CA1043890A - Electrochromic display cell containing substituted bipyridinium salts as redox substances - Google Patents

Abstract

ABSTRACT

The invention relates to an image display cell which forms an image which persists if no longer current flows through the cell. The cell has at least two elec-trodes which a? in contact with a cell liquid in which a colourless viologen salt

Description

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PHN.6924 KTS/AvdV
`, ~ 2~ . 74 ~ , "Image display cell"
~. . .
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j '- The invention relates to an image display ~: d~ .
~ cell having a transparent wall part and provided '~ with an aqueous solution of a N,N'-disubstituted-4,4'-'; blpyridinium salt as a reverslbly reducible redox substance and a halide as a reversibly oxidizable redox substance, which solution is in contact with at least two inert electrodes.
' Such a cell ~rhen used as an indicator is descrlbed~ln~Chemical Abstracts 74, 48964x (1971).
10 ~ ~ In this cell, N,N'-dimethyl-4,4~-bipyridiniumdichloride is used as the reversibly reducible sub tance and pota _ium chloride as the reversible oxidlzable substance.
; If a voltage higher than the threshold value is applied across the electrodes, the bip~ridinium ion is con-s: ~ ~ verted at the oath~ode into a blue-coloured radical ion which colours the electrode surface and thus produces an image. Simult'aneously, at the 'anode chloride is converted lnto chlorine.
The known cell when used a.s an imaee display ~ c ll ha~s tha di.sadvantage that the blue-coloured radical '' ion~l dissolved in the medium and henc~ diffuses away rom th~ electrode. As a result, contrast of the image i reduced. Furthermore the chlorine produced at the - anode :is dissolvcd in t'he medium and by d:iffusion comes ' ' ' ~
into contact with the blue-coloured radical ion, causing ~ the lat~er to be converted to its leuco-form. Although ,, this chemical reaction can be counteracted by incorpora- , ;1 ting a diaphragm between the cathode and anode spaces o~
$ 5 the cell, diffusion of the blue-coloured radical ion from, the cathode to the medium of the cathode space cannot be ~-, prevented, with a resulting loss in contrast of the image.
The properties of the cell further require a potential difference greater than the threshold value ~o be main-.. . .
~ 10 tained across the electrodes as long as an image is to be '-. ... :
maintained.
~-i Our Canadian Patent 950,653 which issued on July 9, 1974 also describes an image display cell in whic,h N, N'-disubstituted-4,4'-bipyridinium compounds are used.
In this cell the reversibly oxidizable substances are ~`- ~ either tetrachlorohydroquinone or ferrous salts. This cell :': ~ :
has the same dlsadvantages as the above-descrlbed one.
.
Pre-published German Patent Appllcation 2,043,5~2 by Imperial Chemical Industries Ltd. (I.C.I.) and which was published on March 11, 1971 describes an opt~cal light filter in which N.N'-disubstltuted-4,4'-blpyrldinium compounds are used. In many cases the bipyridinlum compounds used produce soluble radlcal cation salts which diffuse away from the cathode. All the oxldation products formed at the anode are soluble in ~he medlum. Maintal,n~,ng an ~ obtalned optlcal denslty of the filter consequently 5~ requl,res an electr1c current flow such that the loss ,,,,~i,,~:

