4-phenylenevinylene) Derivatives Having Bulky Phenyl
and Fluorenyl Pendant Groups: Synthesis, Characterization,
and Electro-Optical Properties
SHENG-HSIUNG YANG, HSING-CHUAN LI, CHIEN-KAI CHEN, CHAIN-SHU HSU Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, Republic of China
Received 14 August 2006; accepted 5 September 2006 DOI: 10.1002/pola.21773
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Two series of poly(2,3-diphenyl-1,4-phenylenevinylene) (DP-PPV) deriva-tives containing multiple bulky substituents were synthesized. In the first series, two different groups were incorporated on C-5,6 positions of the phenylene moiety to increase steric hindrance and to obtain blue-shifted emissions. In the second series, bulky fluorenyl groups with two hexyl chains on the C-9 position were introduced on two phenyl pendants to increase the solubility as well as steric hindrance to prevent close packing of the main chain. Polymers with high molecular weights and fine-tuned electro-optical properties were obtained by controlling the feed ratio of different mono-mers during polymerization. The maximum photoluminescent emissions of the thin films are located between 384 and 541 nm. Cyclic voltammetric analysis reveals that the band gaps of these light-emitting materials are in the range from 2.4 to 3.3 eV. A double-layer EL device with the configuration of ITO/PEDOT/P4/Ca/Al emitted pure green light with CIE01931 at (0.24, 0.5). Using copolymer P6 as the emissive layer, the maximum luminescence and current efficiency were both improved when compared with the homopolymer P4.VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6738–6749, 2006
Keywords: conjugated polymers; fluorescence; light-emitting diodes
INTRODUCTION
Semiconducting polymers have been intensively investigated for their potential applications in light-emitting diodes,1 thin film transitors,2 or-ganic laser,3 and solar cells.4 Among them, poly (1,4-phenylene vinylene) (PPV) has attracted a great deal of attention in recent years because of its unique structure and highly electrolumines-cent (EL) properties.5 Long alkyl chains and/or
bulky substitutents have been incorporated onto the PPV main chain to improve its solubility to cast thin films by solution process. Electron donating/withdrawing groups have also been introduced to adjust the optical and electrical properties. Until now many PPV derivatives have been synthesized to investigate their potential applications, for example, poly[2-methoxy-5-(20 -ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV) is an orange-red emissive polymer and soluble in
common organic solvents.6 Thin film of
MEH-PPV can be obtained from a spin-coating process. Cyano-substituted poly(2,5-dialkoxy-1,4-phenyl-ene vinylpoly(2,5-dialkoxy-1,4-phenyl-ene) (CN-PPV) is a red emissive polymer with high electron affinities.7,8 Silyl-substituted Correspondence to: C.-S. Hsu (E-mail: [email protected].
edu.tw)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 6738–6749 (2006) V
PPV is a greenish emissive material with a tend-ency to be easily charged by electrons rather than holes.9,10
Hsieh et al. first proposed a synthetic route to poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV), which exhibits high photoluminescence (PL) efficiency in the solid state.11Different sub-stituents were introduced at C-5 position of the phenylene moiety to modify its properties. For example, highly phenylated DP-PPV was
synthe-sized to further improve PL efficiency.12 Long
alkyl chains were incorporated to improve the sol-ubility of the polymer.13 Liquid crystalline side chains were also incorporated to achieve polar-ized emissions.14,15 By following this synthetic route, monomers containing diverse functional groups are easily synthesized and therefore solu-ble DP-PPV derivatives with high molecular weights are also easily obtained. Despite the advantages mentioned above, low device perform-ance by using DP-PPVs as active layers is obtained and thus restricts their potential use for display applications. 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.16
