CA1081447A - High efficiency electrochromic display device - Google Patents

Abstract

HIGH EFFICIENCY ELECTROCHROMIC DISPLAY DEVICE
ABSTRACT OF THE DISCLOSURE
This invention relates to a reversible display device based upon the electrochromic properties of triaryl pyrazoline compounds when used in conjunction with a complementary redox material.

Description

7 Field of the Invention ' 8The present invention is concerned with a reversible g display device. The device utilizes the electrochromic effect obtained using a triaryl pyrazoline compound and a 11 complementary redox material. The triaryl pyrazoline com-. .~
12 pounds useful in the present invention have the formula 15A - 5 C - (C~ = CH)n ~ A
I ' 11 ' , 16A1 _ N N

18 wherein N is 0 or 1, and A, Al and A2 are each aryl radicals.
19 The Prior Art Pyrazoline compounds have been known for some time and 21 their preparation has been described in the literaturè. The 22 prior art, for example U.S. patents 3,180,729 and 3,549,362, 23 teaches the photoconductive nature of pyrazolines. The 24 anodic oxidation and electro-chemical luminescence of pyrazo-line is taught in the Journal fur Praktische Chemie Band 315 . _ ..
26 Heft 3, 1973, pages 549-564, and Band 316 Heft 2, 1974, ; 27 pages 267~285. The use of pyrazoline compounds as charge 28 transport layers in electrophotography is taught in U.S.
- 29 patent 3,824,099 and 3,837,851. As far as we are aware, - 30 however, there is no prior art teaching of~the use of triaryl pyrazoline compounds in an electrochromic display device.
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1~81~4`7 The prior art teaches several types of electrochromic

2 ,display devices. Among other things, they differ in the , 3, nature of the materials used therein. U.S. patent 3,806,229 4 describes a device based upon the use of viologen compounds.
', -' S Another system is based upon the inorganic material tungsten '' 6 trioxide. U.S. patent 3,451,741 describes an electrochromic .. . .
7 display device using any of several different types of ; 8 organic materials, including anthraquinones, hydroxyaryl 9 arylamines, diphenoquinone compounds, indigo and thioindigo materials, and, in particular, hydroxyaryl imidazole mater-; 11 ials. This latter patent provides a good description of the 12 operation of a reversible electrochromic device utilizing 13 oxidant/reductant pairs.
,i L4 The display devices obtained according to the present ,; 15 invention have advantages o~er those of the prior art. In ''I 16 particular, the present invention provides devices having ~~
~,,l 17 eIectrochromic e-fficiencies at least a factor of two greater 18 than those known to the prior art, in some cases, an order ' 19 f magnitude greater.
'' 20 Summary of the Invention 21 According to the present invention, an,electrochem~cal '`~ 22 reaction is used to form a color absorbing species. This 23 color forming process is utilized as a display device by ~ 24 containing the reactive medium betwee,n electrically con-:¦ 25 ductive electrodes, at least one of which must be transparent.
'! 26 In such a configuration, information is selectively displayed 27 by segmenting the electrodes into a suitable pattern and 28 applying a pot-ntial across the proper electrodes to produce 29 coloration in the desired areas.
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~08~447 1 In the present invention, the electrochromic coloration 2 reaction takes place due to the oxidation of the triaryl

