/m.sup.2) versus thickened solution viscosity using polyethylene oxide as the thickener; and

#FIG. 3 is a graph of leakage current (Amps/m.sup.2) versus thickened solution viscosity using polymethylmethacrylate as the thickener.

#In the preferred embodiment, the electrochemichromic solutions of the present invention include a redox chemical pair which colors in the presence of an applied voltage and which bleaches to a colorless condition in the absence of an applied voltage dissolved in an electrochemichromic solvent system that contains a linear polyether such as polyethylene oxide which is soluble in the electrochemichromic solvent system.

#The electrochemichromic solutions of the preferred embodiment can utilize conventional or equivalent redox systems such as the viologens combined with phenazines, diamines or benzidines, dissolved in a suitable solvent or mixture of solvents that are known to be useful as solvents in the electrochemical arts. Such solvents include acetonitrile, dimethylformamide, propylene carbonate, and gamma-butyrolactone. Particularly preferred solvents include: 3-hydroxypropionitrile (HPN), 3,3'-oxydipropionitrile (ODPN), glutaronitrile (GNT), 3-methylsulfolane (MS), 2-methylglutaronitrile (MGNT) and 2-acetylbutyrolactone (ABL) or mixtures thereof in which linear polyethers are sufficiently soluble. Electrolytes may optionally be used.

#Viologens are preferred cathodic materials for the redox pair. Methylviologen, ethylviologen, benzylviologen and heptylviologen are all satisfactory, with 0.025 to 0.05 molar solutions of methylviologen and ethylviologen being preferred. Higher concentrations up to the solubility limits are also operable. In the structural formulas set forth below, X.sup.-- represents the anion of the viologen salt. Various anions are disclosed in the literature, though we have discovered that the most preferred anion is hexafluorophosphate (PF.sub.6.sup.--) because it surprisingly enhances viologen solubility. This preferred embodiment is the subject of a copending U.S. patent application entitled ELECTROCHEMICHROMIC VIOLOGENS. ##STR2## Hexafluorophosphate counter ion is listed below with other acceptable, though less preferred, counter ions for use on the viologens: