Efficient electroluminescencefr om a newn-type conjugatedpolyquinoline

X.Zhang, A.S.ShettyandS.A.Jenekhe*

Departmentsof Chemical EngineeringandChemistry andCenter for Photoinduced ChargeTransfer, Universityof Rochester, Rochester,NY 14627-0166,USA

The synthesisof a new conjugatedrigid-rod polyquinoline, poly(2,29-(p-phenylene)-6,69-bis(4-(p-tert-butylphenyl)quino-line)), andits incorporationin light-emitting diodesasthe emissivelayer are reported.Theseelectroluminescentdevices,containing 1,1-bis(di-4-tolylaminophenyl)cyclohexane(TAPC) dispersed in polystyreneas the hole-transportlayer, emitbrightyellow light (kmax = 554nm)with aquantumefficiency of 0.26%photons/electronanda luminanceof 280cd/m2 atacurrent densityof 100mA/cm2. Theseresultsalsodemonstratethat thenewpolyquinolineis a goodn-type(electrontrans-port)electroluminescentmaterial.

Conjugated rigid-rod polyquinolines have excellentthermal stability and high mechanical strength [1, 2].Thesen-type semiconducting polymers[3] haveinterest-ing electronic [2b], photoconductive [4], and nonlinearoptical [5] properties.Recently, somepolyquinolineswereusedas both theelectron-transport layerandanemissionlayer in polyquinoline/poly(p-phenylenevinylene) hetero-junction light-emitting diodes (LEDs) [6]. The attractivecombination of excellentthermal, mechanical, andopto-electronic propertiesof theconjugatedpolyquinolineshasmotivatedour synthesis of new derivatives for electrolu-minescent deviceapplications.

The conjugatedrigid-rod polyquinolines have limi tedsolubility in commonorganicsolvents.They are,however,highly soluble in commonorganicsolventsasLewis acid(GaCl3, AlCl 3) or alkyl/aryl phosphate complexes [1a,2a]. To simplify thin film processing and improve opto-electronic properties of the polyquinolines for deviceapplications,we are currently exploring the synthesisofconjugated polyquinoline derivatives that may have aninherent solubility in organic solventsthrough sidegroupsubstitution, which could also influence the electronicstructure and optical properties. A previous attempt atimproving the solubility of rigid-rod polyquinolines byappending aryl ethergroupsto thepolymerbackbonevia afairly involvedsynthesisof themonomerswasunsuccess-ful [7]. In this paper, we report the synthesis,photolumi-nescence, and electroluminescence of a new conjugatedpolyquinoline, poly(2,29-(p-phenylene)-6,69-bis(4-(p-tert-butylphenyl)quinoline)) (Bu-PPQ, 2). The relatedpoly(2,29-(p-phenyl)-6,69-bis(4-phenylquinoline)) (PPPQ,1) is known to besoluble in protonicacidsolvents(metha-nesulfonic acid, trifluoroaceticacid, and formic acid),di-m-cresyl phosphate/m-cresol, and Lewis acid/nitro-methane solutionsfrom which thin films canbe prepared[2]. In Bu-PPQ(2), two tert-butyl groups were incorpo-rated into the pendant phenyl groups of the polymer asshownin Scheme1.

The new monomer 3,39-di(499-tert-butylbenzoyl)benzi-dine (10) wassynthesizedaccording to a modified litera-ture procedure(Scheme 1) [1b]. Treatmentof bromide 3with potassiumcyanidein acetonitrile atroomtemperatureaffordednitrile 4 [8] in quantitative yield, which on con-

densation with 5 in basicmethanolyieldedanthranil 6 [9].Reduction of theanthranil with iron andacetic acid[9] fol-lowed by treatment with trifl uoroacetate group gave 8[1b]. Couplingof 8 with half anequivalentof hexamethyl-ditin in dry tolueneandusingapalladium catalystaffordedbiphenyl9 (70%yield),whichonrefluxingwith potassiumcarbonateandethanol for 8 h to allow for thedeprotectionof the amino groups, yielded monomer 10 in quantitativeyield [1b].

