RAPID COMMUNICATION

Novel Bipolar Light-Emitting Copolymer Containing Triazole andTriphenylamine Moieties

ZE LIU, YANGUANG ZHANG, YUFENG HU, GUANGPING SU, DONGGE MA, LIXIANG WANG, XIABIN JING,FOSONG WANG

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

Received 20 August 2001; accepted 18 January 2002Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pola.10176

Keywords: LEDs; bipolar; copolymer; triazole (TAZ); triphenylamine (TPA); conju-gated polymers

INTRODUCTION

Polymeric light-emitting diodes have attracted much at-tention because of their potential for applications in de-veloping larger and flexible displays since the discovery ofthe electroluminescence (EL) of poly(p-phenylenevi-nylene) (PPV) as a conjugated polymer.1 Some typicalconjugated polymer systems, such as poly(p-phenylene),polythiophene, and polyfluorene, have been developed,and some important results have been obtained.2–4 How-ever, the search for new polymers that possess combineddesirable properties, such as high brightness and EL ef-ficiency, low turn-on voltage, long operating lifetime, andgood processability, remains a big challenge. It is wellknown that most of the light-emitting polymers tend to bep-type polymers with a much greater tendency for inject-ing and transporting holes than for injecting and trans-porting electrons.5–8 This charge imbalance is one of thekey limits for increasing the EL quantum efficiency ofpolymer light-emitting diodes. To solve this problem,many research groups have reported the use of light-emitting polymers containing both electron- and hole-transporting moieties to achieve efficient device perfor-mance.9–20 However, to the best of our knowledge, abipolar light-emitting PPV-based polymer containing

both triazole (TAZ) and triphenylamine (TPA) moietieshas not been reported so far.

In this communication, we present the synthesis andprimary characterization of a novel conjugated copolymercontaining TAZ and TPA moieties, poly[4-phenyl-3,5-bi(p-vinylenephenyl)-1,2,4-triazole-alt-4,4-bi(p-vinylene-phenyl)-p-2�-ethylhexyloxylphenylamine] (TAZ–TPA–PPV), which can be regarded as the conjugated combina-tion of TPA and TAZ moieties and can be expected to havefunctions of both electron and hole affinity in one polymerchain because TAZ and TPA have been widely used asexcellent electron-transporting/hole-blocking materialsand hole-transporting materials in the fabrication ofsmall-molecule EL devices, respectively.21–25

EXPERIMENTAL

The synthetic route for TAZ–TPA–PPV is outlined inScheme 1. The monomer 4,4�-dimethyl benzoylhydrazide(1) was synthesized from p-toluic acid as the startingmaterial. p-Toluic acid was first reacted with thionyl chlo-ride to yield p-toluoyl chloride; this was reacted withhydrazine dihydrochloride in pyridine to yield 1 (76%yield).15 The ring-closure reaction of 1 with aniline in thepresence of phosphorus trichloride at the reflux temper-ature was carried out to yield 4-phenyl-3,5-bi(4�-methyl)phenyl-1,2,4-triazole (2; 56% yield); this was chlo-romethylated with SO2Cl2 and then converted into thecorresponding tributyl phosphine salt by the reaction of 2with excess tributyl phosphine. Monomer 2 was synthe-

Correspondence to: L. Wang (E-mail: lixiang@ciac.jl.cnJournal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 1122–1126 (2002)© 2002 Wiley Periodicals, Inc.

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sized from 4-iodo-2�-ethylhexyloxybenzene by an Ull-mann reaction with diphenylamine (62% yield) followedby a Vilsmeier reaction. The polymerization betweenmonomers 1 and 2 was carried out through a Wittigreaction in the presence of EtONa in a CHCl3–EtOHmixed solvent at room temperature. The polymer wasobtained as a yellow solid after two precipitations from achloroform solution into methanol (44% yield). The chem-ical structures of the monomers and the resulting poly-mer were confirmed by elemental analysis.26

TAZ–TPA–PPV is fully soluble in common organicsolvents such as chloroform, tetrahydrofuran, and

1,2-dichloroethane. The thermal properties of TAZ–TPA–PPV were investigated with PerkinElmer 7 dif-ferential scanning calorimetry and thermogravimet-ric analysis instruments under nitrogen. TAZ–TPA–PPV possesses good thermal stability and has aglass-transition temperature of 129 °C and a decom-position temperature of 400 °C. The molecularweight of TAZ–TPA–PPV was determined by gel per-meation chromatography with tetrahydrofuran as aneluent and polystyrene as the standard. TAZ–TPA–PPV has a weight-average molecular weight of21,109 and a polydispersity index of 1.9.

Scheme 1. Syntheses of the polymers via the Wittig condensation reaction.

