Containing Poly(2,3-diphenyl-1,4-phenylenevinylene)
Derivatives
SHENG-HSIUNG YANG, SHIANG-YING CHEN, YU-CHUN WU, CHAIN-SHU HSU
Department of Applied Chemistry, National Chiao Tung University, 1001, Ta-Hsueh Rd., Hsinchu 30010, Taiwan, Republic of China
Received 11 April 2007; accepted 3 May 2007 DOI: 10.1002/pola.22207
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A new series of poly(2,3-diphenyl-1,4-phenylenevinylene) derivatives con-taining dendritic side groups were synthesized. Different generations of dendrons were integrated on the pendant phenyl ring to investigate their effect on optical and electrical properties of final polymers. Homopolymers can not be obtained via the Gilch polymerization because of sterically bulky dendrons. By controlling the feed ra-tio of different monomers during polymerizara-tion, dendron-containing copolymers with high molecular weights were obtained. The UV–vis absorption and photoluminescent spectra of the thin films are pretty close; however, quantum efficiency is significantly enhanced with increasing the generation of dendrons. The electrochemical analysis reveals that hole-injection is also improved by increasing dendritic generation. Dou-ble-layer light-emitting devices with the configuration of ITO/PEDOT:PSS/polymer/ Ca/Al were fabricated. High generation dendrons bring benefit of improved device per-formance.VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3440–3450, 2007 Keywords: conjugated polymers; light-emitting diodes (LED); luminescence
INTRODUCTION
Semiconducting polymers have been intensively investigated for their potential applications in light-emitting diodes,1,2 thin film transitors,3 organic laser,4 and solar cells.5 Among them, poly(1,4-phenylene vinylene) (PPV) has at-tracted a great deal of attention in recent years because of its unique structure and highly elec-troluminescent (EL) properties.6 Long alkyl chains and/or bulky substitutents have been incorporated onto the PPV main chain to improve its solubility to cast thin films by solu-tion process. Electron donating/withdrawing
groups have also been introduced to adjust the optical and electrical properties. Till now many PPV derivatives have been synthesized to inves-tigate their potential applications, for example, poly[2-methoxy-5-(20 -ethylhexoxy)-1,4-phenylene-vinylene] (MEH-PPV) is an orange–red emissive polymer and soluble in common organic sol-vents.7 Thin film of MEH-PPV can be obtained from a spin-coating process. Cyano-substituted poly(2,5-dialkoxy-1,4-phenylene vinylene) is a red emissive polymer with high electron affin-ities.8,9Silyl-substituted PPV is a greenish emis-sive material with a tendency to be easily charged by electrons rather than holes.10,11
PPV, originally, is a yellow–green emissive polymer. Pure blue, green, and red emissions are not easy to achieve for fully conjugated PPV. It is generally thought that increase of steric hindrance can reduce the conjugation length of
Correspondence to: C.-S. Hsu (E-mail: [email protected]. edu.tw)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 3440–3450 (2007)
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VC2007 Wiley Periodicals, Inc.
the polymer chain and tune the emission to blue light region. For this reason, many bulky groups have been incorporated and blue-shifted emis-sions were obtained.12,13 Hsieh et al. first pro-posed a synthetic route to poly(2,3-diphenyl-1, 4-phenylene vinylene) (DP-PPV), which exhibits high photoluminescence (PL) efficiency in the solid state.14 Different substituents were intro-duced at C-5 position of the phenylene moiety to modify its properties. For example, highly phe-nylated DP-PPV was synthesized to further improve PL efficiency.15 Long alkyl chains were incorporated to improve the solubility of the polymer.16 Liquid crystalline side chains were also incorporated to achieve polarized emis-sions.17,18 By following this synthetic route, monomers containing diverse functional groups are easily synthesized and therefore soluble DP-PPV derivatives with high molecular weights are also easily obtained. Despite the advantages mentioned earlier, low device performance using DP-PPVs as active layers is obtained and thus restricts their potential use for display applica-tions. Recently we have reported two series of DP-PPV derivatives containing long branched alkoxy and fluorenyl substituents. Both the brightness and current efficiency are highly improved.19
For most of the conjugated polymers, their luminescent quantum efficiency is substantially lower in the solid state than that in the solution state because of intermolecular interactions, such as aggregation and excimer formation, which lead to a self-quenching process of exci-tons. To minimize such intermolecular interac-tions and increase the luminescent quantum efficiency in solid state, an effective strategy is introducing the bulky dendritic side groups to the conjugated polymer backbone. Many dendron-con-taining polymers and their self-assembled proper-ties have been reported by Percec and cow-orkers.20–29 Conjugated polymers with dendritic side groups have also been reported in the litera-tures, such as poly(p-phenylene vinylene),30–22 pol-yfluorene,33–35and polythiophene.36In this study, we synthesized the first example of DP-PPV con-taining dendron side groups. Four copolymers derived from copolymerizing with 1,4-bis(chlor-omethyl)-2,5-dimethoxybenzene and 1,4-bis-(chloromethyl)-2-[40-(3,7-dimethyloctoxy)phenyl] -3-phenylbenzene19 were synthesized. The elec-trical and spectroscopic properties of theses poly-mers were systematically investigated. In addi-tion, double-layer light-emitting devices were
also fabricated to study EL properties of the polymers.
