Synthesis of laterally attached side-chain liquid crystalline poly(p-phenylene vinylene) and polyfluorene derivatives for the application of polarized electroluminescence

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Synthesis of laterally attached side-chain liquid crystalline

poly(p-phenylene vinylene) and polyfluorene derivatives for the

application of polarized electroluminescence

Yung-Hsin Yao, Sheng-Hsiung Yang, Chain-Shu Hsu

*

Department of Applied Chemistry, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan, ROC Received 16 August 2006; received in revised form 5 October 2006; accepted 6 October 2006

Available online 27 October 2006

Abstract

Two series of poly(p-phenylene vinylene) and polyfluorene derivatives (PPV1ePPV4 and PF1ePF5) containing laterally attached penta-(p-phenylene) mesogenes were synthesized and characterized. These polymers show nematic liquid crystalline behavior. The optical properties of the polymers were investigated by UVevis absorption and photoluminescence spectrometers and these polymers were fabricated to form the polarized electroluminescent devices using poly(ethylenedioxythiophene)epoly(styrene sulfonic acid) (PEDOTePSS) as an alignment layer. In the series of poly(p-phenylene vinylene) derivatives, polymer PPV4 offered the best EL device performance. It emitted yellow light at 588 nm at 4 V. The maximum brightness was about 1337 cd/m2at 9 V with a polarized ratio of 2.6. In another series of polyfluorene derivatives, PF4 offered the best EL device performance with the polarized ratio of 12.4 and a maximum luminescence of 1855 cd/m2. In the case of polarized white light, as a consequence of blending small amount of PF4 and PF5 with a host polymer PF2, polarized ratio of up to 10.2 and a maximum brightness of 2454 cd/m2 have been attained. The aligned films exhibited pronounced polarized ratio, implying that the polymers exhibit potential for linearly polarized LED application.

Keywords: Poly(p-phenylene vinylene); Polyfluorene; Polarized light

1. Introduction

Since the first report on polymer light-emitting diodes (PLED) [1], a number of p-conjugated polymers have been intensively investigated in order to fabricate devices for indus-trial applications [2e4]. Among them, poly(1,4-phenylene vinylene) (PPV) and polyfluorene (PF) as well as their deriv-atives show great promise for PLED applications[5].

In 1995, Inganas et al. demonstrated a PLED device based on aligned conjugated polymers which directly emitted polarized light and realized that such devices would be particularly useful as backlights for conventional liquid crystal displays (LCDs) [6]. This will simplify LCD manufacturing and reduce cost. In principle, the liquid crystalline oligomers and conjugated poly-mers which can be aligned under a mesomorphic phase to

achieve monodomain thin film, are the best candidates for the manufacturing of highly polarized electroluminescence (EL) devices [7e21]. Among them, oligo(fluorene)s show very promising properties in polarized emission. Chen et al. synthe-sized some oligo(fluorene)s with different chain lengths and chromophores[17,18]. These materials show liquid crystalline behavior and polarized white light emission can be obtained by blending method. It showed a polarization ratio of 16 and a lu-minance yield of 4.5 cd/A; however, no brightness data were re-ported in that work. Besides, the preparation of these state-of-art materials strongly depends on synthetic skills. Relatively, PFs made by well-known Suzuki-coupling polymerization are easier to obtain. With proper side chains, PFs can also exhibit liquid crystallinity and be used for the fabrication of polarized EL de-vices. Very recently Wen et al. demonstrated blue-light polar-ized PLED using aligned polyfluorene [19]. The best device showed a high dichroic ratio of 25.7 (at 451 nm emission wave-length), while the brightness approached 1000 cd/m2at a low

* Corresponding author. Tel./fax:þ886 3 5131523. E-mail address:[email protected](C.-S. Hsu).

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biased voltage. An additional rubbed PVK layer was used to align PF. Our group has also demonstrated polarized white emis-sion from blending three PF derivatives[20], yet the EL dichroic ratio was not very high (up to 4.6). It seems that the liquid crys-tallinity of common PF polymers is less sufficient to align thin films. A further modification by incorporating laterally attached liquid crystalline moieties on the main chain to improve dichroic ratio is thus proposed and executed in this work.

In 2000 our laboratory reported poly(2,3-diphenyl-1,4-phenylene vinylene) derivatives having liquid crystalline side groups [21]. These polymers exhibit a nematic phase and have potential application for polarized electroluminescence. When these polymers were rubbed at a liquid crystalline state, the mesogenic side groups were aligned along the rubbing direction and the conjugated polymer backbones were forced to align more or less perpendicular to the rubbing direction

(Fig. 1). Nevertheless, under such situation, the alignment of

polymer backbones was very poor since the polymer back-bones were not involving in the formation of mesophases and the polarized ratio of EL emission was very low (up to 2.1). To solve this problem, the best strategy is to synthesize conjugated polymer with laterally attached mesogens. It is well-documented that the side-chain liquid crystalline poly-mers with the laterally attached mesogens have tendency to form a nematic phase[22e26]. The large extended mesogens force a polymer backbone to take extended conformation due to the mesogen-jacket surrounding it.

In this paper, we synthesized two series of PPV and PF deriv-atives containing laterally attached penta(p-phenylene) meso-gens. To our best knowledge, this is the first example of such kinds of liquid crystalline conjugated polymers with laterally attached mesogens. The mesomorphic behaviors as well as po-larized PL and EL properties of these polymers were studied.

2. Experimental section 2.1. Materials

4-Butyl-40-iodobiphenyl, 4-methoxyphenyl, 1,8-bromooc-tane, n-butyl lithium,

2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and the other reagents were purchased from Aldrich and used as received. Tetrahydrofuran (THF) and dichloromethane were distilled and dried from sodium/ benzophenone and calcium hydride, respectively. Monomers 1,4-bis(bromomethyl)-2-methoxy-5-octyloxybenzene (M3), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (M4), 2,7-dibromo-9,9-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (M5), 4,7-dibromo-2,1,3-benzothiadiazole (M6) and 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (M7) and the end capping agents (1-bromo-4-(tert-butyl) benzene and 1-(tert-butyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) benzene) used in the Suzuki coupling for polyfluorenes, were prepared accord-ing to the previously published procedures[27e30].