3 pyrazoline compound at the anode and simultaneous reduction

4 of a suitable redox material at the cathode. The pyrazoline -~ 5 compound has the formula described above. The useful comple-6 mentary redox materials are electron acceptors and include 7 such compounds as, for example, phenylhydroquinone, fluoren-_ ones, fluorenes, carbazoles which are polynitro substituted, g and-benzene compounds substituted with electron withdrawing groups. The electron acceptor compound serves as a comple-11 mentary material in an oxidation/reduction process with the 12 pyrazoline. In that oxidation/reduction reaction, the 13 pyrazoline compound is oxidized, while the electron acceptor 14 material is reduced, thereby tending to balance the display cell electrochemically. This electrochemical balance results 16 in good reversibility of cell operation. Still another ad-17 vantage is a reduction of electrode degradation. Increased 18 color change may also be obtained due to color produced by 19 the reduced form of the complementary redox material, in addition to the oxidized pyrazoline. Erasure of the image 21 is obtained in a symmetrical cell by short-circuiting the 22 cell or by momentary application of the reverse polarity 23 potential.
24 As is known to the prior art, the electrochromic reaction is carried out in an anhydrous solvent. Useful 26 solvents include, for example, methyl ethyl ketone, dimethyl-27 formamide, dimethylsulfoxide, N, N-dimethylacetamide, tetra-;l 28 hydrofuran, and acetonitrile.
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10814~`7 L From the-list of solvents shown above, tetrahydrofuran 2 is the best solvent for dissolving large amounts of pyrazo-^ 3 Lines. Acetonitrile is the poorest for dissolving pyrazo-4 lines, but in terms of solution conductivity, the most -_5 conductive solutions are obtained with acetonitrile, and the 6 least conductive with tetrahydrofuran. Methyl ethyl ketone 7 is a fair solvent for both solubility and conductivity.
8 The choice of optimum solvent depends on solubility required, -g conductivity, stability, etc.
It is desirable to add a sal~ to increase the conduc-11 tivity of the solution, since the passage of current is 12 dependent on ions. The choice of optimum electrolyte is 13 dictated by the solubility in the solvent used, the dissoc-14 iation constant, the mobility and the discharge potential.
Useful salts include, for example, tetraalkylammonium salts, 16 such as tetraethylammonium perchlorate, tetrabutylammonium 17 perchlorate, tetraethylammonium fluoborate, and tetrabutyl-18 ammonium fluoborate, ammonium perchlorate, ammonium flu-19 oborate, lithium perchlorate and lithium chl~ride.
. _ The electrochromic ef~iciency of a material is a param-21 eter of prime importance in determining the utility of that ` 22 material in a display device. In an electrochemical display 23 device as considered here, an individual molecule becomes 24 colored as the result of the gain or loss of an integral number of electrons.Assuming that the molecules do not 26 become decolored rapidly by another process, the number of 27 colored molecules produced per unit area in a display device 28 will be proportional to the charge per unit area passed 29 through the device. Since, according to Beer's law, the number of colored molecules is linearly related to the .' . .

10~1447 . ' 1 optical density of the display device through the extinction 2 coef~icient, it is convenient to define the electrochromic 3 efficiency of a material as the induced optical density 4 obtained as the result of the passage of a given charge --- 5 per unit area, usually as mC/cm2.
6 The effect of the electrochromic efficiency on the 7 operation of a display device is now apparent. For identical 8 devices operating at the same voltages but using materials .- g with different electrochromic efficiencies, the device using the material with the higher electrochromic efficiency will 11 consume less power. Alternatively, a larger display panel 12 can be operated at the same power consumption by using the 13 higher efficiency material. In addition to these obvious L4 advantages in terms of power consumption, higher efficiency materials make possible display devices which would otherwise ; 16 not function properly. Since transparent conductors are _ 17 usually used with electrochromic display devices and there 18 is generally a trade-off with respect to the transparency 19 and conductivity of these conductors, potent~al gradients along the transparent conductor due to IR losses can be 21 troublesome. Such potential gradients can lead to non-22 uniformity of the display coloration or may require that 23 the ~isp}ay be written at reduced rates to reduce the current 24 in the electrodes. A higher efficiency electrochromic mate-rial offers significant advantages in overcoming these prob-26 lems since it can provide the same optical performance as a 27 lower efficiency material, but at lower current levels.
28 Reliable and reproducible measurements of the electro-2g chromic efficiency of various materials can be made since the electroc~romic efficiency does not depend on the concentration ~08~447 1 of the electrochromic material in solution, the applied 2 .~oltage or the geometry of the display device. Measure-~ 3 ments of the electrochromic efficiency of some common 4 electrochromic materials have been reported by I. F. Chang and W. E. ~oward, IEEE Trans. Electron Devices, ED-22, 6 749 (1975). Their results show a linear relationship be-7 tween induced optical density and the charge per unit area 8 passed through the display device, as anticipated. The g slope of the line through the data points gives the elec-trochromic eficiency. Monochromatic light was used in 11 this investigation, so that the electrochromic efficiency, 12 which is in general a function of wavelength, was determined L3 at a specific wavelength or wavelengths. The table below L4 summarizes some of the relevant results obtained by Chang and Howard.
J 16 Material Wavelength Electrochromic (nm) Efficien~y 17 (OD/mC.cm . . . .

19 h~ptyl viologen dibromide 513.5 0.075 heptyl viologen dibromide 544 0.13 21 For heptyl viologen dibromide, the two wavelengths listed 22 correspond to the maxima in the absorption spectra, where 23 the electrochromic eficiency will also be highest.
To determine the electrochromic efficiency of a pyrazo-line containing solution, the optical density of a cell of 26 known area was monitored by measuring the attenuation of 27 the light from a He-Ne laser at 632.8 nm passing through 28 the cell. The cell consisted of two glass plates with a 29 transparent conductive coating, separated by a 5 mil ~ylar ~o spacer. (Mylar is duPont's brand of polyethylene terphthalate).