Polymerization of 10 with diacetyl benzene(11) wasconducted in a m-cresol/diphenylphosphatemedium, at1408C for 48 h underargon atmosphere, to Bu-PPQ (2).Af ter cooling, the polymerization dopewas precipitatedinto an ethanol-tri ethylamine solution [2b]. The precipi-tatedpolymer wascollectedby suctionfilt rationandcon-tinuously extractedfor 24h with anethanol-triethylaminesolutionand driedin vacuo.Thenew polymer, obtained in

52 Acta Polymer., 49, 52–55 i WILEY-VCH VerlagGmbH,D-69451Weinheim1998 0323-7648/98/0101-0052$17.50+.50/0

Fax:+1-716-442-6686E-mail: jenekhe@che.rochester.edu

Scheme1.

Acta Polymer., 49, 52–55 (1998) Efficient electroluminescencefrom a newn-typeconjugatedpolyquinoline 53

essentially quantitative yield (A95%), with high thermalstability, wascharacterizedby Fourier transform infrared(FT-IR), UV-vis,and1H NMR spectroscopies,and by ther-mogravimetric analysis (TGA). Disappearanceof carbonylabsorption bandsof themonomersandappearanceof newstrongbandsbetween 1600 and 1400 cm–1 in the FT-IRspectraof the polymersconfirmed the complete cycliza-tion to form the quinoline rings [1, 2]. The low intrinsicviscosity of Bu-PPQ (1.28dl/g in methanesulfonic acidat308C) impliedalow molecularweight; however, excellentfilm-f orming properties were obtained. The tert-butylgroups were intendedto disrupt the coplanar array in thepolymer chain through steric interference of the tert-butylgroups,therebypreventing thedensepacking of thechainsin thesolid stateandconsequently influencingthesolubil-ity andoptical properties.However, Bu-PPQ(2) hadsimi-lar solubility properties to PPPQ(1). The photolumines-cenceand electroluminescence of Bu-PPQwere signifi-cantlydifferent from thoseof PPPQ(1).

Figure 1 shows the optical absorption, steady-statephotoluminescence (PL), and electroluminescence (EL)spectraof Bu-PPQ.This polymer shows a strongabsorp-tion with a p-p* transition at 399nm. Theoptical absorp-tion edgebandgap is 2.78eV (446nm). Bu-PPQthus hasabsorption peak andbandgapidentical to thoseof PPPQ,which were previously reported[2b]. This suggeststhatthe ground stateelectronic structure of PPPQ doesnotchangewith the introduction of the tert-butyl groups inBu-PPQ.

Also shown in Fig. 1 is thesteady-statePL spectrumofBu-PPQthin film excited at 399nm. The PL spectrumshowsan emissionpeak at 554nm. The yellow emissioncorrespondsto a large Stokes shift of 155nm (0.87eV),which is characteristic of excimer emission of the solidfilm of manyconjugated polymers[10]. ThePL emissionof Bu-PPQshoweda 20 nm blue shift comparedto PPPQ(1), which hasa PL emissionpeak at 574nm. This indi-catesthat the introduction of the tert-butyl groups in Bu-PPQmodifiesthe excited state electronic structure of theparentpolymer PPPQ. That chain packing of conjugated

polymersshould significantly influence their excitedstateproperties more thantheir ground stateelectronic proper-tieshasbeen predicted[10].

TheEL spectrumof thedeviceITO/TAPC:PS/Bu-PPQ/Al atabiasvoltageof 8 V is shown in Fig. 1.TheEL emis-sion peak is at554nm.It canbeseenthattheEL spectrumis identical to the PL spectrum, indicating that the ELemissionis from theBu-PPQlayeronly andthat theelec-troluminescenceandphotoluminescenceof Bu-PPQorigi-natefrom thesameexcitedstates.

Figure 2 shows the current–voltage and luminance–voltagecharacteristics of theEL device. The turn-on vol-tage of the ITO/TAPC:PS(50nm)/Bu-PPQ(50nm)/Aldevicewas 8 V. It showed bright yellow color emission,whichcanbeclearlyseenunderroomlight. Theluminanceof the device was 280cd/m2 at a current density of100mA/cm2. The EL efficiency of the device was esti-mated to be 0.26% photons/electron. Thesevalueswereaboutsix timeshigher than thoseof PPPQ(1) underthesame conditions. For example, the device ITO/TAPC:PS(50nm)/PPPQ(50nm)/Al had a luminance of45 cd/m2 at thesamecurrentdensity of 100mA/cm2. Thelarge enhancement of electroluminescence in the tert-butyl-substitutedBu-PPQ(2) comparedto theparentpoly-quinoline1 canbeunderstood in termsof reducedconcen-trationquenching of luminescencefacilitated by increasedinterchainpacking distances [10, 11]. Observation of effi-cient electroluminescence in the ITO/TAPC:PS/Bu-PPQ/Al devicesimplies that the emissive Bu-PPQlayer alsoexhibits goodelectrontransport (n-type)characteristics,inaccordwith prior findingsfor otherpolyquinolines[3, 6].

