Novel blue-light-emitting poly(terphenylenevinylene) derivative

Optical Materials 21 (2002) 175–180 www.elsevier.com/locate/optmat

Novel blue-light-emitting poly(terphenylenevinylene) derivative Yun-Hi Kim, Jun-Hwan Ahn, Dong-Cheol Shin, Hyung-Sun Kim, Soon-Ki Kwon * Department of Polymer Science and Engineering and RIIT, Gyeongsang National University, Chinju 660-701, South Korea

Abstract The blue electroluminescent polymer, poly(terphenylenevinylene) derivative which has advantages of poly(p-phenylene) and poly(p-phenylenevinylene), was prepared by the Suzuki coupling reaction. The structure and properties of the polymer, was analyzed by various spectroscopic methods. The obtained polymer has good solubility and thermal stability. The polymer film showed maximum absorption and emission at 340 and 450 nm, respectively. A blue electroluminescence (kmax ¼ 450 nm) was obtained from the light-emitting diode of ITO/PDHPPV/Al–Li with turn on voltage of 8 V. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 85.60.J Keywords: Suzuki coupling; Good solubility; Good thermal stability; Electroluminescent; Blue LED; Poly(terphenylenevinylene)

1. Introduction Efficient thermally stable blue-light-emitting materials are needed both to complete the color spectrum and serve as energy transfer media for incorporated fluorophores. The extended delocalization lengths of most fully conjugated polymers, however, result in small electronic band gaps and red shifted emissions. The poly(p-phenylenevinylene) (PPV) [1–6], which is the first reported electroluminescent polymer, and PPV derivatives have several advantages as an emitting material; high thermal stability, good film quality, suitable color tun*

Corresponding author. Tel.: +82-55-751-5296; fax: +82-55753-6311. E-mail address: [email protected] (S.-K. Kwon).

ability, etc. However, it has low oxidative stability and some difficulties in processability and emitting blue color. Poly(p-phenylene) (PPP), and its derivatives [7–12] have also been extensively investigated for light-emitting materials because they are thermally and oxidatively stable polymers. PPP derivatives show large band gaps since the aromatic rings are twisted to relive unfavorable steric interactions in the backbone, which limits the effective conjugation length. Unsubstituted PPP is highly insoluble, limiting the molecular weights and processability. Large, solubilizing substituents may also be incorporated, resulting in improved processability, but this usually exacerbates the steric interactions in the polymer main chain. Thus, PPP derivatives are intrinsically violet–blue emitter to have some difficulties in color tunability [13].

0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 1 3 2 - 5

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Solubility, oxidative stability, low turn-on voltage, and color tunability (especially blue emission) are desirable properties for conjugated polymers used in LEDs. Moreover, good thermal stability is needed because heat is generated when current passes through the device. Thus, we tried to synthesize a blue-light-emitting polymer, which has the advantages of both PPP and PPV. Recently, we reported synthesis and properties of poly(biphenylenevinylene) (PBPV) derivatives with a controlled conjugation length of biphenylenevinylene unit, which is composed of alternating PPP and PPV units [14,15]. Here, we report the synthesis and characterization of blue-light-emitting, poly(terphenylenevinylene) derivative. And, on the basis of poly(terphenylenevinylene), our research also involved introducing substituents into the vinyl bridge and/or phenyl ring. Introducing substituent group to a vinyl bridge and/or phenyl ring, may expect to lead to enhanced solubility, oxidative and thermal stability, and reduced formation of excimers owing to interchain interactions.

drous hexane was cooled to )10 °C, n-BuLi was added in the solution. The mixture was stirred for 3 h, and then the solution was cooled to )60 °C. The trimethyl borate was slowly added into the solution and stirred for 12 h. The reaction was terminated by 150 ml of 2 N HCl and ice. The mixture was extracted with ether and filtered. The crude product was recrystallized from acetone. (Yield ¼ 15%, m.p. ¼ 170 °C) 1 H-NMR (500 MHz, CDCl3 ) (ppm): d 7.1(s, 2H), d 2.5 (t, 4H), d 1.6–1.2 (m, 16H), d 0.8 (t, 6H) FT-IR (KBr) (cm1 ): 3030(aromatic C–H), 2846(aliphatic C–H). 2.3. Preparation of (4-bromobenzyl)triphenylphosphonium bromide

2. Experimental

A mixture of 50 g (0.20 mol) of 4-bromobenzylbromide and 63 g (0.24 mol) of triphenylphosphine in 400 ml of dry benzene was stirred at 70 °C for 12 h. After the mixture was allowed to room temperature, the white powder was filtered. The crude product was washed with dry diethylether. (Yield ¼ 98%) 1 H-NMR (500 MHz, CDCl3 ) (ppm): d 7.2–7.3(d of d, 4H), d 5.3(s, 2H), FT-IR (KBr) (cm1 ): 3030(aromatic C–H), 2846(aliphatic C–H).

