IE904194A1 - Electrochemichromic viologens - Google Patents

Abstract

The specification discloses electrochemichromic solutions and devices based thereon in which viologen 5 solubility is surprisingly enhanced by using hexafluorophosphate (PFg~) or hexafluoroarsenate (AsFg”) or mixtures thereof as the counterion.

Description

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 (United States Patent 3,280,701, October 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).

United States Patent 4,090,782 to Bredfeldt et al., United States Patent 4,572,119 to Ueno et al. (June 1988), Chemical Abstract 86:196871c, 72-Electro. Chemistry, Vol. 86, 1977, i.v. Shelepin et al. in Electrokhimva. 13(3). 404-408 (March 1977), O.A. Ushakov et al., Electrokhimva. 14 (2) . 319-322 (February 1978), U.S.S.R. Patent 566863 to Shelepin (August 1977), United States Patent 3,451,741 to Manos, European Patent Publication 240,226 published October 7, 1987 to Byker, United States Patent 3,506,229 to Schoot et al., United States Patent 4,093,358 to Shattuck et al., European Patent Publication 0012419 published June 25, 1980 to Shattuck, and United States Patent 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 anodid and cathodic coloring components comprise redox couples selected to exhibit the following reaction: “D1 + 0X2 ^voltage > OX1 + RED2 (Colorless) (Colored) (Low energy pair) (High Energy Pair) 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^, and the cathodically coloring oxidant species 0X2 are colorless. To cause a color change, voltage is applied and the normally colorless RED^ is anodically oxidized to its colored antipode OX^ while, simultaneously, OX2 is cathodically reduced to its colored antipode, RED2· 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 RED2 and OX^ come together, they revert to their lower energy form.

This means the applied potential need only suffice to drive the above reaction to the right. On removing the potential, the system reverts to its low energy state and the cell spontaneously self-erases.

Such redox pairs are placed in solution in an inert solvent. Typically, an electrolyte is also added.

This solution is then placed into a relatively thin cell, between two conductive surfaces. In most applications, at -2IE 904194 least one of the conductive surfaces comprises a very thin layer of a transparent conductor such as indium tin oxide .(ITO), doped tin oxide or, doped js^Lnc oxidq deposited on a glass substrate so that the cell is transparent from at least one side. If the device is to be used in a mirror, the second surface is typically defined by a relatively thin layer of transparent conductor such as indium tin oxide, doped tin oxide or doped zinc oxide deposited on another glass substrate, which is silvered or aluminized or otherwise reflector coated on its opposite side. In the case of solar control windows, the second glass substrate would of course not be silvered on its opposite side so that when the redox pair is colorless, the window would be entirely transparent.

In some applications, as for example as outside rearview mirrors or as windows, such electrochemichromic devices must withstand substantial temperature extremes.

This creates a problem for the viologens, which are often recommended as the cathodically coloring ingredient. It is desirable when formulating electrochemichromic solutions that utilize viologens as the cathodically coloring species that the viologen concentration be high so that devices filled with said viologen based electrochemichromic solutions color deeply and rapidly when voltage is applied. Viologen solubility in typical electrochemichromic solutions is sufficiently marginal that there is a tendency for the viologens in concentrated solutions to precipitate out when the temperature drops. It would be desirable to increase viologen solubility in such electrochemichromic solutions.

SUMMARY OF THE INVENTION In the present invention, it has been surprisingly -3IE 904194 found that viologen solubility can be strikingly increased J using hexafluorophosphate or hexa.fluoroarsenate as the viologen counterion. This enables one to increase the viologen concentration in the solution and obtain more deeply coloring solutions, without havirig to worry about excessive precipitation of the viologen as the environmental temperature decreases. ' These and other objects, advantages and features of the invention will be more fully understood and appreciated by reference to the written specification.

BRIEF PE5CRIPTI9N QF THS DRAWING Fig. 1 is a cross-sectional view of an electrochemichromic cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment, viologen hexafluorophosphate or hexafluoroarsenate is dissolved in a suitable solvent along with an anodically coloring compound and preferably an electrolyte. UV stabilizers, thickeners and other additives may optionally be included.

