Synthesis and Characterization of Color-Stable Electroluminescent Polymers: Poly(dinaphtho [1,2-a:10,20-g]-s-indacene)s

Synthesis and Characterization of Color-StableElectroluminescent Polymers: Poly(dinaphtho[1,2-a:10,20-g]-s-indacene)s

XIN GUO,1,2 BING YAO,1 GUOXIN JIANG,1,2 YANXIANG CHENG,1 ZHIYUAN XIE,1

LIXIANG WANG,1 XIABIN JING,1 FOSONG WANG1

1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, People’s Republic of China

2Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Received 21 August 2007; accepted 18 April 2008DOI: 10.1002/pola.22821Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Two new stepladder conjugated polymers, that is, poly(7,7,15,15-tetraoc-tyldinaphtho[1,2-a:10,20-g]-s-indacene) (PONSI) and poly(7,7,15,15-tetra(4-octylphe-nyl)dinaphtho[1,2-a:10,20-g]-s-indacene) (PANSI) with alkyl and aryl substituents,respectively, have been synthesized and characterized. In comparison with poly(inde-nofluorene)s, both polymers have extended conjugation at the direction perpendicularto the polymer backbone because of the introduction of naphthalene moieties. Theemission color of the polymers in film state is strongly dependent on the substituents.While PONSI emits at a maximum of 463 nm, PANSI with the same backbonebut aryl substituents displays dramatically redshifted emission with a maximum at494 nm. Both polymers show stable photoluminescence spectra while annealing at200 8C in inert atmosphere. The PONSI-based devices with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al turn on at 3.7 V, and emit at a maximum of 461 nm withthe CIE coordinates of (0.19, 0.26), a maximum luminance efficiency of 1.40 cd/A, anda maximum brightness of 2036 cd/m2 at 13 V. Meanwhile, the emission color of thedevices is independent of driving voltage and keeps unchanged during the continuousoperation. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4866–4878, 2008

Keywords: color stability; conjugated polymers; light-emitting diodes; luminescence

INTRODUCTION

Conjugated polymers have attracted muchattention for utilization in polymeric light-emit-ting diodes (PLEDs).1,2 In particular, blue elec-troluminescent (EL) polymers with largebandgap have been studied intensively sincethey not only emit one of the three primary col-ors (blue) but can also be used as host materials

to realize emission covering whole visibleregion.3,4 Much research into blue polymers hasfocused on p-phenylene-based polymers,5,6 espe-cially, on the stepladder polymers that possess astructure between poly(p-phenylene)s (PPPs)and ladder-type poly(p-phenylene)s (LPPPs),7

for example, polyfluorenes (PFs). PFs and theirderivatives are regarded as the most promisingcandidates for blue PLEDs because of structuralfeatures for the facile functionalization andpromising properties such as excellent chemicalstability, great thermal stability, and highfluorescence quantum yield in solid state.8–10

However, the human eyes are insensitive to the

Correspondence to: Z.-Y. Xie or L.-X. Wang (E-mail: [email protected] or [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 4866–4878 (2008)VVC 2008 Wiley Periodicals, Inc.

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emission of typical PF homopolymers. Bridgingmore phenylene units can result in redshiftedemission, to which the eyes are more sensitive.For instances, poly(indenofluorene)s (PIFs)11

and the polymers based-on ladder-type penta-phenylenes12 exhibit emission maximum at 430and 445 nm, respectively, in comparison with420 nm of PFs. Meanwhile, attributed to theaggregations/excimers formation13–16 and theketone-defects formation,17–20 both PFs andPIFs suffer from the tendency to generate theundesired long-wavelength emission bands dur-ing device operation, and upon heat treatmentor photoirradiation, which result in the redshiftof the emission spectra and the reduction of thedevice efficiency. To avoid the detrimental long-wavelength emission, several approacheshave been reported for PFs21–29 and PIFs,30–32

respectively.The optoelectronic properties of conjugated

polymers are dependent on the nature of thebuilding blocks, the pattern in which they arelinked, and the type and positions of the sub-stituents.33 In principle, the polymers with moreextended conjugation emit at a longer wave-length. For example, PFs, PIFs, and ladder-typepoly(pentaphenylenes)s whose conjugationlength of the building blocks is extended alongthe polymer backbone exhibit gradually red-shifted emission as mentioned earlier. In this ar-ticle, we report the synthesis and propertiesof two new EL polymers, that is, poly(7,7,15,15-tetraoctyldinaphtho[1,2-a:10,20-g]-s-indacene)(PONSI) and poly(7,7,15,15-tetra(4-octylphenyl)-dinaphtho[1,2-a:10,20-g]-s-indacene) (PANSI) asshown in Chart 1. In comparison with PIFs, twophenyl units in indenofluorene are replacedwith naphthylenes in the two polymers, which