2.2. Characterizations

1_{H and} 13_{C NMR spectra were recorded on a Varian 300}

spectrometer using tetramethylsilane as internal references. Gel permeation chromatography (GPC) was obtained through a VE-2001 GPC in THF using a calibration curve of styrene standards. Thermal transitions of monomers and poly-mers were determined on a PerkineElmer Pyris 1 differential scanning calorimeter (DSC) with a heating/cooling rate of 10C/min. A Carl-Zeiss Axiphot polarized optical microscope (POM) equipped with a Mettler FP82 hot stage and an FP80 central processor were used to analyze the anisotropic textures. The polarized ultravioletevisible (UVevis) spectra were measured on a Shimadzu UV-1601 spectrophotometer with a polarizer placed between sample and detector. Spec-tra were measured with the polarizer aligned parallel and perpendicular to the rubbing direction. Polarized ratios were defined as the parallel to perpendicular intensity. Polarized photoluminescence (PL) and electroluminescence (EL) were measured, respectively, on a Shimadzu RF-5301 PC spectrofluoro-photometer and a Photo Research PR-650 spectrophotometer with a polarizer placed between sample and detector.

2.3. Synthesis of monomers M1 and M2, and model compound LC1

Scheme 1 outlines the synthetic routes for monomers M1

and M2, and model compound LC1.

2.3.1. 2-(40 -Butyl-biphenyl-4-yl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (1)

To a solution of 4-butyl-40-iodobiphenyl (5.0 g, 14.9 mmol) in THF (50 mL) at 78_{C, 10.2 mL of} _{n-butyl lithium} (16.4 mmol, 1.6 M in hexane) was added. The mixture was stirred at78_{C for 1 h and then} 2-isopropoxy-4,4,5,5-tetra-methyl-1,3,2-dioxaborolane (3.3 mL, 16.4 mmol) was added slowly to the solution. The resulting mixture was warmed to room temperature and stirred at room temperature for 12 h and then 50 mL of ethyl acetate was added. The resulting solution was washed with brine and dried over magnesium sulfate. The crude product isolated by evaporating the solvent

Fig. 1. Alignment of LC side groups and polymer backbones due to rubbing treatment.

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and was purified by recrystallization from hexane to yield 2.55 g (51%) of white crystals; mp 76e77C. MS (EI,m/z): 336 (Mþ). 1H NMR (CDCl3, d ppm): 0.95 (t, J¼ 7.5 Hz,

3H, eC3H6eCH3), 1.39 (m, 2H, eCH2eCH3), 1.40 (s,

12H, eBeOeC(CH3)2eC(CH3)2eOe), 1.58e1.66 (m, 2H,

eCH2eCH2eCH2eCH3), 2.66 (t, J¼ 7.5 Hz, 2H, eCH2e C3H7), 7.26 (d, J¼ 9.0 Hz, 2H, aromatic-H), 7.54 (d, J¼ 9.0 Hz, 2H, aromatic-H), 7.61 (d, J ¼ 9.0 Hz, 2H, aro-matic-H), 7.88 (d, J¼ 9.0 Hz, 2H, aromatic-H). 13_{C NMR} (CDCl3, d ppm): 13.6, 22.4, 24.6, 33.9, 35.5, 83.6, 107.4, 127.1, 128.3, 129.2, 138.3, 139.4, 142.4, 146.5, 167.9. 2.3.2. 1-(8-Bromoctyloxyl)-4-methoxybenzene (2)

4-Methoxyphenol (5.0 g, 40.3 mmol), 1,8-dibromooctane (21.9 g, 80.5 mmol), potassium carbonate (11.1 g, 80.5 mmol), and potassium iodide (1.34 g, 8.05 mmol) were dissolved in 200 mL of acetonitrile under nitrogen atmosphere. The mixture was heated to 80C for 24 h. Excess potassium carbonate was filtered off. The solvent was evaporated and the solid was dis-solved in ethyl acetate. The ethyl acetate solution was extracted with 5% hydrochloric acid and then washed with water, satu-rated NaCl solution and dried over anhydrous MgSO4. After

evaporating the solvent, the crude product was purified by silica

+ Br Br LC1 B O O H9C4 _H 9C4 I H₃CO H₃CO H3CO H3CO H3CO H₃CO H3CO OH K2CO3 / KI CH₃CN K2CO3 CH3CN 1 CHCl3 B O O O n-BuLi THF Br Br BrH16C8O OCH3 OCH3 OCH3 OCH3 OCH3 OCH3 OCH3 H₃CO 2 4 KI OH O OB 3 Br Br Aliquat 336 K2CO3 / H2O Pd(PPh3)4 toluene Aliquat 336 K2CO3 / H2O Pd(PPh3)4 toluene B O O H9C4 H9C4 C4H9 C₄H₉ C4H9 C4H9 C4H9 C4H9 C4H9 C4H9 H₃CO 5 BrC8H16Br OC8H16Br OC8H16O OC8H16O OC8H16O OC8H16O Br2 BrCH2 CH2Br M1 5 CHCl3 Br Br M2 5 Br2 HBr CH3COOH HCHO

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gel column chromatography using ethyl acetate/hexane (1:4 by volume) as eluent to yield 11.8 g (93%) of white crystals; mp 85e86C. MS (EI, m/z): 315 (Mþ). 1H NMR (CDCl3,

d ppm): 1.34e1.44 (m, 8H, eOCH2CH2e(CH2)4e), 1.73e

1.88 (m, 4H, eOCH2CH2e(CH2)4eCH2CH2Br), 3.41 (t,

J¼ 6.0 Hz, 2H, eCH2Br), 3.77 (s, 3H, eOCH3), 3.90 (t,

J¼ 7.5 Hz, 2H, eOCH2e), 6.83 (s, 4H, aromatic-H). 13C

NMR (CDCl3, d ppm): 25.2, 27.2, 27.9, 28.3, 28.8, 33.1, 33.3,

55.6, 68.3, 114.5, 116.3, 154.7, 156.8.