Viologens which are useful in electrochemichromic solutions have the following general formula: R—^N^H^N—R 2X” where R is an aryl or alkyl group such as methyl, benzyl or heptyl. X shown in the above structural formula is the so-called counterion to the positively charged viologen ion. We have found in the present invention that by using hexafluorophosphate (PFg“) or hexafluoroarsenate (AsFg”) as the counterion, the viologen solubility in electrochemichromic solvents can be strikingly increased. -4IE 904194 Suitable anodic coloring compounds for use in I conjunction with the cathodically. coloring viologens include DMPA - 5,10-dihydro-5,10-dimethylphenazine -Λ DEPA - 5,10-dihydro-5,10-diethylphenazirie DOPA - 5,10-dihydro-5,10-dioctylphenazine R=CH R= TMPD - Ν,Ν,Ν',N'-tetramethylphenylenediamine Npt TMBZ - N,Ν'-tetramethylbenzidine CH CH CH CH S.

•S TTF - Tetrathiafulvalene Most preferred is a .025 molar solution of 5,io-dihydro5, 10-dimethylphenazine (DMPA).

Numerous electrolytes can be used in the present invention. Some which are often suggested for electrochemichromic cells and which is acceptable in accordance with the preferred embodiment of the invention are a tetrabutylammonium hexafluorophosphate and tetrabutylammonium hexafluoroarsenate. We prefer a .025 molar solution.

UV stabilizers such as Uvinul™ 400, Cyasorb 24™, Tinuvin 327™, Uvinul D-49™, Uvinul D-50™, Uvinul M-40™, Tinuvin 328™, Tinuvin 1130™, Tinuvin P™ and similar commercially available UV stabilizers can also be used in the solutions of the present invention at concentrations up to 15% weight/volume or thereabouts. -5_z\nn _ivii It The best mode electrochemichromic solution contemplated for practicing the invention comprises a solvent containing from about . 0’2M to about . 14M, most preferably .075M molar methylviologen hexafluorophosphate or » hexafluoroarsenate, from about .02M to about .14M, most preferably .075M molar tetrabutylammonium hexafluorophosphate or hexafltioroarsenate, and from about .02M to about .14M, most preferably .075M molar ,10-dihydro-5,10-dimethylphenazine (DMPA).

Fig. 1 illustrates a typical electrochemichromic cell 1 into which solutions of the present invention are typically filled. Cell 1 comprises a pair of glass plates 10 and 11 each coated on its inwardly facing surface with a half wave indium tin oxide (ITO) coating 12 of about 15 ohms/sguare sheet resistance. Plates 10 and 11 are separated by peripheral seal 13 so that the interior of the cell has a thickness of 150 microns. Cell 1 is sealed at its perimeter by peripheral seal 13. Seal 13 comprises an epoxy material, to which 150 micron diameter spacers are added, and silk-screened to a thickness of about 150 microns. Glass beads are used as spacers. As shown, cell 1 is intended to be used as a mirror, and thus the rear surface of glass plate 11 is coated with a silver reflector layer 14. If the device were used as a window, layer 14 would be deleted. The conductive indium tin oxide layers 12 are connected to electrical terminals 15 and 16 so that a voltage can be established across a solution located between plates 10 and 11 in cell 1.

In Table 1 below, the solubility of methylviologen employing various alternative counterions has been determined in various solvents suggested for use in -6IE 904194 electrochemichromic solutions. Propylene carbonate (PC) is a commonly recommended solvent.'for use in electrochemichromic solutions. We have also used in Table 1 2- acetylbutyrolactone (ABL), 3,3’-oxydipropionitrile (ODPN), and 3-hydroxypropionitrile (HPN), and in Table 2 3- methylsulfolane (MS) and 2-methylglutaronitrile (MGNT), all of which are the subject of a separate copending patent application, Serial No. 07/443,113 (attorney docket Donnelly P-456) entitled HIGH PERFORMANCE ELECTROCHEMICHROMIC SOLUTIONS AND DEVICES THEREOF. Other solvents have been suggested in the art for use in electrochemichromic solutions. These are also applicable in the broader aspects of the present invention. The counterions compared to hexafluorophosphate and hexafluoroarsenate were perchlorate (C1O4~), tetrafluoroborate (BF4~), trifluoromethylsulfonate (CF3SO3~) and chloride (Cl-) .

