Synthesis and Electroluminescent Properties of Disubstituted Polyacetylene Derivatives Containing Multi-Fluorophenyl and Cyclohexylphenyl Side Groups

Synthesis and Electroluminescent Properties

of Disubstituted Polyacetylene Derivatives

Containing Multi-Fluorophenyl and

Cyclohexylphenyl Side Groups

Sheng-Hsiung Yang, Chun-Hao Huang, Chiu-Hsiang Chen, Chain-Shu Hsu*

Introduction

Conjugated polymers have been extensively studied for their potential applications in light emitting diodes,[1] organic lasers,[2]thin film transistors,[3]and solar cells.[4] Polyacetylene (PA) is a prototype among many promising conjugated polymers, which exhibits high conductivity, and n- and/or p-type properties upon doping.[5]

The conductivity of this polymer can reach as high as 105S
cm1after doping.[6]Although PA shows a very high electrical conductivity by doping, it has rarely been used as the light-emitting layer for electroluminescent (EL) devices. Non-substituted PA is insoluble, infusible, and unstable in air; almost no photoluminescence (PL) is observed in the visible region.

In the past few years, Masuda et al.,[7–10]Tang et al.,[11–14] and our laboratory[15–17]have synthesized several series of substituted PA derivatives that are stable in air and soluble in common organic solvents. Although the conductivity of these PA derivatives is not as high as that of non-substituted PA, they show both PL and EL properties in the

Full Paper

A new series of disubstituted polyacetylene derivatives that contain multi-fluorine atoms on

the pendent phenyl ring have been synthesized and characterized. The results reveal a greater

red-shift in UV-vis absorption and PL emission upon incorporating more fluorine atoms on the

pendent phenyl ring. Among them, disubstituted polyacetylene with a difluorophenyl group

(PDPA-2F) showed the highest luminescent efficiency. The

device performance can be promoted by blending a

hole-transporting material TM-TPD into PDPA-2F as the active layer

or by using a light-emitting copolymer in which PDPA-2F was

copolymerized with a carbazole group (PDPA-2Fcab). A

light-emitting diode of ITO/PEDOT/PDPA-2Fcab/Ca/Al revealed a

maximum luminescence of 4230 cd

m

at 14 V and a

maxi-mum current efficiency of 3.37 cd

A

at 7 V.

S.-H. Yang, C.-H. Huang, C.-H. Chen, C.-S. Hsu

Department of Applied Chemistry, National Chiao Tung University, 1001, Ta-Hsueh Road, Hsinchu 30010, Taiwan R.O.C. Fax: +886-3-5131523; E-mail: [email protected]

visible wavelength region. Alkyl and phenyl groups are the most commonly used substituents for miscellaneous PA derivatives. These alkyl- and/or phenyl-substituted PAs emit blue to red light and have been used as active materials in polymer light emitting devices (PLED).[16]For example, a single-layer device using poly(diphenylacety-lene) (PDPA) as an emissive layer showed a stable green EL spectrum. However, the EL intensity is quite low.[8]

Recently Masuda et al. have synthesized carbazole-substituted PAs using rhodium and tungsten as catalysts. These polymers exhibit photoconductivity and EL properties upon doping with an iridium complex.[10] The best performance was reported with a multilayer EL device using poly[1-phenyl-5-(a-naphthoxy)pentyne] blended with poly(vinylcarbazole) (PVK) as the emitting layer, bath-ocuproine as a hole blocking layer, and 8-hydroxyquinoline aluminum (Alq3) as an electron transporting layer.[12] _A

similar multilayer EL device using silole-containing PA as the active layer emitted blue-green light of 496 nm with a maximum brightness and current efficiency of 1118 cd
m2 and 1.45 cd
A1, respectively.[13] Since then, the high-efficiency PA has not been explored for PLED application.

In this work, we aimed to synthesize a new series of disubstituted PAs with multifluorine substitution on the pendent phenyl ring. The effect of the number and substitution position of these fluorine atoms on energy levels and luminescent efficiency were examined. To further improve the device performance of these synthesized PAs, two approaches were proposed as well. One is based on a blend of PA and a hole transport material N,N,N0,N0 -tetra(4-methylphenyl)(1,10-biphenyl)-4,40-diamine) (TM-TPD) as the emissive layer, while the other one is by

incorporating a carbazole unit into the PA backbone by copolymerization.