causes that the conjugation was extended at thedirection perpendicular to rather than along thepolymer backbone. The replacement of the ben-zene by a naphthalene moiety should offer thepolymers new optoelectronic properties andsome advantages: (i) the conjugation extensioncan give rise to redshifted emission that is moresensitive to the human eyes; (ii) the steric hin-drance between the adjacent monomer unitsmay result in larger dihedral angles betweenrepeating units to reduce the p-p stacking ofpolymer chains and depress the crystalline tend-ency of the polymers so as to endow them anamorphous property, and thereby enhance thespectral stability of the polymers in solid state.34

EXPERIMENTAL

Instruments

1H NMR and 13C NMR spectra were recordedon a Bruker Avance 300 NMR spectrometer, andchemical shifts were recorded in ppm units,with tetramethylsilane as an interval standard.Elemental analysis was performed on a Bio-Radelemental analysis system. Molecular mass wasmeasured by LDI-1700 MALDI-TOF mass spec-troscopy (Linear Scientific Inc.) with 2,5-dihy-droxylbenzoic as the substrate. Molecularweights and polydispersity (PDI) of the polymerswere determined by gel permeation chromatog-raphy (GPC) on a Water 510 system, with poly-styrene as the standard and tetrahydrofuran(THF) as the eluent. UV–vis absorption spectrawere obtained on a Perkin-Elmer Lambda35UV/Vis spectrometer. Photoluminescence (PL)spectra were recorded on a Perkin–ElmerLS50B spectrofluorometer. Thermal properties

Chart 1. The chemical structures of the polymers PONSI and PANSI.

COLOR-STABLE ELECTROLUMINESCENT POLYMERS 4867

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

of the polymers were measured using Perkin–Elmer DSC7 and TGA7 equipments at a heatingrate of 10 8C/min under nitrogen atmosphere.The nature of the phase transition was charac-terized with an Olympus BX51 polarizing opticalmicroscope (POM) equipped with a LTS 350 hotstage and a TMS 94 temperature programmer(Linkam). Cyclic voltammetry (CV) was carriedout on an EG and G 283 potentiostat/galvano-stat system. Single-crystal X-ray diffraction datawere obtained at 187(2)K using a Bruker SmartAPEX diffractometer with a charge-coupled de-vice detector and graphite monochromator andMo Ka radiation (k ¼ 0.71073 A). Data collectionwas executed using the SMART program. Cellrefinement and data reduction were undertakenusing the SAINT program. The crystal structurewas solved using the SHELXTL program andrefined using full-matrix least squares.

Devices Fabrication and Measurement

The indium-tin oxide (ITO)-covered glass sub-strates for the PLEDs were degreased in an ultra-sonic solvent bath and dried in a heating cham-ber at 120 8C. The poly(styrene sulfonic acid)doped poly(ethylenedioxythiophene) (PEDOT:PSS) layer was spin-coated onto the treated ITOglass at a speed of 3000 rpm for 60 s and dried at120 8C for 30 min to give a film with an approxi-mate thickness of 40 nm. The emissive polymerfilm was then spin-cast from toluene solutionunder ambient conditions. The thickness of theemissive layer was determined by DEKTAK 6 MSTYLUS PROFILER. Metal electrodes were ther-mally deposited in a vacuum thermal evaporatorthrough a shadow mask at a pressure of 1.53 10�4 Pa. The active area of the devices was 10mm2. The EL spectra and current density-volt-age-brightness characteristics of the EL deviceswere measured by a Spectra Scan PR650 spectro-photometer and a Keithley 2400 current/voltagesource unit under ambient condition, respectively.

Materials

All reagents and chemicals were purchased fromAldrich and Acros and used for synthesiswithout further purification unless otherwisespecified. THF, diethyl ether, and toluenewere distilled over sodium/benzophenone. 2,5-Dibromo-dimethylterephthalate (1),35,36 a-naph-thyl boronic acid (2),37 and n-octyllithium38,39

were synthesized according to the modified

literature procedures. Poly(9,9-dioctylfluorene)(POF)27 and poly(6,6,12,12-tetraoctylindenofluor-ene) (POIF)11 were synthesized according to thereferences, with the number-average molecularweight (Mn)/PDI of 5.91 3 104/2.08 and 1.743 104/2.09, respectively.