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of coloured compound due to oxidation ;s compensated for -by the products formed at the anode. When the filter is used as an image display cell ,it moreover suffers from . .:
the disadvantage that the contrast of the image decreases -~
in all the cases in which the coloured substance is not insoluble in the medium.
It is an object of the present invention to pro-vide an image display cell in which the image is main-tained even if no current flows through the cell.
.~ .. . .
Netherlands Patent Application 7,116,637 by ~ -Imperial Chemical Industries Ltd. (I.C.I.) and laid open to public inspection and published on June 12, 1972 . i . .
describes a cell in which N,N'-di-(p-cyanophenyl)-4,4'-;;l bipyridiniumchloride is used as the reversibly reducible substance. In this cell the chloride ~on whlch precipi-tates at the silver anode in the form of AgCl acts as the reversible oxidizable component. The application as an alternation mentions the use of the bipyridinium radical ~h~ cation chloriide as the reversibly oxidlzable substance.
In the latter case, prior to assembly of the cell the anode is electrolytiically coa~ed wiith the sa,d radlcal catiion, which means an additional compliication of cell manufacture.
Although the cell according to the latter patent applircatiion satlsf-es the requ,rement that the image should be maintained in the absence o~ current flow, it has d1sadvantages which are avoided by the cell according to the present invention.
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. :1 , ,: ~: ,, ~ ., ,; 1 . :., _ 4 ~4 ', ~ 3~ P~.6921 ;-' f One of these disadvantages is that the j: :
solubility of the bipyridinium compound used is lower by a factor of 10 than the minimum solubility , required to obtain~a high concentration of the sub-stance in the medium and hence to enable a high-contrast image to be formed in a very short time. -In addition, in thG known cell the redox potential at which the pyridinium compound takes up one electron with the formation of a green-coloured f~ 10 radical oation salt differs by only 0.2 volt from the potential at ~hich the compound takes up a second electron and is converted into a substantially colour-less sabstance. Consequently the voltage applied across the electrodes is allowed to ~-ary within very narrow limits only, with the result that this factor ^ 3~ also seriously limits the speed at which an image can be formed.
Anoth0r reason why in ths known cell maintaining ,, ~ ~ the correct potential difference across thc electrodes is of importance is that the substantially colourless compound produced by the taking up of two electrons ~ reaots very slowly only with the initial substance with ;~ the formation of the radical cation salt.
Another disadvantage of the kno~f~n cell is that thc disp1ay electrode is not homogeneously coloured but . ~
;-~3 ~ the colourlng is greatly dependcnt upon the shape and disposition of the anode.
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~ The image display cell according to the ~.
' invention ~hich avoids all the said disadvantages is characterized in that the reversibly reducible -. redox substance either is a compound of the formula .~ ...

2 ( m ~ ~ 5 R1 ~ ~ ~ - l(CH2)n~N X N~2 .2(m-~1)X (1) ~- m ~'i'.:
`:~ where m = 0, in which case R1 and R2 represent a linear or bra~ohed alkyI, alkenyl, alc~oxycarbonylalkyl~or phenyl-alkyl group containlng up to ~10 carbon atoms.which may 10~ be substltuted with a halogen~or nltrlle, or a~morpho-linooarbonylmethyl group in whlch the morphollno~group may be~ substituted with one or two methyl groups, ~and X ~epreseDts a monovalent anion seleoted from the group:
~ comprising ClOI, , BFI~ , N3 , Cl ,~Br , F and C6H5S03 ;~L~ 1;5 ~ so, that the solubility of the substance in water exceeds mol/l and the solubility product in water of the : radl;cal cation salt formed by reduction of the oompound : ..
is les~ than 5 times 10 5 (concentrations in mol/l), -i or m = I, i~n ~hich case R1 and R2 are the same and each ~ 20~ ; represent~an:alkyl eroup containing ~rom 1 to 5 carbon ..

-~ ~ atoms or a benzyl group and n - 4, ,!'` .~ X represents a ClOI~ ion, if R1 and R2 represent an .alkyl group containing from 1 to 3 carbon atoms, or -:

a BF4 ion, if R1 and R2 represent an alkyl group con-25~ taining fronl 2 to ~ carbon atoms or a benzyl group, 6- . :
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PHN. 6924. ' '~L~ ;3 or m = 2 or 3, in which case n = 4 and Rl and R2 each represent ' an ethyl or propyl group and X represents a BF4 or C104 ; -ion, or the reversibly reducible redox substance is the '~ 5 reaction product of 4,4'-bipyridyl and an CxJO-dibromoalkane ''~
.~
~ containing from 4 to 7 carbon atoms, obtained in a polar '~
~ . ~
aprotic solvent at temperatures between room temperature and ~ the boiling point of the mixture, and the reversibly oxidiz-- '~ able redox substance is an alkali metal bromide or quaternary ' ammonium brom;de having a minimum solubility of 10 2 mol/l -` ~i as far as the cation of the bromide does not precipitate with the anion of the reducible redox substance.
One skilled in the art will know which of the '.~ cat~ons used precipitate with perchlorate or borontetra- ' - ' fluoride ions. In general this is not the case with ' ~ ~
:- 1: .
lithium, sodium and quaternary ammonium ions.
It should be mentioned that compounds of the formula l in wh~ch m = O are also used ;n the image display ;1 cell described in our Canadian Patent 984,604 which issued on March 2,'1976. According to said Canadian Patent, however, the revers~bly oxidizable substance is not a bromide, as in the present inventiGn, but the'radical cation salt produced by reduction of a compound of the formula 1. A disadvantage ; ;
o~ the'relevant cell is the'complexity of assembly, similarly ; 25 to wha~ was mentioned above with ~ . ~
. i . .
~ ; 7 ,''.'~'~ ~ ''' ;~3~ PHN. 6924. ~ -respect to an embodiment of the cell described in previou~ly mentioned Netherlands Patent Application 7,116,637.
- The structure of the reaction product of CX, C~-d;bromo-alkane and bipyridyl is not known. It may be -assumed that the structure of a part of the reaction product can be represented by the formula l in which n = 4 to 7, --X = Br and Rl and/or R2 represent an ~-bromoalkyl group. -Attempts to determine the value of m by determining the amount .~ ......
, of covalently bound bromine in the reaction product yielded ~ lO values of a height such that it must be assumed that part of ,~ the reaction product consists of compounds having a cyclic structure. :- ;
l~ The compounds of the formula l in which m = 2 or 3 are novel compounds. Hence the invention also relates to :;
S 15 these compounds and to their preparation--. Thé compounds - ;
are obtainable by methods known for synthesizing such com~
pounds~and by analogous methods. ~5 For example, the compounds of the formula l in which ,, - ~: : . .. . .
~ m = 2 are obtainable by reacting a~compound of the formula 2 + ' '~
Rl ~ N ~ N+- (CH2)4 - Br. 2Br (2) -with a compound of the'formula 3 Rl - N ~ R - (C~2)~- ; ~ ; . 3Br (3) :. .. .
,~ - 8 -~, . . . .. .
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~3~ P~lN;6924 . ~ , ~-~ Compounds of the formula 1 in whi.ch m = 3 are obtainable by reacting a compound of the formula 3

3 . with a compound of the formula 4 1 ~ ~ (CH2)4 ~ ~ N -(CH2)4Br-4Br (~J
... .
.. : 5 Compounds of the formula 2 are obtainable by . ~ . reacting N-ethyl of N-propyldipyridylbromide with ... : 1,4-dibromobutane, whllst a reaction of a compound of the formula 2 ~ith bipyridyl yiolds a compound of the formula 3.~Compounds of the formu~ 4~are~obtained ~ ~ by~ reacting a compound o~ the formula 3 with 1,4-dlbromobutane.
All the abo~e reaction~s are performed in à
polar aprotio~solvent at temperatures whlch lie bet~een room temperature and the boiling point of the~reaction ;15~ mlxture. ~ ~
Examples of polar aprotic solvents are dlmebhylformamide, aoetonltrlle, sulfo~lane, dimethyl-sul~oxide, propylenecarborlate and the like.
It is o:f essential importance ~or the cell : :according to the invention that the compounds of the formula~1 are satisfacborily ~soluble in the medlum so that the medium contains sufficient substance for fast~.image formation. On the other hand, it is of partioular lmportance for the reduction product formed ~t; :~ 25 : at the display electrode to be hardly soluble in the ~, ~, , :

. ' . pH~T .6924 .~" ' ' 2~ ,.74 ,3~

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' ~, medium so that the contrast of the irnage is not ~. . .
reduced by diffusion of the coloured compound to ~i the medium.
. Compounds of the formula 1 in which m = 0 .i 5satisfy the above requirements by a suitable combination of the substituents R1 and R2 and the anion X .
It ~as found that the solubility of these compounds in water increases as R1, R2 and X are smaller. It was also found that the solubility product z~
10of the reduction products of the said~compounds de-creases as R1- R2 and X are greater.
A~branched alkyl group R1~ or Rz~ causes a compound~of the formula 1 to have a lower solubility and !-`
thè reduction product obtained from it to have a smaller 15~solubility product than a straight-ohain alkyl group containing the same number of carbon atoms. :
~'S ; :~ If R1 and R2 represent alkyl groups, th~
~ follo~qln~ combinations of R1, Rz~and X may be chosen:

;i, ~?,,:~ ' R1 and R2 ~ --1 ~ C6 Cll~- . :.
::20 ~ C - C7 BF4-"~ :C5 - Cg Br ~:
6 ~ C8 3 .
6 ~ C9 ~ : ~l-C G ~ 0 -10~

