baek 2004 synthesis and photol

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Synthesis and Photoluminescence of Linear andHyperbranched Polyethers Containing PhenylquinoxalineUnits and Flexible Aliphatic Spacers

JONG-BEOM BAEK,1,2 L.-C. CHIEN2

1School of Chemical Engineering, Chungbuk National University, Cheongju, Chungbuk, 361-763 South Korea

2Liquid Crystal Institute and National Science Foundation Center for Advanced Liquid Crystal Optical Materials,Kent State University, Kent, Ohio 44242

Received 15 January 2004; accepted 2 March 2004

DOI: 10.1002/pola.20163

Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Ultrahigh-molecular-weight linear polyethers were prepared through a

reaction between the phenylquinoxaline monomers 2,3-bis(4-hydroxyphenyl)-6-fluoro-

quinoxaline and 2,3-bis(4-hydroxyphenyl)-6-(,,-trifluoromethyl)quinoxaline and

1,12-dibromododecane. A new hyperbranched polyether containing a phenylquinoxa-

line moiety was also prepared from a new self-polymerizable AB2 monomer, 2,3-bis(6-

bromohexyloxyphenyl)-6-(4-hydroxyphenyloxy)quinoxaline. All the polyethers were

amorphous and soluble in polar aprotic solvents. Their solution-cast thin films were

light yellow, ductile, and optically transparent. The polymers were thermally stable up

to 350 C and had glass-transition temperatures in the range of 2583 C, which

depended on the architecture and monomer structure. The monomers and polymers

displayed fluorescence maxima in the blue-light region in the range of 431 449 nm with

relatively narrow peak widths; this indicated that they had pure and intense fluores-

cence. 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 35873603, 2004

Keywords: luminescence; hyperbranched; polyphenylquinoxaline (PPQ); linear

INTRODUCTION

Fluorescent polymers, which are very interestingbecause of their potential applications in new dis-play technology, emit light when they are excitedby either the flow of an electric current or ultra-

violet (UV) light. Conjugated polymers are partic-ularly known as electroluminescence (EL) andphotoluminescence (PL) materials.1 Although in-organic and organic molecular EL materials have

been known for many years and have been com-mercially used,2 the devices from such materialshave to be deposited as thin films by the relativelyexpensive techniques of sublimation and vapor

deposition, which are not well suited to the fabri-cation of large-area devices. For these reasons,the possibility of using fluorescent conjugatedpolymers, which can be readily deposited fromsolution as thin films over large areas by spin-coating or doctor-blade techniques, is most attrac-tive.3 However, there are also some limitations onthese conjugated materials for practical applica-tions. For example, the inherent rigidity of theconjugated polymers causes poor solubility in

common organic solvents, and the vinyl moiety inthe main chain is easy to oxidize; this reduces thedevice operation lifetime.4

As a solution to the poor solubility of conju-gated polymers, fluorescent polymers have beenreported that avoid the use of fully conjugatedsystems. One approach to making a material sol-uble is the introduction of flexible spacers into themain chains, side chains, or both.5Another exam-

Correspondence to: J.-B. Baek (E-mail: [email protected])or L.-C. Chien (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 35873603 (2004)

2004 Wiley Periodicals, Inc.

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ple is chain-growth polymers with chromophoresas side chains; this avoids conjugation and helpswith solubility.6 One exciting approach is liquid-crystalline chromophores self-assembling intodiscotic columnar supramolecular structures,which have a tendency to orient parallel to the

film surface; this results in high chargemobilities.6(b) A similar approach has been previ-ously applied to cyano-substituted poly(phenyl-enevinylene) (PPV) polymers,7 which display im-proved solubility in organic solvents and EL effi-ciencies. Because this approach preventscrystallization, the properties of film formationfrom spin coating or self-assembly processes areenhanced. It is also a versatile way of confiningthe degree of conjugation, increasing thebandgap, and generating a fine-tuned blue-emis-sion spectrum.