2.3.3. 1,4-Dibromo-2-(8-bromoctyloxy)-5-methoxybenzene (3) Bromine (1.47 mL, 28.5 mmol) was added dropwise to a chloroform (50 mL) solution of 1 (3.0 g, 9.5 mmol). The reaction mixture was stirred at room temperature for 12 h, and excess bromine was quenched with sodium thiosulphate aqueous solution. The organic layers were collected, washed with water, and then dried over anhydrous magnesium sulfate. After organic solvent was evaporated, 4.13 g of yellowish liquid was obtained; yield 92%. MS (EI, m/z): 473 (Mþ).1H NMR (CDCl3, d ppm): 1.37e1.47 (m, 8H, eOCH2CH2e (CH2)4e), 1.78e1.88 (m, 4H, eOCH2CH2e(CH2)4e CH2CH2Br), 3.41 (t, J¼ 6.0 Hz, 2H, eCH2Br), 3.77 (s,

3H, eOCH3), 3.90 (t, 2H, J¼ 7.5 Hz, 2H, eOCH2e), 7.10

(s, 2H, aromatic-H). 13C NMR (CDCl3, d ppm): 25.6, 27.3, 28.0, 28.8, 33.0, 33.2, 56.6, 67.2, 115.1, 115.3, 153.7, 154.2. 2.3.4. 1,4-Dibromo-2-methoxy-5-[8-(4-methoxyphenoxy)-octan-1-yloxy]benzene (4) 4-Methoxyphenol (0.44 g, 3.5 mmol), K2CO3 (1.0 g,

7.0 mmol), KI (0.12 g, 0.7 mmol), and compound 2 (2.0 g, 4.2 mmol) were dissolved in 50 mL of acetonitrile under nitro-gen atmosphere. The mixture was heated to 80C and stirred at that temperature for 24 h. Excess potassium carbonate was fil-tered off. After solvent was evaporated, the solid was dissolved in ethyl acetate. The ethyl acetate solution was extracted with 5% hydrochloric acid, washed with water, saturated NaCl aque-ous solution and dried over anhydraque-ous MgSO4. After solvent

was removed, the crude product was purified by column chro-matography (silica gel, hexane as eluent) to yield 1.65 g (91%) of white crystals; mp 52e53C. MS (EI, m/z): 516 (Mþ).

1

H NMR (CDCl3, d ppm): 1.40e1.49 (m, 8H, eOCH2CH2e (CH2)4e), 1.73e1.88 (m, 4H, eOCH2CH2e(CH2)4eCH2e),

3.77 (s, 3H, eOCH3), 3.84 (s, 3H, eOCH3), 3.87e3.97 (m,

4H, eOCH2e(CH2)6eCH2Oe), 6.83 (s, 4H, aromatic-H),

7.26 (s, 2H, aromatic-H). 13C NMR (CDCl3, d ppm): 25.6,

28.8, 28.9, 55.6, 55.8, 67.2, 68.1, 114.5, 115.2, 115.3, 116.1, 153.7, 154.1, 154.7, 157.0.

2.3.5. 1,4-Bis(40 -butylbiphenyl)-2-methoxy-5-[8-(4-methoxy-phenoxy)octan-1-yloxy]benzene (5)

Compounds 3 (0.31 g, 0.6 mmol) and 4 (0.5 g, 1.5 mmol), K2CO3 (0.55 g), Aliquat 336 (0.1 g), and Pd(PPh3)4(0.01 g,

0.01 mmol) were dissolved in a mixed solvent of toluene (20 mL) and degassed water (4 mL). The reaction mixture was refluxed with vigorous stirring for 48 h under argon atmosphere. Ethyl acetate (50 mL) was added into the mixture.

The obtained solution was washed with brine and dried over magnesium sulfate. After evaporating the solvent, the crude product was purified by silica gel column chromatography using ethyl acetate/hexane (1:10 by volume) as eluent to yield 0.30 g (65%) of yellow crystals; mp 92e93C. MS (EI,m/z): 775 (Mþ).1H NMR (CDCl3, d ppm): 0.96 (t,J¼ 7.5 Hz, 6H,

eC3H6eCH3), 1.31e1.44 (m, 12H, eOCH2CH2e(CH2)4e and two eCH2eCH3), 1.60e1.71 (m, 8H, eOCH2CH2e (CH2)4eCH2e and two eCH2eCH2eCH3), 2.64e2.69 (t,

J¼ 7.5 Hz, 4H, eCH2eC3H7), 3.76 (s, 3H, eOCH3), 3.84

(s, 3H, eOCH3), 3.94e3.98 (m, 4H, eOCH2e(CH2)6e CH2Oe), 6.80 (s, 4H, H), 7.05 (s, 2H,

aromatic-H), 7.26e7.29 (m, 4H, aromatic-aromatic-H), 7.57e7.59 (m, 4H, aromatic-H), 7.67e7.72 (m, 8H, aromatic-H). 13C NMR (CDCl3, d ppm): 13.5, 22.3, 25.6, 28.9, 33.9, 35.4, 55.6,

55.8, 68.1, 68.3, 114.5, 115.3, 116.1, 120.0, 127.6, 128.7, 129.4, 134.4, 136.5, 139.5, 146.4, 152.3, 154.8, 157.1. 2.3.6. 1,4-Bis(40 -butylbiphenyl)-2-methoxy-5-[8-(2,5-dibromo-methyl-4-methoxyphenoxy)octan-1-yloxy]benzene (M1)