Experimental Part

Characterization Methods

1

H NMR spectra were measured with a Varian 300 MHz spectrometer. Gel permeation chro-matography (GPC) data assembled from a Viscotek T50A Differential Viscometer and a LR125 Laser Refractometer and three columns in series were used to measure the molecular weights of polymers relative to polystyrene standards at 358C. Infrared spectra were obtained by using a Perkin–Elmer Spectrum One spectrophotometer in the range of 400– 4 000 cm1. Differential scanning calorimetry (DSC) was performed on a Perkin–Elmer Pyris Diamond DSC instrument at a scan rate of 108C
min1. Thermal gravimetric analysis (TGA) was undertaken on a Perkin–Elmer Pyris 1 TGA instrument with a heating rate of 108C
min1. UV-Vis absorption spectra were

obtained with an HP 8453 diode array spectrophotometer. PL emission spectra were obtained using ARC SpectraPro-150 luminescence spectrometer. Cyclic voltammetric (CV) measure-ments were made in acetonitrile (CH3CN) with 0.1M tetrabutyl-ammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a scan rate of 50 mV
s. Platinum wires were used as both the counter and working electrodes, and silver/silver ions (Ag in 0.1MAgNO3solution, from Bioanalytical Systems, Inc.) were used as the reference electrode, and ferrocene was used as an internal standard. The corresponding highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels were estimated from the onset redox potentials.

Synthesis of Monomers

All reagents and chemicals were purchased from commercial sources (Aldrich, Merck, Lancaster or TCI) and used without further purification. Anhydrous tetrahydrofuran (THF) and toluene were dried by distillation from sodium/benzophenone and calcium hydride, respectively. Scheme 1 depicts the syntheses of mono-mers M1–M6. Monomono-mers M2, M3, and M6 were synthesized according to the previous reports in the literature.[18–20]

1-(4-(Pentylcyclohexyl)phenyl)-2-phenylacetylene (M1)

Iodobenzene (5.0 g, 24.5 mmol), PdCl2(PPh3)2(0.17 g, 0.25 mmol), CuI (0.24 g, 1.23 mmol), and PPh3 (0.48 g, 1.84 mmol) were dissolved in triethylamine (50 mL) and the reaction mixture was stirred at room temperature under nitrogen. After all the catalysts were dissolved, 1-ethynyl-4-(4-pentylcyclohexyl)benzene (6.86 g, 27.0 mmol) was added and the mixture was refluxed at 708C for 12 h. After cooling to room temperature, triethylamine was removed under reduced pressure, and the crude product was

extracted with ethyl acetate several times. The crude product was isolated by evaporating the solvent and purified by column chromatography (silica gel, hexane as eluent) to yield 6.87 g (86%) of white crystals; m.p. 54–558C.1_{H NMR (300 MHz, CDCl} 3): d ¼ 0.88 (t, J¼ 7.2 Hz, 3H), 1.04–1.47 (m, 13H), 1.87 (d, J ¼ 10.5 Hz, 4H), 2.4 (t, J¼ 12 Hz, 1H), 7.19 (d, J ¼ 7.2 Hz, 2H), 7.34 (m, 5H), 7.38 (d, J ¼ 7.2 Hz, 2H). 13_{C NMR (75 MHz, CDCl} 3): d ¼ 12.38, 22.71, 26.63, 32.19, 33.45 (2C), 34.09 (2C), 37.25, 37.72, 44.09, 89.5 (C C), 90.4 (C C), 119.38 (1C, Ar), 125.6 (1C, Ar), 127.02 (2C, Ar), 128.35 (2C, Ar), 131.55 (2C, Ar), 131.64 (2C, Ar), 148.56 (1C, Ar).

IR (KBr): 3 030, 2 954, 2 924, 2 846, 2 221, 1 597, 1 511, 1 467, 1 442, 1 375, 1 214, 1 069, 966, 896, 828, 754 cm1. MS (EI-MS): m/z¼ 330.5. C25H30: Calcd. C 90.85, H 9.15; Found C 90.8, H 9.2. 1-(3,4,5-Trifluorophenyl)-2-(4-(pentylcyclohexyl) phenyl)acetylene (M4)

By following the synthetic procedure for M1 and using 3,4,5-trifluoroiodobenzene as starting material, the compound M4 was obtained as white crystals (83% yield); m.p. 60–628C.1H NMR (300 MHz, CDCl3): d ¼ 0.87 (t, J ¼ 7.2 Hz, 3H), 1.04–1.47 (m, 13H), 1.86 (d, J¼ 10.5 Hz, 4H), 2.46 (t, J ¼ 12 Hz, 1H), 7.08 (m, 2H), 7.12 (d, J ¼ 7.2 Hz, 2H), 7.42 (d, J¼ 7.2 Hz, 2H). 13_{C NMR (75 MHz, CDCl} 3): d ¼ 12.38, 22.71, 26.63, 32.19, 33.45 (2C), 34.09 (2C), 37.25, 37.32, 44.09, 85.61 (C C), 91.2 (C C), 115.61 (1C, Ar), 115.91 (1C, Ar), 119.38 (1C, Ar), 127.02 (2C, Ar), 131.64 (2C, Ar), 138.38 (1C, Ar), 141.96 (1C, Ar), 143 (1C, Ar), 149.16 (1C, Ar), 152.3 (1C, Ar). IR (KBr): 3 030, 2 957, 2 922, 2 852, 2 682, 2 217, 1 909, 1 609, 1 529, 1 447, 1 428, 1 379, 1 255, 1 206, 1 202, 1 070, 1 046, 980, 910, 856, 832. MS (EI-MS): m/z¼ 384.4. C25H27F3: Calcd. C 78.1, H 7.08; Found C 77.9, H 7.12. 1-(2,3,4,5,6-Pentafluorophenyl)-2-(4-(pentylcyclohexyl) phenyl)acetylene (M5)