In general, chromophores with relatively

shorter conjugation lengths have higher PL quan-tum yields and better EL quantum yields.8 Thisapproach also brings the dilution of chro-mophores, minimizing self-quenching probabili-ties. The solubility, efficiency, and processabilityof hyperbranched polyphenylquinoxaline (PPQ)could be further enhanced because of the three-dimensional molecular architecture of the hyper-branched polymer.9 For operational stability (ordevice lifetime), PPV polymers, which are bestknown and have been well studied for PL and ELdevice applications, should be avoided becausethey contain vinyl moieties that do not have oxi-

dative stability.10

For both the processability and lifetime of con-jugated materials, new competitive materials areneeded to substitute for PPV polymers, and theoptoelectronic efficiency and necessary colors canbe tuned by molecular engineering of the chemi-cal structures. For examples, nitrogen-containingheterocycles such as 1,2,4-triazoles,11 1,3,5-tria-zines,12 and quinoxalines13 are particularly inter-esting because they display surprisingly unal-tered EL properties without an electron-trans-porting layer in the case of polymers containing1,2,4-triazole units.11 Aromatic heterocyclic com-

pounds are also known to be stable and efficientenough for practical applications.11

In this article, we report on the synthesis andcharacterization of a series of phenylquinoxalinemonomers for the preparation of linear and hy-perbranched polyethers containing phenylqui-noxaline moieties as the chromophores. The chro-mophores are isolated from one another through anonconjugated flexible aliphatic spacer in the

main chain. The phenylquinoxaline unit has awell-defined conjugation length that emits bluelight.

EXPERIMENTAL

Materials

All the reagents and solvents were purchasedfrom Aldrich Chemical Co. and used as received,unless otherwise specified. N-Methyl-2-pyrrolidi-none (NMP) was distilled under reduced pressurein the presence of phosphorous pentoxide beforeuse. Acetone was purified by fractional distilla-tion in the presence of potassium carbonate beforeuse. The ortho-phenylenediamines4-fluoro-1,2-phenylenediamine (1),14 3,4-diamino-,,-tri-fluorotoluene (2),15 and 3,4-diamino-4-hydroxy-

phenylether (9)16

were prepared according toliterature procedures and were purified by recrys-tallization from toluene or aqueous ethanol togive off-white powders (mp 8991, 5961, and220221 C, respectively). The 1,12-dibromodode-cane used in this study was recrystallized twicefrom hexane to give colorless, white flakes (mp 4042 C). 4,4-Dihydroxybenzil (3) was alsoprepared according to a literature procedure andwas purified by recrystallization from benzene togive yellow needles (mp 254256 C).17

Instrumentation

1H and 13C NMR spectra were obtained with aVarian Gemini 200 NMR spectrometer. Differen-tial scanning calorimetry (DSC) analyses wereperformed with a PerkinElmer DSC7 in nitrogenat a heating rate of 20 C/min. Thermogravimet-ric analysis (TGA) was performed with a TA Hi-Res TGA 2950 thermogravimetric analyzer in anair atmosphere at a heating rate of 20 C/min.High-performance liquid chromatography (HPLC)was performed on a Shimadzu LC-600 liquidchromatograph with a Shimadzu SPD-6A UVspectrophotometric detector, a Shimadzu CD 501

Chromatopac, and a YMC HPLC reverse-phasecolumn. HPLC samples were run with a 7/3 (v/v)acetonitrile/water mixed solvent system. Gel per-meation chromatography (GPC) was carried outon a Waters 150-CV equipped with a refractive-index detector and calibrated against polystyrenestandards (Shodex Standard SM-105 series kitsranging from 1350 to 3,600,000 Da). Tetrahydro-furan (THF) was used as the elution solvent. THF

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solutions of polymer samples were filteredthrough a 0.45-m syringe filter. Ultravioletvis-ible (UVvis) spectra were obtained with aHewlettPackard 8453 UVvis spectrophotome-ter. Fluorescent studies were conducted with aFluorolog FL 3-11 fluorometer. The applied exci-

tation wavelength was the UV-absorption maxi-mum of each sample. Wide-angle X-ray diffrac-tion (WAXD) film patterns were recorded with aRigaku RU-200 diffractometer with Ni-filtered CuK radiation (40 kV and 100 mA; 0.15418nm). The energy-minimized structures and dihe-dral angles of the monomers were determinedwith the CS Chem 3D Std computational package(version 5.0; CambridgeSoft Corp., Cambridge,MA).