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 hin-drance can reduce the conjugation length of the polymer chain and tune the emission to blue light region. For this reason, many bulky groups have been incorporated and blue-shifted emissions were obtained.17,18Hsieh et al. ever investigated 2,3,5,6-tetraphenyl PPV in their synthetic route. However, they met the problem of reducing steri-cally hindered ester groups, and the predictably insoluble chlorine precursor polymer discouraged further investigation.12
In this study, two series of DP-PPV derivatives were designed to modify luminescent properties. In the first series, asymmetrically bulky substitu-ents were incorporated on C-5,6 positions of the phenylene moiety to further increase steric hin-drance and to decrease the conjugation length of the main chain. The obtained DP-PPV derivatives were predicted to show blue-shifted emission. In the second series, a novel DP-PPV structure with two fluorenyl substituents on phenyl pendants was synthesized. This fluorenylphenyl group is essentially bulky to increase steric hindrance and to prevent close packing of polymer chains. Two
hexyl chains at C-9 position can also help to increase the solubility. The homopolymer and two copolymers derived from copolymerizing with bis(chloromethyl)-2,5-dimethoxybenzene and 1,4-bis(chloromethyl)-2-[40 -(3,7-dimethyloctoxy)phenyl]-3-phenylbenzene16were also synthesized. The elec-trical and spectroscopic properties of these polymers were systematically investigated. In addition, dou-ble-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 chromatogra-phy data assembled from a Viscotek T50A Differen-tial Viscometer and a LR125 Laser Refractometer and three columns in series were used to measure the molecular weights of polymers relative to poly-styrene standards at 35 8C. 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 10 8C
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) measurements were made in acetonitrile (CH3CN) with 0.1 M
tetrabutylammo-nium hexafluorophosphate (TBAPF6) as the
sup-porting electrolyte at a scan rate of 50 mV/s. Plati-num wires were used as both the counter and working electrodes, and silver/silver ions (Ag in 0.1 M AgNO3solution, from Bioanalytical Systems, Inc.) 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 M1–M4
All reagents and chemicals were obtained from commercial sources (Aldrich, Merck, or TCI) and used without further purification. Tetrahydrofu-ran (THF) and dichloromethane (CH2Cl2) were dried by distillation from sodium/benzophenone and calcium hydride, respectively. Schemes 1 and
2 outline the synthetic routes for monomers M1– M4. The starting materials 2,5-dicarbethoxy-3,4-diphenylcyclopentadienone, (1) 1-phenyl-1hexyne (2b), and 4,40-dibromobenzil (5) were obtained from Aldrich Corp. The compound 2-(1-ethynyl)-9,9-dihexylfluorene (2c) was synthesized as described previously in the literature.19
1-(2-Ethylhexoxy)-4-(2-phenyl-1-ethynyl) benzene (2a)
To a solution of phenylacetylene (7.3 g, 71.7 mmol) in triethylamine (350 mL) was added 1-(2-ethylhexoxy)-4-iodobenzene (22.6 g, 68.1 mmol), triphenylphosphine (1.43 g, 5.46 mmol), CuI (0.54 g, 2.87 mmol), and bis(triphenylphosphine) palla-dium(II) chloride (0.48 g, 0.68 mmol). The mix-ture was refluxed at 858C for 12 h. After cooling to room temperature, the crude product was fil-tered, washed with a large amount of n-hexane, and dried. It was purified by recrystallization
from methanol to yield 17.7 g (85%) of white crys-tals; mp 518C. 1H NMR (CDCl 3, d, ppm): 0.9–0.96 (t, J ¼ 7 Hz, 6H, OCH2CH(CH2CH3)(CH2)3CH3), 1.28– 1.52 (m, 8H, OCH2CH(CH2CH3)(CH2)3CH3), 1.69 (m, 1H,OCH2CH), 3.78 (d, J ¼ 6.4 Hz, 2H, OCH2), 6.88 (d, J ¼ 6.8 Hz, 4H, CBCC6H4O), 7.33–7.57 (m, 5H, C6H5 CBC). Diethyl 2-(40-(2-ethylhexoxy)phenyl)-3,5,6-triphenyl terephthalate (3a)
To a 100 mL round flask was added a mixture of 1 (9.72 g, 25.8 mmol) and 2a (3.0 g, 10.3 mmol) and refluxed at 170 8C for 12 h. After cooling to room temperature, the crude product was purified by gel chromatography (silica gel, n-hexane as the eluent), following recrystallization from n-hexane to give 4.7 g (71%) of white crystals; mp 1228C.
1
H NMR (CDCl3, d, ppm): 0.58–0.66 (m, 6H,
CO2CH2CH3), 0.81 (t, J ¼ 7 Hz, 6H,
OCH2CH(CH2CH3)(CH2)3CH3), 1.2–1.66 (m, 9H, OCH2CH(CH2CH3)(CH2)3CH3), 3.55–3.61 (m, 4H, CO2CH2), 3.72 (d, J ¼ 6.4 Hz, 2H, OCH2), 6.57 (d, J ¼ 7.2 Hz, 2H, aromatic-H), 6.91 (d, J ¼ 7.2 Hz, 2H, aromatic-H), 7.03–7.08 (m, 15H, aromatic-H).