Compound 5 (0.5 g, 0.6 mmol), paraformaldehyde (0.1 g, 3.3 mol), and 30% HBr in acetic acid (2 mL) were dissolved in 5 mL of acetic acid. The mixture was heated to 70C and kept at that temperature with stirring for 16 h. After cooling to room temperature, the reaction mixture was diluted with chloroform followed by extraction with water and 5% NaHCO3aqueous solution. The chloroform solution was dried

over MgSO4followed by the removal of the chloroform under

reduced pressure. The crude product was recrystallized from hexane to yield 0.33 g (54%) of M6; mp 97e98C. Element Anal.: calculated 69.93% C, 6.66% H; found 70.36% C, 6.53% H. 1H NMR (CDCl3, d ppm): 0.96 (t, J¼ 7.5 Hz, 6H, two

eC3H6eCH3), 1.31e1.44 (m, 12H, eOCH2CH2e(CH2)4e and two eCH2eCH3), 1.58e1.71 (m, 8H, eOCH2CH2e (CH2)4eCH2CH2Oe and two eCH2eCH2eC2H5), 2.67 (t,

J¼ 7.5 Hz, 4H, two eCH2eC3H7), 3.75 (s, 3H, eOCH3), 3.85

(s, 3H, eOCH3), 3.88e3.98 (m, 4H, eOCH2e(CH2)6e CH2Oe), 4.50 (s, 4H, two eCH2Br), 6.80 (s, 2H,

aromatic-H), 7.05 (s, 2H, aromatic-aromatic-H), 7.26e7.29 (m, 4H, aromatic-aromatic-H), 7.57e7.59 (m, 4H, H), 7.67 (m, 8H, aromatic-H). 13C NMR (CDCl3, d ppm): 13.8, 22.4, 25.6, 28.6, 28.9, 33.9, 35.5, 55.6, 55.9, 68.2, 115.3, 117.5, 120.0, 125.8, 127.6, 128.7, 129.3, 134.3, 136.5, 139.6, 146.4, 152.3, 153.1, 153.8. 2.3.7. 1,4-Bis(40 -butylbiphenyl)-2-methoxy-5-[8-(2,5-di-bromo-4-methoxyphenoxy)octan-1-yloxy]-benzene (M2)

Bromine (0.14 mL, 2.7 mmol) was added dropwise to a solution of compound 5 (0.7 g, 0.9 mmol) in chloroform (20 mL). The solution was stirred at room temperature for 12 h, and the excess bromine was quenched with sodium thio-sulphate aqueous solution. The organic layers were combined, washed until neutral, and then dried over magnesium sulfate. The organic solvent was evaporated and obtained 0.73 g (87%) of a yellow powder; mp 103e104C. Element Anal.: calculated 69.53% C, 6.44% H; found 69.87% C, 6.26% H.

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eC3H6eCH3), 1.31e1.44 (m, 12H, eOCH2CH2e(CH2)4e and two eCH2eCH3), 1.58e1.71 (m, 8H, eOCH2CH2e (CH2)4eCH2CH2Oe and two eCH2eCH2eC2H5), 2.65e2.70

(t,J¼ 7.5 Hz, 4H, two eCH2eC3H7), 3.76 (s, 3H, eOCH3),

3.85 (s, 3H, eOCH3), 3.88e3.98 (m, 4H, eOCH2e(CH2)6e CH2Oe), 6.80 (s, 2H, H), 7.05 (s, 2H,

aromatic-H), 7.26e7.29 (m, 4H, aromatic-aromatic-H), 7.57e7.59 (m, 4H, aromatic-H), 7.67e7.73 (m, 8H, aromatic-H). 13C NMR (CDCl3, d ppm): 13.3, 22.1, 25.2, 28.8, 33.7, 35.6, 55.8,

55.5, 68.2, 68.6, 115.3, 120.2, 127.4, 128.5, 129.5, 134.6, 136.3, 139.4, 146.4, 152.2, 154.7, 157.4.

2.3.8. Model compound (LC1)

Compound 4 (0.5 g, 1.5 mmol), 1,4-dibromo-2,5-dimethoxy-benzene (0.18 g, 0.6 mmol), K2CO3 (0.55 g), Aliquat 336

(0.1 g) and Pd(PPh3)4(0.01 g, 0.01 mmol) were dissolved in a

mixed solvent of toluene (20 mL) and degassed water (4 mL). The reaction mixture was refluxed with vigorous stirring for 48 h under argon atmosphere. Ethyl acetate (50 mL) was added. The obtained solution was washed with brine and dried over magnesium sulfate. After solvent was removed by rotary evaporation, the crude product was purified by silica gel column chromatography using ethyl acetate/hexane (1:10 by volume) as eluent to yield 0.24 g (72%) of white crystals; mp 148e149C. MS (EI,m/z): 554 (Mþ). Element Anal.: calculated 86.52% C, 7.57% H; found 86.34% C, 7.46% H. 1H NMR (CDCl3,

d ppm): 0.96 (t, J¼ 7.5 Hz, 6H, two eC3H6eCH3), 1.36e1.45

(m, 4H, two eCH2eCH3), 1.61e1.68 (m, 4H, two eCH2e CH2eC2H5), 2.67 (t,J¼ 7.5 Hz, 4H, two eCH2eC3H7), 3.84

(s, 6H, eOCH3), 7.05 (s, 2H, aromatic-H), 7.26e7.29 (m, 4H,

aromatic-H), 7.56e7.59 (m, 4H, aromatic-H), 7.70 (s, 8H, aromatic-H).13C NMR (CDCl3, d ppm): 13.6, 22.3, 33.8, 35.5,

55.8, 115.6, 120.1, 127.6, 129.8, 134.6, 136.5, 137.4, 146.3, 152.3.

2.4. Preparation of polymers PPV1ePPV4

Scheme 2 outlines the synthetic routes for polymers

PPV1ePPV4. Polymers PPV1ePPV4 were prepared by polymerization of different ratios of monomers M1 and M2 via Gilch route[27]. The synthesis of PPV1 was given as an example. Monomer M1 (0.2 g, 0.2 mmol) was dissolved in 5 mL of THF and flushed with N2. A THF solution of

potas-siumtert-butoxide (2 mL, 1.0 M) was then added slowly. After complete addition of the base, the reaction mixture was stirred for additional 16 h. The solution was poured into rapidly stirred methanol, and the obtained polymer was collected by suction filtration. The polymer was dissolved in THF, repreci-pitated several times into methanol, collected, and dried under reduced pressure to give 0.06 g (32%) of PPV1.