2,3-Bis(4-hydroxyphenyl)-6-fluoroquinoxaline (4)

A 500-mL, round-bottom flask equipped with amagnetic stirring bar, a reflux condenser, aDeanStark trap, and a nitrogen inlet wascharged with 3 (12.0 g, 4.95 mol), 1 (6.25 g, 4.96mmol), toluene (75 mL), and deoxygenated aceticacid (250 mL). The reaction mixture was stirredand gently heated at the reflux temperature over-night. The water that was generated was re-moved as a toluene azeotrope. The reaction mix-ture was then allowed to cool to room tempera-ture and was poured into 1 L of a slurry of ice andwater containing 50 mL of concentrated hydro-

chloric acid. The precipitate that formed was col-lected by suction filtration, washed with water,and dissolved in hot aqueous ethanol containingcharcoal. The solution was filtered and then al-lowed to cool to room temperature to give 15.6 g(95% based on 3) of large, yellow crystals [mp 142 (lit.18 mp not reported) and 238 C (DSC)].

ELEM. ANAL. Calcd. for C20H13FN2O2: C,72.28%; H, 3.94%; N, 8.43%. Found: C, 72.21%; H,4.06%; N, 8.19%. Fourier transform infrared(FTIR; KBr, cm1): 1209 (ArOF), 3259 (ArOOH).UV-absorption maximum (1 104 mol/L inTHF): ab 376 nm. Emission maximum (1

104

mol/L in THF): em

443 nm. Mass spec-trometry (m/e): 332 (M, 100% relative abun-dance). 1H NMR (acetone-d6, ppm, ): 6.806.86(d, 4H, Ar), 7.407.46 (dd, 4H, Ar), 7.587.73 (m,2H, Ar), 8.068.13 (t, 1H, Ar), 8.83 ppm (s, 2H,OH). 13C NMR (acetone-d6, ppm, ): 114.54,114.97, 117.70, 121.79, 122.31, 133.21, 133.36,133.84, 134.05, 134.24, 140.82, 156.73, 161.06,161.24, 162.74, 167.68.

2,3-Bis(4-hydroxyphenyl)-6-(,,-trifluoromethyl)quinoxaline (5)

A 500-mL, round-bottom flask equipped with amagnetic stirring bar, a reflux condenser, aDeanStark trap, and a nitrogen inlet was

charged with 3 (12.0 g, 4.95 mol), 2 (8.5 g, 482.6mmol), toluene (75 mL), and deoxygenated aceticacid (300 mL). The reaction mixture was stirredand gently heated at the reflux temperature over-night. The water that was generated was re-moved as a toluene azeotrope. The reaction mix-ture was then allowed to cool to room tempera-ture and poured into 1 L of a slurry of ice andwater containing 50 mL of concentrated hydro-chloric acid. The precipitate that formed was col-lected by suction filtration, washed with water,and dissolved in a large quantity of toluene con-taining charcoal. The solution was filtered andthen allowed to cool to room temperature to give17.1 g (98% based on 3) of large, light browncrystals (mp 158160 C).

ELEM. ANAL. Calcd. for C21H13F3N2O2: C,65.97%; H, 3.43%; N, 7.33%. Found: C, 65.89%; H,3.65%; N, 7.21%. UV-absorption maximum (1 104 mol/L in THF): ab 382 nm. Emissionmaximum (1 104 mol/L in THF): em 449nm. Mass spectrometry (m/e): 382 (M, 100% rel-ative abundance). 1H NMR [dimethyl sulfoxide-d6(DMSO-d6), ppm, ]: 6.746.78 (d, 4H, Ar), 7.357.40 (dd, 4H, Ar), 8.00 8.05 (d, 1H, Ar), 8.23 8.27(d, 1H, Ar), 8.41 (a, 1H, Ar), 9.88 ppm (s, 2H, OH).13C NMR (DMSO-d6, ppm, ): 116.81, 126.63,126.69, 128.11, 128.19, 130.86, 132.09, 133.03,133.11, 140.90, 160.32, 160.42.

