High Performance Anthraquinones

8/2/2019 High Performance Anthraquinones

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED- 30, NO. 5 , MAY 1983 49 9

New Photostable Anthraquinone Dyes WithHigh Order ParametersF. C. SAUNDERS, K. J. HARRISON, E. P. RAYNES, AND DAVID J. THOMPSON

Abstract-New anthraquinone dyes have beendevelopedwith high-order param eters and solubilities in bot h positive and negative dielectricanisotr opy liquid-crystal hosts. The dye s show high stability and allowthe formulation of black mixtures which show good contra st in a rangeof display devices.INTRODUCTIONL QUID-CRYSTAL displays using the twisted-nematicelectro-opticeffect are well know n nmanyconsumer-based applications.Althoug h this ty pe of display is ideallysuited t o application s requiring low voltages and ow powerconsumption, it does suffer rom a numb er of imitationsinherent in itsmod e of opera tion. Alternative devicesuse

pleochroic dyesdissolved in the liquid crystal1 ] Suc h devicesneed not use polarizers [2] (which are expensive, difficult tohandle, degrade inmoisture,an dcreate a dull appearance),have a significantly better angle of view, and can be use di-rectly on active substrates used t o switch complex displays.The choice of pleochroic dye is crucial to the performanceof the display. It should have a transition mom ent which lignsalong the local molecular axis of the liquid crystal, it must bereadily soluble in the liquid crysta l,ossess high photochemicalstability, and show range of colors.The first systems investigated were azo dyes [2], [3] whichshow xcellent alignment with he liquid-crystal molecules(high order parameter) bu t a very poor photochemical stabilitywhich restricts their use in displays. Pellatt, Roe, and C onstant[4] showed tha t t is possible to achieve excellentstabilityusing anthraqu inone dyes, but the order parameters they re-ported were marginal for atisfactory display performance.More recentlysomewh at higher orde r parameters have beenfound in anthraquino ne dyes [5], [6] throu gh the additio n of1 or 2 pendent groups, but the solubility and range of colorare still inadequate.We have investigated new types of anthraquinone dyes, andhave succeeded in synthesizing dyes with all theattributesnecessary fo r good display p erformance.

EXPERIMENTALA . Materials1 ) Dyes: The blue dyes were synthesized fromeither 1,5or 1,8 dihydroxyanthraquinone through substi tut ionwith theManuscript received October 7, 1982.F. C. Saunders, K. J. Harrison, and E. P. R aynes are with the Roy alSignals and Radar Establishment, Great Malvern, Worcestershire WR143PS, U.K.D. J. Thom pson is with Imperial Chemical Industries, Organics Divi-sion, Hexagon House, Blackley, Manchester M9 3DA, U.K.

appropriate alkyl group and nitration/reduction to give the re-quired structure. The sulphur yes were sythesized by substittion of a suitable anthraquinon e precursorith the appropriatethiophenol. In all cases, the crud e products were recovered byfiltration and purified by cry stallization and colum n chro matography where necessary. Purities were chtecked b y HPLC an delemen tal analysis.2 ) Liquid Crystal Hosts: Three nematichosts were usedfor testing the dyes. Two of these havepositive dielectricanisotropy, thecyanobiphenyl h ost E43 (BDH Chemicals, Ltd.)and thecyanophenylcyclohexane mix ture ZL1 11 32 (E. Merck)and are both broad-range mixtures with clearing points of 80"and 70" C, respectively. The third hos t h as negative dielectricanisotropy , can b e used t o produce displays with positive con -trast, and is a mixture of two fluorinated icyclo-octane esters[7] (FBCO).

This mixture is nematic from - 4" to 64"C, and at 20" C has aviscosity of 46 cP, a birefringence of 0.08 and a dielectric an-isotropy of - 1 l .B. Order Parameter

The optical order parameters (S) of the dye-host mixtureswere deduced fro m the absorbance measured parallel (A ,) andperpendicular (A l) to th e directorof parallel homogeneo uslayers using the expression