By following the synthetic procedure for M1 and using 2,3,4,5,6-pentafluoroiodobenzene as starting material, the compound M5 was obtained as white crystals (84% yield); m.p. 73–758C.1

H NMR (300 MHz, CDCl3): d ¼ 0.87 (t, J ¼ 7.2 Hz, 3H), 1.01–1.48 (m, 13H), 1.86 (d, J¼ 11.5 Hz, 4H), 2.48 (t, J ¼ 12 Hz, 1H), 7.18 (d, J ¼ 7.2 Hz, 2H), 7.48 (d, J¼ 7.2 Hz, 2H). 13_{C NMR (75 MHz, CDCl} 3): d ¼ 14.09, 22.63, 26.64, 30.87, 32.21 (2C), 34.45 (2C), 37.25, 37.33, 44.68, 72.36 (C C), 100 (1C, Ar), 101.97 (C C), 118.8 (1C, Ar), 126.82 (2C, Ar), 131.47 (2C, Ar), 136.1 (2C, Ar), 139.31 (2C, Ar), 142.82 (1C, Ar), 145.17 (1C, Ar), 148.93 (1C, Ar), 149.91 (1C, Ar). IR (KBr): 3 030, 2 957, 2 920, 2 854, 2 230, 1 917, 1 622, 1 525, 1 496, 1 445, 1 412, 1 365, 1 109, 1 096, 1 029, 984, 967, 835. MS (EI-MS): m/z¼ 420.4. C25H25F5: Calcd. C 71.41, H 5.99; Found C 71.9, H 5.82. Synthesis of Polymers

Scheme 2 depicts the synthetic routes for homopolymers PDPA-P, 1F, 2F, 3F, 5F, and a copolymer

PDPA-2Fcab. The polymerization was carried out using TaCl5as catalyst and n-Bu4Sn as co-catalyst in anhydrous toluene. All experimental operations were performed in a glove-box, except for the purification of the polymers, which was done in the open atmosphere. An experimental procedure for the polymerization of M1 is given below. A mixture of TaCl5(0.158 g, 0.4 mmol) and n-Bu4Sn (0.28 mg, 0.8 mmol) in anhydrous toluene (10 mL) was heated at 808C for 30 min. A solution of monomer M1 (0.9 g, 4.12 mmol) in anhydrous toluene (10 mL) was then added. The resulting mixture was continuously stirred and heated at 808C for 24 h. After cooling to room temperature, the solution was poured into methanol and the product filtered off. The polymer was purified by dissolving the crude product in THF and re-precipitated from methanol several times. After drying under vacuum for 24 h, the final polymer PDPA-P was obtained as a yellow solid (0.61 g, 81%).1_{H NMR (300 MHz, CDCl} 3): d ¼ 0.88 (t, J ¼ 7.2 Hz, 3H), 1.04– 1.47 (m, 13H), 1.87 (d, J¼ 10.5 Hz, 4H), 2.4 (t, J ¼ 12 Hz, 1H), 7.19 (d, J¼ 7.2 Hz, 2H), 7.34 (m, 5H), 7.38 (d, J ¼ 7.2 Hz, 2H). 13 C NMR (75 MHz, CDCl3): d ¼ 12.38, 22.71, 26.63, 32.19, 33.45 (2C), 34.09 (2C), 37.25, 37.32, 44.09, 119.38 (1C, Ar), 125.6 (1C, Ar), 127.02 (2C, Ar), 128.35 (2C, Ar), 131.55 (2C, Ar), 131.64 (2C, Ar), 148.56 (1C, Ar), 156.02 (2C, C––C).

C25H30: Calcd. C 90.85, H 9.15; Found C 89.55, H 8.89.

S.-H. Yang, C.-H. Huang, C.-H. Chen, C.-S. Hsu

Scheme 2. Synthesis of homopolymers P, 1F, PDPA-2F, PDPA-3F, PDPA-5F, and the copolymer PDPA-2Fcab.