General Procedure of Linear Polymerization

A three-necked, round-bottom flask containing astirrer bar was dried and flushed thoroughly withnitrogen. The quinoxaline monomer (1 equiv),1,12-dibromododecane (1 equiv), and potassiumcarbonate (2.4 equiv) were charged into the flask.The solids were carefully washed with NMP. Themixture was then heated to 110 C and main-

tained at the same temperature until the deep redcolor disappeared. This usually took about 8 h.After the mixture was filtered while it was stillwarm, the filtrate was poured into a large excessamount of water containing 5% hydrochloric acid.The light yellow, fibrous polymer that coagulatedwas collected by filtration, washed with metha-nol, and Soxhlet-extracted with water for 72 hand with methanol for 72 h. The polymer was

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then collected and dried. It was dissolved in meth-ylene chloride and precipitated into hexane again.The porous, spongy polymer that coagulated wascollected and dried at 50 C under reduced pres-sure (1 mmHg) for approximately 48 h. The poly-mer yield was essentially quantitative.

ELEM. ANAL. Calcd. for C32H35FN2O2 (6): C,77.08%; H, 7.07%; N, 5.62%. Found: C, 76.98%; H,7.12%; N, 5.64%. UV-absorption maximum (1 104 mol/L of repeating units in THF): ab 372 nm. Emission maximum (1 104 mol/L ofrepeating units in THF): em 440 nm.

ELEM. ANAL. Calcd. for C33H35F3N2O2 (7): C,72.24%; H, 6.43%; N, 5.11%. Found: C, 72.11%; H,6.51%; N, 5.14%. UV-absorption maximum (1 104 mol/L of repeating units in THF): ab 379 nm. Emission maximum (1 104 mol/L ofrepeating units in THF): em 442 nm.

6,6 -Dibromohexyloxybenzil (9)

A 500-mL, round-bottom flask equipped with amagnetic stirring bar, a reflux condenser, aDeanStark trap, and a nitrogen inlet wascharged with 3 (9.0 g, 37 mmol), 1,6-dibromohex-ane (54.4 g, 222.9 mmol), potassium carbonate(12.3 g, 89.0 mmol), and acetone (300 mL). Thereaction mixture was stirred and gently heatedunder reflux for 18 h. The mixture was dilutedwith an additional 500 mL of acetone and filteredwhile it was warm to remove the acetone-insolu-ble portion. The filtrate was concentrated andpoured into 1 L of a slurry of ice and water con-

taining 50 mL of concentrated hydrochloric acid.The precipitate that formed was collected by suc-tion filtration, washed with water, and dissolvedin boiling acetone and was allowed to cool to roomtemperature to give 14.7 g (70% based on 3) ofwhite needles (mp 9395 C).

ELEM. ANAL. Calcd. for C26H32Br2O4: C, 54.95%;H, 5.68%; O, 11.26%. Found: C, 54.92%; H, 5.73%;O, 11.18%. Mass spectrometry (m/e): 568/2 (M,100% relative abundance). 1H NMR (CDCl3, ppm,): 1.471.55 (m, 8H, CH2), 1.801.90 (m, 8H,CH2), 2.152.18 (d, 4H, CH2), 3.393.46 (t, 4H,CH2Br), 4.004.07 (t, 4H, CH2O), 6.916.97 (d,

4H, Ar), 7.907.95 ppm (d, 4H, Ar). 13C NMR(CDCl3, ppm, ): 27.19, 29.84, 30.84, 34.59, 35.75,70.17, 116.69, 128.13, 134.37, 166.37, 195.53.