2.1. Materials

2.4. 1,2-[40 ,400 -dibromophenyl]-1-phenylethylene

Diethylether, THF, CH2 Cl2 and toluene were purified by distillation from CaH2 and used immediately. Triphenylphosphine was purchased from Aldrich, recrystallized from degassed ethanol, and sublimed under vacuum prior to use. 4-Bromobenzophenone, 4-bromobenzaldehyde, 4-bromobenzylbromide, trimethylborate, 2-ethylhexylbromide, 4-methoxyphenol, and tetrakis(triphenylphosphine)palladium(0) were purchased from Aldrich. Spectroscopic grade CHCl3 (Aldrich) was used for all absorption and emission experiments. All other compounds were used as received.

(4-Bromobenzyl)triphenylphosphonium bromide 20 g (39 mmol) and sodium hydride 2.5 g (192.9 mmol) dissolved in toluene (150 ml) under nitrogen. The solution was stirred for 3 h and cooled to )10 °C. 4-Bromobenzophenone 10.2 g (39 mmol) was added and refluxed for 24 h at 110 °C. After the reaction mixture was cooled to room temperature, the organic layer was extracted by methylene dichloride. The product was obtained by chromatography with eluent of hexane. (Yield ¼ 30, m.p. ¼ 75–76 °C) 1 H-NMR (500 MHz, CDCl3 ) (ppm): d 7.6–6.9(m, 2H), FT-IR (KBr) (cm1 ): 3020(aromatic and vinylic C–H), 1065(aromatic C–Br).

2.2. Preparation of 2,5-dihexylbenzene-1,4-diboronic acid

2.5. Synthesis of polymer

After a solution of 10 g (24.8 mmol) 1,4-dihexyl-2,5-dibromobenzene in the 100 ml anhy-

All handling of catalysts and polymerization was done in dry box under a nitrogen atmosphere.

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To a stirred solution of 2,5-dihexylbenzene1,4-diboronic acid 0.5 g (1.496 mmol), 1,2-[40 ,400 dibromophenyl]-1-phenylethylene 0.62 g (1.496 mmol) in 8 ml THF and 3 ml 2 M K2 CO3 aqueous solution was added catalysts of Pd(PPh3 )2 0.0103 g (0.6 mol%). The reaction mixture was heated at 80 °C under nitrogen atmosphere for 8 h. After bromobenzene 0.05 g (0.32 mmol) was added, phenyl boronic acid 0.05 g (0.41 mmol) was added with small amounts of catalysts for end-capping. After 2 h, the reaction mixture was poured into methanol (50 ml) and filtered with glass-filter. The residue was dissolved in CHCl3 and washed with waters. After being dried over MgSO4 , precipitation was twice repeated to methanol. (Yield ¼ 55%) 1 H-NMR (500 MHz, CDCl3 ) (ppm): aromatic(C–H), d 7.4–7.1 (m, 10H), aliphatic(C–H) d 2.5 (t, 4H), d 1.6–1.2 (m, 16H), 0.8 (t, 6H) FT-IR (KBr) (cm1 ): 3020(aromatic and vinylic C–H), 2846(aliphatic C–H). Anal. Calcd for C38 H38 : C, 92.31; H, 7.69, Found: C, 92.19; H, 7.55. 2.6. Fabrication of the LED An indium tin oxide (ITO) coated glass substrate which had been washed with water, acetone, and isopropyl alcohol sequentially. A thin polymer ) was spin-coated (2200 rpm, 50 s) film (800–1100 A from a filtered (0.2 lm filter) 2.0 wt.% of polymer solution in chlorobenzene on a ITO layer. An aluminum/lithium alloy (Al:Li ¼ 99.95:0.05 wt.%) ) was deposited on top of the electrode (1300 A device at a high vacuum (<1 105 T). Wires were attached to the respective electrodes with a conductive epoxy adhesive. All fabrication steps were performed in clean room conditions. Measurements were done at room temperature in air. 2.7. Instrument Melting points were determined using an Electrothermal Mode 1307 digital analyzer. 1 H-NMR and 13 C-NMR spectral data were expressed in ppm relative to the internal standard and were obtained on a DRX 500 MHz NMR spectrometer. FT-IR spectra were obtained with a Bomem Michelson series FT-IR spectrometer and the UV–vis absorption spectra were obtained in chloroform

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on a Shimadzu UV-3100 spectrophotometer. Molecular weight and polydispersity of the polymer were determined by gel permeation chromatography analysis with polystyrene standard calibration. Elemental analyses were performed by Leco Co. CHNS-932. TGA measurements were performed on a Perkin–Elmer Series 7 analysis system under N2 at a heating rate of 10 °C/min. The photoluminescence spectra were recorded on a Perkin–Elmer LS-50 fluorometer utilizing a lock-in amplifier system with a chopping frequency of 150 Hz. Thickness of films was determined with a Sloan Dektak.