The layers were constructed from 3-mm thick glass spacedto (12 f 1) ,um by mylar and were aligned by using a rubbe dpolyvinyl alco hol coating. The absorbances were measured bya thermos tated Perkin-Elmer 55 4 double-beam spectrometerwith o ne beam containing a cell filled with the dye-host mix-ture and an id entical cell filled with the host alon e in the othbeam. The baselinewas checked to be zero at wavelengthswell away from the dye absorp tion. The eams were polarizedvertically using HN3 2 linear polarizers ;and bo th cellswererotated throug h 90" to obtain the spectra. for A l an d A l l . Therepeatability of the order arameters was generally within 0.01provided the absorbances measured fell within the range0.1 < A < 3.0.0018-9383/83/0599-0499$01 OO 0 ritish Crown Copyright 1983


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50 0 IEEERf'.:l?SACTIONSNLECTRONEVICES,OL .D-30, NO. 5 , MAY 1983

C S o h bilityThe solubilities of the dyes in the various hosts were mea.-sured spectroscopically. A saturated solution was obtained by

adding excess dye to he liquid crystal host, leaving to m.xthoroughly on a spiromixer for a eek at 20 "C, and thenpalis-ing throughaO.l-pmfilter. A measured quantity (typical..y50 mg) of the solution was dissolved in a standard volum e o fchloroform and ts absorbance was measured and comparc t lwith the absorbance of a standard solution f the dye ,D. Viscosity

The bulk viscosities of th e hosts and dye-host solutions wel'emeasured using a rotating cone viscometer (Brookfield), wilhthe tem perature controlled o within 1 "C.E. Life Testing

All the lifetime data quo ted were o btai ned using the dylEsdissolved inE43.The cells used in theseexperiments we:'(:parallel-aligned 30" evaporated silicon monoxide -coated plat(::;spaced and sealed with a screen printable material containir:!;9 pm spacers. Thesecells werevacuum filled with he dyr:mixture and their peak absorbance, dye order parame ter, ant/resistivity were m easur ed befor e being placed in th e life-testir.1:equipm ent. This consisted of a Microscal accelerated lighfastness tester [Mk IV] with a I-kW m ercury-fluorescent larrlpwhich simulates approximately 10 X sunlig ht. The cells were:held in cooled holders and their temperature throughout t1.k:course of he life testmaintained atabout 30C. The cet.parameters were monitored every hundre d hours to build u pa full picture of the fading proces s.

DYE STRUCTURESTwo classes ofdy ebased on he general nthraquinonestructure

were investigated, the difference being n the ype of aux o-chromic group X and in one class the presence of 2 penderllgroups A . A seriesof bluedyes (h,,,590 to 640 nm) W:IIE ,studiedwhere heauxochromicgroups are 2 amino (NH,;an d 2 hydroxy (OH) substituents, a combination common f(:a.blue extile dyes. The present series of dyes are additionally.substituted with 2 alkyl groups in the 2,6 and ,7 positions c ;Ithe anthraquinone ring.

2.6 2 , 1

Both straight-chain andbranched-chainalkylgroups we~:eused and in all cases these substituen ts were ortho to the h),.droxy group. The ch oice and position of the alkyl groups ha.donly a small effect on the absorption spectrum.

4 0 0 5 0 0 600 7 0 0W a v e l e n g t h i n m i

Fig. 1. Spectra of yellow, red, and blue dyesin E43.TABLE IORDER ARAMETERSN D SOLUBILITIESF S O M E ,6 A N D 2, 7 BLUE

DYES I N E43

OrderParameter I Solubility( w t% )

1.6

A second series of dyes was investigated containing 2 , 3 , o r4 auxochromic groups of the type

th esubstituent R being H , alkyl,alkoxy,or aryl. When 2such groups are present, the y are in he 1 ,5 positions givingyellow dyes (hma,-465 nm) and if 3 or 4 are used, the dyesare orang e (h,,490-520 nm )or red (h,,,520-550 nm),respectively. All theemaining available positions in theanthraquinone ring contain hydrogen. The nature of the sub-stituenton he sulphurhad virtually no effect on he dyeabsorption. Fig. 1 shows the spectra of a yellow, red, and bluedye in the host E4 3.