In summary, a new electroluminescent, yellow light-emitting n-typeconjugatedpolymer, Bu-PPQ(2), hasbeensynthesizedand characterized. Although the substitutionof tert-butyl groups in Bu-PPQdid not changetheopticalabsorption propertiesof theparent PPPQ, a more efficientelectroluminescence with an EL quantumefficiency of0.26% photons/electronandaluminanceof 280cd/m2 wasobserved. The brightness andefficiency of light-emittingdiodesfrom Bu-PPQ (2) weresix timeshigher that those

Fig. 1. Optical absorption,photoluminescence(excitedat399nm)andelectroluminescence(8 V) spectraof Bu-PPQthin films.

Fig. 2. Current–voltageandvoltage–luminance characteristics oftheelectroluminescentdeviceITO/TAPC:PS/Bu-PPQ/Al.

54 Zhang,Shetty, Jenekhe Acta Polymer., 49, 52–55 (1998)

of PPPQ(1) with no tert-butyl substitution. Theseresultsalsodemonstratethat thin films of thenewpolyquinolineexhibit goodelectron transport (n-type) characteristics inEL devices.

Experimental

4-tert-Butyl benzylnitrile (4) wassynthesizedaccordingto a literature procedure[8]. Theotherintermediatesweresynthesizedby modifying literatureproceduresas is out-linedbelow.

3-(p-tert-Butylphenyl)-5-bromoanthranil(6) [9]

To a solution of 360g KOH in methanol (500ml) andTHF (100ml) was addedwith stirring and cooling, in anice bath,40 g (0.23mol) of 4-tert-butyl benzylnitri le (4)andasolutionof 44g (0.23mol) 4-bromonitrobenzene(5)in methanol(300ml) andTHF (50 ml). The mixture wasstirredfor 16 h at roomtemperatureand 1.5 l of water wasthenaddedwith stirring. The precipitate was isolatedbyfiltration, washed with water, anddried. The product wasfurther purified uponcrystallization from hexane aspaleyellow needles(50 g, 70%).1H NMR (CDCl3, 300MHz):d 8.06 (s, 1H), 7.95 (d, 2H), 7.61 (d, 2H), 7.55 (d, 1H),7.40 (dd, 1H), 1.50 (s, 9H). MS (EI) 331 (71%), 329(64%),57.10 (100%).

2-Amino-5-bromo-49-tert-butylbenzophenone(7) [9]

A solutionof 16.8g (51mmol) of 6 in 300ml of aceticacidwasheatedat 808C, and24 g (8.5eq.) of iron fili ngswas addedover 4 h, during which 30ml of water alsoadded.The mixture was then cooled, diluted with 1 l ofwaterand extractedwith ether (5 6 100ml). Theextractwaswashed with dilute sodiumcarbonatesolution (2 6100ml) andbrine(26 100ml) anddriedovermagnesiumsulfate. Theetherwasremovedunderreduced pressure toafford the product asbright yellow crystals(14 g, 83%).1H NMR (CDCl3, 300MHz): d 8.58(d, 1H), 7.85(d, 1H),7.80(dd,1H), 7.75(d,2H), 7.59(d,2H), 1.40(s,9H). MS(EI) 333(80%),331(100%), 276(60%).

