Thin Solid Films 417(2002) 215–220

0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž02.00577-1

Synthesis and characterization of a novel conjugated polymer containingPPV oligomer and fluorene

Su Lu, Qu-Li Fan, Yang Xiao, Soo-Jin Chua, Wei Huang*

Institute of Materials Research and Engineering (IMRE), National University of Singapore, 3 Research Link, Singapore 117602, Singapore

Received 11 January 2002; accepted 30 May 2002

Abstract

A novel conjugated polymer containing fluorene and oligo-PPV units which are alternatingly arranged in the backbone wassynthesized by using Suzuki coupling reaction. The chemical structure of the polymer was characterized by H-NMR and FT-IR.1

The polymer shows good solubility in CHCl , Tetrahydrofuran and CH Cl etc. TGA revealed good thermal stability of the3 2 2

polymer. Optical properties were characterized by using Ultraviolet–visible and Photoluminescence spectroscopies, indicating blueemission of the polymer.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Conjugated polymer; Emission; Copolymer

1. Introduction

Since Poly(p-phenylenevinylene) (PPV) was initiallyreported to be the electroluminescent material in 1990w1x, great progress has been made in developing newpolymers for light-emitting diode usesw2,3x. Polymericlight-emitting materials share a lot of advantages withtheir organic molecular counterparts such as low drivingvoltage for operation and high quantum efficiency fordevices, while offer the ease of processing into largeand flexible displays over the latterw2–6x. Therefore,polymeric lighting-emitting materials have attracted con-siderable attention in recent years because of the poten-tial applications in the future lighting, back light sourceand full color flat panel displays.Emission colors covering the whole range of the

visible spectrum have so far been realized for thepolymeric electroluminescent materials. Tuning emissioncolors of the polymer materials at a molecular level isbased on adjusting the HOMO–LUMO band gap whichis one of the crucial factors to determine the emissionwavelength for the conjugated polymers, to a value

*Corresponding author. Tel.:q86-21-65103617; fax:q86-21-65640293.

E-mail address: wei-huang@fudan.edu.cn(W. Huang),chehw@nus.edu.sg(W. Huang).

which corresponds to a certain electron transition energy.Practically two strategies have been developed success-fully. The first is modification of the polymer backboneswith suitable functional substitutes or building blocks ofdifferent electron affinities, and based on their inductiveor steric effects or both, the molecular electronic struc-ture and effective conjugation length can be tailoredeffectively, resulting in the desired emission colorsw7–11x. The second employs the organic dyes and well-defined or statistical oligomers as the chromophores,which can be either attached as side groups to orintegrated in the polymer main chainw12–14x. Theobtained emission upon excitation will be the character-istic ones of the prefabricated emitters in the polymers.Based on the last approach, a great variety of chro-

mophores have been incorporated into the polymerbackbones, and non-conjugated chains are usuallyemployed as linkages. These flexible segments haveseveral favorable effects on the performance of thematerials, including controlled emission wavelength andenhanced photoluminescence and electroluminescenceefficienciesw15,16x. However, in such an approach onehas to face some negative effects. For instance, theintroduced non-conjugated spacers dramatically decreasethe rigidity of the polymer main chain, which affectsthe molecular order of the polymerw17x. Furthermore,

216 S. Lu et al. / Thin Solid Films 417 (2002) 215–220

Scheme 1. Chemical structure of the polymer PF-PPV.

higher turn-on voltages will occur for the devices fab-ricated using these materials.In this paper, we report on the design, synthesis, and

characterization of a new blue light-emitting polymerPF-PPV (Scheme 1) containing fluorene units, whichare alternatingly connected with the middle benzenerings of PPV oligomers with defined length. As theconjugated spacers, fluorene units may provide thepolymer unusual physical properties, while are expectedto have minimum effect on the emission color.