X Ο Ο < · Φ C 3 ft οε ο ο Ο Ο X ο • • • • X *- VI 04 rv rv • -¼ < Χ> ο VI Ό Ό ** — Ό -4 ο — “ΤΊ Ο -tl rt ο Μ —» Μ* -* #~κ σ Χ« Ο 04 04 S <✓» • • • • Η Ο ο ** -4 -4 < <— φ C π σ Φ — 3 χ*\ a rt —. ο Ο X ο • • ί <— X Ο < νι Ο Ο Ο • < <» Ο ο -4 «Α *— η χ* C ο — -η 0» 3 ·*, rt 04 X* rt rv Λ «Λ χ-* Φ X X* 04 X Ο 3 φ • X 04 ft Ο ο Ο rv < Ο y W 3 X ω r— < a ο ο ο ο ο _* X ο f- ί- • • • • <“ *— X ο ο 04 -4 04 ο • χ < ** 4 (Ο χ* fV X» 04 —» *— > V» ♦ φ — 0 Ο 3 ft X» ·« -Λ X* -» VI <Χ ο *-» Ό ο Ο Ο -4 X X •η —* X* —· S2 Γ\Ι Ο Ο ΟΟ-4 rv rv νι -* in ιη ο χ» Ο Ο «-* X» Μ Ο -4 < w 03 Ο η Ο X* *·* Γν X* 04 ο» • · » · X -» IV -4 Ο >> VI VI ο ** *— X Ν < V» — ο χ*· ο ο ο X *> < > Γ> η *-* ο a Ό ST οτ τ ® 904194 Table 1 reports solubility for the various ·,' *C r viologens both in terms of moles per liter and percent weight/volume. It can be, seen that methylviologen using either hexafluorophosphate or hexafluoroarsenate as the counterion is strikingly more soluble at 25* C. than any of the other methylviologen-counterion combinations. This is true regardless of the particular solvent used. In the commonly recommended propylene carbonate solvent, the solubility of methylviologen hexafluorophosphate is about twice as great as is the solubility of methylviologen tetrafluoroborate. That contrast is even more striking in the solvents ODPN and ABL.

The practical benefit that results from use of electrochromic viologens that utilize hexafluorophosphate or hexafluoroarsenate as the counterion can be appreciated by reference to Table 2. Electrochemichromic window cell 1 (Fig. 1) was used for this data. Cell area was about 110 cm and thickness was about 150 microns. Sheet resistance of the ITO transparent conductors used in the cell was 15 ohms per square. The electrochemichromic solutions used the solvents indicated in column 1 of Table 2. The anodic electrochemichromic species was ,10-dihydro-5,10-dimethylphenazine and the cathodic electrochemichromic species was methylviologen utilizing the counterions indicated in column 2. The concentration (molarity) of the cathodic and anodic species (individually) in the solution is given in column 3. The % transmission (measured at the cell center and using Standard Illuminant A as a light source and a detector that reproduced the eye's photopic response) at zero applied potential is given in column 4. Column 5 gives the % transmission level to which the cell dims when 1 volt is applied across the ITO transparent conductors that sandwich the electrochemichromic solution in the window cell. Column 6 reports the status of the solution after storage for several days at room temperature. Column 7 reports'the status of the solution i, after cold storage at -20’ C. for several days. ”No ppt” indicates that no precipitate was noted after several days storage at the indicated temperatures. Slight ppt indicates only slight precipitation was seen after storage.

More ppt indicates that more precipitation was seen when stored at -20’ C. relative to when that cell filled with that same solution was stored at room temperature. Very slight ppt indicates that only very slight precipitation was seen after storage. i i r if i xi 041/1 IC T3 > VI I DOT Ο Ό II II II It It It II II II ζ ζ *ο rv 04 04 04 rv 0 0 0 . . * . “) . X X -) 3 3 04 ζ O 0 £0 Q> η 0 0 “ X *D n -t» 3“ rt rt · Ο. < 0 — — 3“ 3“ Ο “1 — <-r > Ό Γ) c c Ο X X X O 0 (ft ΊΠ — ο ο “1 — — X X □ >— TO* © “1 “I α> 0 CT O' X* Ο ο rr — C —Ό c ο *ο 0 C — Ό “) O Γ* -ι 3* rr -*» -1 O Q> X (ft ο 0)0 0*0 “Ί -1 0 (ft -» <—-o — σ O 3 Ό O fi) — O ο &> 3* 3 3 0 3 3 0) rt Q) — 03 — 0» o 0 rt rr — rr rt r+ 0 3 rr -) 0 O ι 3 -. r- 0 o o © 0 — 0 • .