EXPERIMENTAL
Characterization Methods 1
H NMR spectra were measured with a Varian 300 MHz spectrometer. Gel permeation chroma-tography (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 35 8C. Differential scanning calorimetry (DSC) was performed on a PerkinElmer Pyris Diamond DSC instrument at a scan rate of 10 8C/min. Thermal gravimetric analysis (TGA) was under-taken on a PerkinElmer Pyris 1 TGA instru-ment with a heating rate of 10 8C/min. UV–vis absorption spectra were obtained with an HP 8453 diode array spectrophotometer. PL emis-sion spectra were obtained using ARC Spec-traPro-150 luminescence spectrometer. Cyclic voltammetric (CV) measurements were made in acetonitrile (CH3CN) with 0.1 M tetrabutylam-monium 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 0.1 M AgNO3solution, from Bioanalytical Systems) as the reference electrode, and ferro-cene was used as an internal standard. The cor-responding highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular or-bital (LUMO) energy levels were estimated from the onset redox potentials.
Synthesis of Monomers and Polymers
All reagents and chemicals were purchased from commercial sources (Aldrich, Lancaster, or TCI) and used without further purification. Tetrahy-drofuran (THF) was dried by distillation from sodium/benzophenone. Scheme 1 outlines the synthetic routes for Dendrons 1, 3, and 5. (3,5-Bis(10-undecenyloxy)phenyl)methanol (1) To a solution of 11-bromo-1-undecene (5.0 g, 21.44 mmol), potassium carbonate (3.22 g, 23.3 mmol), and potassium iodide (0.39 g, 2.33 mmol) in acetonitrile (50 mL) was added (3,5-dihyd-roxyphenyl)methanol (1.31 g, 9.32 mmol). The
mixture was refluxed at 100 8C for 24 h. After cooling to room temperature, the solution was filtered and concentrated by evaporating the sol-vent. The crude product was dissolved in 50 mL of ethyl acetate and extracted with water (50 mL) twice. The organic phase was then dried over magnesium sulfate, isolated by evaporating the solvent, and purified by gel chromatography (silica gel, n-hexane:ethyl acetate ¼ 5:1 as the eluent) to give 3.61 g (87%) of colorless liquid.
1 H NMR (CDCl3, d, ppm): 1.28–1.52 (m, 24H, (CH2)6), 1.74 (m, 4H, OCH2CH2), 2.01 (q, J ¼ 6.4 Hz, 4H, CH2CH¼¼CH2), 3.91 (t, J ¼ 7 Hz, 4H, OCH2), 4.59 (s, 2H, PhCH2OH), 4.94 (dd, J ¼ 8.4 Hz, CH¼¼CH2), 5.78 (m, 2H, CH¼¼CH2), 6.35 (s, 1H, aromatic-H), 6.47 (s, 2H, aromatic-H). (3,5-Bis(10-undecenyloxy)phenyl)methyl Bromide (2)
To a 50 mL two-necked round flask was added 1 (2.0 g, 4.5 mmol), carbon tetrabromide (2.23 g,
Scheme 1. Synthesis of Dendrons 1, 3, and 5.