2,3-Bis(6-bromohexyloxyphenyl)-6-(4-hydroxyphenyloxy)quinoxaline (10)

A 250-mL, round-bottom flask equipped with amagnetic stirring bar, a reflux condenser, a

DeanStark trap, and a nitrogen inlet wascharged with 9 (9.0 g, 15.8 mmol), 3,4-diamino-4-hydroxydiphenylether (3.6 g, 16.6 mmol), chloro-form (100 mL), and deoxygenated acetic acid (100mL). The reaction mixture was stirred and gentlyheated at the reflux temperature overnight. The

water that was generated was removed as a chlo-roform azeotrope. The reaction mixture was thenallowed to cool to room temperature and pouredinto 1 L of a slurry of ice and water containing 50mL of concentrated hydrochloric acid. The organiclayer was separated, concentrated, subjected tocolumn chromatography with ethyl acetate/hex-ane (25/75, v/v), and recrystallized from heptaneto give 7.6 g (64% yield) of clear needles [mp 96C (DSC)].

ELEM. ANAL. Calcd. for C38H40Br2N2O4: C,60.97%; H, 5.39%; N, 3.74%; O, 8.55%. Found: C,60.95%; H, 5.43%; N, 3.71%; O, 8.48%. UV-ab-

sorption maximum (1 104

mol/L in THF): ab 382 nm. Emission maximum (1 104 mol/L inTHF): em 431 nm. Mass spectrometry (m/e):748, 749 (M, 100% relative abundance). 1H NMR(CDCl3, ppm, ): 1.511.52 (s, 8H, CH2), 1.781.94 (m, 8H, CH2), 3.403.67 (t, 4H, CH2Br),3.934.02 (t, 4H, CH2O), 6.796.87 (dd, 6H, Ar),6.987.02 (d, 2H, Ar), 7.357.46 (m, 5H, Ar),7.527.58 (d 1H, Ar), 8.098.14 ppm (d, 1H, Ar).13C NMR (CDCl3, ppm, ): 27.28, 29.92, 31.03,34.66, 35.82, 69.77, 112.18, 116.32, 118.59,124.07, 125.05, 132.31, 132.86, 133.20, 139.54,143.64, 149.84, 153.32, 155.21, 155.54, 161.55,

161.74, 162.66.

Synthesis of the Hyperbranched Polymer (11)

A three-necked, round-bottom flask containing astirrer bar was dried and flushed thoroughly withnitrogen. Monomer 10 (1 equiv) and potassiumcarbonate (1.2 equiv) were charged into the flask.The solids were carefully washed with acetone.The mixture was then heated under reflux for24 h. Afterward, the mixture was allowed to coolto room temperature and was filtered for the re-moval of insoluble salts. The filtrate was poured

into a large excess amount of water containing 5%hydrochloric acid. The light yellow, spongy poly-mer that coagulated was collected by filtration,washed with methanol, and Soxhlet-extractedwith water for 72 h and with methanol for 72 h.The polymer was then collected and dried. It wasdissolved in methylene chloride and precipitatedinto hexane again. The porous, spongy polymerthat coagulated was collected and dried at 50 C

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under reduced pressure (1 mmHg) for approxi-mately 48 h. The polymer yield was essentiallyquantitative.

ELEM. ANAL. Calcd. for C38H39BrN2O4 (11): C,68.36%; H, 5.89%; N, 4.20%. Found: C, 68.27%; H,5.93%; N, 4.16%. UV-absorption maximum (1 104 mol/L of repeating units in THF): ab 380 nm. Emission maximum (1 104 mol/L ofrepeating units in THF): em 436 nm.

Stock Solution Preparation

To estimate the sample concentration and rela-tive PL intensity, we prepared stock solutions (1.0 104 mol/L). Each sample (0.5 104 mol) wasdissolved in a volumetric flask containing 500 mL

of THF. The polymer concentration was calcu-lated on the basis of the molar mass of its repeat-ing unit.