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Byker in European patent publication 240,226 published October 7, 1987 teaches use of viologens combined with a counterion selected from chloride, bromide, iodide, BF4”, PFg”, AsFg”, Cl04~ and N03”. In his teaching, he fails to distinguish PFg”, and AsFg” from the six other counterions he cites. Byker fails to appreciate the substantial increase in viologen solubility which can be achieved using these specific counterions. The highest viologen concentration even suggested by Byker is .1 molar and the highest concentration actually reported by Byker is .05 molar. As indicated in Table 2 of our invention, practical benefit, such as dark coloring windows and their use at low temperatures, can be achieved when hexafluorophosphate or hexafluoroarsenate are used as counterions. -12IE 904194 Also, we find that these benefits are found with whatever viologen hexafluorophosphate or h®^afluoroarsenate is combined with as counterion. As shown in Table 3, higher solubility was achieved.'using hexafluorophosphate and hexafluoroarsenate regardless of whether-methyl, benzyl or ethylviologen was used.

Table 3 Solubility (moles/liter) MVPFg EVPFg BVPFg HVAsFg EVAsFg BVAsFg PC 0.29 0.99 0.83 1.03 1.0 1.25 HPN 0.50 0.45 0.36 0.34 0.63 0.42 ABL 0.35 0.56 0.50 0.72 0.71 1.00 ODPN 0.29 0.35 0.42 0.34 0.56 0.62 MVPFg = Methylviologen hexafluorophosphate EVPFg = Ethylviologen hexafluorophosphate BVPFg = Benzylviologen hexafluorophosphate MVAsFfi = Methylviologen hexafluoroarsenate EVAsFg = Ethylviologen hexafluoroarsenate BVAsF& = Benzylviologen hexafluoroarsenate o Of course, it is understood that the above is merely a preferred embodiment of the invention and that various changes and alterations can be made without departing from the spirit and broader aspects thereof.

Claims (5)

1. The embodiments of the invention in which an exclusive property or privilege is claimed,are defined as follows. -11 An electrochemichromic solution· comprising: a solvent; a redox chemical pair in solution in said solvent which colors in the presence of an applied voltage and which 5 bleaches to a colorless condition in the absence of an applied voltage, one of said redox pair comprising one of a viologen hexafluorophosphate and viologen hexafluoroarsenate salt and mixtures thereof. -22 The electrochemichromic solution of claim 1 in which said viologen salt comprises methylviologen. -31 The electrochemichromic solution of claim 2 in which the viologen salt is in solution concentration of greater than 0.05M. -42 The electrochemichromic solution of claim 1 in which the viologen salt is in solution concentration of greater than 0.05M. -52 The electrochemichromic solution of claim 4 in which the solvent includes at least 25% by volume of one of the group consisting of propylene carbonate, 3-hydroxypropionitrile, 3,3’-oxydipropionitrile, 5 2-acetylbutyrolactone, 3-methylsulfolane,

2. -methylglutaronitrile, and mixtures thereof. -14IE 904194 -61 The electrochemichromic solution of claim 4 wherein the solvent is substantially entirely 2- acetylbutyrolactone. -71 The electrochemichromic solution of claim 4 wherein the cathodic member of said redox chemical pair in solution is selected from the group consisting of methylviologen hexafluorophosphate and hexafluoroarsenate, 5 ethylviologen hexafluorophosphate and hexafluoroarsenate, and mixtures thereof and the anodic member is 5,10-dihydro-5,10-dimethylphenazine. -81 The electrochemichromic solution of claim 1 in which the solvent includes at least 25% by volume of one of the group consisting of propylene carbonate,

3. - hydroxypropionitrile, 3,3'-oxydipropionitrile,

4. 5 2-acetylbutyrolactone, 3-methylsulfolane, 2-methylglutaronitrile, and mixtures thereof. -92 The electrochemichromic solution of claim 1 wherein the solvent is substantially entirely 2-acetylbutyrolactone. -ΙΟΙ The electrochemichromic solution of claim 1 wherein the cathodic member of said redox chemical pair in solution is selected from the group consisting of methylviologen hexafluorophosphate and hexafluoroarsenate, 5 ethylviologen hexafluorophosphate and hexafluoroarsenate, and mixtures thereof and the anodic member is 5,10-dihydro-5,10-dimethylphenazine. -15IE 904194 -111 An electrochemichromic cell comprising: spaced plates, each having an inwardly facing conductive surface, and an electrochemichromic solution located in said cell between said inwardly facing conductive 5 surfaces, said solution comprising: a solvent; a redox chemical pair in solution in said solvent which colors in the presence of an applied voltage and which bleaches to a colorless condition in the absence of an