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~ - SA975070 -6-108144~7 1 The spacer had a l/2 inch diameter circle punched out to con-2 tain the electrochromic solution, corresponding to a 1.27 3 s~. cm. area. This cell was filled with a solution o~ 0.09 4 molar l-p-methoxyphenyl-3-~-diethyl-aminostyryl-5-~-diethyl-aminophenyl-~2-pyrazoline (Me0-DEASP), 0.04 molar phenyl 6 quinone and 0.13 molar tetrabutylammoniumperchlorate dis-7 solved in tetrahydrofuran (THF). This cell was subjected to 8 a serles of voltage pulses ranging in duration from 10 to 500 9 msec. and in amplitude from 0.5 to 20 volts. For each voltage pulse the induced optical density at 632.8 nm was obtained 11 by measuring the decrease in the laser light passed through 12 the cell, and the charge required per ~lnit area was determined 13 from the current, the pulse time and the area of the cell. When 14 the induced optical density at 632.8 nm is plotted as a function of charge per unit area, the result indicates a linear relation-16 ship between induced optical density and charge per unit area, 17 independent of pulse duration and amplitude. The slope of ., .
18 the line drawn through the experimental points indicates an 19 electrochromic efficiency of 0.68 OD/mC cm2. The efficiency of the electrochromic solution used in this test is thus more 21 than a factor of five higher than the most efficient material 22 reported by Chang and Howard. It should be noted that ~he . .
23 ~e-Ne laser was used as a light source as a matter of conven-24 ience. The peak of the absorption spectrum of the MeO-DEASP
cation is at approximately 650 nm, so that measurements made 26 with light of this wavelength would result in a stil~ higher 27 electrochromic efficiency.
28 While the electrochromic efficiencies determined at 2g specific wavelengths can be used to estimate the efficacy of an electrochromic material in a display de~ice, a more .- ' , ~

~08144~7 1 meaningful determination should take into account the 2 electrochromic response at all wa~elengths to which the 3 human eye is sensitive and weight the response according to 4 the sensitivity of the human eye at each wavelength. For this reason, the apparatus described abo~e was modified.
The He-Ne laser was replaced with a tungsten-halogen lamp 7 filtered by an infrared absorbing 1-69 filter. This combina-tion of lamp and filter results-in nearly uniform intensity 9 of illuminatio~ throughout the visible spectrum. The silicon photodetector used to measure the attenuation of the light 11 passed through the display cell was covered with a photopic 12 filter, so that the sensitivity of the detector as a function - --13 of wavelength closely matched that of the human eye. Fur`ther 14 measurements were carried out using this modified apparatus on the solution described above. In this series of measure-16 ments, an electrochromic efficiency of 0.24 OD/mC-cm 2 17 obtained.
18 The same apparatus was used to measure the electrochromic 19 efficiency of a solution of 0.025 grams of heptylviologen dibromide in 0.5 grams of water. An efficiency of 0.10 ~~-21 OD/mC cm was obtained.
22 In like manner, the efficiency o a solution of 0.8 23 pts. by weight of Z-(4-hydroxy-3,5-dimethylphenyl)-4,5-24 bis~methoxyphenyl) imidazole, 1.1 parts ditertiary butyl benzoquinone, 2.7 parts aluminum p-toluene sulfonate, 4.8 26 parts dimethylformamide was evaluated. An electrochromic 27 efficiency of 0. 025 OD/mC cm was obtained. This is the 28 same formulation as described in Example 10 of U.S. Patent 29 3 r451~ 741.

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1 In addition, electrochromic solutions containing various 2 .G)ther pyrazolines as color producing agents were evaluated 3 for electrochromic efficiency using the apparatus with photopic 4 r.esponse. The results are summarized below:
. Compound Electrochromi~ Efficiency 6 (OD/mC cm 7 l-phenyl~ 3-2-aminophenyl~ 5-phenyl- 0.065 8 ~2-pyrazoline;
g l-phenyl-3-p-dimethylaminophenyl 0.055