2,5-Dinaphthylterephthalate (3)

A mixture of 2,5-dibromo-dimethylterephthalate(1, 10.6 g, 30.0 mmol), a-naphthylboronic acid(2, 11.4 g, 66.0 mmol), K2CO3 (11.5 g,83.0 mmol), and Aliqut 336 (1.21 g, 3.00 mmol)in toluene (300 mL) and water (42 mL) waspurged with argon for 10 min, and then tetra-kis(triphenylphosphine)palladium (380 mg, 0.01equiv) was added, and the mixture was heatedwith stirring at 90 8C for 12 h. The cooled solu-tion was extracted with toluene, and the organiclayer was washed with brine and dried overNa2SO4. The crude product was purified bychromatography on silica gel with dichlorome-thane/hexane (1:1) to give 3 as a white solid(12.3 g, yield: 92%).

1H NMR (300 MHz, CDCl3): d (ppm) 8.06 (s,2H); 7.94 (d, J ¼ 7.47 Hz, 4H); 7.67–7.41 (m,10H); 3.40 (s, 6H). 13C NMR (75 MHz, CDCl3): d(ppm) 167.45, 140.94, 138.61, 134.61, 133.82,132.31, 128.83, 128.55, 126.74, 126.27, 125.80,125.74, 125.62, 52.53. Elemental analysis: Calcd.for C30H22O4: C, 80.70; H, 4.97. Found: C, 80.80;H, 4.78.

7,7,15,15-Tetraoctyldinaphtho[1,2-a:10,20-g]-s-indacene (4)

A solution of n-octyllithium (17.8 mL, 0.510 M,9.00 mmol) in anhydrous THF (30 mL) in a dryflask was cooled in an ice/salt bath. Then a solu-tion of 3 (0.670 g, 1.50 mmol) in anhydrous THF(20 mL) was added dropwise with stirring, andthe solution was slowly allowed to warm toroom temperature. The mixture was stirredovernight, and the brine was added to quenchthe reaction. The mixture was extracted withdiethyl ether, and the organic layer was washedwith water and dried over Na2SO4. The crudeproduct was chromatographed on silica gel withdichloromethane/hexane (1:3) to give the diol in-termediate. Then the intermediate was dissolvedin dichloromethane (30 mL) and then BF3 ether-ate (0.21 mL) was added under stirring at roomtemperature. The mixture was stirred for 6 hand then methanol (10 mL) was added. After re-

4868 GUO ET AL.


moval of the solvent, the crude product waspurified by chromatography on silica gel withhexane to give 4 as a white solid (0.550 g, yield:46% in two steps).

1H NMR (300 MHz, CDCl3): d (ppm) 8.86 (d,J ¼ 8.46 Hz, 2H), 8.27 (s, 2H), 7.99 (d, J ¼ 8.04Hz, 2H), 7.86 (d, J ¼ 8.28 Hz, 2H), 7.72 (t, J¼ 7.17 Hz, 2H), 7.57–7.52 (m, 4H), 2.23–2.12 (m,8H), 1.09–1.00 (m, 40H), 0.70 (t, J ¼ 6.9 Hz,12H), 0.61–0.57 (m, 8H). 13C NMR (75 MHz,CDCl3): d (ppm) 151.42, 150.49, 141.28, 136.30,134.11, 129.70, 128.08, 126.74, 125.23, 124.41,121.51, 117.02, 55.19, 40.90, 32.14, 30.37, 29.58,29.52, 23.96, 22.90, 14.34. Elemental analysis:Calcd. for C60H82: C, 89.71; H, 10.29. Found: C,89.77; H, 10.32. Molecular mass: m/z 801.7.

7,7,15,15-Tetra(4-octylphenyl)dinaphtho[1,2-a:10,20-g]-s-indacene (5)

A solution of 4-octylbromobenzene (1.31 g, 4.80mol) in dry THF (20 mL) in a dry flask wascooled in an acetone/dry ice bath. And then n-butyllithium in hexane (3.00 mL, 1.60 M,4.80 mmol) was added with stirring. After30 min, a solution of dinaphthyl diester 3(0.440 g, 1.00 mmol) in dry THF (10 mL) wasadded dropwise with stirring. The mixture wasslowly allowed to warm up to room temperatureand stirred overnight. After quenched withbrine, the mixture was extracted with diethylether and the organic layer was washed withwater and dried over Na2SO4. The crude prod-uct was purified by chromatography on alumi-num oxide with dichloromethane/hexane (1:2) togive the diol intermediate. Then the intermedi-ate was added to acetic acid (10 mL). The mix-ture was heated to reflux and then two drops ofaqueous concentrated HCl solution were added.The mixture was refluxed for additional 2 h.The solution was cooled down to room tempera-ture, and the precipitated pale yellow solid wascollected by filtration and washed with water.The crude product was recrystallized with etha-nol to give the title compound as a white solid(0.530 g, yield: 48% in two steps).