RESULTS AND DISCUSSION

Monomer Synthesis

Dihydroxyl monomers were synthesized for thepreparation of the linear polymers (6 and 7) ac-cording to the reaction sequence shown in Scheme1. Monomers 4 and 5 were afforded in almost

quantitative yields by double condensation be-tween compounds 1 and 3 and 2 and 3, respec-tively. As described in the Experimental section,the identities and purities of monomers 4 and 5were ascertained by conventional organic charac-terization methods before the polymerization ex-periments were carried out, including 1H NMR[Figs. 1(a) and 2(a)], 13C NMR [Figs. 1(b) and2(b)], FTIR, HPLC, elemental analysis, and mass

analysis. The purities of both monomers were ap-proximately 100% (HPLC). The comonomer 1,12-dibromododecane was recrystallized twice fromhexane (100% pure, HPLC). Although there areseveral commercially available ,-aliphatic di-bromides, 1,12-dibromododecane was chosen be-cause it was cheap and easy to purify by recrys-tallization for maximum-molecular-weight poly-mers.

Linear Polymers

The dihydroxyl monomers 4 and 5 were reactedwith 1,12-dibromododecane in NMP in the pres-ence of potassium carbonate to afford linear poly-

ethers 6 and 7 containing a phenylquinoxalineunit as a chromophore and a flexible aliphaticspacer (Scheme 2). Both contained electron-defi-cient fluorine and ,,-trifluoromethyl groupsattached to the 6- or 7-position on the quinoxalinering. The resulting polymers were carefullyworked up to remove the residual solvent (see theExperimental section and TGA). There was noresidual impurity monitored after a carefulworkup. Linear polyethers 6 and 7 were soluble inmost organic solvents such as ether solvents(ethyl ether and THF), chlorinated solvents (di-chloromethane and chloroform), and polar aproticsolvents (N,N-dimethylamide, N,N-dimethylac-etamide, NMP, and dimethyl sulfoxide). Qualityflexible films were obtained from the solutioncasting of both polymers in dichloromethane.

AB2 Monomer Synthesis

The preparation of the AB2 monomer is shownin Scheme 3. 9 was prepared by the reaction

Scheme 1. Synthesis of the AA monomers (4 and 5): (a) AcOH and reflux.



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between 3 and an excess amount of 1,6-dibro-mohexane in the presence of potassium carbon-ate and a catalytic amount of 18-crown-6 inNMP in a good yield. The corresponding com-pound (9) was prepared by the reduction of3-amino-4-nitro-4-hydroxyphenylether, whichwas afforded by the condensation of 5-fluoro-2-nitroaniline with an excess amount of hydroqui-

none. The AB2 monomer 10 was synthesized bydouble condensation between compounds 8 and9 in an almost quantitative yield. The identityand purity of 10 were also ascertained with 1HNMR [Fig. 3(a)], 13C NMR [Fig. 3(b)], FTIR,HPLC, elemental analysis, and mass analysis.The purity of monomer 10 was 100% accordingto an HPLC experiment.

Figure 1. 1H NMR (acetone-d6) and13C NMR (acetone-d6) spectra of monomer 4.

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Hyperbranched Polymer

Similar to the linear polymers, AB2 monomer 10

was polymerized in acetone in the presence ofpotassium carbonate to afford hyperbranchedpolyether 11, which contained a phenylquinoxa-line unit and a flexible aliphatic spacer (Scheme4). The resulting polymer was carefully workedup in the same manner as the linear polymers(see the Experimental section and TGA). 11 dis-

played solubility (in organic solvents) similar to

that of its linear versions (6 and 7), except inacetone.

GPC Analysis

The molecular weights and molecular weightdistributions (MWDs) of the polymers were de-termined by GPC against polystyrene stan-

Figure 2. 1H NMR (DMSO-d6) and13C NMR (DMSO-d6) spectra of monomer 5.



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dards (13503,600,000 Da; Fig. 4 and Table 1).The polymer was dissolved in THF and filteredthrough a 0.45-m syringe filter before injectioninto a GPC instrument. The number-averagemolecular weight (Mn) and weight-average mo-lecular weight (Mw) of polymer 6 were 655,000and 1,445,000 Da, respectively (MWD 2.21).