6.75 mmol), and THF (5 mL). A solution of tri-phenylphosphine (1.77 g, 6.75 mmol) in THF (5 mL) was then added dropwise at 0 8C. The mix-ture was stirred at room temperamix-ture for 30 min. After the reaction was completed, the solution was concentrated by evaporating the solvent. The crude product was dissolved in 30 mL of ethyl acetate and extracted with water (30 mL) twice. The organic phase was then dried over magnesium sulfate, isolated by evaporating the solvent, and purified by gel chromatography (silica gel, n-hexane:dichloromethane ¼ 2:1 as the eluent) to give 2.12 g (93%) of colorless liquid.
1 H NMR (CDCl3, d, ppm): 1.23–1.42 (m, 24H, (CH2)6), 1.74 (m, 4H, OCH2CH2), 2.03 (q, J ¼ 6.4 Hz, 4H, CH2CH¼¼CH2), 3.9 (t, J ¼ 7 Hz, 4H, OCH2), 4.38 (s, 2H, PhCH2Br), 4.94 (dd, J ¼ 8.4 Hz, CH¼¼CH2), 5.78 (m, 2H, CH¼¼CH2), 6.35 (s, 1H, aromatic-H), 6.48 (s, 2H, aromatic-H). 3,5-Bis(3,5-bis(10-undecenyloxy)benzyloxy)phenyl Methanol (3)
To a solution of 2 (2.0 g, 3.94 mmol), 18-crown-6 ether (0.12 g, 0.47 mmol), and potassium car-bonate (0.65 g, 4.7 mmol) in acetone (20 mL) was added 3,5-dihydroxyphenylmethanol (0.26 g, 1.88 mmol). The mixture was refluxed at 80 8C for 24 h. After cooling to room temperature, the solution was filtered and concentrated by evapo-rating the solvent. The crude product was dis-solved in 50 mL of ethyl acetate and extracted with water (50 mL) twice. The organic phase was then dried over magnesium sulfate, isolated by evaporating the solvent, and purified by gel chro-matography (silica gel, n-hexane:ethyl acetate ¼ 5:1 as the eluent) to give 1.59 g (85%) of pale-yellow liquid. 1 H NMR (CDCl3, d, ppm): 1.24–1.41 (m, 48H, (CH2)6), 1.74 (m, 8H, OCH2CH2), 2.03 (q, J ¼ 6.4 Hz, 8H, CH2CH¼¼CH2), 3.91 (t, J ¼ 7 Hz, 8H, OCH2), 4.61 (s, 2H, PhCH2OH), 4.93 (s, 4H, PhCH2OPh), 4.94 (dd, J ¼ 8.4 Hz, 8H, CH¼¼CH2), 5.75 (m, 4H, CH¼¼CH2), 6.37 (s, 2H, aromatic-H), 6.52 (s, 5H, aromatic-H), 6.58 (s, 2H, aromatic-H). 3,5-Bis-(3,5-bis(10-undecenyloxy)benzyloxy)benzyl Bromide (4)
By following the synthetic procedure for 2 and using 3 as starting material, the compound 4 was obtained as pale-yellow liquid (90% yield).
1 H NMR (CDCl3, d, ppm): 1.24–1.41 (m, 48H, (CH2)6), 1.74 (m, 8H, OCH2CH2), 2.03 (q, J ¼ 6.4 Hz, 8H, CH2CH¼¼CH2), 3.91 (t, J ¼ 7 Hz, 8H, OCH2), 4.38 (s, 2H, PhCH2Br), 4.94 (s, 4H, PhCH2OPh), 4.99 (dd, J ¼ 8.4 Hz, 8H, CH¼¼CH2), 5.78 (m, 4H, CH¼¼CH2), 6.38 (s, 2H, aromatic-H), 6.52 (s, 5H, aromatic-H), 6.6 (s, 2H, aromatic-H). 3,5-Bis(3,5-bis(3,5-bis(10-undecenyloxy)benzyloxy)-benzyloxy)phenyl Methanol (5)
By following the synthetic procedure for 3 and using 4 as starting material, the compound 5 was obtained as yellow liquid (83% yield).