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PHN.6924 ~ 2l~.4.7 ',~ .
In the case oP an anion X , as a rule an ¦ alkenyl group is chosen which has the same number ~; of carbon atoms as, or 1 carbon atom more than, the alkyl group which is combined with said anion.
In general, an alkoxycarbonylalkyl group ~`j when combined with a given anion X has one carbon ` ~ atom less than analkyl group combined with said anion X .
As a rule, a phenylallcyl group is chosen so as to have two carbon atoms more than an alk~l group.
. . ~ , , ;10~ ~ If a group R1 or R3 carries a nitrlle group, the carbon skeleton o~ R1~or R2 may as a rule~ contain two carbon~atoms less than an~unsubstl~tuted group. ~ -a group R1 or R2 carries a halogen~atom, the number of oarbon Qtom3 oi the group may ln general 15 ~ be equal to tha-t of an unsubstitutad group.
A aompound used in the cell~according to ~ the invention as a reversible reduclble substance '. .~3 ,~ b ~ (hereinafter also referred to as a viologen~f at the cathode is converted into a blue-ooloured radiaal 20 ~ ~ oatlon by taking up an aleotron. The number o~ electrons which a compound oan take up from the electrode is equal to the number o~ bipyridyl groups contained in the~molaoule. ~~
Compounds o~ the formula 1 in which m = 0 X5~ ocoupy a particular position amongst the reversibly redu~clble compounds~used according to the invention, beoause when reduced they form a non-porous film on P~N . 692l~
24.4,7~ , . ii 3~
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-~'` the electrodes ~hich can be erased at a particularly high rate even ~ith potenticstatic control.
The bromine formed from bromide at the anode j ~
is, combined with a quaternary ammonium bromide, deposited on the anode as an insoluble salt.
~- The image display cell ~ccording to the i ~ invention has a casing at least one wall or wall portion - - of which is transparent, as to allow for the visual .~,.,~ , .
~ perception of an image formed. It may be made from a .;; ~ . . ..
synthetic materlal, a glass or the like, The electrodes may consist of~stannic oxide, ndiumoxide, plati~num, palladium, gold and other inert electrode materials. ObviousIy it i5 not necessary for all the electrodes to be made~o~ the same material.
15 ~ The electrodes~may alternabively co~nsist~o~carbon.
If the imaBe is~to be visually~perceptible, a carbon electrods cannot be used as the display electrode because the colour is not perceptible against a black background.
The electrccles may be formed in a variety of s~apes, Also, more than one anode and/or cathode may be~used.
The lmage produced may have th~ same shape ~7,'~ a the sùrface of the display electrode or as the visible part of thi~ surface. When~a transparent electrode 25~ a~used as the display electrode, it may be deposited on t~e inner surface of the transparent wall of the ca~ing. Part o~ the electrode may be coa-ted with an eleotrically non-ccnducting materlal, for example a , ~ ,.

'- . "f, ~' :

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~ PHN 692~1 , ~, . .
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~ synthetic material, in which case the image will have . the same shape as the uncoated part of the electrode . surface. As an alternative, part of the ~all of the cell may be screened by an external opaque mask, in which case the image produced has the same shape as ~ ~ .
. the visible part of the display electrode.
~, ` The display electrode may alternatively be applied to the rear wall of the casing so that the image is viewed through the transparent wall and the~cell liquid. However, it is nGt necessary for the lmage electrode to be disposed on a wall of the ca.sing, but ~ A`~ t may also be disposed at another location where it is surrounded by the cell liquid.~In this case lt may :be preferably for the rear surface of thé eleotrode to 15~ ~ be coated with an electrically non-conducting material.
If the image is to be viewed froln two directions, ; : the display electrode may advantageously be located ; inside the cell not in contact with a wall because thus in the case of a ~lat electrode - an image is produced 20 ~ on both major sur~aces.
Beoause in the cell according to the invention the coloured substance adheres to~the electrode and :~ does not disperse in the cell contents by diffusion, : tha colour ~ormed may rapidly be removed by reversing electrode polarity. This provides the important ad-vantage that the response is not determined by the ,~;;- ;~æ~ spacing between the anode and the cathode. --~',,' ~ :` ' ` ' ' :','~
~ 13-,.s~ ~ .

:~! PHN.6921.~ ~
24.l~ .7lJ, .