Mn and Mw of polymer 7 were 712,000 and1,456,000 Da, respectively (MWD 2.04). Al-

though the unusually high molecular weights ofthe polymers obtained from GPC against poly-styrene standards could not be directly com-pared, extremely high-molecular-weight poly-mers were achieved according to the high vis-cosities during the polymerization, the fibrousprecipitation when the solutions were pouredinto methanol, and the good-quality film forma-tion.

Scheme 2. Synthesis of the linear polymers (6 and 7): (a) NMP, K2CO3, and 110 C.

Scheme 3. Synthesis of the AB2 monomer (10): (a) acetone, K2CO3, and reflux and (b)

AcOH and reflux.

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Mn and Mw of hyperbranched polyether 11were 823,000 and 1,507,000 Da, respectively(MWD 1.83). According to a comparison withthe GPC results for the linear polymers, a high-molecular-weight hyperbranched polymer wasalso prepared. Because the molecular architec-

ture of the hyperbranched polymer prevented in-termolecular entanglement, it was impossible toform rigid freestanding films out of the hyper-branched polymer. In this case, we obtained quiteductile thin films cast from a hyperbranched poly-mer solution in dichloromethane.

Figure 3. 1H NMR (CDCl3) and13C NMR (CDCl3) spectra of monomer 10.



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Thermal Analysis

Morphological information about the polymerfilms was obtained by WAXD measurements. The

diffraction patterns of all the samples were broad,

amorphous halos. DSC scans confirmed that both

linear polymers were amorphous. Their films

were light yellow and optically transparent; this

Figure 4. GPC curves of polymers: (a) 6, (b) 7, and (c) 11. THF was used as the elution

solvent.

Scheme 4. Synthesis of the hyperbranched polymer (11): (a) acetone, K2CO3, and

reflux.

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Figure 5. DSC thermograms of the polymers at a heating rate of 20 C/min: (a) 6, (b)

7, and (c) 11.

Table 1. Average Molecular Weights and MWDs of the Polymers

Polymer Mn (g/mol) Mw (g/mol) Mz (g/mol) MWD

655,000 1,445,000 2,056,000 2.21

712,000 1,456,000 2,044,000 2.04

823,000 1,507,000 2,057,000 1.83



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could be useful for practical applications. The

glass-transition temperatures (Tgs) of polymers6, 7, and 11 were 37, 25, and 83 C, respectively(Fig. 5 and Table 2). Although the Tgs of thelinear polymers appeared to be low, the effect ofphase separation (either partial or complete) wasundetected from DSC thermal analysis (no addi-tional first-order phase-transition peak or base-line change was observed on either heating orcooling scans). The amorphous nature of the poly-ethers containing phenylquinoxaline with pen-dant fluorine and ,,-trifluoromethyl groupscould be explained in two ways. First, the phenylrings on the 2- and 3-positions were not on the

same plane with the quinoxaline ring. The dihe-dral angle to the quinoxaline ring was about 44[Fig. 6(a,b)], and so the phenylquinoxaline unitswere difficult to pack to form crystals. Second, thependant fluorine or ,,-trifluoromethyl groupson the quinoxaline ring were randomly placed atthe 6- or 7-position during polymerization, as de-scribed in Figure 6(c). The 5% weight loss tem-peratures (Td5s) measured with TGA for poly-

mers 6, 7, and 11 were 351, 390, and 401 C,

respectively (Fig. 7).

UV-Absorption and PL Properties

Stock solutions (1.0 104 mol/L) of each samplewere prepared in THF. UVvis spectra of mono-mer 4 and polymer 6 are shown in Figure 8. Theabsorption maxima for both monomer 4 and poly-mer 6 were almost the same (376 and 372 nm,respectively; Fig. 8 and Table 3). The absorbancewas related to the * transition leading to asinglet exciton, and this indicated that the mono-mer and polymer had similar excitation energies.