Diethyl 2-butyl-3,5,6-triphenyl terephthalate (3b) By following the synthetic procedure for 3a and using 2b as starting material, the compound 3b was obtained as white crystals (61% yield); mp 1338C. 1 H NMR (CDCl3, d, ppm): 0.61–0.67 (t, J ¼ 7 Hz, 3H, (CH2)3CH3), 0.87 (t, J ¼ 7 Hz, 6H, CO2CH2CH3), 1.04 (m, 2H, phenyl-CH2CH2 CH2CH3), 1.33–1.42 (m, 2H, phenyl-CH2CH2CH2 CH3), 2.43–2.45 (m, 2H, phenyl-CH2CH2CH2 CH3), 3.51–3.94 (m, 4H, CO2CH2), 7.01– 7.12 (m, 10H, aromatic-H), 7.29–7.37 (m, 5H, aro-matic-H). Diethyl 2-(9,9-dihexylfluoren-2-yl)-5,6-diphenyl terephthalate (3c)
By following the synthetic procedure for 3a and using 2c as starting material, the compound 3c was obtained as light-brown liquid (80% yield).
1 H NMR (CDCl3, d, ppm): 0.61–1.26 (m, 28H, fluoreneCH2 (CH2)4CH3, CO2CH2 CH3), 1.82 (t, J¼ 6.8 Hz, 4H, fluoreneCH2), 6.99–7.14 (m, 10H, aromatic-H), 7.32–7.34 (m, 3H, aromatic-H), 7.44 (d, J ¼ 7.4 Hz, 2H, aro-matic-H), 7.73 (t, J ¼ 7.8 Hz, 2H, aromatic-H), 7.86 (s, 1H, aromatic-H). 1,4-Bis(hydroxymethyl)-2-(40 -(2-ethylhexoxy)-phenyl)-3,5,6-triphenylbenzene (4a) A solution of 3a (3.0 g, 4.7 mmol) in THF (30 mL) was added dropwise with stirring to a suspension of LiAlH4 (1.02 g, 26.8 mol) in THF (40 mL) at
0 8C under nitrogen atmosphere. The solution
was then refluxed at 708C for 4 h. After the solu-tion was cooled in an ice-bath, a saturated
Na2SO4 aqueous solution was added dropwise until the solution became white. The white pre-cipitate was removed by filtration. The crude product was isolated by evaporating the solvent and purified by recrystallization from n-hexane to yield 2.33 g (90%) of white crystals; mp 1758C.
1 H NMR (CDCl3, d, ppm): 0.83 (t, J ¼ 7 Hz, 3H, OCH2CH(CH2CH3)(CH2)3CH3), 1.22–1.32 (m, 9H, OCH2CH(CH2CH3)(CH2)3CH3), 3.68 (t, J ¼ 6.8 Hz, 2H, OCH2), 4.11 (d, J ¼ 6.4 Hz, 2H,CH2OH), 6.62 (d, J¼ 7.2 Hz, 2H, aromatic-H), 6.95 (d, J¼ 7.2 Hz, 2H, aromatic-H), 7.05–7.13 (m, 15H, aromatic-H). 1,4-Bis(hydroxymethyl)-2-butyl-3,5,6-triphenyl-benzene (4b)
By following the synthetic procedure for 4a and using 3b as starting material, the compound 4b was obtained as white crystals (62% yield); mp 2348C. 1 H NMR (CDCl3, d, ppm): 0.68 (t, J ¼ 7 Hz, 3H, (CH2)3CH3), 1.11–1.41 (m, 4H, phenyl-CH2 CH2CH2CH3), 2.54 (m, 2H, phenyl-CH2 CH2CH2CH3), 4.23 (d, J¼ 7.4 Hz, 4H, CH2OH), 7.02–7.15 (m, 10H, aromatic-H), 7.32–7.45 (m, 5H, aromatic-H). 1,4-Bis(hydroxymethyl)-2-(9,9-dihexylfluoren-2-yl)-5,6-diphenylbenzene (4c)
By following the synthetic procedure for 4a and using 3c as starting material, the compound 4c was obtained as white crystals (85% yield); mp 1408C.