3. Results and discussion The synthetic route of polymer was shown in Scheme 1. 2,5-Dihexylbenzene-1,4-diboronic acid was obtained by reaction of lithiated 1,4-dihexyl2,5-dibromobenzene and trimethylborate. 1,2-(40 , 400 -dibromophenyl)-1-phenyl ethylene was synthesized by Wittig reaction of (4-bromobenzyl)triphenylphosponium bromide with 4-bromobenzophenone. The polymerization was carried out using a typical Suzuki coupling reaction with good yield. After the polymerization, the end-capped reaction of the bromine end group and boronic acid end group, which hamper thermal stability and efficiency of PL, were accomplished by phenyl boronic acid and bromobenzene, respectively. The polymer structure shown was consistent with the elemental analysis and the spectroscopic data from 1 H-NMR and FT-IR. The obtained polymer was readily soluble in common organic solvents such as chloroform, dichloromethane, toluene and THF. The number average molecular weight Mw and polydispersity index of polymer is 9500 and 1.5, respectively. Thermal characterization of the polymer was accomplished by DSC and TGA. The DSC thermogram obtained from the second heating of the polymer (Fig. 1). The polymer shows narrow endotherm at 142 °C and no additional melting transition. The TGA thermogram under nitrogen shows that the weight loss of polymer is 5% on heating 320 °C. Fig. 2 shows the optical absorption and photoluminescence spectra of a dilute solution of the

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Scheme 1.

Fig. 1. DSC thermogram of the polymer.

Fig. 2. UV–vis absorption and photoluminescence spectra of polymer in chloroform.

polymer in chloroform. The maximum absorption of polymer showed 340 nm. The maximum ab-

sorption peak of the polymer are 70–90 nm blue shifted as compared with that of PPV [16], 10–20

Y.-H. Kim et al. / Optical Materials 21 (2002) 175–180

nm blue shifted as compared with that of PBPV [14,15] and 10–20 nm red shifted as compared with that of PPP [17]. These shifts may originate from the terphenylenevinylene units composed of PPP and PPV units. The band-gap energy of the new polymer estimated from extrapolation of the low energy absorption spectra was about 3.2 eV. The polymer emitted strong blue fluorescence under ultraviolet irradiation in chloroform solution. The maximum PL of the polymer showed at 450 nm, when the PL spectra was recorded with an excitation wavelength corresponding to the absorption maximum wavelength of the polymer. Although the emission in solid state is 10 nm red shifted relatively to those of the solution state, it is blue shifted by 10–20 nm from those of poly(biphenylenevinylene) derivatives. It is also explained that the main chain is composed of terphenylenevinylene (Fig. 3). Fig. 4 shows the voltage–current density curve of the simple ITO/emitting polymer/Al–Li device and ITO/(polmer:PVK ¼ 1:5)/Al–Li device. The current increases with increasing forward bias voltage. The turn-on voltage of the devices are 8– 12 V. The electroluminescent spectrum of the ITO/ emitting polymer/Al–Li device is similar to the PL spectra of polymer except a small shoulder appeared around 530 nm (Fig. 5). This small shoulder may have resulted from interchain excimer emission. When the polymer was blended with the poly(vinylcarbazole) (PVK) which serves several

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Fig. 4. Luminescence–voltage characteristic of the polymer (r) and blended polymer (d).

Fig. 5. Electroluminescent spectra of polymer (- - -) and polymer:PVK(1:5) (––).

Fig. 3. Solid state absorption and photoluminescence spectra of the polymer.

functions in the blend of emitting polymer materials [18,19] the electroluminescent spectrum shows a more suppressed shoulder at 530 nm (Fig. 5). As the ratio of PVK to polymer is increased to (polymer:PVK ¼ 1:5), the maximum brightness and efficiency of the device increased though the turn on voltage of the device slightly increased. This suggests that the increase of maximum brightness and efficiency is caused by the dilution effect of PVK, which suppressed the interchain excimer, and/or by hole transporting role of PVK.

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4. Conclusions A new blue-light-emitting conjugated polymer, poly(terphenylenevinylene) derivative, having advantages of both PPP and PPV, has been designed and synthesized. The synthesized polymer has good solubility and thermal stability. The LEDs based on the polymer showed to have bright blue emission and low turn on voltage.

Acknowledgement This work was supported by KOSEF(R052000-00016).

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