When the same solutions were excited at theirabsorption maxima, the fluorescent spectra ofmonomer 4 and polymer 6 displayed maxima at443 and 440 nm, respectively. Redshifts of boththe absorption and emission of monomer 4 withrespect to those of polymer 6 were due to solventchromism19 because the monomer with two hy-droxyl groups could strongly interact with a polarsolvent such as THF and/or possible intermolec-

Table 2. Thermal Properties of the Polymers

Polymer Tg (C)a Td5 (C)

b

37 350

25 390

83 401

a Determined at a heating rate of 20 C/min.b Determined at a 5% weight loss at a heating rate of 20 C/min in air.

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Figure 6. Energy-minimized structures of the monomers: (a) 4, (b) 5, and (c) possible

random locations of pendant groups of polymers 6 and 7. The dihedral angle of the 2-

and 3-position phenyl rings onto the quinoxaline ring is 44, and the probability of the

X group on the 6- or 7-position of the quinoxaline ring is random.

Figure 7. TGA thermograms of the polymers at a heating rate of 20 C/min: () 6,

(- -) 7, and (- -) 11.



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Figure 8. UV-absorption and fluorescence spectra of 1.0 104 mol of () monomer

4 and (- -) polymer 6 in THF solutions. Both the absorbance and PL intensity are

normalized.

Table 3. UV-Absorption and Fluorescence Properties of the Monomers and Polymers

Monomer Polymer ab em

Mmr:

376

Pmr:

372

Mmr:

443

Pmr:

440

Mmr:

382

Pmr:

379

Mmr:

449

Pmr:

442

Mmr:

382

Pmr:

380

Mmr:

431

Pmr:

436

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ular hydrogen bonding between monomer 4 re-sulting in a longer effective conjugation length.Similar solvent chromism was monitored frommonomer 5 with respect to that of polymer 7.Monomer 5 displayed a major UV-absorption

peak with a maximum at 382 nm and a fluores-cence emission peak with a maximum at 449 nmin THF. Corresponding polymer 7 had an absorp-tion maximum at 379 nm and an emission maxi-mum at 442 nm (Fig. 9 and Table 3). Both theabsorption and emission maxima of polymer 7were slightly blueshifted 3 and 7 nm from those ofits monomer. The absorption and emission max-ima of Ab2 monomer 10 were at 382 and 431 nm,respectively (Fig. 10 and Table 3). The absorptionand emission maxima of polymer 11 preparedfrom 10 were at 380 and 436 nm, respectively(Fig. 10). In this case, the absorption maximum of

the polymer was also almost slightly blueshiftedto that of its monomer, whereas the emissionmaximum was redshifted.

Both the absorption and emission intensities ofall the polyethers containing phenylquinoxalineunit in THF were linearly dependent (Beers Law)on the polymer concentrations from 1.00 106

to 1.00 104 g/L. At concentrations above 1.00 104 g/L, the intensity of the fluorescence emis-

sion was off the scale of the instrumentation. Thisbehavior indicates that the phenylquinoxalinechromophores in each repeating unit were physi-cally well isolated from each other and not subjectto electron quenching.

Surprisingly, all the polymer films emitted astrong blue-green fluorescence when excited witha 360-nm UV lamp in the solid state. This is goodevidence that the phenylquinoxaline chro-mophores were well isolated by flexible aliphaticspacers and not subject to internalquenchingbecause of intermolecular and intramolecular in-teractions between the phenylquinoxalines.

CONCLUSIONS

A novel series of fluorescent linear polyethers con-

taining phenylquinoxaline units were preparedfrom the reaction of 4 and 5 with 1,12-dibromo-dodecane. A hyperbranched polyether from 10was also prepared. The polyethers were ultra-high-molecular-weight and were soluble in mostcommon organic solvents. All the polymers wereamorphous and optically transparent. On the ba-sis of the fluorescence studies, the monomers andpolymers containing phenylquinoxaline units had



normalized.



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very intense and pure blue fluorescence around431 and 449 nm. All the polyethers containingphenylquinoxaline units had intense fluorescencein both solution and solid states. These polymerswere thermally stable up to 350 C. Therefore,

these polymers could be good candidates for thefabrication of optoelectronic devices emitting bluelight.

This work was supported by the National Science

Foundation Center for Advanced Liquid Crystal Opti-

cal Materials (DMR 89-20147).

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