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, Consequently the position of the anode relative to ~ the cathode may be chosen at will. The anode may, -~ for example, be at an angle of 90 to the cathode or bo co-planar with it.
In the above an image has been referred to .
which has the same shape as (the visible part of) the .f~ display electrode, but obviously the image may alter-~
natively be built up from a number of components which are provided by an equal number Or display electrodes 10; of the cell which are rendered operative.
If the or~each display electrode is disposed on a transparent wall~of the cell, the contrast of~thè~
image may be increased by dispersing an lnsolublej for example white, substance in the cell liquid. Suitable 5$ such substances are titanium oxide and b,~rium sulfate.
The cell according to the invention may be used not only for dis~playing images but also as a voltage dioatar. If a ~oltage higher than the minimum voltage of about 1.5 volts required to cause the oell to respond 20;~ is, or has been, sot up across the electrodes, colour is observed on the displa~ eleotrode.
A signal may, howe;ver, be observed by o-ther than vlsual ~leans. Since in~the formation of colour the surface~of tho electrode is coated with a viologon 25~ radioal~, ~the potential of the electrode relative to the cell liquid changes. This change in potential can be ohserved by means of a raference elec-trode.
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`j Even if the amount of electrical charge ~1~ which has passed through the cell is not enough to form .: an amount of colouring substance sufficient to be visually perceptible, the change in potential of the electrode relative~ to the cell liquid can be observed ,;~ by.means of a reference electrode.
. Hence the cell may be used as a means of ~ ~ recording that a voltage has been set up across the ,~ . electrodes, causing a small amount of charge to pass through the cell, in other words, the cell may ba used . as a storage element. Obviously in such use of the cell the transparent wall portion of the cell need not be optically transparent. ~ \
Referenoe electrodes may be mads of the same \
materials as:the display electrodes, however, as an alternative glass electrode, calomel electrodes and ~ the like may be used.
: S~ Reference slectrodes may also be used.to reduce to a mi.nimum the time required for the formation of ~`; 20 ~ the image. If a oomparatively.high potential di.fference . i9 set ùp across a oathode and an anode ofa~cell, thé
~ viologen in the viclnlty o~ the cathode will rapidly .~ be reduced~ Further increase of the amaunt of reduced viologen at the cathode will then depend upon the rate at which viologen from the medium is conveyed to the cathode. However, side reactions, for example the formation of hydrogen, may occur, and this is undesirable.
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~ To prevent such side reactions the potential ; . , .
;, difference across the electrodes is chosen sothat always a sufficient amount of viologen can diffuse to -~! the cathode. Thus the rate at which initially colouring matter is formed on the cathode is lower than in the above case.
If a reference electrode disposed near the cathode i9 used, the advantage of a rapid reaction due to a large potential difference may be combined with the advantage provided by the use of a smaller potential ~ difference in that side reactions are prevented.
.`~';'!1~, ` The potential difference produced across the cathode and the reference electrode by the appllcation of~a potential di~ferenoe across the cathode and the anode may con-tinuously be c~ompared with a potential difference which is~lower than, or equal to9 the maximum permlssible potential difference between the cathode and the reference electrode.`If9 for example, the measured potentlal difference i9 greater, the potantial difference across the cathode and the anode may be reduced until the ~ j. : . , :
` de~ired potential difference across the cathode and the ~' ~dr ~ refercnce electrode is reached.
;,,, ~: :
In ~neral the image display cell is operated with a potential difference between 1.; volts and 10 volt9 Z5 ~ acro99 the display electrode and the counter electrode.
` ; A~ a rule a potential difference between 1.5 volts and , . . .
~ 4 volts i9` applled.