1H NMR (300 MHz,CDCl3): d (ppm) 8.64 (d, J¼ 8.46 Hz, 2H), 8.43 (s, 2H), 7.90 (d, J ¼ 7.98Hz, 2H), 7.76 (d, J ¼ 8.49 Hz, 2H), 7.63–7.56(m, 4H), 7.49 (t, J ¼ 7.50 Hz, 2H), 7.29 (d, J¼ 8.10 Hz, 8H), 7.09 (d, J ¼ 8.22 Hz, 8H), 2.55(t, J ¼ 7.62 Hz, 8H), 1.58–1.53 (m, 8H), 1.28–1.24 (m, 40H), 0.86 (t, J ¼ 6.6 Hz, 12H). 13CNMR (75 MHz, CDCl3): d (ppm) 153.10, 151.56,

143.19, 141.83, 140.69, 134.88, 134.10, 129.82,129.58, 128.72, 127.12, 125.77, 124.56, 124.43,121.25, 65.21, 35.98, 32.28, 31.73, 29.91, 29.87,29.64, 23.06, 14.50. Elemental analysis: Calcd.for C84H98: C, 91.08; H, 8.92. Found: C, 91.01;H, 8.87. Molecular mass: m/z 1107.2.

5,13-Dibromo-7,7,15,15-tetraoctyldinaphtho[1,2-a:10,20-g]-s-indacene (6)

Into a solution of 4 (0.520 g, 0.650 mmol) in drychloroform (30 mL) was added Br2 (0.250 g,1.54 mmol) in chloroform (10 mL) dropwise at0 8C in 30 min, and the reaction mixture wasstirred for 8 h. The mixture was washed withsaturated aqueous solution of sodium hydrogensulfite, water and then dried over Na2SO4. Afterremoval of the solvent, the crude product wasrecrystallized from the mixture of methanol anddichloromethane. The dibromide monomer 6 wasisolated as light yellow needles (0.610 g, yield:97%).

1H NMR (300 MHz,CDCl3): d (ppm) 8.84 (d, J¼ 8.40 Hz, 2H), 8.43 (d, J ¼ 8.40 Hz, 2H), 8.22(s, 2H), 7.86 (s, 2H), 7.78 (t, J ¼ 7.62 Hz, 2H),7.67 (t, J ¼ 7.68 Hz, 2H), 2.27–2.09 (m, 8H),1.04–1.00 (m, 40H), 0.70 (t, J ¼ 6.9 Hz, 12H),0.60–0.55 (m, 8H). 13C NMR (75 MHz, CDCl3): d(ppm) 151.06, 150.53, 140.54, 135.89, 131.63,130.42, 128.58, 127.24, 126.34, 125.46, 124.38,122.46, 116.77, 55.16, 40.34, 31.75, 29.86, 29.14,29.11, 23.49, 22.52, 13.97. Elemental analysis:Calcd. for C60H80Br2: C, 74.98; H, 8.39. Found:C, 74.97; H, 8.54.

5,13-Dibromo-7,7,15,15-tetra(4-octylphenyl)dinaphtho[1,2-a:10,20-g]-s-indacene (7)

The procedure for synthesis of the monomer 7was the same as that of the monomer 6. Thedibromide monomer 7 was isolated as light yel-low solid (0.460 g, yield: 90%).

1H NMR (300 MHz, CDCl3): d (ppm) 8.61 (d,J ¼ 7.95 Hz, 2H), 8.36 (s, 2H), 8.34 (d, J ¼ 8.07Hz, 2H), 7.86 (s, 2H), 7.67–7.57 (m, 4H), 7.25 (d,J ¼ 8.31 Hz, 8H), 7.10 (d, J ¼ 8.28 Hz, 8H),2.55 (t, J ¼ 7.65 Hz, 8H), 1.58–1.53 (m, 8H),1.28–1.24 (m, 40H), 0.85 (t, J ¼ 6.9 Hz, 12H).13C NMR (75 MHz, CDCl3): d (ppm) 153.15,151.88, 142.41, 142.19, 140.25, 134.84, 132.13,130.79, 128.90, 128.64, 127.95, 127.19, 124.88,123.25, 121.34, 65.31, 35.98, 32.28, 31.70, 29.92,29.86, 29.63, 23.05, 14.49. Elemental analysis:



Calcd. for C84H96Br2: C, 79.73; H, 7.65. Found:C, 79.41; H, 7.46.