PROPERTIES O F TH E BLUE DYESTable I shows the order parameters and solubilities of fourblue dyes, measured in the host E43 and illustrates the effects

of different chain length and the two substitution patterns.The presence of the two pe ndent alkyl groups increases thevalue of both parameters com pared with he parent compounds(1,8 diamino 4,5 dihydroxy S = 0.64 sol. = 0.6 wt %,1 ,5 di-amino 4,8 dihydroxy S = 0.64 sol. =


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SAUNDERS et aI.: PHOTOSTABLE ANTHRAQUINONE DYES 5 0 1

TABLE I1PROPERTIESF BLUE DYESN DIFFERENTOSTST 20"CDYE SOLUBILITY (w t %)rder Parameter I

E43 I ZLI 1132 1 FBCO E43 1 ZLI 1132 1 FBCO1 I I I 1 I

havior of three dyes in differen t hosts, b ot h positive and neg-ative dielectric anisotropy, is show n in Table11.In a positive host, the order param eter may be increased to0 .80 and solubilities well in excess of 10 wt % were achieved.However, the highest values may not be achieved in the samecom pound and for practical use acomprom ise is necessary,e.g. dye B3. The extin ction coefficient of these dy ess -20 000and fo r display applications 1-2 wt % is necessary for the dif-ferentdisplay effects. In general the solubilities are slightlyhigher in a cyanophenylcyclohexane host.Although their absolute values are lower in the negative ho stFBCO, the dyes show a similar behaviour and dye B3 shows agood order parameter combined with he required solubilityfo rdisplayapplications even at low emperatures.Dye B1shows tha t a high order param eter is achievable in a n egativehost, a significant development as this is a pa rame ter which hasbeen retarding progress in positive-contrast displays.Viscosity of the liquid crystal is an important parameter indetermining esponse imesof the display and he presenceof a dye is know n to increase the viscosity. In the cya nobi-phenyl host E6 1 dy e B3 only increases the host viscosity (at20 C) slightly from 3 8 to 41 cP .

PROPERTIES F THE YELLOW,O R A N G E , ND RED DYESThe presence of differenthio substituents inh e 1 , 5 positionsgave yellow dyes of consistently high order parametersn E43 .However, the solubilitywas found to be much higher when hetwo substituents on the sulphur ere different (Table 111). Thisdifference may b e small, e.g. a tertia ry buty l gr oup eplacing ahydrogen. High order parameters were found with a ryl, cyclo-alkyl, and alkyl thio substituents n he 1,s positions whenmeasured in the different ositive hosts (Table IV).

Again the presence of two different substituents ncreasedsolubility compared with the corresponding symm etrical dyes.

TABLE 111COMPARISONF PROPERTIESF YELLOW DYESIT H S Y M M E T R I C A LN D~JNSYMMETRICAL SIJBSTITUENTSN 1E43 AT 20" C

H 1.7.79iH

2.0.80t -CqHq-CqHg7.8.79-cawg

TABLE IVPROPERTIESF SULPHUR DYESN DIFFERENTOSTS T 20 C-.________-

iSOLU8lLln

/It %I

-.The advantage of such substituents was further illustrated b ythehighorderparameters in the negat.ive host FBCO.Thevalueof 0.71-0.76 make uchdyesextremelyvaluable forpositive-contrast displays. Thepresenceof 3- (orange) or 4-(red) thio substituents again resulted in high order parametersand also significantly improved solubilities w hen one of thesubstituents was different. In the negative host FBCO theorderparameterswere again highand itwas ageneralcon-clusion that this property of the yellow, orange, and red dyeswas much less sensitive to the host.The imp ortan t influence of asymme try on solubility is theresult of such structures hindering crystallization of the dyefrom the liquid crystal host. Where 3 su.bstituents are presentthe molecule itself is asymmetric and th.e additional presenceof one different substituent r einforce s the ffect. Conseque ntlythe orangedyes have good solubilities even when the hiogroups are the same.Contrary to hesituation n he bluedyes, the ulphurdyes howa slightly highe r solubility incyanobiphenyl hosts. Increasing the number of thio substitu-entsproducesa slight change n extin ction coefficient from9000 (yellow) to 10 000 (red). These vdu es require th e presence of 1-3 wt % ofdye n thehost, depending upon hedisplay type and application.

S T A B I L ~ T YF THE DYESTh ephotochemicalstability of representativeexamples ofyellow, red, and blue dyeswas determined from measurementof the change in absorbance and resistivity of dye-host cellsFig. 2 shows the change in absorbance a fter1000 h.