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~f~38~3~f A visually perceptible image will as a rule be obtained in about 0.01 second.
. The concentration of the viologen in the cell liquid will usually be between 0.01 and 0.2 equivalent/
litre. In this connection, an equivalent of viologen -means an amount of viologen which contains the same number of bipyridyl groups as a mole of bipyridyl. - ~
Some of the compounds of the formula 1 in which ~ :
. m = 0 are known. They are obtainable by coupling . .
bipyridyl with a halide by means of a Menskhutkin ~
reaction. The preparation of a number of compounds was -~` described inter alia by Michaelis in J. Gen. Phys;ol. ; No. 16,859 (1933).
The resulting halides may simply be converted into other salts. For example, the addition of silver sulfate enables silver ha1ide to be precipi~ated, a ~-solution of the viologen sulfate being obtalned. From this, for examp1e, bariumsulfate may be precipitated by the addition of a barium salt, for example barium 20~ nitrate, a solution of viologen n1trate being obtained.
The compounds o~ khe formula 1 in wh~lch m - 1 are descrifbed ln United K;ngdom patent specification h: 1,054,397 by Imperial Chemical Industries Ltd. (I.C.I.) and publ~shed on November 30~ 1966.
~ ~ ~The concentration of the alkall metal or quater-nary ammonium bromide, for example tetraalkyl ammonium brom~de, contained by the cell as a reversibly ox~dizable substance 11s of the same order of magnltude as that of . il ~e , .
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the viologen compound. In order not to retard operation "
of the cell, as a rule the concentration will be made at least equal tothat of the viologen compound. ;-The invention will npw be described more -fully ~ith reference to the following Examples.
EXAMPI,ES
;, ,; 1) A-glass cuvette two opposed walls of which were internally coated with:an SnO2 electrode was rinsed wlth nitrogen for 24 hours. The surface area o-f either electrode WQS 1.5 cm2, the electrode spacing was 1 cm.
The resistance of the SnOz, which was measured by applying a voltage to two conductive strips provided along two opposite sides of an electrode, was 10 ohms~ ~he cell was filled wlth an oxygen-free solution of tetramethylene bis [4(1-éthylpyridin-4-yl)pyridiniu ~ tetraborofluoride ; (0.05 mol/litre) and NaBr (0.05 mol/litre3 in water which~
~" ~ previously had been boiled out whilst passing through nltrogen.
A direct voltage of 2.5 volts was applied ., ,j ZO across the electrodes ~or 250 milliseconds. As a result ,Z~ the cathode was ~oloured violet. The bromine formed at the anode preoipitated as Br3 togethar with the tetra-methylene bis ~4(1-ethyl-pyridin-4-yl)pyridiniunl~ ion on the anode.
A~ter no voltage had been applied to the cell for 1 minute9 a voltag¢ of reversed polarity was applied '~ for 250 milliseconds, with the result that the coloured ~:';,~::
~' ~electrode wa~ decolourized.
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PHN. 692 4 24~4.74 : ` .
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: 2) Example 1) was repeated with the difference that a direct voltage of 2.5 volts was applied for 500 milliseconds. After 1 hour the coloured electrode ; was decolouri~ed by applyin~ a Yoltage of reversed polarity for 500 milliseconds.
3) Example 1) was repea-ted with the same result, using a cell, a side wall and part of the front 1~all of which were coated with SnO2 electrodes.

4) Example 1) was repeated~wlth -the difference ~10 that the cell was filled with an aqueous solution of tetramethylene bisL4 ( . -ethylpyrldin-4-yl)pyridl~lum}
tetraborofluoride (0.05 mol/litre) and tetramethyl-ammoniumbromide (0~05 mol/litre)0

5) ~ ~ Example 1) was repeated, using aqueous solutions 15~ o~ eaoh~of the oompounds mentioned in Table~I and of NaBr ~(0.05 mol/litre).

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~ TABLE I-, . . . .
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p~.692l~
~I~.4,74 .-~ ' ' '.

R - ~ N+- ~(CH2)4- + ~T+ ~ R.2(m-~1)X

m .
- ~ :

:~ R X~u~c :~o~jlltr-_ ~ ..
, CH3 1` Cl04 0.05 . C2H5 1 Cl04 0.05 5 ~: . C2~5 1 BF4 ~ o.05 3 7 1 C104 ~0.05 : 3 7 1 ~BF4 0.05 ;; C~Hg 1 B~4~: 0.05 :: 5 1~1 ~ 1 BF4 0.05 10: ~6 ~ 2 1 BF4~ :~ ~: -5 ~2H5 -:~2 : BF4 0.05:
2 5~ 3 BFL~ ~5 2 5 2 ~ ClO4 ~0.02 C2H5 ~ 3 :Cl04 ~ 0.02 5~ C5H11 ~ ~ 0~ : Br~: : 0.05 C~6H13~ .~ ~ ~Br ~ 0.05 C7H15 0 Br 0.05 : C6H5CH2 ~Br~ - ~0.05 _ _ : . ~ ~ ~

. ,~ 6) ~ Example 1):was repe~ted ~ith the diffte,renc~
20 ~ that the cell was fllleid with an aqueous solution i of tetramethylene bis ~ benz~lpyridin-4-yl)p~ridlniumJ
; ~ tetraborofluoride (0.04 mol~lltr~e) and tetrabutylammonium-:, ~ :broml:de (0.04 mol/litre).
7) 1 ~Exampl~ 1) was repeated with the dif~erenc~
25 ~ that~a direct voltage of 3 volts was appl:Led for 20~

;