Yield of PPV1: 32%. Element Anal.: calculated 83.99% C, 7.88% H; found 83.29% C, 7.81% H. 1H NMR (CDCl3,

d ppm): 0.95e1.02 (m, alkyl protons), 1.26e1.33 (m, alkyl protons), 1.60e1.74 (m, alkyl protons), 2.60e2.67 (m, ePheCH2e), 3.75 (s, eOCH3), 3.83 (s, eOCH3), 3.92e

3.98 (m, eOCH2e), 6.80 (s, vinyl-H), 7.04 (s, aromatic-H),

7.24e7.28 (m, aromatic-H), 7.50e7.72 (m, aromatic-H). 13C NMR (CDCl3, d ppm): 13.8, 24.4, 26.1, 29.5, 37.8, 55.8,

56.7, 68.9, 70.1, 114.9, 115.7, 117.0, 126.5, 126.9, 127.3, 128.9, 129.9, 130.9, 137.3, 138.5, 140.0, 141.8, 150.7, 151.2. Yield of PPV2: 48%. Element Anal.: calculated 82.56% C, 8.29% H; found 81.73% C, 8.47% H. 1H NMR (CDCl3,

d ppm): 0.94e1.00 (m, alkyl protons), 1.24e1.32 (m, alkyl protons), 1.66e1.70 (m, alkyl protons), 2.59e2.62 (m, ePheCH2e), 3.74 (s, eOCH3), 3.82 (s, eOCH3), 3.94e3.99

(m, eOCH2e), 6.70 (s, vinyl-H), 6.79 (s, vinyl-H), 7.04

(s, aromatic-H), 7.24e7.27 (m, aromatic-H), 7.54e7.65 (m, aromatic-H). 13C NMR (CDCl3, d ppm): 13.6, 24.2, 26.2, 29.6, 37.5, 55.7, 56.6, 68.8, 70.1, 114.8, 115.6, 117.1, 126.3, BrH2C CH2Br H17C8O OCH3 + t-BuOK THF M1 M3 x y x y H17C8O OCH3 OCH3 C4H9 H9C4 OC₈H₁₆O H3CO Polymer 100 0 50 50 25 75 PPV1 PPV2 PPV3 PPV4 10 90 x (M1%) y (M3%)

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126.8, 127.3, 128.8, 129.9, 130.8, 137.1, 138.4, 140.1, 141.9, 150.7, 151.3.

d ppm): 0.94e1.01 (m, alkyl protons), 1.23e1.31 (m, alkyl protons), 1.68e1.73 (m, alkyl protons), 2.57e2.61 (m, ePhe CH2e), 3.76 (s, eOCH3), 3.82 (s, eOCH3), 3.91e3.98 (m,

eOCH2e), 6.72 (s, vinyl-H), 6.81 (s, vinyl-H), 7.02 (s,

aromatic-H), 7.24e7.28 (m, aromatic-aromatic-H), 7.53e7.64 (m, aromatic-H).

13_{C NMR (CDCl}

3, d ppm): 13.7, 24.4, 26.5, 29.5, 37.4, 55.9,

56.4, 68.7, 70.2, 114.5, 115.7, 117.2, 126.3, 126.5, 127.3, 128.9, 129.7, 130.8, 137.2, 138.3, 140.1, 141.7, 150.6, 151.6.

(m, eOCH2e), 6.71 (s, vinyl-H), 6.79 (s, vinyl-H), 7.05

(s, aromatic-H), 7.23e7.27 (m, aromatic-H), 7.54e7.66 (m, aromatic-H). 13C NMR (CDCl3, d ppm): 13.7, 24.4, 26.4,

29.7, 37.6, 55.8, 56.6, 68.7, 70.3, 114.5, 115.2, 117.3, 126.1, 126.6, 127.4, 128.8, 129.7, 130.6, 137.3, 138.5, 140.2, 141.8, 150.8, 151.5.

2.5. Preparation of polymers PF1ePF5

Scheme 3outlines the synthetic routes for polymers PF1e

PF5. Polymers PF1ePF5 were prepared via Suzuki coupling of monomer M4 with M2 and/or M5eM7 [28e30]. The synthesis of PF1 was given as an example. Monomers M2 (0.1 g, 0.11 mmol), M4 (0.17 g, 0.27 mmol), M5 (0.09 g, 0.16 mmol), K2CO3(0.55 g), Aliquat 336 (0.1 g) and Pd(PPh3)4

(0.01 g, 0.01 mmol) were dissolved in a mixed solvent of tolu-ene (10 mL) and degassed water (2 mL). The reaction mixture was refluxed with vigorous stirring for 72 h under argon atmo-sphere. At the end of polymerization, the end capping reagents (1-bromo-4-(tert-butyl) benzene and 1-(tert-butyl)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) benzene) were added and the resulting solution was stirred for 24 h. The solution was poured into methanol to precipitate the polymer. A pow-dered solid was obtained by filtration. The polymer was dis-solved in THF and reprecipitated several times into methanol and then dried under vacuum to yield 0.16 g (45%) of PF1.