General Procedure for Yamamoto Polymerization

A mixture of bis(1,5-cyclooctadiene) nickel(0)(Ni(COD)2) (2.0 equiv), 2,20-dipyridyl (2.0 equiv),1,5-cyclooctadiene (2.0 equiv), and DMF (2 mL)was heated at 80 8C under argon for half anhour, and then the monomer (1.0 equiv,0.30 mmol) in anhydrous toluene (3 mL) wasadded. After 3 days heating at 80 8C, 1-bromo-benzene (4.0 equiv), the end-capper, in anhy-drous toluene (1 mL) was added. The mixturewas further heated at 80 8C for 24 h. After cool-ing to room temperature, the mixture waspoured into a mixture of concentrated HCl (100mL), acetone (100 mL), and methanol (100 mL).After the mixture was stirred for 4 h, the solidwas collected by filtration and then dissolved indichloromethane. The solution was washed withaqueous ammonia solution, water, and driedover Na2SO4. After removal of the solvent, theresidue was dissolved with a minimum amountof chloroform and precipitated in methanol. Thepolymers were further purified by Soxhletextraction with acetone and then were reprecipi-tated in methanol. Finally, the polymers wereobtained after drying in vacuum with yields of60–80%.

PONSI: yellow fiber. 1H NMR (300 MHz,CDCl3): d (ppm) 9.03 (br, 2H), 8.45 (br, 2H), 7.73(m, 6H), 7.37 (br, 2H), 2.29 (br, 8H), 1.15 (br,48H), 0.79 (t, J ¼ 6.9 Hz, 12H). Elemental anal-ysis: Calcd. for C60H80: C, 89.94; H, 10.06.Found: C, 89.27; H, 9.35.

PANSI: yellow fiber. 1H NMR (300 MHz,CDCl3): d (ppm) 8.71 (br, 2H), 8.50 (br, 2H), 7.62(br, 4H), 7.48 (br, 4H), 7.29 (br, 8H), 7.05 (br, 8H),2.55 (br, 8H), 1.57 (br, 8H), 1.26 (br, 40H), 0.86(br, 12H). Elemental analysis: Calcd. for C84H96:C, 91.25; H, 8.75. Found: C, 90.55; H, 8.05.

RESULTS AND DISCUSSION

Synthesis and Characterization

The synthetic route toward the desired polymersPONSI and PANSI is outlined in Scheme 1. Thecompound 7,15-dihydrodinaphtho[1,2-a:10,20-g]-s-indacene without any substituents on bridge-head positions was first synthesized by under-going cyclodehydration with phosphorus pentox-

ide by Saint-Ruf et al. in 196040 and its deriva-tive with tolyl as side chains was prepared byWong et al. recently.41 In this work, we first pre-pared a kind of new conjugated polymer utiliz-ing the conjugated and rigid dinaphtho-s-inda-cene molecule as main chain. For the synthesisof monomers, the key intermediate diester 3was prepared by Suzuki coupling between the2,5-dibromo-dimethylterephthalate (1) and thea-naphthyl boronic acid (2) in 92% yield. The n-octyllithium was prepared in a modified way forsynthesis of n-butyllithium.38 The concentrationand the yield were determined by the double-ti-tration method to be 0.51 M and 35.5%, respec-tively.39 Addition of the excess n-octyllithiumand 4-octylphenyllithium to diester 3 producedthe corresponding diols, and then they werering-closed by two different methods to generate4 and 5. BF3 etherate was used in preparationof 4. For synthesizing aryl-substituted 5, theAcOH/HCl was used instead to avoid the forma-tion of byproducts. It should be noted that inthe aforementioned procedures the substituentswere introduced to bridgehead carbon atomsprior to ring closure, which can ensure the fullmethine carbons substituted and reduce theprobability of ketone defects formation.20,42

Therefore, the synthetic approach we employedcould as greatly as possible avoid the appear-ance of undesirable long-wavelength emissioncaused by ketone defects in the resulting poly-mers. The bromination of 4 and 5 with Br2 inCHCl3 gave the two monomers 6 and 7 at theexpected positions in high yields.