1 H NMR (CDCl3, d, ppm): 1.23–1.39 (m, 96H, (CH2)6), 1.73 (m, 16H, OCH2CH2), 1.99 (q, J ¼ 6.4 Hz, 16H, CH2CH¼¼CH2), 3.9 (t, J ¼ 7 Hz, 16H, OCH2), 4.6 (s, 2H, PhCH2OH), 4.88 (s, 12H, PhCH2OPh), 4.92 (dd, J ¼ 8.4 Hz, 16H, CH¼¼CH2), 5.78 (m, 8H, CH¼¼CH2), 6.37 (s, 2H, aromatic-H), 6.47 (s, 5H, aromatic-H), 6.52 (s, 10H, aromatic-H), 6.63 (s, 4H, aromatic-H).
Synthesis of Monomers G1M-G3M: General Procedure
Scheme 2 shows the synthetic route for den-dron-containing momomers G1M–G3M with dif-ferent generations. An experimental procedure for G1M is given below. To a 50 mL round flask was added 1 (0.57 g, 1.28 mmol), 6 (0.5 g, 1.16 mmol), triphenylphosphine (PPh3) (0.4 g, 1.51 mmol), and THF (10 mL). Diethyl azodicarboxy-late (DIAD) (0.31 g, 1.51 mmol) was slowly added to the solution at 0 8C. The mixture was stirred at room temperature for 12 h. The crude product was isolated by evaporating the solvent and further purified by gel chromatography (silica gel, n-hexane:ethyl acetate ¼ 5:1 as the eluent) to give the product of pale-yellow liquid. 1,4-Bis(bromomethyl)-2-(3,5-bis(10-undecenyloxy)-benzyloxy)phenyl-3-phenyl Benzene (G1M) Yield 63%. 1H NMR (CDCl3, d, ppm): 1.28–1.4 (m, 24H, (CH2)6), 1.74 (m, 4H, OCH2CH2), 2.01 (q, J ¼ 6.4 Hz, 4H, CH2CH¼¼CH2), 3.91 (t, J ¼ 7 Hz, 8H, OCH2), 4.24 (s, 4H, PhCH2Br), 4.89 (s, 2H, PhCH2OPh), 4.94 (dd, J ¼ 8.4 Hz, 4H, CH¼¼CH2), 5.11 (s, 2H, diphenyl-OCH2), 5.78 (m, 2H, CH¼¼CH2), 6.35 (s, 1H, aromatic-H),
6.47 (s, 2H, H), 6.52 (s, 10H, aromatic-H), 6.87 (d, J ¼ 7.4 Hz, 2H, aromatic-aromatic-H), 7.02 (d, J ¼ 7.4 Hz, 2H, aromatic-H), 7.12 (d, J ¼ 7.6 Hz, 3H, aromatic-H), 7.17 (d, J ¼ 7.6 Hz, 2H, aromatic-H), 7.41 (s, 2H, aromatic-H). 1,4-Bis(bromomethyl)-2-(3,5-bis(3,5-bis(10- undecenyloxy)benzyloxy)benzyloxy)phenyl-3-phenylbenzene (G2M) Yield 61%. 1H NMR (CDCl3, d, ppm): 1.23–1.33 (m, 48H, (CH2)6), 1.74 (m, 8H, OCH2CH2), 2.03 (q, J ¼ 6.4 Hz, 8H, CH2CH¼¼CH2), 3.91 (t, J ¼ 7 Hz, 8H, OCH2), 4.21 (s, 4H, PhCH2Br), 4.88 (s, 4H, PhCH2OPh), 4.92 (dd, J ¼ 8.4 Hz, 8H, CH¼¼CH2), 5.22 (s, 2H, diphenyl-OCH2), 5.78 (m, 4H, CH¼¼CH2), 6.38 (s, 2H, aromatic-H), 6.47 (s, 5H, aromatic-aromatic-H), 6.52 (s, 2H, aro-matic-H), 6.9 (d, J ¼ 7.4 Hz, 2H, aroaro-matic-H), 7.09 (d, J ¼ 7.4 Hz, 2H, aromatic-H), 7.17 (d, J ¼ 7.6 Hz, 3H, aromatic-H), 7.2 (d, J ¼ 7.6 Hz, 2H, aromatic-H), 7.41 (s, 2H, aromatic-H). 1,4-Bis(bromomethyl)-2-(3,5-bis(3,5-bis(3,5-bis(10- undecenyloxy)benzyloxy)benzyloxy)benzyloxy)-phenyl-3-phenylbenzene (G3M) Yield 59%. 