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5 0 2 IEEE TRAY'lllACTIONS O N ELECTRON DEVICES, VOL. ED-30, NO . 5 , M A Y 1983r 1 I I I

1 0 0 2 0 0 50 0 1000

Fig. 2. Ab sorb anc e of anthraquinone dyes on exposure t o "microscal'lamp.

To provide comparative data, wo well-known anthraquinonedyes D l6 and D37 (BDH Chemicals, Ltd.) were mon itored,These d yes have been widely tested n display devices, D37being widely acknow ledged as extrem ely stable and suited tooutdo or applications. The present dyes have shown compar-able stability to D37, a decrease of only 10 percent in absor.,bance occurring after 10 00 h. Resistivities were found to fallinitially from a value of 10" SZ - cm and the n remain constant.at lo9 52 * cm over the period of exposure. AU dyes, excepl:D1 6, show ed no change in their X,,over the 10 00 h. How,,ever, Dl 6 hanged color from blue to neutral.

Exposure T i m e I h o u r s )

BLACKGUEST-HOST IXTURESThe two classes ofdyesmak e available a range of colortisuitable ormixing into blackdye-hostsolutions. Many ap.plicationsdemandsuchmixturesan doftenunderdifferentilluminations. A mixture suitable for use in daylight illumina,tion may be formulated using yellow (1.75 w t %), ed (1.2 W I:%) and blue (0.7 wt %) dyes. The spectrum in the host E 43 s;shown in Fig. 3. By addition of one of the orange dyes theminimum around 500 nm may be remove d and if a suitabledye absorbing above 650 nm is further a dded, ablack mixturesuited to a wider range of illuminations is obtained.The three-component dye mixture (3.7 wt %) howed an in.

crease in th e viscosity of the cyanobiphenyl host E61 (BDHChemicals , Ltd.) from 38 to 53 CP at 20".P E R F O R M A N C EF POSITIVECONTRASTCELL

The cell used to demonstrate a positive contrast dye displaywas produced using a hybrid alignment process to give a near,homeotropic alignment with a small uniform pretilto product:a preferred alignment direction, This was achieved by using :5" evaporated silicon-monoxide coating followed by applyin!;either a ecithin solution or a chrome complex in the usua:manner. The resulting alignment of the liquid crystal n tht:cellwas found to be very close to home otropic when theconoscopic figure was investigated. The cell (12 pm spacing:was filled with the FBCO host containing one of the blue dye:!(1 wt F6 and the electro-optic performance measured in transmission using a single polarizer. At 20 " C, the threshold wa!4.5 V and at 13.5 V the response imes Tonan d Teff wereeach 0.5 s. Fig. 4 shows the spectra of th e OFF an d ON (9 V:lstates, a contrast ratio of 5 1 being measured at thewavelengtlof maximum absorbance.

4 3 0 5 0 0 60 0 700Wav e l e n g t h l nm i

Fig. 3. Spectrum of a black mixtu re inE63.

1 , 1450 30 5 3 ~,,, ,600 650 730 X10Fig. 4. Spectra of th e ON an d OFF states in a positive contra st displaycontaining a blue dye.

CONCLUSIONTwo new classes of anthraq uinone dyes ave been developed,bot h being capable of m anufacture on a commercial scale to ahigh level of purity, and thus immediately available for use indye-host mixtures fo r a wide range of applications. By choiceof pendent alkyl groups, blue dyes with high order parameters

and solubilities in bo th positive and negative dielectric aniso-trop y hosts have been prepared, substitution inhe 2 ,7positionbeing shown to be a new and useful structural va riation.ellow,orange, and red dyes of consistently high order parameters areavailableusing 2, 3, or 4 thio substituents. High solubilitiesare achieved using asymmetry in these substituents. All dyesshow high photochemical stability and may besed to formulateblack mixtures. The goodperformanceof hesedyes nanegative host is reflected in a good contrast positive contrastdisplay.A range of colors can now be provided by anthraquino nedyes, all showing the requiredphysicalproperties for manyguest-host effects. These dyes are a crucials tep forward to thepractical applications of thisype ofdisplay.A C K N O W L E D G M E N T

The authors gratefully acknowledg e the technical assistanceof colleagues at heRo yal Signals andRadarEstablishmentand ImperialChemical Industries, Organics Division, n synthesisand physical assessment of th e dyes.REFERENCES

[1 G. H. Heilmeier an d L. A. Zanoni, Appl . Phys. Lett . , vol. 13, p. 91,[21 D. L. White and G. N. Taylor, J. App l . P h y s , vol. 45, p. 4718,[3 ] J. Constant, J. Kirton, E. P. Raynes, I. A. Shanks, D. Coates, G. W.