5. 10 applied voltage, one of said redox pair comprising one of viologen hexafluorophosphate and viologen hexafluoroarsenate salt and mixtures thereof. -121 The electrochemichromic device of claim 11 in which said viologen salt comprises methylviologen. -131 The electrochemichromic device of claim 12 in which the viologen salt is in solution concentration of greater than 0.05M. -141 The electrochemichromic device of claim 11 in which the viologen salt is in solution concentration of greater than 0.05M. -151 The electrochemichromic device of claim 14 in which the solvent includes at least 25% by volume of one of the group consisting of propylene carbonate, 3-hydroxypropionitrile, 3,3’-oxydipropionitrile, 5 2-acetylbutyrolactone, 3-methylsulfolane, 2-methylglutaronitrile, and mixtures thereof. -16IE 904194 -161 The electrochemichromic device of claim 14 wherein the solvent is substantially entirely 2-acetylbutyrolactone. -171 The electrochemichromic device Of claim 14 wherein the cathodic member of said redox chemical pair in solution is selected from the group consisting of methylviologen hexafluorophosphate and hexafluoroarsenate, ethylviologen 5 hexafluorophosphate and hexafluoroarsenate, and mixtures thereof and the anodic member is 5.10- dihydro-5,10-dimethylphenazine. -181 The electrochemichromic device of claim 11 in which the solvent includes at least 25% by volume of one of the group consisting of propylene carbonate, 3-hydroxypropionitrile, 3,3'-oxydipropionitrile, 5 2-acetylbutyrolactone, 3-methylsulfolane, 2-methylglutaronitrile, and mixtures thereof. -192 The electrochemichromic device of claim 11 wherein the solvent is substantially entirely 2-acetylbutyrolactone. -201 The electrochemichromic device of claim 11 wherein the cathodic member of said redox chemical pair in solution is selected from the group consisting of methylviologen hexafluorophosphate and hexafluoroarsenate, ethylviologen 5 hexafluorophosphate and hexafluoroarsenate, and mixtures thereof and the anodic member is 5.10- dihydro-5,10-dimethylphenazine. -17IE 904194 -21I A method for increasing the solubility of viologen salts in electrochemichromic solutions comprising: employing one of hexafluorophosphate (PF g ), hexafluoroarsenate (AsF ”) and mixtures thereof as the 6 X 5 viologen salt counterion. -221 The method of claim 21 in which said counterion is combined with methylviologen. -231 The method of claim 22 in which the concentration of said viologen salt is increased to in excess of .05 molar. -242 The method of claim 21 in which the concentration of said viologen salt is increased to in excess of .05 molar. -252 The method of claim 24 in which the solvent includes at least 25% by volume of one of the group consisting of propylene carbonate, 3-hydroxypropionitrile, 3,3’-oxydipropionitrile, 2-acetylbutyrolactone, 5 3-methylsulfolane, 2-methylglutaronitrile, and mixtures thereof. -262 The method of claim 24 wherein the solvent is substantially entirely 2-acetylbutyrolactone. -272 The method of claim 24 wherein the cathodic member of said redox chemical pair in solution is selected from the group consisting of methylviologen hexafluorophosphate and hexafluoroarsenate, ethylviologen' hexafluorophosphate and 5 hexafluoroarsenate, and mixtures thereof and the anodic member is 5,10-dihydro-5,10-dimethylphenazine. -281 The method of claim 21 in which the solvent includes at least 25% by volume of one of the group consisting of propylene carbonate, 3-hydroxypropionitrile, 3,3'-oxydipropionitrile, 2-acetylbutyrolactone, 5 3-methylsulfolane, 2-methylglutaronitrile, and mixtures thereof. -291 The method of claim 21 wherein the solvent is substantially entirely 2-acetylbutyrolactone. -301 The method of claim 21 wherein the cathodic member of said redox chemical pair in solution is selected from the group consisting of methylviologen hexafluorophosphate and hexafluoroarsenate, ethylviologen hexafluorophosphate and 5 hexafluoroarsenate, and mixtures thereof and the anodic member is 5,10-dihydro-5,10-dimethylphenazine. -31An electrochemichromic solution according to claim 1, substantially as hereinbefore described. -32An electrochemichromic cell according to claim 11, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. -33A method according to claim 21 for increasing the solubility of viologen salts in electrochemichromic solutions substantially as hereinbefore described.