2. Experimental

2.1. Materials and measurements

2,5-Dibromo-p-xylene, N-bromosuccinimide(NBS),triethyl phosphite, 4-methoxyphenol, potassiumtert-butoxide (1 M in Tetrahydrofuran(THF)), n-octylbromide and tetrakis(triphenylphosphine)palladiumfrom Aldrich Chemical Co. were used without anyfurther purification. THF was dried and distilled fromsodium. All other solvents and reagents were purchasedcommercially, and used without any further purification.Nuclear magnetic resonance(NMR) spectra were

collected on a Bruker ACF 400 spectrometer usingchloroform-d as a solvent and tetramethylsilane as aninternal standard. Fourier transform infrared spectra wererecorded on a Bio-Rad FTS 165 spectrometer by dis-persing samples in KBr discs. Ultraviolet–visible(UV–vis) and Fluorescence spectra were obtained using aShimadzu UV 3101PC UV–vis–near infrared spectro-photometer and a Perkin-Elmer LS 50B luminescencespectrometer with a xenon lamp as light source, respec-tively. GPC analysis was conducted with a Waters 2690separation module equipped with a Waters 410 differ-ential refractometer HPLC system and Waters StyragelHR 4E columns using polystyrene as the standard and

chloroform as the eluant. TGA analysis was performedon TA instruments, TGA 2950, and DSC was carriedout on DSC 2920. TGA was run at a heating rate of 108Cymin while that of DSC was 208Cymin.

2.1.1. Compound 1To a 500-ml two-necked flask containing 2,5-dibro-

mo-p-xylene (26.4 g, 0.10 mol), NBS (35.6 g, 0.20mol) and a catalytic of AIBN, 200 ml benzene wasadded. The mixture was refluxed over night. After beingcooled to room temperature, the mixture was filtered,and the excess of benzene in the filtrate was removedby rotary evaporation in a 500-ml flask, where anhy-drous potassium acetate(30 g, 0.306 mol), 200 mlacetic acid and 5 ml acetic anhydride were then intro-duced successively. The reaction mixture was refluxedfor another 5 h, and most of the solvent was removedunder reduced pressure. The residue was poured into300 ml water and the resulting mixture was extractedwith chloroform for three times. The combined chloro-form solutions were washed with water and saturatedsodium hydrogencarbonate solution successively andthen were dried over anhydrous sodium sulfate andfiltered. After removal of the solvent the resulting solidwas recrystallized from ethanolychloroform to yield 23g (61% yield) of white crystals. H NMR(CDCl ): d1

3

7.63 (s, 2H, Ar–H), 5.17 (s, 4H, CH), 2.19 (s, 6H,2

CH ). Anal. calcd for C H Br O : C, 37.93; H, 3.18.3 12 12 2 4

Found: C, 38.09; H, 3.21.

2.1.2. Compound 2A mixture of compound 1(10.0 g, 26.3 mmol) and

potassium hydroxide(4.43 g, 79.0 mmol) in 100 ml ofethanol was refluxed for 2 h. After cooling, the mixturewas concentrated and was poured into 200 ml of distilledwater. Hydrochloric acid of 2 M was added dropwise tothe above mixture until it turned neutral. It was thenextracted with ether for three times, the organic layerwas collected and dried over anhydrous sodium sulfate.After removal of the solvent, the crude product waspurified by recrystallization from ethanolychloroform toafford 5.5 g (70% yield) white crystals. H NMR1

(DMSO-d6): d 7.71 (s, 2H, Ar–H), 5.50 (s, 2H, OH),4.42(s, 4H, CH). Anal. calcd for C H Br O : C, 32.47;2 8 8 2 2

H, 2.72. Found: C, 32.61; H, 2.61.

2.1.3. Compound 3The mixture of compound 2(2.66 g, 9.00 mmol),

SOCl (5 ml, 68.5 mmol) and 20 ml benzene was stirred2

at 50 8C until no bubble was observed. After stirringfor another 4 h, the mixture was cooled to roomtemperature and then poured slowly into a beakercontaining 100 ml ice-water. The aqueous phase wasextracted with chloroform for three times, and thecombined organic layers were washed with water andsaturated sodium hydrogencarbonate successively, fol-

217S. Lu et al. / Thin Solid Films 417 (2002) 215–220

Scheme 2. Synthetic route to the polymer PF-PPV: reagents and conditions:(i) NBS, CCl ; (ii) NaOAc, HOAc;(iii ) KOH, EtOH; (iv) SOCl ;4 2