The monomers 6 and 7 for polymerizationwere purified carefully by twice recrystalliza-tion. Their structures were confirmed by 1HNMR spectra, 13C NMR spectra, and single crys-tal structure analysis. Figure 1 showed the 1HNMR spectra of monomers 6 and 7. In the 13CNMR spectra, the signals at d ¼ 55.16 and 65.31ppm indicated the presence of the bridged car-bon atoms in the two monomers.

For the single crystal X-ray analysis, asshown in Figure 2, although the middle benzenering and two naphthalene rings in the mono-mers were both fastened by two methine-bridges, the coplanarity of the two monomerswas different. In monomer 6, the dihedralangles between the benzene ring and the twonaphthalene rings were 1.98 and 4.48, respec-tively, while the two corresponding dihedralangles in monomer 7 were both 2.88. In addi-tion, there existed the p-p stacking between the

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naphthalene ring of neighboring molecules inboth monomers 6 and 7 [Fig. 2(c,d)]. The mole-cules of monomer 6 adopted stereoscopicarrangement. Only the pairwise stacking ofneighboring molecules was present with thenearest distance of 3.592 A (C6 and C60 fromthe naphthalene rings of neighboring molecules)and the overlap area was fairly small. For mono-mer 7, the neighboring molecules employed away of mutually parallel arrangement with thenearest distance of 3.558 A (C10 and C100) andthe naphthalene rings of neighboring moleculesalmost overlay one another, which indicated thestronger p-p interaction than former.

The polymers PONSI and PANSI were synthe-sized by the nickel (0)-mediated Yamamoto poly-merization.43 The polymerization was performedat 80 8C for 3 days, and at the end of the polymer-ization, 1-bromobenzene was added for end-cap-ping the polymeric chain-terminals. The resultingpolymers PONSI and PANSI are both soluble in

common organic solvents such as chloroform, THF,toluene, and chlorobenzene. The Mn and PDI are6.14 3 104 and 2.06 for PONSI, and 4.89 3 104

and 2.11 for PANSI, respectively, as measured bymeans of GPC, with THF as the eluent and poly-styrene as the standard. Thermal properties wereinvestigated by differential scanning calorimetry(DSC) and thermogravimetric analysis (TGA). Incontrast to the liquid crystalline POF, no phasetransitions in temperature range from 50 to300 8C were observed in DSC scans. The amor-phous characteristic of the polymers was con-firmed by POM observation. TGA measurementsindicated that both polymers are very stable with5% weight loss at a temperature of 420 8C.

Optical Properties

The UV–vis absorption and PL spectra ofPONSI and PANSI in solution and film were

Scheme 1. Synthetic routes of the two polymers PONSI and PANSI. The reagentsand conditions were as follows: (i) Pd[(PPh3)4], K2CO3, toluene, 90 8C; (ii) (1)C8H17Li, THF, ice/salt bath; (2) BF3�Et2O; (iii) (1) C8H17PhLi, THF, �78 8C; (2)AcOH, HCl, reflux; (iv) Br2, CHCl3; (v) Ni(COD)2, BPY, COD, DMF/Toluene, 80 8C,3 days, 1-bromobenzene.



shown in Figure 3. All spectral data were sum-marized in Table 1. PONSI exhibited almost nodifference in absorption from solution tofilm with a maximum of 416 nm, but displayeda 5-nm redshifted emission, with the maximumwavelength in solution and film at 458 and463 nm, respectively. In contrast to POIF, whichexhibited both absorption and emission spectrawith well-defined vibronic structures and smallStokes shift, both absorption and emission spec-tra of PONSI were almost featureless along witha larger Stokes shift. This implied that PONSIhad less rigid main chain, possibly due to largerdihedral angles of 1,10-binaphthyl subunits (1038for trans-conformation and 688 for cis-conforma-tion reported in literatures)44,45 in the polymerchain. Almost unchanged absorption spectrumand only a small bathochromic shift of the emis-sion spectrum from solution to film state indi-cated the absence of obvious aggregation andconformation-change of PONSI polymer chain.