1968 .1974 .


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IEEE TRA NSACTIONS ON ELECTRON DEVICES, VOL. ED-30, NO. 5 , MAY 19830 3Gray , and D. G. McD onnell, Electron. Lett . , vol. 12, p. 514,1 97 6. [6 G. Heppke, B. Knippenberg, A.Mofler, andG.Scherowsky, n[ 4 ] M. G. Pella tt, I.H.C. R oe an d J. Consta nt, MOL Cryst. Liq. Cryst., Proc.Eurodisplay 1981 (Munich,Germany)Sept. 16-18,1981,voL 59, p. 299, 1980. voL 25.[5] J. C ognard nd T. HieuPhan, Mol. Cryst. Liq. Cryst., vol.70 , [7] G. W. G r a y a n d S . M. Kelly,Mol. Cryst. Ljiq. Cryst.,vol. 75, p. 109p. 127 9, 1981 . 1982.

A Full-Color Matrix Liquid-Crystal Display with ColorLayers on the E ectrodes

Absrracr-A full-color m atrix liquid-crystal display panel with colorstr ipeayers ofed ,reen, andlu e on the Y-electrodes is proposed. 6 k ~ ~ ~ P ! k ~ ~ ~LECTRODEThe color str ipe layers of 300-pm pitch are made by photoli thography.The color purities of greenish, bluish, and purplish colors obtained by . R : G , .k :.G ,E..:G:/ COLOR LAYER. . TRANSPARENTLECTRODEthe liquid-crystalisplayanelre as good as thosefypicalrinted -----.-LASSLATEinks,hilehosefellowish and reddisholors areo o rt present. j i;;:N]discussed.

, , , , , . , . , , . ~ L l Q U l D R YST AL

In addition, effects of the color layers on the display properties areFig. 1. Cross section of the multicolorLCD.

T 11 I. I NTR ODUC TI ONWISTEDNEMATIC liquid-crystal displaysTN-LCDs)are increasingly being used for numeric displays andma trix displays. According to expansion of their application s,color LCDs are desired in some fields because novelty, view-ability, and variety of display are expec ted. Especially for thelatter two item s, multicolor or full-color display is consideredto be necessary. Among various color LCDs, however, anelectrically co ntro lled birefringenc e cell (ECB-cell) [2 ] - [4] isthe only LCD tha t can display multicolor by itself, while theoth er LCDs such as a TN-cell using a dichroic filter or a bire-fringent film [SI, 6 ] , guest-host cell (GH-cell) [7 ] , [8] anda choleste ric-nem atic phase change cell using optic al rotato rydispersion [9] are all mo noc hrom e or two-color displays. Th eECB-cell has the disadvantages of narro w viewing angle, poorpurity in some colors, and difficulty in controlling displayedcolor because it is sensitive to cell gap and tem pera ture. There-fore he uthors [lo] have investigated a multicolor LCDwithcolor layers on the elec trode s as shown in Fig. 1. Theliquid crystal itself acts as a light value, so that he wisted

Manuscript received September 19, 198 2; evised December 20, 198 2.T. Uchidaan d Y . Shibataar ewith heDepartment of ElectronicS. Yamamoto is with the Dainippon Screen Manufacturing Company,Engineering, Tohoku University, Sendai, 980 Japan.Ltd. , Kyoto, 620 Japan.

POLARIZER L ~ Q U I DCRYSTAL(a )

PI

/ / \ IPOLAR IZ ER LIQUID DYECRYSTAL

(b 1Fig. 2 . The multic olor LCD using (a) TN-mode and (b) GH-mode.

nematic mode (TN-mode) with parallel polarizers or the guesthost mode (GH-mode) with black color can be used for thisdevice as shown in Fig. 2 . In both c a m , incident light is cuoff at off state. When a voltage s applied to a segment, thelight passes throu gh the segment and is colored by the cololayer onhe lectrode . By applying this system to usua0018-9383/83/0500-0S03$01.00 0 1983 IEEE

High Performance Anthraquinones

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