(v) P(OEt) ; (vi) CHCl , KOH; (vii) K CO , n-C H Br, DMF; (viii ) t-BuOK; (ix) Pd(PPh) , 2 N Na CO .3 3 2 3 8 17 3 4 2 3

lowed by being dried over anhydrous sodium sulfate.The crude product was recovered by rotary evaporation,and 2.7 g(90% yield) white crystals were obtained afterrecrystallization from chloroformyhexane. H NMR1

(CDCl ): d 7.73 (s, 2H, Ar–H), 4.66 (s, 4H, CH).3 2

Anal. calcd for C H Br Cl : C, 28.87; H, 1.82. Found:8 6 2 2

C, 29.01; H, 2.10.

2.1.4. Compound 4A solution of 2.60 g compound 3(7.81 mmol) and

15.0 ml triethyl phosphite(87.5 mmol) was refluxedover night under argon atmosphere, after which theexcess of triethyl phosphite was removed under reducedpressure. The residue was solidified and recrystallizedfrom chloroformyhexane to afford 3.7 g(89% yield)white crystals. H NMR(CDCl ): d 7.66 (d, 2H, Ar–1

3

H), 4.07 (q, 8H, OCH), 3.33 (d, 4H, Ar–CH ), 1.292 2

(t, 12H, CH ). Anal. calcd for C H Br O P : C, 35.84;3 16 26 2 6 2

H, 4.89. Found: C, 36.08; H, 4.79.

2.1.5. Compound 5To a 500-ml three-necked flask was added 16.1 g 4-

methoxyphenol(0.130 mol) and 60 g(1.1 mol) potas-sium hydroxide solution in 50 ml water with stirringuntil a clear phenol solution was obtained. The mixturewas kept at 708C while chloroform(31 g, 0.26 mol)was added dropwise at a speed that the chloroform wasgently refluxed. The reaction mixture was turned to darkbrown and stirring was continued at room temperaturefor 1 day. The aqueous phase was separated and extract-ed with chloroform. The combined organic layers werewashed with water, 5% hydrochloric acid and saturated

218 S. Lu et al. / Thin Solid Films 417 (2002) 215–220

Fig. 1. FT-IR spectrum of polymer PF-PPV in KBr pellet.

Fig. 2. TGA thermogram of PF-PPV recorded in air at a heating of 108Cymin.

sodium hydrogencarbonate solution successively, anddried over sodium sulfate. The chloroform was thenremoved by rotary evaporation. The residual dark oilwas further purified by silica gel column(ethyl ace-tate:hexanes1:15) to afford 7.9 g colorless liquid(40%yield). H NMR (CDCl ,): d 10.60 (s, 1H, Ar–OH),1

3

9.80 (s, 1H, CHO), 7.10(m, 1H, Ar–H), 6.95(m, 1H,Ar–H), 6.87 (m, 1H, Ar–H), 3.78 (s, 3H, CH). Anal.3

calcd for C H O : C, 63.15; H, 5.30. Found: C, 63.24;8 8 3

H, 5.25.

2.1.6. Compound 6A mixture of 2-hydroxy-5-methoxybenzaldehyde

(3.04 g, 20.0 mmol), 1-bromo-n-octane(4.25 g, 22.0mmol), and potassium carbonate(4.15 g, 30.0 mmol)in N,N-dimethylformamide(20 ml) was heated to 708C for 12 h. The reaction mixture was cooled to roomtemperature and poured into ice-water(400 ml). The

aqueous mixture was extracted with chloroform severaltimes. The combined chloroform layers were thenwashed with distilled water and dried over sodiumsulfate. After evaporation of chloroform under reducedpressure, the oily product was obtained. It was purifiedby silica gel column(ethyl acetate:hexanes1:10) toafford white crystals 4.20 g(79% yield). H NMR1

(CDCl ,): d 10.49(s, 1H, CHO), 7.33(m, 1H, Ar–H),3

7.13 (m, 1H, Ar–H), 6.94(m, 1H, Ar–H), 4.05(t, 2H,OCH ), 3.82 (s, 3H, CH), 1.83 (m, 2H, CH ), 1.492 3 2

(m, 2H, CH ), 1.31 (m, 8H, (CH ) ), 0.91 (m, 3H,2 2 4

CH ). Anal. calcd for C H O : C, 72.69; H, 9.15.3 16 24 3

Found: C, 72.43; H, 9.24.