PHN~6924 2~.4.74 '''', ' ' ' 3~ 3 100 ~illisecondi. Then an equal voltage with reversed polari-ty was applied for an equal period.
~ 8) Example 1) was repeated with the difference `I that the SnO2 electrodes were replaced by In203 ,~ 5 electrodes with a sheet resistance of 5 ohms.
9) Example 1) was repeated with the difference that the cell was filled with an aqueous solution ~ of NaBr (0.05 mol/litre) and a polymeric quaternlzed ;`J`~ bipyridyl compound (0.05 equivalent/litre) obtained 10~ by reacting 1,4-dibromobutane and 4,4'-bipyridyl in a polar aprotic solvent.
10)~ Example 1~) was re~p~eated~with~the difference ~
that the quaternized~polymeric~blpyridyl~oompound was ~ ;
pr~oduced~from bi~pyridyl and~one of~the~followin~g~
dibromoalkanes~
a~ 5-di~romop~ntane~
b);~ ,6-dibromo~exane~
c)~ 7-dibromoheptane.
Example 1) was r~p~eated with aqueous solutions 20~ of~(formula 1, m = 0) TABLE-~,$: ~

PIlI~.692 L'J
2 4 . 4 ~ 7 4 ,3~
~ .
_ . . _ _ _ .
n.C3H7 n.C3H7 Cl0 i-C3~I7 i.C3H7 BF4 i.C3H7 i.C3H7 ClO
n.C4H9 ~n.C~H9 BF4 n.C4H9 n.C4H9 Cl0 n-C5H11 5 11 Br CHC35H11 CH 5 1 ClO

C2H5 C2H5 Cl0 5 11 5 11 Cl0 3 3 4 8 3-CH3Cl~H~ BF~
3-CH3C,4H8 3 3 4 8 Br 'C6H13 n-~6H13 Cl0 n'C6 13 6 13 BF4 n'C6H13 n'C6H13 Br n'C6H13 n- 6 13 Cl n.cf5H13 n- 6 13 7 15 n-C7H15 BF4 'C7H15 7 15 Br 7 15 n.C7H15 ~ Cl n'C7H15 7 15 F
7 15 .7 15 n'C7H15 7 15 6 n-C8H17 n.C8 17 Br nC8H17 8 17 Cl

6 5 2 ~,6H5CH2 Br -~ C6H5C3H6 6 5 3 6 Br n.C H11 n-C7H15 Br n.C5H11 3.5-dimethyl- BF
morpholino-carbonylmethyl 3.5-dime-thyl- 3.5~dimethyl- F
morpholino- morpholino~
carbonylmet-hy~ carbonylme'hy]
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The present invention will be described in more de-tail with reference to the accompanying drawings in which:
Figure 1 shows a nickel plate having a pattern formed -by grooves which can be used to form electrodes on a substrate for an image display cell according to the present invention and Figure 2 shows a transparent plate which is to be cemented to ~ype of plate shown in Figure l to form an image ~' display cell according to the present invention.
.. ,:, .
~ l0. Figure 3,which is on the same sheet as Figure l, shows ,, g ~ I p O /y r~ /~ 6 i~ a c r y ~
'' ''f'~ a Perspex, a registered trade mark for'~e~ys~re~e>substrate ~; coated with a pattern of gold film electrodes and -i Figure 4, which is also on the same sheet as Figure 1, i ~ shows the same type of substrate as shown in Fi~gure 3 and in-. ~ , ... . .
dicates parts of the gold film pattern which~are covered with an electrically insulating transparent lacquer.
12) Grooves having a depth of 0.4 mm and a width of 0.1 mm ;~;
were formed according to the pattern shown in Figure l in a nickel plate 0.5 mm thick. A nickel wire was welded to each re-sulting segment. The plate was then placed in a mould which was ~illed w1th a synthetic resin in an amount such that the ...~, grooves were filled with the resin and the attached wires pro~
I ~ truded from the resin. After the resin had hardened the as-~ sembly was ground untll so much nickel and resin were removed -.' 25~ from the plate that the segments were electrically disconnected 1 ~ .. . ,.~
from one another. The plate was polished and then coated with a layer of gold by electrodeposition. A transparent plate of Perspex formed with a recess as shown in Figure 2 was cemented - ;
to the plate. The cell was filled through the recess 12 with ~.............. . .
the same solutions as described in Example l), after which the ;