5-Bromo-2-(trifluoroacetamido)-49-tert-butylbenzophenone(8)[1b]

To a solution of 9.5g (28.6mmol) of 7 in 350ml offreshly distilled (Na/benzophenone) diethyl ether wasadded35 g (0.21mol) of anhydrous sodium carbonate.Thereaction mixturewascooledin anice bath,and30 ml(0.21mmol) of freshly distilled trifl uoroacetic anhydridewasadded dropwiseasrapidly aspossible while maintain-ing a gentlereflux. Upon completion of the addition, theice bathwasremovedandthereaction mixture stirred for1 h. 500ml of waterwas addedandtheorganic layerwasseparated andwashedwith brine (2 6 100ml) anddriedwith magnesium sulfate.The solventwasremovedunderreduced pressureto afford the product (8) as light brown

needles (11.7g, 95%). 1H NMR (CDCl3, 300MHz): d8.58 (d, 1H), 7.85 (d, 1H), 7.80 (dd, 1H), 7.75 (d, 2H),7.59(d, 2H), 1.40(s,9H). MS (EI) 429(30%),427(35%),294(100%).

4,49-Bis(trifluoroacetamido)-3,39-di(para-tert-butyl)benzoylbi-phenyl(9) [4]

A heavy-walled flask (250ml) waschargedwith a sol-utionof 0.64g (0.54mmol, 2 mol%) of tetrakis(triphenyl-phosphine)palladium(0) in 50 ml of freshly distilled (Na/benzophenone)deoxygenatedtolueneunderargon.To theabovewasadded a solution of 11.7g (27.3mmol) of 8 in60 ml of dry toluene. The reactionmixture wasdegassedandback-filled with argonthreetimes,and stirredat808C.To this mixture wasaddeddropwiseover1 h a solution of4.47g (13.6mmol) of hexamethylditin in 50 ml of drytoluene. After heating for an additional 10 h the reactionwascooledandtheproduct isolatedby suctionfiltration toafford theproduct9 asa paleyellow powder (7.1g 75%).1H NMR (CDCl3, 300MHz): d 8.70(d, 2H), 7.89(d, 2H),7.80(dd,2H), 7.72(d, 4H), 7.52(d, 4H), 1.40(s,18H).

3,39-Di(para-tert-butyl)benzoylbenzidine(10)

A mixture of 5.37g (7.7mmol) of 9 and 20 g(144mmol) of anhydrouspotassium carbonate in 200mlof ethanol and 100ml of waterwasrefluxedfor 6 h. Af tercooling, the precipitate was isolatedby suctionfiltrationandthenstirredin 150ml of water to dissolveanyremain-ing potassium carbonate. Filtration and drying yielded3.38g (87%)of 10. 1H NMR (CDCl3, 300MHz): d 7.6(m,6H), 7.89(m,6H), 6.80(d,2H), 6.0(s,4H), 1.40(s,18H).

Polymer 1 was synthesized according to the literatureprocedure[2]. ThenewpolymerBu-PPQ(2) wassynthe-sizedas follows. Equimolar amounts of both 3,39-bis(p-tert-butyl)benzoyl benzidine (0.28g) andp-diacetylben-zene (0.09g) were addedalong with 5.0g of diphenylphosphate(DPP)and 10 ml of freshly distilled m-cresol inaglassreactor fitted with mechanicalstirrer, two gasinlets,and a side arm. The reaction mixture was purged withargon for 15min, andthenthe temperaturewasraisedto1408C underpositive pressureof argon.The temperaturewasmaintainedfor 48 h,during whichtimesmall amountsof m-cresol to facilitate efficient stirring of the reactionmixture were addedwhenever it becamehighly viscous.Af ter cooling, the polymerization mixture was slowlypoured into a stirring solution of 500ml of ethanol/50mlof triethylamine. The precipitatedpolymer wascollectedby suctionfiltration: [g] = 1.28dl/g (308C, methanesulfo-nic acid). FT-IR (free standing film , cm–1): 3056, 3028,1585, 1572, 1541,1486, 1353, 1182,826,587.

TheLED fabrication andcharacterizationaresimilar topreviously reported procedures [6,12]. EL quantum effi-cienciesof thediodeswere estimatedby usingproceduressimilar to thosepreviously reported[13]. All the fabrica-tion andmeasurements weredoneunderambient labora-tory conditions.

Acta Polymer., 49, 52–55 (1998) Efficient electroluminescencefrom a newn-typeconjugatedpolyquinoline 55

Acknowledgement

This research was supported by the Office of NavalResearch and the National Science Foundation (CTS-9311741,CHE-9120001).

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ReceivedSeptember23,1997