PDPA-1F Yield 92%. 1H NMR (300 MHz, CDCl3): d ¼ 0.88 (t, J ¼ 7.2 Hz, 3H),1.04–1.47 (m, 13H), 1.85 (d, J¼ 10.5 Hz, 4H), 2.41 (t, J ¼ 12 Hz, 1H), 7.11 (m, 2H), 7.17 (d, J¼ 7.2 Hz, 2H), 7.43 (d, J ¼ 7.2 Hz, 2H), 7.55 (m, 2H). 13 C NMR (75 MHz, CDCl3): d ¼ 12.38, 22.71, 26.63, 32.19, 33.45 (2C), 34.09 (2C), 37.25, 37.32, 44.09, 115.69 (1C, Ar), 115.85 (1C, Ar), 117.2 (2C, Ar), 119.38 (1C, Ar), 127.02 (2C, Ar), 131.64 (2C, Ar), 141.96 (1C, Ar), 143 (1C, Ar), 149.16 (1C, Ar), 154.52 (2C, C––C).

C25H29F: Calcd. C 86.16, H 8.39; Found C 86.01, H 8.35. PDPA-2F Yield 96%.1H NMR (300 MHz, CDCl3): d ¼ 0.87 (t, J ¼ 7.2 Hz, 3H), 1.08–1.47 (m, 13H), 1.87 (d, J¼ 10.5 Hz, 4H), 2.57 (t, J ¼ 12 Hz, 1H), 7.09 (m, 2H), 7.13 (d, J¼ 7.4 Hz, 2H), 7.16 (s, 1H), 7.41 (d, J ¼ 7.2 Hz, 2H). 13_{C NMR (75 MHz, CDCl} 3): d ¼ 12.38, 22.61, 26.63, 32.19, 33.24 (2C), 34.12 (2C), 37.25, 37.32, 44.09, 115.75 (1C, Ar), 115.85 (1C, Ar), 116.1 (1C, Ar), 119.38 (1C, Ar), 126.82 (2C, Ar), 131.34 (2C, Ar), 137.38 (1C, Ar), 141.96 (1C, Ar), 143.3 (1C, Ar), 149.26 (1C, Ar), 155.24 (2C, C––C). C25H28F2: Calcd. C 81.93, H 7.7; Found C 81.54, H 7.4. PDPA-3F Yield 90%.1_{H NMR (300 MHz, CDCl} 3): d ¼ 0.87 (t, J ¼ 7.2 Hz, 3H), 1.04–1.47 (m, 13H), 1.92 (d, J¼ 10.3 Hz, 4H), 2.48 (t, J ¼ 12 Hz, 1H), 7.09 (m, 2H), 7.14 (d, J¼ 7.2 Hz, 2H), 7.46 (d, J ¼ 7.1 Hz, 2H). 13_{C NMR (75 MHz, CDCl} 3): d ¼ 12.38, 22.71, 26.63, 32.19, 33.45 (2C), 34.09 (2C), 37.25, 37.32, 44.09, 115.61 (1C, Ar), 115.9 (1C, Ar), 119.38 (1C, Ar), 127.02 (2C, Ar), 131.64 (2C, Ar), 138.78 (1C, Ar), 141.96 (1C, Ar), 143 (1C, Ar), 149.2 (1C, Ar), 152.5 (1C, Ar), 154.64 (2C, C––C). C25H27F3: Calcd. C 78.1, H 7.08; Found C 77.61, H 6.83. PDPA-5F Yield 31%.1H NMR (300 MHz, CDCl3): d ¼ 0.88 (t, J ¼ 7.2 Hz, 3H), 1.1–1.48 (m, 13H), 1.86 (d, J¼ 11.5 Hz, 4H), 2.46 (t, J ¼ 12 Hz, 1H), 7.16 (d, J¼ 7.2 Hz, 2H), 7.45 (d, J ¼ 7.2 Hz, 2H). 13_{C NMR (75 MHz, CDCl} 3): d ¼ 14.09, 22.63, 26.64, 30.87, 32.21 (2C), 34.45 (2C), 37.25, 37.33, 44.68, 100 (1C, Ar), 118.8 (1C, Ar), 126.82 (2C, Ar), 131.47 (2C, Ar), 136.1 (1C, Ar), 139.31 (1C, Ar), 142.82 (1C, Ar), 145.17 (1C, Ar), 148.93 (1C, Ar), 149.91 (1C, Ar), 154.1 (2C, C––C). C25H25F5: Calcd. C 71.41, H 5.99; Found C 70.94, H 5.53. PDPA-2Fcab Yield 90%.1H NMR (300 MHz, CDCl3): d ¼ 0.87 (t, J ¼ 7.2 Hz, 3H), 1.08–1.47 (m, 13H), 1.87 (d, J¼ 10.5 Hz, 4H), 2.57 (t, J ¼ 12 Hz, 1H), 7.09 (m, 2H), 7.13 (d, J¼ 7.4 Hz, 2H), 7.16 (s, 1H), 7.41 (d, J ¼ 7.2 Hz, 2H). 13_{C NMR (75 MHz, CDCl} 3): d ¼ 12.38, 22.61, 26.63, 32.19, 33.24 (2C), 34.12 (2C), 37.25, 37.32, 44.09, 115.75 (1C, Ar), 115.85 (1C, Ar), 116.1 (1C, Ar), 119.38 (1C, Ar), 126.82 (2C, Ar), 131.34 (2C, Ar), 137.38 (1C, Ar), 141.96 (1C, Ar), 126.82 (2C, Ar), 131.34 (2C, Ar), 137.38 (1C, Ar), 141.96 (1C, Ar), 143.3 (1C, Ar), 149.26 (1C, Ar), 155.24 (2C, C––C). C249H273F18N: Calcd. C 82.53, H 7.65, N 0.39; Found C 80.81, H 7.91, N 0.78.