2.1.7. Monomer 1To a 150-ml round-bottom flask was placed diphos-

phonate 5(2.68 g, 5.00 mmol) and 2-octoxy-5-meth-oxybenzaldehyde(2.64 g, 10.0 mmol) under argon and

219S. Lu et al. / Thin Solid Films 417 (2002) 215–220

Fig. 3. UV–vis and PL spectra of PF-PPV in CHCl and as film.3

anhydrous THF(40 ml) was added by syringe. After aclear solution was obtained, 12 ml of potassiumtert-butoxide solution(1 M in THF) was added dropwise,and the reaction mixture was stirred for 24 h at roomtemperature. The reaction mixture was concentrated to10 ml and poured into 200 ml water. The mixture wasthen extracted with chloroform(50=4 ml). The com-bined chloroform layers were washed with water anddried over sodium sulfate. The crude product wasrecrystallized from chloroformyethanol to give yellowcrystals 2.6 g(69% yield). H NMR (CDCl ,): d 7.911

3

(s, 2H, Ar–H), 7.47 (d, 2H, vinyl H), 7.37, (d, 2H,vinyl H), 7.16 (d, 2H, Ar–H), 6.87 (m, 4H, Ar–H),4.03 (t, 4H, OCH ), 3.85 (s, 6H, OCH), 1.90(m, 4H,2 3

CH ), 1.55 (m, 4H, CH ), 1.3–1.4(b, 20H, (CH ) ),2 2 2 5

0.88 (t, 6H, CH ). C NMR (CDCl ): d 154.2, 151.8,133 3

138.2, 130.9, 127.9, 127.2, 127.1, 123.6, 115.3, 114.2,113.1, 70.0, 56.2, 32.4, 29.9, 29.8, 29.6, 26.8, 23.1,14.4. Anal. calcd for C H Br O : C, 63.49; H, 6.93.40 52 2 4

Found: C, 63.56; H, 7.01.

2.1.8. PolymerizationMonomer 1(0.757 g, 10 mmol), monomer 2(0.502

g, 10 mmol) were dissolved in 3 ml of degassed toluene.To this solution was added 2 N degassed Na CO2 3

solution 3 ml. The reaction mixture was bubbled withargon for 30 min and catalystic of tetrak-is(triphenylphosphine)palladium was added. The reac-tion was heated under vigorous stirring at 958C for 2days. The mixture was then cooled to room temperatureand poured into 300 ml methanol. The precipitatedpolymer was collected and re-dissolved in chloroform.After being washed with diluted HCl solution and watersuccessively, the chloroform solution was dried oversodium sulfate, concentrated and precipitated into meth-anol to give yellow solid. The polymer was furtherpurified by Soxhlet extraction with acetone and repre-

cipitated into methanol one more time to give finalproduct 0.61 g(66% yield). H NMR (CDCl ,): d1

3

7.75–7.9(b, 4H), 7.35–7.6(b, 6H), 7.2–7.3(b, 2H),6.93 (s, 2H), 6.8 (d, 2H), 6.73 (m, 2H), 3.9 (b, 4H,OCH ), 3.8 (b, 6H, OCH), 1.95 (b, 4H, CH ), 1.752 3 2

(b, 4H, CH ), 0.5–1.6(b, 48H). C NMR (CDCl ): d132 3

154.4, 153.7, 151.8, 141.5, 140.4, 136.0, 134.4, 131.9,129.1, 128.6, 125.4, 119.8, 116.5, 114.4, 114.0, 112.6,112.1, 70.1, 56.0, 55.5, 40.5, 32.5, 32.0, 30.2, 30.0,26.7, 24.5, 23.2, 14.6, 14.4. FT-IR(KBr cm ): 3049.5,y1

2926.0, 2852.7, 1605.4, 1581.3, 1496.8, 1464.7, 1428.3,1282.5, 1210.6, 1045.4, 970.2, 819.7, 795.7. Anal. calcdfor C H Br O : C, 84.00; H, 9.11. Found: C, 83.07;40 52 2 4

H, 9.34.