5-phenyl-~ -pyrazoline;
11 1-phenyl-3-~-dimethylaminostyryl-5-~- 0.23 12 dimethylaminophenyl-~2-pyrazoline;
13 1-phenyl-3-~-dipropylaminostyryl-5-p-'~ 0.20 L4 dipropylaminophenyl-~2-pyrazoline;
1-~-methoxyphenyl-3-~-dipropylaminostyryl- 0.22 16 5--p-dipropylaminophenyl-~2-pyrazoline;
17 1-phenyl-3-~-methoxystyryl-5-p-methoxy- 0.035 18 phenyl-~2-pyrazoline;
19 1-~-methoxyphenyl-3-diphenylaminophenyl- . 0.12 'J 5-phenyl-~2-pyrazoline;
21 1-phenyl-3-p-diethylaminostyryl-5-~- 0.24 22 diethylaminophenyl-~2-pyrazoline;
23 1-~-methoxyphenyl-3-~-methoxystyryl_5_~_ 0.17 24 . methoxyphenyl-~2-pyrazoline;
1-~-bromcphenyl-3-~-diethylaminostyryl- 0.11 - 26 5-~-diethylaminophenyl-~2-pyrazoline;
27 1-~-methoxyphenyl-3 ~-dimethylaminostyryl- 0.17 .. .
28 5-~-dimethylaminophenyl-~2-pyrazoline;
. 29 1-phenyl-3-~-methoxyphenyl-5-o-methoxyphenyl- 0.03 ~ -pyrazoline:

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1 Compound (Continued~ Electrochromic Ef icienc~
2 .~ (OD/mC cm 2) 3 1-~o-methoxyphenyl-3-~-diethylamino- 0.18 , 4 styryl-5--~-diethylaminophenyl-~2-pyrazoline.

- 6 ' 7 The following examples are given solely for purposes .
, 8 of illustration and are not to be considered limitations 9 on the invention, many variations of which are possible , 10 without departing from the spirit or scope thereof.
11 Example I. A cell was prepared by filling the space 12 between parallel plate conductive electrodes with an ~", 13 electrochromic solution consisting of 0.4 mole l-p-met'hyl 14 phenyl-3-~-diethylamino5tyryl-5-p-diethylamino phenyl-,r~ 15 ~2-pyrazoline; 0~ 4 mole phenyl ~-benzoquinone; 1.0 mole .. , 16 tetrabutylammonium fluoborate in a 1:1 mixture o~ tetra-17 hydrofuran and acetonitrile. One of the electrodes con-'` 18 sisted of a thin indium oxide layer on glass while the 19 opposite electrode was gold on glass. The space between ~, 20 the plates was approximately 40 microns and was maintained 21 by a non-conductive spacer. Voltage was applied to the cell '; 2~ by means of a D.C. power supply. A positive voltage of 1.5 '' -' ' 23 volt~ was applied to the indium oxide electrode. The nega-24 tive lead was connected to a gold electrode. Application '~' 25 of a 10 millisecond pulse to the ceil produced a color change ', ` - 26 'from an initially light yellow to a blue green.
27 Example II. A cell consisting of conductive indium , ' 28 oxide coated glass plates was arranged with the conductive . . .
29 surfaces facing each other. The plates were separated ~o approximately 75 microns with a non-conductive spacer. The . .
,' ~. .... _ . _ 1~8144q 1 space between the plates was filled with a solution of .09 - Z ~ole l-p-methoxyphenyl-3-p-diethylamino styryl-5-~-diethyl-3 amino phenyl-~2-pyrazoline; .13 mole tetrabutylammonium 4 perchlorate; .09 mole 2,7.dinitrofluorene in N,N-dimethyl-acetamide. A potential of 6 volts was applied across the 6 cell by means o a power supply. A 100 millisecond pulse

7 produced an optical density change of 0.5 with a color

8 change from yellow to dark green.