Device Fabrication and Measurement

Double-layer EL devices were fabricated as sandwich structures between a calcium (Ca) cathode and an indium-tin oxide (ITO) anode. An ITO-coated glass substrate was pre-cleaned and treated with UV/ozone before use. The poly(ethylene 3,4-dioxythiophene): polystyrene sulfonate (PEDOT:PSS) purchased from Bayer was then spin-coated and annealed at 1508C for 1 h under vacuum as the hole injunction layer. The thickness of the PEDOT:PSS layer was ca. 50 nm. An emissive layer was then spin-coated from its chloroform solution on top of the PEDOT layer to give a thin film with a thickness of70 nm. Finally, a layer of Ca (35 nm) and aluminum (100 nm) was thermally evaporated on top of the emissive layer at base pressures lower than 6 107 _{Torr. The} active device area was 4 mm2. The current density–voltage– luminance characteristics were measured under ambient condi-tions using a Keithley 2400 source meter and a PR-650 spectro-photometer.

Results and Discussion

Synthesis of Polymers

As mentioned in the experimental part, all polymeriza-tions were carried out at 808C in anhydrous toluene, using TaCl5 as catalyst and n-Bu4Sn as co-catalyst. The

poly-merization conditions are mild and the molecular weights of the obtained polymers are relatively large. Table 1 summarizes the molecular weights and polydispersity index (PDI) of the resulting polymers. The weight-average molecular weights (Mw) of these polymers are in the range

from 7.5 104_{to 141.7 10}4_{, and the PDI (M}

w=Mn) is less

than 2. Among them, PDPA-5F has the lowest yield and

Table 1. Polymerization results and thermal properties of poly-mersPDPA-P, PDPA-1F, PDPA-2F, PDPA-3F, and PDPA-5F.

Polymer _{Mn (T 10}4_{) M} w (T 104) PDI Tg Td -C -C PDPA-P 30.6 42.6 1.39 202 415 PDPA-1F 76.2 127.2 1.67 220 401 PFPA-2F 89.1 141.7 1.59 233 410 PFPA-3F 72.6 123.4 1.70 236 425 PFPA-5F 4.2 7.5 1.78 231 400

molecular weights; this could be a result of the pentafluoro substitution which withdraws partial electron densities from the triple bond and thus affects the polymerization. All the obtained polymers can be dissolved in common organic solvents, such as chloroform, toluene, and chlorobenzene. Transparent and self-standing films can be cast from their solutions.

Figure 1 shows the FT-IR spectra of monomer M3 and its corresponding polymer PDPA-2F. The characteristic band centered at 2 220 cm1was assigned to the C C stretching in M3, and it totally diminished in the spectrum of PDPA-2F. Instead, a new absorption band located at 1 650 cm1 was formed, which was assigned to C––C stretching in the main chain. The GPC and FT-IR observations revealed that PDPA-2F was successfully synthesized.

Thermal Properties

The glass transition temperature (Tg) and thermal

decom-position temperature (Td) of the synthesized PAs are also

summarized in Table 1. The Td value is defined as the

temperature for 5% weight loss. These polymers show good thermal stabilities with a high Tg(>200 8C) and a high

Td (>400 8C), which are important advantages in the

fabrication of light-emitting devices. The Tgof fluorinated

polymers PDPA-1F, PDPA-2F, PDPA-3F, and PDPA-5F is higher than that of PDPA-P, which indicates that fluorine substitution can improve thermal stabilities. In addition, PDPA-3F has the highest Tdup to 4258C.

Optical Properties

Figure 2 shows the UV-vis absorption spectra of five PA derivatives in the thin film state. The UV-vis spectrum of

PAs often shows two absorption bands that belong to a p–p inter-band transition along the conjugated main chain.[15] It is seen that a continuous red-shift of the absorption band occurs as the number of fluorine atom increases, from 423 to 473 nm. A similar observation is also observed in the PL emission spectra, as shown in Figure 3. The maximum emission band is red-shifted from 517 nm (PDPA-P) to 568 nm (PDPA-5F). The UV-vis absorption and PL emission maxima of all polymers in different states are summarized in Table 2. It is well-known that the fluorine atom has a high electron affinity and is often regarded as an electron-withdrawing group; however, these fluoro-substituted PAs show a gradual red-shift from their UV-vis absorption and PL emission measurements, especially in the thin film state. In other words, the conjugation length S.-H. Yang, C.-H. Huang, C.-H. Chen, C.-S. Hsu

Figure 2. UV-vis absorption spectra of polymers PDPA-P (*), PDPA-1F (



Figure 1. FT-IR spectra of monomer M3 and the corresponding polymerPDPA-2F.