Yield of PF1: 45%. Element Anal.: calculated 88.09% C, 10.25% H; found 87.34% C, 9.46% H. 1H NMR (CDCl3,

(m, eOCH2e), 6.67e6.78 (m, aromatic-H), 7.04e7.08 (m,

aromatic-H), 7.24e8.01 (m, aromatic-H). 13C NMR (CDCl3,

d ppm): 13.9, 22.5, 23.9, 29.7, 31.5, 32.3, 34.7, 37.5, 55.5, 66.4, 114.5, 120.1, 121.8, 126.6, 129.8, 140.8, 146.5, 152.1.

d ppm): 0.82e0.88 (m, alkyl protons), 0.94e1.24 (m, alkyl protons), 1.54e1.82 (m, alkyl protons), 2.58e2.66 (m, epheCH2e), 3.74 (s, eOCH3), 3.83 (s, eOCH3), 3.86e3.97

aromatic-H), 7.22e8.02 (m, aromatic-H).13C NMR (CDCl3,

d ppm): 13.8, 22.6, 23.9, 29.6, 31.4, 32.4, 34.6, 37.3, 55.7, 66.1, 114.3, 120.2, 121.7, 126.5, 129.7, 140.6, 146.5, 152.0.

d ppm): 0.82e0.87 (m, alkyl protons), 0.94e1.23 (m, alkyl protons), 1.53e1.81 (m, alkyl protons), 2.56e2.64 (m, epheCH2e), 3.73 (s, eOCH3), 3.83 (s, eOCH3), 3.87e3.98

aromatic-H), 7.23e8.02 (m, aromatic-H).13C NMR (CDCl3,

d ppm): 13.9, 22.8, 23.7, 29.4, 31.2, 32.7, 34.5, 37.1, 55.8, 66.4, 114.2, 120.1, 121.6, 126.6, 129.8, 140.5, 146.4, 152.1.

Yield of PF4: 42%. Element Anal.: calculated 85.39% C, 9.39% H, 0.98% N; found 84.47% C, 8.98% H, 0.82% N.1H NMR (CDCl3, d ppm): 0.80e0.87 (m, alkyl protons), 1.23e

1.26 (m, alkyl protons), 1.57e1.85 (m, alkyl protons), 2.55e 2.64 (m, epheCH2e), 3.74 (s, eOCH3), 3.83 (s, eOCH3),

3.77e3.87 (m, eOCH2e), 6.67e6.78 (m, aromatic-H), 7.04e

7.08 (m, aromatic-H), 7.24e8.01 (m, aromatic-H).13C NMR (CDCl3, d ppm): 13.9, 22.5, 24.0, 29.7, 31.5, 32.3, 34.7, 37.5,

52.8, 66.4, 114.2, 120.0, 121.8, 126.3, 129.9, 140.8, 146.7, 151.9.

Yield of PF5: 39%. Element Anal.: calculated 84.88% C, 9.29% H, 0.96% N; found 84.82% C, 8.81% H, 1.05% N.

1

H NMR (CDCl3, d ppm): 0.80e0.88 (m, alkyl protons),

1.22e1.26 (m, alkyl protons), 1.56e1.84 (m, alkyl protons), 2.54e2.66 (m, epheCH2e), 3.73 (s, eOCH3), 3.83 (s,

eOCH3), 3.78e3.89 (m, eOCH2e), 6.66e6.78 (m,

aromatic-H), 7.03e7.08 (m, aromatic-aromatic-H), 7.23e8.02 (m, aromatic-H).

13_{C NMR (CDCl}

3, d ppm): 13.8, 22.4, 24.1, 29.6, 31.7,

32.5, 34.7, 37.4, 52.8, 66.4, 114.3, 120.1, 121.7, 126.2, 129.8, 132.5, 140.5, 146.4, 152.2.

2.6. Fabrication of polarized EL devices

Polarized EL devices were fabricated on indium tin oxide (ITO)-coated glass substrates cleaned sequentially in ultra-sonic baths of detergent, ionized water, 2-propanol, de-ionized water, and acetone. UVeozone treatment was taken for 3 min as the final cleaning step to improve the contact angle just before film forming. Onto the ITO glass a layer of PEDOTePSS film was spin-cast from its aqueous dispersion. After baking at 120C for 1 h, the PEDOT was rubbed by a rubbing machine and used as an alignment layer. The poly-mers were dissolved in toluene and the obtained solutions were spin-coated on the top of rubbed PEDOT layer. After annealing for 1 h, 35 nm calcium and 100 nm aluminum cathodes were deposited onto the top of an aligned emitting film by thermal evaporation through a shadow mask.

3. Results and discussion

3.1. Synthesis and thermal properties

Table 1lists the polymerization results and phase transition

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C8H17 C₈H₁₇ H17C8 C8H17 H17C8 H17C8 B O O B O O M4 + Br _R Br Pd(PPh₃)₄ Aliquat 336 toluene K2CO3 H2O random copolymer R n m OCH3 C4H9 H9C4 OC8H16O H3CO M2 M5 N SN NSN S S M6 M7 R = Polymer PF1 10 50 40 - - PF2 20 50 30 - - PF3 40 50 10 - - PF4 20 50 15 15 - PF5 20 50 15 10 5 M2(%) M4(%) M5(%) M6(%) M7(%)

Scheme 3. Synthesis of polymers PF1ePF5.

Table 1

Polymerization results and phase transition temperatures of copolymers

Copolymer Yield (%) Mn Mw PDI Phase transition temperaturesa(C) Tdb(C)

PPV1 32 8600 13 100 1.52 G 48 K 112 N 327 I 340 PPV2 48 35 800 63 200 1.73 G 54 K 116 N 322 I 357 PPV3 51 39 300 70 900 1.81 G 61 K 124 N 306 I 372 PPV4 57 85 600 185 300 1.85 G 65 K 128 N 308 I 388 PF1 45 8100 12 800 1.58 G 75 N 296 I 374 PF2 46 8300 12 100 1.46 G 76 N 302 I 368 PF3 35 6500 9300 1.43 G 73 N 258 I 340 PF4 42 7400 9800 1.32 G 80 N 297 I 416 PF5 39 6200 9600 1.55 G 81 N 300 I 422 a

The temperatures were observed by DSC. G: glassy; K: crystalline; N: nematic; I: isotropic.

b

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number average molecular weights of PPV1ePPV4 are in the range from 8.6 103_{to 8.56}₁₀4_{. The bulky mesogenic side}

group affects dramatically the degree of polymerization. Ho-mopolymer PPV1, with 100% of M1 units, shows a lowest molecular weight while PPV4, with the lowest content (10%) of M1 units, achieves the highest molecular weight.

Fig. 2A presents the DSC thermograms of LC1. The heating

scan shows a crystal to nematic phase transition at 161C, and a nematic to isotropic phase transition (TNeI) at 204C.