1H NMR (CDCl3, d, ppm): 1.23–1.39 (m, 96H, (CH2)6), 1.73 (m, 16H, OCH2CH2), 1.99 (q, J ¼ 6.4 Hz, 16H, CH2CH¼¼CH2), 3.9 (t, J ¼ 7 Hz, 16H, OCH2), 4.2 (s, 4H, PhCH2Br), 4.88 (s, 12H, PhCH2OPh), 4.92 (dd, J ¼ 8.4 Hz, 16H, CH¼¼CH2), 5.22 (s, 2H, diphenyl-OCH2), 5.78 (m, 8H, CH¼¼CH2), 6.37 (s, 2H, aromatic-H), 6.47 (s, 5H, aromatic-H), 6.52 (s, 10H, aromatic-H), 6.63 (s, 4H, aromatic-H), 6.92 (d, J ¼ 7.4 Hz, 2H, aromatic-H), 7.12 (d, J ¼ 7.4 Hz, 2H, aromatic-H), 7.17 (d, J ¼ 7.6 Hz, 3H, ar-omatic-H), 7.2 (d, J ¼ 7.6 Hz, 2H, arar-omatic-H), 7.41 (s, 2H, aromatic-H). 1,4-Bis(bromomethyl)-2,5-dimethoxybenzene (M1) and 1,4-bis(bromomethyl)-2-[40 -(3,7-dimethyloctox-y)phenyl]-3-phenylbenzene (M2)
Monomers M1 and M2 were synthesized as described previously in the literatures.19,37
Synthesis of Polymers: General Procedure
Schemes 3 outlines the synthetic route for poly-mers P1–P4. An experimental procedure for polymer P3 is given below. To a mixture of G3M (1.0 g, 0.4 mmol), M1 (3.2 3 102 g, 0.1 mmol), M2 (0.24 g, 0.5 mmol) in THF (25 mL) was added a solution of potassium tert-butoxide (t-BuOK, 12 equiv) in THF (15 mL). The resulting mixture was stirred at room temperature for 24 h under nitrogen atmosphere. A solution of 2,6-di-tert-butylphenol (6 equiv) as end-capping agent in THF (10 mL) was then added and stirred for additional 8 h. The polymer was obtained by pouring the mixture into methanol and filtered. It was purified by dialysis using Mw 50,000 membrane in THF. After drying under vacuum for 24 h, the polymer was obtained as bright-yellow solid (0.65 g, 51%).
Scheme 2. Synthesis of dentron-containing monomers G1M–G3M.
Device Fabrication and Measurements
Double-layer devices were fabricated as sand-wich structures between calcium (Ca) cathodes and indium-tin oxide (ITO) anodes. ITO-coated glass substrates were cleaned sequentially in
ultrasonic baths of detergent, 2-propanol/ized water (1:1 volume) mixture, toluene, deion-ized water, and acetone. A 50 nm-thick hole-injection layer of poly(ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate)
Scheme 3. Synthesis of polymers P1–P4.
Table 1. Feed Ratio and Polymerization Results of Polymers P1–P4
Polymer Ga R x y z Mn(3105) Mw(3105) PDI b P1 G1 Methyl 40 10 50 2.6 4.38 1.68 P2 G2 Methyl 40 10 50 2.47 4.09 1.66 P3 G3 Methyl 40 10 50 1.5 2.93 1.95 P4 G3 3,7-Dimethyloctyl 75 25 1.74 1.84 1.06 a Generation of dendrons. bPolydispersity index ¼ (Mw/Mn).