Figure 3. Photoluminescent spectra of polymers PDPA-P (*), PDPA-1F (



is elongated for the PAs that contain more fluorine atoms. We speculate that this phenomenon is a result of a stacking effect. For PDPA-P, the pendent phenyl rings possess a torsional twist from the PA main chain that prevents close packing of neighboring polymer chains. For PDPA-1F to PDPA-5F, the fluorine atoms show a polariza-tion force to attract each other and cause the pendent phenyl rings to stack together, which results in close packing of the main chains. With more fluorine substitu-tions, the extent of chain stacking increases and a red-shift in optical properties takes place.

The PL quantum efficiencies (FPL) of these PAs were also

measured and are listed in Table 2. It can be seen that the FPLvalue decreases from 55% (PDPA-P) to 4% (PDPA-5F) as

the number of fluorine atoms increase. The decreasedFPL

value can be explained by a similar stacking effect. Since a pendent fluorophenyl group helps to stack the polymer chains together, the formation of chain aggregation results in a red-shift in the solid state. In this situation energy transfer from neighboring polymers occurs and decreases theFPLvalues.

Electrochemical Analysis

Cyclic voltammetry (CV) was employed to investigate the electrochemical behaviors of the synthesized polymers and to estimate their energy levels. The oxidation process is clear and directly associated with the conjugation structure of the polymer. Figure 4 shows cyclic voltammo-grams of some polymers in the oxidation process. The HOMO energy level is determined from the onset of the oxidation curve (Eox), which is given by

HOMOðeVÞ ¼  Ej oxþ 4:4j

which are in the range from5.3 to 5.73 eV. The energy gaps (EG) of materials are determined from the edge of their UV-vis absorption spectra (lonset), which is given by

EGðeVÞ ¼ 1 240=lonset

The EG values of the synthesized PA derivatives are in the range from 2.29 to 2.67 eV. Combining the electro-chemical data and UV-vis characteristics gives an estimate of the LUMO energy levels. Table 3 summarizes the HOMO, LUMO, and EG values of the five PA derivatives. By incorporating strong electron-withdrawing fluorine atoms onto the conjugated main chain, the LUMO and HOMO levels of fluorinated polymers 1F, 2F, PDPA-3F, and PDPA-5F were all lowered as compared to PDPA-P. Similar observations were also reported in the previous literature.[21–23] Furthermore, the incorporation of more fluorine atoms caused a larger decrease in LUMO levels, and consequently resulted in smaller EG values. The results are in accordance with optical observations, i.e., the extent of red-shift increases with the introduction of more fluorine atoms.

Device Performance

Double-layer light-emitting diodes with the configuration of ITO/PEDOT/polymer/Ca/Al were fabricated to evaluate the EL performance of synthesized PA derivatives. The EL emission spectra of these polymers are shown in Figure 5,

Figure 4. Cyclic voltammograms of polymers P (*), PDPA-2F (



Table 3. Electrochemical properties of polymers PDPA-P, PDPA-1F, PDPA-2F, PDPA-3F, and PDPA-5F.

Polymer Eox HOMO UV edge EG LUMO

V eV nm eV eV PDPA-P 0.9 5.3 464 2.67 2.63 PDPA-1F 1.1 5.5 464 2.67 2.83 PDPA-2F 1.2 5.6 471 2.63 2.97 PDPA-3F 1.3 5.7 481 2.58 3.12 PDPA-5F 1.33 5.73 541 2.29 3.44

Table 2. Optical properties and PL efficiencies of polymers PDPA-P, PDPA-1F, PDPA-2F, PDPA-3F, and PDPA-5F.

Polymer UV-vis PL FPL

nm nm %

Solution Film Solution Film Film

PDPA-P 430 492 423 517 55

PDPA-1F 430 494 429 519 53 PDPA-2F 437 500 433 536 44 PDPA-3F 438 533 441 547 31

and their emission maxima are listed in Table 4. The EL emissive maxima are located from 528 to 620 nm. The CIE coordinates of five polymers are also shown in Table 4, referring to green P, PDPA-1F), green-yellow (PDPA-2F, PDPA-3F), and orange light (PDPA-5F). It is noted that the emission maximum of PDPA-5F is located at 620 nm, which normally refers to red light; however, its EL spectrum is pretty broad and covers a certain part of the yellow region, as shown in Figure 5. Here again the EL spectra of these PA derivatives shows a greater red-shift upon incorporating more fluorine atoms.