3. Results and discussion

Synthesis of the monomer and the polymer is shownin Scheme 2. Starting fromp-methoxyphenol, 2-hydroxy-5-methoxybenzaldehyde was prepared by usingwell-known Reimer–Tiemann reactionw18x. Monomer1 was synthesized at high yield by the Wittig–Hornerreaction between compound 6 and 4. The large alkenecoupling constants of 17 Hz occurred in the H-NMR1

spectrum indicated the exclusivetrans-olefin bonds. Theother monomer, 9,9-Dihexylfluorene-2,7-bis(trimeth-ylene boronate), was prepared following the literatureprocedurew19x. The polymer was synthesized by thePd-catalyzed Suzuki coupling reaction of the dibromocompound with the fluorene-based diboranate.The final polymer PF-PPV were highly soluble in

common organic solvents such as THF, chloroform,methylene chloride, 1,2-dichloroethane, and so on. Aneven thin film was obtained simply by spin-casting. Thenumber-average molecular weights(M ) and the weight-n

average molecular weights(M ) of the polymers, deter-w

mined by gel permeation chromatography using

220 S. Lu et al. / Thin Solid Films 417 (2002) 215–220

polystyrene standards were 16 863 and 45 298, respec-tively, with polydispersity index of 2.68.The chemical structure of the polymer was proved by

FT-IR and H-NMR respectively. Fig. 1 shows the FT-1

IR spectrum of PF-PPV. A moderately strong peak atapproximately 970.2 cm corresponding to the out-of-y1

plane bending mode of thetrans-vinylene in the PF-PPV was observed, indicating the successful integrationof the oligo-PPV unit into the polymer backbone.Thermal properties of the synthesized polymers were

evaluated by the means of TGA and DSC under air andnitrogen atmosphere respectively. Fig. 2 shows that thepolymer exhibited good thermal stability. The weightlosses of the polymer were less than 5% on heating toapproximately 3408C. DSC thermogram reveals anendo peak with onset at 175.58C in the heating run andan exothermic peak occurring at 1208C in the coolingrun. These results may indicate the presence of crystal-linity of the polymer.Fig. 3 shows the UV–vis absorption and fluorescent

spectra of PF-PPV in diluted chloroform solution andas films of spin-cast onto the quartz plate. The solutionsample gives a main absorption peak at 340 nm, whereasthe maximum absorption peak of the thin films slightlyblueshifts to 335 nm. For both of the cases, a shoulderat approximately 310–320 nm was observed. The almostidentical absorption spectra of the polymer in solutionand as thin film, indicate the little difference betweenthe conformation of polymer chain in these two statesw20x. However, the emission spectra of the polymer insolution and as solid films are different. In comparisonwith its solution emission peak at 440 nm with ashoulder at 460 nm, the main emission peak in the solidfilms redshifted approximately 29 nm toward longerwavelength. The emission peak is only slightly broad-ened in the film state than that in the solution state,indicating weaker intermolecular aggregation whichresults in the splitting of the excited states and morecomplicated electron transitions. The peak at 440 nm ismost likely the emission from the PPV oligomer seg-mentsw21,22x, and the fluorene units contribute little tothat emission. This is based on that the considerablesteric effects of the styrene groups which result in thetwisting conformation in polymer backbones, will leadto dramatic blueshifted emission from the polymerbackbones alone(should be considerably-408 nmw23x). In this regard, self-absorption will occur inevitablyand result in the lower fluorescent quantum yield.

4. Conclusion

A new soluble blue light-emitting polymer containingfluorene and oligo-PPV segments has been synthesizedand characterized. The polymer shows good thermal andenvironmental stability. The similar absorption betweenthe solution and film of the polymer indicates littledifference in the conformation of the polymer chain inthese two states. Photoluminescence spectrum shows theemission is most likely from the PPV segments, and thepolymer main chain plays a less important role in thatemission.

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