9 Example III. A cell constructed as in Example II was filled with a solution of .09 mole l-~-methoxyphenyl-11 3-E~-diethylaminostyryl-5-~-diethylamino phenyl-~2-pyrazo-12 line;-.09 mole 1,5 dinitronapthalene; .13 mole tetrabutyl-13 ~ ammonium perchlorate in N,N-dimethylacetamide. With 6 volts ;
14 applied, the cell was driven to a dense green image. The background color was a pale yellow.
16 Example IV. A cell constructed of indium oxide coated 17 glass plates as in Example II was filled with the following 18 solution and the indium oxide plates were separated by a 125 lS micron non-conductive spacer. .09 mole 1-phenyl-3-~-methoxy phenyl-5-~-methoxy phenyl-~2-pyrazoline; .04 mole 21 phenyl p-benzoquinone; .13 mole tetrabutylammonium perchlorate 22 in dimethylformamide. Using 50 millisecond pulses from a 23 power supply set at 2.1 volts, a color ~change occurred from 24 very pale yellow to deep orange.
Example V. A formulation consisting of .09 mole 1-26 phenyL-3-p-diethylaminophenyl-5-phenyl-~2-pyrazoline, .04 27 mole phenyl-p-benzoquinone; .13 mole tetrabutylammonium 28 perchlorate in dimethylformamide was placed between con-2g ductive plates as in Example ~V. The cell was activated with a D.C. power supply set at 2.1 volts. The nearly colorless solution turned orange.
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, 108144q ' -1 Example VI. A solution of .09 mole l-~-methoxy phenyl-2 ~-~-dimethylamino styryl-S-p-diethylamino phenyl-~2 pyrazo-3 line; .04 mole phenyl-~-benzoquinone; .13 mole tetrabutyl-4 ammonium perchlorate in dimethylformamide was placed in a _ 5 cell as in Example IV. With 2.1 volts applied, a 50 milli-6 second pulse produced a dense green color from the initiaLly 7 pale yellow background color.
8 Example VII. A solution consisting of .09 mole 1-~'. ' g phenyl-3-E~-aminophenyl-5-o-methoxyphenyl-~2-pyrazoline;
mole phenyl-~-benzoquinone; .13 mole tetrabutylammonium 11 perchlorate in dimethylformamide was placed in a cell as in 12 Example IV. A potential of 2.1 volts across the cell pro-13 duced a black solution from a background color of pale.
14 amber.
; 15 Example VIII. A clock display panel was constructed 16 on two 2 x 2-1/2 inch glass plates coated with 50 ohm per 17 square transparent conductor (NESA glass). The front plate 18 was etched to give three digit positions of seven segment 19 numerals, plus one digit position for the numeral 1. Each segment lead was brought out to the glass plate edge for 21 connection to the drive signals. The back plate was left 22 unetched. This plate was the grounded backplane for the , , .
23 display. The glass plates were assembled with a 5 mil 24 spacer between them. This S mil cavity was then filled 25 with the electrochromic solution described in Example VII.
26 The drive for the electrochromic display panel was of 27 the direct segment drive type. Each segment electrode in 28 the display has one of two voltage levels applied to it 29 with reference to the backplane. The write level was a positive voltage of magnitude greater than the electrochromic , , .
, SA975070 -L2-1081~47 1 threshold. This voltage caused the written segments to 2 ~olor and to maintain that color. The clear level was a 3 negative voltage of magnitude less than the electrochromic 4 threshold. Segment clearing could be accomplished by either an open circuit or by shorting the segment to the backplane, 6 but application of the clear voltage causes the segment to clear more rapidly. The display was driven with a write 8 level of .58 volts at 13 microamperes. The clear voltage g was -.3 volts.

The drive unit to the display panel was a standard i;~
11 integrated circuit digital clock chip with special output 12 drivers giving the previously described voltage levels.

13 Minutes and hours were displayed on the panel with a switch L4 selectable minutes and seconds display mode. In the minutes and seconds mode, the least significant digit segments changed 16 at a one second rate. The clock was run continuously for 17 140 hours. Time was displayed in deep green numerals against , 18 a pale yellow background. The contrast decreased to a low 19 level at 24 hours. At 72 hours the segments had completely ` 20 faded out, but an increase in the drive potential again pro-21 duced a display with good contrast.

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Claims (8)

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

1. In a reversible electrochromic device comprising an oxidant/reductant pair, the improvement according to which the oxidant is a triaryl pyrazoline compound having the formula:

wherein A, A1 and A2 are each aryl groups, and n is zero or one.

2. A device as claimed in claim 1 wherein the oxidant is a 1, 3, 5-triphenyl-.DELTA.2-pyrazoline compound.

3. A device as claimed in claim 1 wherein the oxidant is a 1, 5-diphenyl-3-styryl-.DELTA.2-pyrazoline compound.

4. A device as claimed in claim l wherein the oxidant is 1-p-methoxyphenyl-3-p-diethylaminostyryl-5,p-diethylamino-phenyl-.DELTA.2-pyrazoline.

5. A device as claimed in claim 1 wherein the oxidant is 1-phenyl-3-p-diethylaminostyryl-5-p-diethylaminophenyl-.DELTA.2-pyrazoline.

6. A device as claimed in claim 1 wherein the oxidant is 1-phenyl-3-p-dimethylaminostyryl-5-p-dimethylaminophenyl-.DELTA.2-pyrazoline.

7. A device as claimed in claim 1 wherein the oxidant is 1-p-methoxyphenyl-3-p-dipropylaminostyryl-5-p-dipropyl-aminophenyl-.DELTA.2-pyrazoline.

8. A device as claimed in claim 1 wherein the oxidant is 1-phenyl-3-p-dipropylaminostyryl-5-p-dipropylaminophenyl-.DELTA.2-pyrazoline.