Table 4 also summarizes the turn-on voltage, max brightness, and max current efficiency of these devices. The turn-on voltages are quite close,6–7 volts. The device using PDPA-P as the active layer showed a moderate brightness of 195 cd
m2and current efficiency of 0.06 cd
A1. By introducing one or two fluorine atoms on the pendent phenyl ring, the device performance was improved to some extent. PDPA-2F showed the highest brightness of 828 cd
m2and a current efficiency of 0.78 cd
A1. Upon introducing more fluorine atoms, however, the device performance dropped off. The device using PDPA-5F as the active layer showed a quite low brightness

of 17 cd
m2and current efficiency of 0.01 cd
A1. In the following part we chose PDPA-2F as the active material and proposed different approaches to improve its device performance.

Approaches to Highly Efficient PLEDs

To further improve the device performance of the synthesized PDPA-2F, two approaches were proposed as follows. One is based on a blend of PDPA-2F and a hole transport material TM-TPD as the active layer, while the other one is by incorporating carbazole units into the PDPA-2F backbone.

The synthesis of the copolymer PDPA-2Fcab is shown in Scheme 2. The obtained PDPA-2Fcab also has high molecular weights (Mn¼ 7.38  105, Mw¼ 1.09  106) and

a relatively low PDI value of 1.48. The Tgand Td values

obtained from DSC and TGA are 228 and 4068C, respectively. The result reveals that PDPA-2Fcab is competitive with previous di-substituted PA derivatives.

Figure 6 shows the UV-visible absorption and PL emission spectra of PDPA-2F, TM-TPD, and the blend of S.-H. Yang, C.-H. Huang, C.-H. Chen, C.-S. Hsu

Figure 5. Electroluminescent spectra of polymers PDPA-P (*), PDPA-1F (), PDPA-2F (~), PDPA-3F (~), and PDPA-5F (&) in ITO/PEDOT/polymer/Ca/Al devices.

Table 4. Device performance of polymers PDPA-P, PDPA-1F, PDPA-2F, PDPA-3F, and PDPA-5F in ITO/PEDOT/polymer/Ca/Al devices.

Polymer EL Vturn-on Max. brightness Max. efficiency CIE’ 1931

nm volt cd
mS2 cd
AS1 x y PDPA-P 528 6 195 0.06 0.36 0.54 PDPA-1F 528 6 235 0.45 0.37 0.54 PDPA-2F 540 6 828 0.78 0.39 0.54 PDPA-3F 548 7 153 0.02 0.39 0.52 PDPA-5F 620 7 17 0.01 0.54 0.45

Figure 6. UV-vis spectrum of PDPA-2F (*) and PL spectra of TM-TPD (~), PDPA-2F (



90 wt.-% PDPA-2F and 10 wt.-% TM-TPD in solution or thin film state. As mentioned earlier, the UV-vis spectrum of PDPA-2F shows two absorption peaks at 365 and 435 nm, which are attributed to the p–pinter-band transition of the main chains.[15] The PL spectrum of TM-TPD ranges from 374 to 524 nm with an emission maximum located at 418 nm. Owing to efficient Fo¨rster transfer from TM-TPD to PDPA-2F, the emission spectrum of the blend is essentially identical to that of PDPA-2F; no emission band of TM-TPD is observed.

The energy levels of electronic states were studied by electrochemical analysis. Figure 7 presents the HOMO and LUMO energetic levels of different organic layers. Accord-ing to the energy level diagram, the LUMO levels of PDPA-2F and PDPA-PDPA-2Fcab are both located at –2.97 eV, which is close to the work function of the Ca cathode. The close value implies that electrons can be injected from the Ca cathode into the polymer layer easily. On the other hand, the HOMO level of TM-TPD is5.3 eV, which is quite close to that of the ITO/PEDOT layer; the ease of hole injection is also concluded. In addition, the HOMO/LUMO levels of PDPA-2Fcab are found to be similar to those of PDPA-2F, which implies little effect of the carbazole pendent group on the photophysics of the copolymer.

Figure 8a–c shows the current density, brightness, and current efficiency versus applied voltage for different emissive polymer used in double-layer devices with the configuration of ITO/PEDOT/polymer/Ca/Al. A higher current density and lower turn-on voltage were found with increasing TM-TPD content in the PDPA-2F layer. The turn-on voltages of these devices are 5, 4, 3, and 2 volts with adding 0, 5, 10, and 20 wt.-% of TM-TPD, respectively. The result shows that the incorporation of TM-TPD improves the injection and transport of hole carriers in these devices. The device with 5 wt.-% of TM-TPD revealed a maximum current efficiency of 3.56 cd
A1 under a

current density of 3 mA
cm2 at 7 V, and a maximum brightness of 1 950 cd
m2at 14 V. However, the device with 20 wt.-% of TM-TPD showed a decreased performance, i.e., a maximum brightness of 934 cd
m2 and a maximum current efficiency of 0.21 cd
A1were found.