The cooling scan shows an isotropic to nematic phase transi-tion at 196C, and a nematic to crystal phase transition at

138C. Fig. 2B depicts a DSC thermogram of PPV1 which

shows a glass transition temperature at 48C, a crystal to nematic phase transition at 112C, and a nematic to isotropic phase transition at 327C.Fig. 3A and B shows the typical nematic textures exhibited by PPV1 and LC1. As the se-quence from PPV1 to PPV4, the bulky monomer M1 content decreases while the M2 content increases. All four polymers exhibit nematic liquid crystalline behavior. According to the literature [31], similar PPVs without mesogenic side group show only a glass transition around 50C. However, the syn-thesized PPV1ePPV4 shows very complex thermal behavior,

which contain Tg, Tm and TNeI. These polymers eventually

show microphase separation behavior, which is very common for side-chain liquid crystal polymers. It is well-documented in the literatures[32] that a microphase separated side-chain polymers should contain microdomains exclusively of poly-mer backbone and microdomains exclusively of mesogenic side groups. The glass transition is due to the segmental motion of the polymer backbone microdomains. Both Tm

andTNeIare attributed to the thermal behavior of mesogenic

side groups’ microdomains. The mesogenic side groups show very similar thermal behavior with the model compound, which show bothTmandTNeI. All four polymers are thermally

stable and their thermal degradation temperatures are higher than 340C.

The number average molecular weightsðMnÞ of PF1ePF5 are in the range of 6.2103_e8.3103_{. The reason could be}

due to the bulky monomer unit M2 which affects the degree of polymerization.Table 1also summarizes the thermal tran-sition of PF1ePF5. All five polymers present nematic liquid crystalline behavior. Their glass transition temperatures are around 75C and isotropization temperatures are in the range from 258 to 302C. According to literatures [12e15], poly-fluorenes without the mesogenic side groups exhibit also

0 50 100 150 200 250 300 350 PPV1 Tg Tm TN-I 120 140 160 180 200 220 LC1 Temp (°C) Temp (°C) TN-I TCr-N TI-N TN-Cr (a) (b) B A EXO ENDO EXO ENDO

Fig. 2. (A) DSC thermogram of model compound LC1 (a) heating scan and (b) cooling scan; (B) DSC thermogram of polymer PPV1.

Fig. 3. (A) Polarized optical micrograph of LC1 at 170C. (B) Polarized optical micrograph of polymer PPV1 at 150C.

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nematic liquid crystalline behavior. As consequence, both main-chain and side-chain segments of PF1ePF5 have contri-bution to the formation of a nematic phase. However, due to the complicated combinations of different monomer units, it has difficulty to find the order tendency for both transitions. 3.2. Polarized optical properties

Aligned samples were prepared by annealing the polymers on a glass plate whose surface was pre-coated with PEDOT and rubbed by a mechanical rubbing technique. This peculiar procedure permits the polymer film to be aligned in the rub-bing direction. To check the anisotropy of the aligned polymer films of PPV1ePPV4 and PF1ePF5, polarized UVevis ab-sorption and PL emission spectra of these films were measured and the results are summarized in Table 2. Fig. 4A and B shows polarized UV absorption spectra and PL emission spec-tra of an aligned PPV1 film which exhibited a pronounced op-tical anisotropy as expressed by the polarized ratios (defined as the ratio of parallel to perpendicular UV absorption peak inten-sity) of 5.2 at 350 nm and 5.1 at 517 nm. The absorbance and emission in the parallel direction were obviously higher than those in the perpendicular direction, which indicated that annealing these polymers at their liquid crystalline phase could thus induce alignment predominantly along the rubbing direction. The absorption at 350 nm was corresponded to the expected pep* transition from the penta(p-phenylene) liquid crystalline side group, where as the absorption at 517 nm was corresponded to the expected pep* transition from the PPV backbone.Fig. 4B shows two PL emission bands for polymer PPV1. The yellow emission band (572 nm) is due to the emission from the PPV main chains and blue emission band (406 nm) is due to the emission from penta(p-phenylene) liq-uid crystalline side groups. The polarized ratios show 4.7 at 406 nm and 4.5 at 572 nm for PL emissions, which is superior to the side-chain liquid crystalline PPVs reported previously

[8,21].Fig. 5shows polarized UV and PL spectra of an aligned

PF1 film. An UV absorption maximum was observed at 319 nm, which revealed pep* transition of the PF1 backbone (all absorption wavelengths are listed in Table 2). Since the feed ratio of mesogen-containing monomer M2 was only 10%, the absorption intensity of lateral mesogens was

relatively low as compared with PF1 main chain. This is dif-ferent fromFig. 4A, where a strong absorption band was found at 350 nm, referring to lateral mesogens. According to the data shown in Table 2, the UVevis and PL polarized ratios of PF1ePF5 are higher than 11.2. This means that the aligned PF films showed more pronounced optical anisotropy than

Table 2

The polarized UVevis absorption, polarized PL emission and polarized ratio of copolymers Copolymer Polarized UVmaxevis absorption (nm) Polarized PLmaxemission (nm) Polarized ratio UVevis (UVk/UVt) PL (PLk/PLt) PPV1 350(517) 406(572) 5.2(5.1) 4.7(4.5) PPV2 350(516) 406(574) 3.8(3.5) 3.3(3.1) PPV3 352(517) 408(574) 3.6(3.5) 3.3(2.9) PPV4 352(518) 408(580) 3.2(3.1) 3.1(2.9) PF1 319 416 12.6 11.2 PF2 320 419 13.2 12.7 PF3 319 416 13.1 12.6 PF4 435 536 15.5 14.2 PF5 516 648 13.5 13.3 300 350 400 450 500 550 600 0.0 0.2 0.4 0.6 0.8 1.0 UV⎢⎢ UV_⊥ PL⎢⎢ PL_⊥

UV absorption intensity (a.u.)

Wavelength (nm) 350 400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0

PL emission intensity (a.u.)