(PSS) was spin-coated on top of ITO from a 0.7 wt % dispersion in water and dried at 150 8C for 1 h in a vacuum. Thin films of synthesized poly-mers were spin-coated from toluene solutions onto the PEDOT:PSS layer and dried at 50 8C overnight in a vacuum. The thickness of the active layer was about 50 nm. Finally, 35 nm Ca and 100 nm Al electrodes were made through a shadow mask onto the polymer films by thermal evaporation using an AUTO 306 vacuum coater (BOC Edwards, Wilmington, MA). Evaporations were carried out typically at base pressures lower than 2 3 106 torr. The active area of each EL device was 4 mm2 and the device was characterized following a published protocol.38
RESULTS AND DISCUSSION
Synthesis of Polymers
Scheme 1 outlines the synthetic route for den-drons with different generations. Long alkenyl chains were incorporated onto dendrons to increase solubility. Terminal vinyl bonds serve to identify intermediates and monomers, which shows characteristic peaks at 5–6 ppm in1H NMR spectra. Scheme 2 outlines the synthetic route for monomers G1M–G3M. Dendrons 1, 3, and 5 were reacted with Compound 6 via dehydration in the presence of DIAD/PPh3in THF to form monomers. The polymerization was carried out via Gilch route to obtain soluble PPV derivatives. For a typical Gilch synthetic route, a,a0-dihalo-p-xylene is employed with excess amount of tert-BuOK in organic solvents. Alkyl or alkoxy chains are of-ten incorporated on the aromatic rings to improve the solubility of the resulting polymers. It should be noted that only oligomers were obtained during homopolymerization of dendron-containing monomers; moreover, those oligomers could not provide sufficient thermal and film-forming properties. To effectively lower the steric hindrance during polymerization and
increase polymer molecular weights, copolymer-ization was carried out in this study. Scheme 3 outlines the syntheses of polymers P1–P4, from copolymerizing G1M–G3M with two different monomers M1 and M2. It has been reported that incorporation of M1 increases carrier mobil-ity inside polymer layer.39 M2 is firstly synthe-sized and reported by our group, which shows electron-dominating property.19 We expect that incorporation of M1 and M2 can adjust the opti-cal and electriopti-cal properties of final polymers.
Table 1 summarizes the generation of den-drons, alkyl type on M1, feed ratio of monomers, molecular weights and polydispersity index (PDI) of resulting polymers. The number-aver-age molecular weight (Mn) are in the range from 1.17 3 105 to 2.473 105, while the weight-aver-age molecular weights (Mw) are in the range from 1.58 3 105 to 4.38 3 105. The molecular weight distribution is relatively narrow (PDI < 2). Polymers with high molecular weights can be obtained and soluble in common organic sol-vents, such as chloroform, toluene, and chloro-benzene. Transparent and self-standing films can be cast from their solutions.
Table 2. Thermal and Optical Properties of Polymers P1–P4
Polymer Tg(8C) Td(8C)
UV–vis (nm) PL (nm) FPL(%)
Toluene Film Toluene Film Toluene Film
P1 147 377 443 454 519 545 60 25
P2 145 364 447 453 517 540 68 37
P3 115 339 449 449 516 548 81 50
P4 119 378 454 453 533 552 75 46
Figure 1. UV–vis absorption spectra of polymers P1–P4 in thin film state.
From Table 1 some interesting tendencies are also noticed. By comparing polymers P1–P3, it is found that molecular weights were gradually decreased with increasing generation of den-drons. This is reasonable since bulky pendant groups result in large steric hindrance and hinder polymerization. Polymer P4 has the smallest molecular weights and PDI value of 1.06.
Thermal Properties
Table 2 summarizes the thermal properties of polymers P1–P4. Most polymers show good thermal stabilities with high glass transition temperatures (Tg) over 115 8C and high decom-position temperatures (Td) over 3308C. Polymers P1 and P2 show even higher Tg(>140 8C), which can be attributed to their high molecular weights and low generation dendron.
For polymers P1–P3, it is seen that Tdvalue is gradually decreased with increasing generation of dendrons. It is clear that dendrons with higher generation own more alkyl groups. Polymers with long alkyl side chains are easier to decom-pose thermally than those with short chains.
Optical Properties
Figure 1 shows the UV–vis absorption spectra of polymers P1–P4 in thin film state. Table 2 sum-marizes the UV–vis absorption maxima of all polymers in different states. The absorption maxima of synthesized polymers in toluene are located in the range from 443 to 454 nm, which is attributed to the p–p* transition along the conjugated backbone. The absorption maxima in thin film state show only small redshift (some are very close) compared with those in solution state. For these polymers dendritic groups play the role of preventing interchain interaction, which suggests no tendency toward aggregation in the solid state.
Figure 2 reveals the PL emission spectra of polymers P1–P4 in thin film state. Similar to UV–vis spectra, the PL emission maxima is
Figure 3. Cyclic voltammograms of polymers P1–P4.