Figure 8. a) Current density, b) brightness, and c) current effi-ciency versus applied voltage of2F (*), the blend of PDPA-2F and 5 wt.-% (



We attributed this observation to the result of an excess amount of TM-TPD. The crystallization of TM-TPD was performed and observed by optical microscopy, which resulted in lower device performance. Turning to PDPA-2Fcab, the turn-on voltage of the device is 3.5 V. It showed a maximum current efficiency of 3.37 cd
A1 _{under a}

current density of 7 mA
cm2 at 7 V. A maximum brightness of 4 230 cd
m2under a current density of 228 mA
cm2at 14 V was observed. The device performance of PDPA-2Fcab is approximately 100 times higher than that formerly of PDPA.[7]The device performance of the hybrid PDPA-2Fþ TM-TPD and pristine PDPA-2Fcab is summar-ized in Table 5.

In order to comprehend the difference in the perfor-mance of these devices, we fabricated several hole- and electron-only devices to compare current densities (and thus mobility) of two carriers inside the polymer layer.[17]

The device structure of a typical hole-only device is ITO/ PEDOT/polymer/Au. In this device, hole injection is predominant, while electrons are difficult to inject from the Au cathode into the polymer layer. Moreover, an electron-only device is fabricated with the configuration of ITO/Al/polymer/Ca/Al. In this case electron injection is

relatively major and the hole would be blocked between the ITO and metal Al. The current density–electrical field characteristics of PDPA-2F, the blend of PDPA-2F and 5 wt.-% of TM-TPD, and PDPA-2Fcab are shown in Figure 9. For PDPA-2F, the current density of electrons is higher than that of holes by one order at the same electric field, which indicates that PDPA-2F is an electron-transporting mate-rial. For the blend of PDPA-2F and TM-TPD, the hole current is increased and the difference in current density of the holes and electrons is decreased as compared with pristine PDPA-2F. The results described above help to explain the improvement of device performance of PDPA-2F by adding a hole-transporting material TM-TPD. For PDPA-2Fcab, the hole-transporting moiety of carbazole was incorporated into the polymer chain, and phase separation and crystallization phenomena can be diminished. The current densities of the holes and electrons are close to each other at various electrical fields, which indicates a charge balance of two carriers inside the polymer layer. The polymer PDPA-2Fcab showed the best device performance among the PA derivatives synthesized in this work and in the literature.

Figure 10 illustrates the EL spectra of pristine PDPA-2F, the blend of PDPA-2F and TM-TPD, and PDPA-2Fcab alone. The emissive maxima of three materials are located at same position, and the shape of the three EL spectra are pretty similar, which suggests that the emissive behavior of PDPA-2F is not affected by the incorporation of TM-TPD or carbazole moieties significantly. The CIE’1931 coordi-nates are located at (0.35, 0.54), which refers to a yellowish-green color.

Conclusion

In summary, a novel series of fluorophenyl PA derivatives were successfully synthesized and characterized. These PA derivatives show good thermal stability and solubility in common organic solvents. Optical studies reveal that incorporating more fluorine atoms on the pendent phenyl ring results in a greater red-shift in the UV-vis absorption and PL emission. However, the PL efficiency decreases with increasing the number of fluorine atoms.

S.-H. Yang, C.-H. Huang, C.-H. Chen, C.-S. Hsu

Table 5. Device performance of hybrid PDPA-2F þ TM-TPD and pristine PDPA-2Fcab in ITO/PEDOT/polymer/Ca/Al devices.

Polymer EL Vturn-on Max. brightness Max. efficiency CIE’ 1931

nm volt cd
mS2 cd
A–1 x Y

PDPA-2F þ 5% TM-TPD 536 4 1 950 3.56 0.35 0.54

PDPA-2F þ 10% TM-TPD 532 3 996 0.64 0.35 0.52

PDPA-2F þ 20% TM-TPD 532 3 934 0.21 0.32 0.54

PDPA-2Fcab 532 4 4 230 3.37 0.35 0.54

Figure 10. Normalized EL spectra of the devices using PDPA-2F (*), the blend of PDPA-2F and 5 wt.-% (



We also investigated two approaches to achieve high brightness and efficiency of electroluminescent devices by blending with TM-TPD or by incorporating carbazole moieties into polymer chains. The hole injection and transport can be improved by two approaches according to the observation of lowered turn-on voltage and electrical properties. A well-established charge balance of holes and electrons in PDPA-2Fcab was studied from hole- and electron-only devices. To the best of our knowledge, the synthesized PDPA-2Fcab has shown the best electrolumi-nescent properties so far. The results reveal that these materials are promising candidates for PLED applications.

Acknowledgements: The authors are grateful to the National Science Council of the Republic of China (NSC 95-2221-E-009-161-MY3) and Ministry of Education (MOE ATU Program) for its financial support of this work.

Received: September 24, 2008; Revised: October 30, 2008; Accepted: October 30, 2008; DOI: 10.1002/macp.200800467 Keywords: conjugated polymers; light-emitting diodes; metath-esis; polyacetylenes

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