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OPTOELECTRONIC PROPERTIESOF THE POLYMER

TAZ–TPA–PPV exhibited good film-forming ability,and its uniform transparent thin film on a glass sub-strate (quartz plates) was prepared by spin coatingfrom the polymer solution in chloroform. Upon expo-sure to ultraviolet radiation, the film emitted intensegreen-yellow photoluminescence (PL). The ultraviolet–visible absorption and fluorescence spectra of polymerfilms are displayed in Figure 1. The ultraviolet–visibleabsorption spectra for the TAZ–TPA–PPV film and so-lution samples are quite similar. Three absorptionbands at 390, 310, and 245 nm can be observed; theformer can be attributed to the �–�* transition of theconjugated backbone, and the latter two peaks can beassigned to the �–�* transition of the TPA and TAZunits, respectively.17,27–30 The absorption edge of TAZ–TPA–PPV can be seen around 495 nm, which corre-sponds to a 2.50-eV band gap. The PL spectra obtainedfrom the thin film and solution are also given in Figure1. The PL spectra of TAZ–TPA–PPV both in solutionand film are almost identical, and their maximumpeaks appear at 522 nm, corresponding to green-yellowlight emissions.

Cyclic voltammetry was performed with a polymerfilm on a Pt working electrode (0.5 cm2) with a plati-num wire as a counter electrode and Ag/AgCl as areference electrode (the electrochemical potential wasca. 0.44 V vs FOC). The data are shown in Figure 2. Theoxidative reaction is reversible. The onset of oxidationoccurs at about 0.78 V (vs Ag/AgCl), and an anodic peakappears at 1.24 V with the corresponding reductionpeak at 0.25 V (vs Ag/AgCl). The oxidation potential(EOxd

1/2) is 0.75 V (vs Ag/AgCl). Under a negative scan,a reversible redox curve was obtained. The cathodic

peak appears at �1.75 V (vs Ag/AgCl), and a corre-sponding reoxidation peak appears at �0.7 V (vs Ag/AgCl); the n-doping potential (ERed

1/2) is �1.23 V (vsAg/AgCl). Therefore, the highest occupied molecularorbital and lowest unoccupied occupied molecular or-bital energy levels and the band gap have been esti-mated to be �5.15, �2.17, and 2.98 eV, respectively.31

On the basis of this investigation, we conclude that theintroduction of TAZ and TPA units results in a betterelectron- and hole-transporting ability for the designedpolymer.

A single-layer light-emitting diode with the con-figuration of indium tin oxide/TAZ–TPA–PPV/Mg:Agwas fabricated. The current and light output charac-teristics are given in Figure 3. For the single-layerdevice, the threshold voltage is about 6 V, and abright green-yellow EL was observed, with lumines-cence of about 90 cd/m2 at a bias of 12 V. The EL peakwas found at 520 nm, which agrees well with the PLpeak. It indicated the same EL and PL emissionstate. At present, little work has been directed to-ward optimizing the fabrication processes and devicestructures. We expect that quantitative data on de-vice performances will be reported in the near futureafter optimization.

CONCLUSIONS

We have synthesized a novel TAZ- and TPA-containing,PPV-based copolymer with an alternating structure(TAZ–TPA–PPV) by the Wittig reaction. The polymeryields a bright green-yellow emission and has goodsolubility, excellent film-forming ability, and high ther-

Figure 1. Absorption and PL spectrum of the TAZ–TPA–PPV film (—). The absorption and PL spectrum ofsolution (–) is also given for comparison.

Figure 2. Cyclic voltammograms of the TAZ–TPA–PPV film coated on a Pt sheet in a solution of Bu4NClO4

(0.1 M) in MeCN at room temperature and a scanningrate of 50 mV s�1.

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mal stability, properties which make it a good candi-date for polymeric light-emitting diodes.

This work was supported by NSFC (29725410 and29992530) and CAS (KJ 951-A1-501-01).

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26. 1H NMR (400 MHz) for compound 2 (CDCl3, �,ppm): 7.43 (m, 3H), 7.30 (d, 4H), 7.15 (d, 2H), 7.09(d, 4H), 2.32 (s, 6H). 1H NMR (400 MHz) for com-

Figure 3. Current–luminance–voltage characteristics of a device in the single-layerconfiguration.

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pound 3 (CDCl3, �, ppm): 7.49 (m, 3H), 7.40 (d, 4H),7.32(d, 4H), 7.18 (dd, 2H), 4.55 (s, 4H). 1H NMR(400 MHz) for monomer 1 (CDCl3, �, ppm): 7.52–7.09 (m, 13H), 4.39 (d, 4H), 2.35 (d, 12H),1.43 (s.24H), 0.91 (s, 18H). 1H NMR (400 MHz) for mono-mer 2 (CDCl3, �, ppm): 7.74 (d, 4H), 7.17 (d, 4H),7.08 (d, 2H), 6.94 (d, 2H), 3.86 (d, 2H) 1.4–1.7 (m,10H) 0.92 (m, 6H). ELEM. ANAL. Calcd. for TAZ–TPA–PPV (C50H46N4O1): C, 83.56%; H, 6.41%; N,7.80%. Found: C, 83.21%; H, 6.50%; N, 7.74%.

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