In contrast, in the case of PANSI, the UV andPL spectra in films were dramatically redshiftedwhen comparing with those in solution. PANSIshowed the maximum absorption at 420 nm indilute solution and at 428 nm in film. The maxi-mum emission wavelength of PANSI shiftedbathochromically from 462 nm in solution to494 nm in film. The two polymers possessed thesame conjugated main chain and only differentside-chain substituents, but exhibited signifi-cantly different spectral features. This phenom-enon was opposite to the reported conjugationpolymers systems such as PFs,21 PIFs,31 andladder poly(pentaphenylene)s,46 in which theintroduction of phenyl-type substituents in gen-eral resulted in more stable morphology withouteffect on absorption and emission properties infilm. Such distinct optical properties of our poly-mers could be tentatively attributed to the dif-ference of coplanarity and packing mode of thepolymer chains in film state. As aforementioned,

Figure 1. 1H NMR spectra of monomers 6 (top) and 7 (bottom). Solvent: CDCl3whose peaks are marked with (*). Insets: the enlarged views of the signals in aro-matic area.

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the monomer 7 preferred the packing motif withstronger p-p intermolecular interaction.

To investigate the effect of the naphthalenemoieties on the thermal stability of PL spectra,POF, POIF, PONSI, and PANSI films wereannealed at 200 8C under argon for 9 h. Asshown in Figure 4, after annealing, a strongadditional emission band at 500–600 nmappeared for POF and POIF films, while almostno change of spectral shape for PONSI andPANSI films was observed after the same treat-ment. This clearly revealed the higher spectralstability of PONSI and PANSI than POF andPOIF. Aside from the absence of the ketonedefects, this higher spectral stability could beattributed to the presence of the 1,10-dinaphthyl

moieties in the backbones, which resulted in thegreater torsion of the polymer chain, therebystable morphology.

Electrochemical Properties

CV measurements were carried out in a three-electrode cell at room temperature, with a Ptwire, a saturated calomel electrode, and a glassycarbon electrode (10 mm diameter) coated withpolymer film as the counter electrode, the refer-ence electrode, and the working electrode,respectively. Bu4NClO4 (0.1 M in acetonitrile)and ferrocene (4.8 eV below vacuum)47 wereused as the electrolyte and internal standard,

Figure 2. ORTEP views of monomers 6 (a) and 7 (b) with the ellipsoids represent-ing the 30% probability level and hydrogen atoms omitted, and the molecular pack-ing diagrams of the monomers 6 (c) and 7 (d) in single crystals with the substituentsomitted for clarity.



respectively. The potential scan rate is 50 mV/s.Both polymers exhibited partial reversibility inboth oxidation and reduction processes, asshown in Figure 5. From the redox onset poten-tials, the highest occupied molecular orbital(HOMO)/the lowest unoccupied molecular or-bital (LUMO) energy levels of PONSI andPANSI were estimated to be �5.63/�2.27 and�5.70/�2.45 eV, respectively (Table 1). In com-

parison, the HOMO/LUMO energy levels of POFand POIF were also estimated by the samemethod to be �5.78/�2.18 eV that well coincidedwith those reported in literature48 and �5.66/�2.19 eV. The enhanced HOMO energy levelsand the decreased LUMO energy levels ofPONSI and PANSI compared to POF and POIFwere attributed to the extended conjugation,which resulted in the narrower energy bandg-aps, consistent with the aforementioned discus-sion on optical properties.

EL Properties

To investigate the EL performance of the twopolymers, The PLEDs with the configuration ofITO/PEDOT:PSS (40 nm)/emitting layer (EML)/Ca (10 nm)/Al (150 nm) were fabricated. Figure6(a) showed the EL spectra and the luminous ef-ficiency–current density curves of PONSI- andPANSI-based devices. The device performancedata were summarized in Table 2. It was foundthat the device performance of PONSI was var-ied with the thickness of EML. At EML thick-ness of about 50 nm, PONSI exhibits the ELspectrum identical to its PL counterparts, witha maximum peak at 461 nm and the CIE coordi-nates of (0.19, 0.26) at 100 mA/cm2. Low turn-onvoltage of 3.7 V, the maximum luminous effi-ciency of 1.40 cd/A, the maximum externalquantum efficiency (EQE) of 0.71%, and themaximum brightness of 2036 cd/m2 (13 V) wererealized. Although the EL color of PONSI is notpure blue, it is possibly a good host polymer forconstruction of white light-emitting polymersbecause of its relatively redshifted emission.49

PANSI shows the green EL with the maximumpeak at 520 nm and the CIE coordinates of(0.27, 0.49). The difference of EL spectra

Figure 3. Normalized UV–vis absorption and photo-luminescence spectra of PONSI (a) and PANSI (b) intoluene (concentration of the repeating units: 10�6 M)and in film state.