Wavelength (nm) B

A

Fig. 4. (A) Polarized UVevis absorption spectra of PPV1. (B) Polarized PL emission spectra of PPV1. 300 350 400 450 500 0.0 0.2 0.4 0.6 0.8 1.0 Intensity (a.u.) Wavelength (nm) UV⎢⎢ UV_⊥ PL⎢⎢ PL_⊥

Fig. 5. Polarized UVevis absorption spectra and polarized PL emission spectra of PF1.

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the aligned PPV films. The results are reasonable because the PF backbones are involving in the formation of a nematic phase while PPV main chains are not. PF1ePF3 emit ultravi-olet to blue light at around 416 nm which is associated with both polyfluorene backbones and penta(p-phenylene) side groups PF4 and PF5 revealed two emission peaks at 416 nm and 536 nm. The former is associated with the penta( p-phe-nylene) side groups while the latter is attributed to benzothia-diazole moiety in the main chains.

3.3. Polarized EL property

Double-layers PLED devices with the configuration of ITO/ rubbed PEDOTePSS/emitting polymer/Ca/Al were fabricated for the measurements of polarized EL spectra. The results are summarized in Table 3. Basically PPV1ePPV4 reveal very similar EL behavior.Fig. 6A and B reports the representative polarized EL spectra and device properties of PPV4 which emits yellow light at 588 nm. Its maximum luminance and polarized ratio are 1337 cd/m2 and 2.6, respectively. Again, the low polarized ratios of PPV1ePPV4 devices are basically due to the poor alignment of PPV backbones. Since penta-(p-phenylene) side groups emit ultraviolet to blue light, the EL emission at 588 nm should be attributed to PPV main chains only.

As can be seen fromTable 3, PF1ePF3 emit ultraviolet to blue light at 428 nm while PF4 and PF5 emit, respectively, a green light at 540 nm and an orange-red light at 650 nm. A complete energy transfer from penta(p-phenylene) side groups to PF main chains was observed. Fig. 7 depicts the UVevis and PL spectra of model compound LC1. The PL emission of LC1 is overlapped with the UV absorption peak of PF4. This provides a route for energy transfer [18,19,33]. Instead, PPVs showed a UV absorption around 516e518 nm, which is not overlapped with the emission of LC1. Hence, no energy transfer was found in this case. The polarized ratios of PF1ePF5 are higher than 9.5. The highest luminance and polarized ratio are 1855 cd/m2and 12.4, respectively, achieved by PF4.

To generate a polarized white light, PLED device is the main goal of this study. We use PF2 as a host polymer and PF4 and PF5 as guest polymers. Simply blending small amounts of PF4 and PF5 into PF2 can fabricate a white light

device.Fig. 8A shows the polarized white light EL spectra of a PLED device generated by blending 0.04 and 0.06 wt% of PF4and PF5 with PF2. The maximum luminance and polar-ized ratio are 483 cd/m2and 11.6, respectively, with CIE coor-dinate (0.34,0.35) which is very close to pure white light. The

Table 3

EL properties and polarized ratio using copolymers as active layers Copolymer EL (nm) Vturn on (V) Luminance (cd/m2) Yield (cd/A) Polarized ratio PPV1 584 5 163 0.07 3.6 PPV2 584 5 401 0.16 3.3 PPV3 588 5 534 0.17 3.2 PPV4 588 4 1337 0.33 2.6 PF1 428 6 469 0.06 12.6 PF2 428 6 525 0.07 11.9 PF3 432 7 403 0.06 9.5 PF4 540 5 1855 0.57 12.4 PF5 652 6 1052 0.17 10.7 400 450 500 550 600 650 700 750 EL⎢⎢ EL_⊥ EL Intensity (a.u.) Wavelength (nm) B A 0 2 4 6 8 10 0 200 400 600 800 1.0 0.8 0.6 0.4 0.2 0.0 Voltage (V)

Current density (mA/cm

2) 0 300 600 900 1200 1500 J-V L-V Luminance (cd/m 2)

Fig. 6. (A) Polarized EL spectra of PPV4 in the ITO/aligned PEDOT/PPV4/ Ca/Al device; (B) JeV(,) and LeV(C) curves of PPV4 in the ITO/aligned PEDOT/PPV4/Ca/Al device. 300 350 400 450 500 550 600 0.0 0.2 0.4 0.6 0.8 1.0 LC1-PL PF4-UV Intensity (a.u.) Wavelength (nm)

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best result is achieved in device C (Table 4) with 0.08 wt% of PF4and PF5. Its maximum luminance and polarized ratio are 2454 cd/m2 and 10.2, respectively. For practical application EL polarization ratios of 30e40 are required, but with the use of a clean-up polarizer, EL ratios of 10 or more are ade-quate[34]. At present polarized white light devices fabricated in this work showed highest brightness with moderate polari-zation ratio. With further improvement of luminance efficien-cies of device, these EL devices are believed to be applicable to LCD backlights.

4. Conclusions

Two series of new PPV and PF derivatives containing lat-eral attached penta(p-phenylene) mesogens were synthesized

and characterized. Both series of polymers revealed nematic liquid crystalline behavior and were suitable for the fabrica-tion of polarized EL devices. The PPV derivatives emit a yel-low light while PF derivatives emit RGB three primary lights depending on the composition of monomer units in the back-bones. The PLED devices based on PF derivatives show much higher polarized ratios than those based on PPV derivatives because both PF main chains and penta(p-phenylene) side groups are involving in the formation of a nematic phase while PPV main chains are not. The synthesized PF derivatives were useful for the fabrication of polarized white PLED devices. Blending small amount of PF4 and PF5 with a host PF2 achieved a white PLED device with highest luminance of 2454 cd/m2, polarized ratio of 10.2 and CIE coordinate of (0.35,0.39) which had potential application in LCD backlight. Acknowledgements

The authors are grateful to the National Science Council (NSC92-2216-E009-015) and Ministry of Education (MOE ATU program) of the Republic of China for financial support for this work.

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Table 4

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