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P~IN 6924 ;
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opening 12 was closed with a Perspex, a registered trade mark, stoppPr and then sealed with a cement. The segments designated by the reference numerals l to 7 were the display electrodes, electrodes 8 to lO were the reference electrodes and electrode 11 was the counter electrode. For the operation of the cell a circuit arrangement was used which compares the voltage set up , between the display electrodes and one of the reference elec-trodes with a given signal voltage and which so controls the current flowing through the counter electrode and the display electrodes that the measured voltage becomes equal to the sig-nal voltage. At a signal voltage of l volt all the operative .~ ~ -. . .
display electrodes were coloured a deep purple after 0.01 second, ; whereupon the voltage was removed from the display electrodes.
~ 13) A Perspex~ a registe~ed trade mark, substrate was .... ..
coated with a film of gold by vapour deposition. The pattern of Figure 3 was then etched in ~he film. Part of the film (the ~ part shown shaded 1n Figure 4) was coated with an electrically a insulating transparent lacquer. Subsequently a cell was con-'j structed and operated in the manner described in Example 12). , ) 20 14) A nickel plate was provided wlth grooves in the man-,! ner described in Example 12) with the difference that instead of the figure 8 a hole of 10 mm x 15 mm was formed at the centre.
.' . .
l Into this hole 25 platinum wires were inserted at right angles ....
to the plate surface to serve as display electrodes. The assembly was embedded in resin, ground and polished in the manner described in Example 12). Electrodes lO and ll were coated with a film of gold. The cell was then finished and operated in the manner ! descr~bed in Example 12), with the difference that the operative electrodes were switched off after only lO0 /us. By means of 1 30 a high-resistance voltmeter (input impedance ~ l M Q ) it was .' . , .
~ - 24 - ~ i PHN 6924 ^

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possib1e to detect which electrodes had been turned on and which had not. In the first case a voltage difference of 90 millivolts was measured between the reference electrode 10 , . . . .
- and the relevant display electrode, whereas in the second case no voltage difference was found. The information stored in ~` this manner can be erased in less than 100 /us.
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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Image display cell having a transparent wall part and provided with an aqueous solution of an N.N'-disubstituted-4,4'-bipyridinium salt as a reversibly reducible redox sub-stance and of a halide as a reversibly oxidizable redox sub-stance, which solution is in contact with at least two inert electrodes, characterized in that the reversibly reducible redox substance either is a compound of the formula 1 where m = 0, in which case R1 and R2 represent a linear or branched alkyl, alkenyl, alkoxycarbonylalkyl or phenylalkyl group con-taining up to 10 carbon atoms which may be substituted with a halogen or a nitrile, or a morpholinocarbonylmethyl group in which the morpholino group may be substituted with one or two methyl groups, and X is a monovalent anion selected from the group comprising ClO4-, BF4-, NO3-, Cl-, Br-, F- and C6H5SO3-so, that the solubility of the substance in water exceeds 10-2 mol/litre and the solubility product in water of the radical cation salt formed by reduction of the compound is less than 5 times 10-5 (concentrations in mol/litre) or m = 1, in which case R1 and R2 have the same meanings and each represent an alkyl group containing from 1 to 5 carbon atoms or a benzyl group, and n = 4, X- represents a ClO4-, ion if R1 and R2 each represent an alkyl group containing from 1 to 3 carbon atoms, or a BF4- ion, if R1 and R2 each represent an alkyl group containing from 2 to 5 carbon atoms or a benzyl group, or m = 2 or 3, in which case n = 4 and R1 and R2 each repre-sent an ethyl group or a propyl group and X- represents a BF4- or ClO4- ion, or the reaction product of 4,4'-bipyridyl and an .alpha., .omega.-dibromo-alkane containing from 4 to 7 carbon atoms obtained in a polar aprotic solvent at temperatures between room temperature and the boiling point of the mixture, and that as a reversibly oxidizable redox substance an alkali metal bromide or quaternary ammonium bromide having a minimum solubility of 10-2 mol/litre is used as far as the cation of the bromide does not precipitate with the anion of the reducible redox substance.

2. Image display cell as claimed in Claim 1, characterized in that a compound of the formula 1, where m = 0, is used as the reversible reducible redox sub-stance.

3. Image display cell as claimed in Claim 1, char-acterized in that the or each anode is at an angle to the or each cathode.

4. Image display cell as claimed in Claim 1 or 2, characterized in that the or each anode is co-planar with the or each cathode.

5. Image display cell as claimed in Claim 1, 2 or 3, characterized in that an insoluble compound is dispersed in the cell liquid.

6. Image display cell as claimed in Claim 1, 2 or 3, characterized in that the cell has one or more reference electrodes.