Table 1. Absorption and Photoluminescence Maxima (kabs, max and kPL, max), Reduction and Oxidation Onsets(Eox

onset and Eredonset), HOMO and LUMO Energy Levels, and Energy Bandgaps (Eg) of the Polymers

Polymer

kabs, max (nm)a kPL, max (nm)b

Eoxonset

(V)Ered

onset

(V)HOMO(eV)

LUMO(eV)

Eg

(eV)Solution Film Solution Film

PONSI 416 (395)c 416 (391) 458 (488) 463 (491) 0.83 �2.53 �5.63 �2.27 3.38PANSI 420 (395) 428 462 (488) 494 (515) 0.90 �2.35 �5.70 �2.45 3.25

a Solution spectra were measured in toluene with a concentration of 10�6 M (repeating units); the films were spin-cast fromtoluene solution with a concentration of 8 mg/mL.

b The excitation wavelength for PL measurements is 380 nm.c Peaks that appear as shoulders or weak bands are indicated in parentheses.

4874 GUO ET AL.


between PONSI and PANSI was in agreementwith that of PL spectra. The PANSI-based de-vice turned on at 3.8 V and the maximum lumi-nance efficiency, the maximum EQE, and themaximum brightness were 1.73 cd/A, 0.60%, and4531 cd/m2 (22 V), respectively.

We also preliminarily investigated the colorstability of PONSI-based device with the EMLthickness of 50 nm. As shown in Figure 6(b),PONSI exhibited voltage-independent EL spectra.EL spectra of PONSI-based device at differentoperating times were depicted in Figure 6(c).Compared to POF [Fig. 6(d)] whose EL spectrumdisplayed a strong additional long-wavelengthemission band in several minutes, PONSI showedmuch better color stability during the continuousoperation. Only a slight change in the EL spectrawas observed during the operation process. Theimproved EL color stability in comparison withPOF was assigned to both the absence of ketonedefects and the stable film morphology originat-

Figure 4. Normalized film PL spectra of POF (a), POIF (b), PONSI (c), and PANSI(d) before (solid lines) and after (dash lines) annealing at 200 8C for 9 h under argon.

Figure 5. Cyclic voltammograms of PONSI (top)and PANSI (bottom) in 0.1 M Bu4NClO4 in acetoni-trile at a scan rate of 50 mV/s.



ing from the conjugation extension of PONSI atthe direction perpendicular to backbone.

CONCLUSIONS

In conclusion, we have presented herein thesynthesis and properties of the new EL poly-

mers PONSI and PANSI, with extended conju-gation along the direction perpendicular to themain chain, by replacing two phenyl units inindenofluorene with naphthylenes. Such conju-gation extension makes the emission redshiftedin comparison with POIF. 1,10-Binaphthyl con-nection between the repeating units results inthe greater torsion of the polymer chains, which

Figure 6. (a) EL spectra of PONSI- and PANSI-based devices at 8 V. Inset: lumi-nous efficiency–current density curves of the devices. (b) EL spectra of PONSI-baseddevice at different current density; (c) EL spectra of PONSI-based device at differentoperation times. (d) EL spectra of POF-based device at 0, 5, and 15 min.

Table 2. EL Performance Data of PONSI- and PANSI-Based Devices

Polymer VTa (V) LEmax (cd/A) LEb (cd/A) EQEmax (%) Lmax (cd/m2) CIEb (x, y)

PONSI 3.7 1.40 1.25 0.71 2036 0.19, 0.26PANSI 3.8 1.73 1.59 0.60 4531 0.27, 0.49

a The onset voltage at the brightness of 1 cd/m2.b At the current density of 100 mA/cm2.

4876 GUO ET AL.


is potentially critical for obtaining amorphouspolymers with stable film morphology and emis-sion spectra. The PLEDs based on the polymerPONSI emits sky blue light with the CIE coordi-nates of (0.19, 0.26) and the maximum luminousefficiency of 1.40 cd/A. This polymer with rela-tively broad bandgap and high spectral stabilityshould be used not only as a novel efficient ELmaterial, but also as a host material for con-structing polymers to emit low-energy light orwhite light via controlled energy-transfer pro-cess.

This work is supported by the National Natural Sci-ence Foundation of China (No. 20574067 and No.50633040), Science Fund for Creative ResearchGroups (No. 20621401), and the 973 Project (No.2002CB613402).

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Synthesis and Characterization of Color-Stable Electroluminescent Polymers: Poly(dinaphtho [1,2-a:10,20-g]-s-indacene)s

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