Table 3. Electrochemical Properties of Polymers P1–P4 in Solid Films Polymer Eox (V) HOMO (eV) UV Edge (nm) EG (eV) LUMO (eV) P1 1.13 5.53 546 2.27 3.26 P2 1.08 5.48 529 2.34 3.14 P3 0.86 5.26 535 2.32 2.99 P4 1.18 5.58 538 2.3 3.28
Figure 4. Energy level diagrams of polymers P1– P4.
Figure 2. PL emission spectra of polymers P1–P4 in thin film state.
somewhat close (within 12 nm). The PL emis-sion maxima in different states are also sum-marized in Table 2. The PL quantum efficiency FPL is increased with increasing the generation of dendrons, implying high generation dendrons can suppress the formation of aggregation. These results demonstrate that the incorpo-ration of appropriate dendritic moieties into polymers can bring benefit of high quantum efficiency and less chain aggregation, without perturbing emission color.
Electrochemical Analysis
Cyclic voltammetry (CV) was employed to inves-tigate the electrochemical behaviors of synthe-sized polymers and to estimate their energy levels. The oxidation process is clear and directly associated with the conjugation structure of the polymer. Figure 3 shows cyclic voltammograms of polymers P1–P4 in the oxidation process. One can observe that the onset of oxidation potential is gradually decreased with increasing the gen-eration of dendrons. It is known that lowering
oxidation potential favors hole-injection, which shows advantage for EL applications. The HOMO, LUMO, and energy gap of polymers P1–P4 are estimated according to previous liter-ature19 and summarized in Table 3. The energy level diagram of these materials is illustrated in Figure 4. The smaller energy barrier between polymer P3 and PEDOT:PSS layer shows the benefit for hole-injection. Turning to LUMO lev-els, P3 has smallest energy barriers to the cath-ode Ca, implying good electron injection from cathode to polymer layer.
Device Performance
Double-layer light-emitting diodes with the con-figuration of ITO/PEDOT:PSS/Polymer/Ca/Al were fabricated to evaluate the potential use of synthesized DP-PPV derivatives. The device per-formance of these DP-PPV derivatives is sum-marized in Table 4. The maximum EL emission bands of these devices are located between 540 and 548 nm. The CIE coordinates demonstrate
Table 4. Device Performance of Polymers P1–P4 in ITO/PEDOT: PSS/Polymer/Ca/Al Devices
Polymer EL (nm) Vturn-on (volt) Max. Brightness (cd/m2) Max. Yield (cd/A) CIE’ 1931 x y P1 540 5 1190 0.68 0.39 0.58 P2 540 5 1721 0.88 0.39 0.58 P3 544 6 2114 4.53 0.43 0.56 P4 548 5 5068 0.64 0.43 0.55
Figure 5. Brightness–voltage characteristics of poly-mers P1–P4 in ITO/PEDOT:PSS/Polymer/Ca/Al devices.
Figure 6. Efficiency–current density characteristics of polymers P1–P4 in ITO/ PEDOT:PSS/Polymer/Ca/ Al devices.
yellowish green color mostly. Figure 5 shows brightness–voltage characteristics of devices using polymers P1–P4 as active layers. Figure 6 reveals efficiency-current density characteristics of same devices. The device of polymer P1 showed maximum brightness of 1190 cd/m2 and maximum luminescent efficiency of 0.68 cd/A at 10 V. With increasing generation of dendrons, the corresponding devices showed enhanced per-formance. The device of polymer P2 showed improved brightness of 1721 cd/m2 and maxi-mum luminescent efficiency of 0.88 cd/A. Using polymer P3 as active layer, its device showed even higher brightness of 2114 cd/m2 and lumi-nescent efficiency of 4.53 cd/A. Here again high generation dendrons bring benefit of improved device performance. Polymer P4 showed an improved brightness of 5068 cd/m2. However the luminescent efficiency dropped off.
CONCLUSIONS
In this work, four DP-PPV derivatives containing bulky dendritic side groups were synthesized. Homopolymerization of these dendron-containing monomers only gave oligomers, while high molec-ular weights of copolymers were obtained by con-trolling the feed ratio of different monomers. The luminescent quantum efficiency in solid state was gradually increased with increasing the generation of dendrons. The hole-injection ability, as well as device performance, were both im-proved with introducing high-generation dendri-tic groups.
The authors thank the National Science Council (NSC) of the Republic of China (NSC95-2221-E-009-161-MY3) and Ministry of Education (MOE ATU Program) for financial support of this research.
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