1 H NMR (CDCl3, d, ppm): 0.64–1.11 (m, 22H, fluorene-CH2 (CH2)4CH3), 1.92 (t, J¼ 6.8 Hz, 4H, fluorene-CH2), 4.41 (t, J ¼ 7 Hz, 4H, CH2OH), 7.01–7.18 (m, 10H, aromatic-H), 7.28–7.35 (m, 3H, aromatic-H), 7.49–7.6 (m, 3H, aromatic-H), 7.7–7.78 (m, 2H, aromatic-H). 1,4-Bis(chloromethyl)-2-(40 -(2-ethylhexoxy)phenyl)-3,5,6-triphenylbenzene (M1)
To a solution of 4a (1.0 g, 1.56 mmol) in anhy-drous CH2Cl2 (30 mL) was slowly added 5 mL of thionyl chloride and stirred overnight under a nitrogen atmosphere. Water was then added dropwise into the solution to destroy excess thi-onyl chloride. The mixture was extracted with
10% NaHCO3(aq) and organic phase was
concen-trated in vacuo. The crude product was purified by recrystallization from n-hexane to give 0.5 g (46%) of white crystals; mp 1398C. 1 H NMR (CDCl3, d, ppm): 0.85 (t, J ¼ 7 Hz, 6H, OCH2CH(CH2CH3)(CH2)3CH3), 1.1–1.32 (m, 9H,OCH2CH(CH2CH3)(CH2)3CH3), 3.72 (t, J ¼ 6.8 Hz, 2H, OCH2), 4.1 (d, J ¼ 6.4 Hz, 2H, CH2Cl), 6.66 (d, J ¼ 7.2 Hz, 2H, aromatic-H), 7.03 (d, J ¼ 7.2 Hz, 2H, aromatic-H), 7.05–7.13 (m, 15H, aromatic-H). 1,4-Bis(chloromethyl)-2-butyl-3,5,6-triphenyl-benzene (M2)
By following the synthetic procedure for M1 and using 4b as starting material, the compound M2 was obtained as white crystals (42% yield); mp 1848C. 1 H NMR (CDCl3, d, ppm): 0.69 (t, J ¼ 7 Hz, 3H, (CH2)3CH3), 1.13–1.45 (m, 4H, phenyl-CH2 CH2CH2CH3), 2.54 (m, 2H, phenyl-CH2CH2CH2 CH3), 4.02 (d, J ¼ 7.4 Hz, 4H, CH2Cl), 7.03– 7.17 (m, 10H, aromatic-H), 7.35–7.47 (m, 5H, aro-matic-H). 1,4-Bis(chloromethyl)-2-(9,9-dihexylfluoren-2-yl)-5,6-diphenylbenzene (M3)
By following the synthetic procedure for M1 and using 4c as starting material, the compound M3 was obtained as white crystals (53% yield); mp 608C. 1 H NMR (CDCl3, d, ppm): 0.65–1.02 (m, 22H, fluorene-CH2(CH2)4CH3), 1.98 (t, J¼ 6.8 Hz, 4H, fluorene-CH2), 4.29 (t, J ¼ 7 Hz, 4H, CH2Cl), 7.04–7.16 (m, 10H, aromatic-H), 7.29– 7.35 (m, 3H, aromatic-H), 7.43–7.64 (m, 3H, aro-matic-H), 7.72–7.78 (m, 2H, aromatic-H).
Diethyl 4,5-bis(40 -bromophenyl)-2-oxo-3,5-cyclo-pentadiene-1,3-dicarboxylate (6)
To a mixture of 5 (10 g, 26.04 mmol) and diethyl 1,3-acetonedicarboxylate (5.26 g, 26.04 mmol) in ethanol (100 mL), a solution of KOH (1.46 g, 26.1 mmol) in ethanol (20 mL) was added and stirred at room temperature for 24 h. The resultant yel-low precipitate was filtered off and dried. H2SO4 was then added dropwise with stirring to slurry of the yellow precipitate in acetic anhydride (50 mL) until the solution became dark-red. After 30 min, 50 g of ice was added slowly to decompose excess acetic anhydride, and the crude product was collected and dried. It was purified by recrys-tallization from methanol to give 11.2 g (78%) of white crystals; mp 1188C.
1
H NMR (CDCl3, d, ppm): 1.43–1.47 (t, J ¼ 7 Hz, 6H, CO2CH2CH3), 4.31–4.36 (q, J ¼ 7 Hz, 4H, CO2CH2), 7.58–7.62 (m, 4H, aro-matic-H), 7.66–7.69 (m, 4H, aromatic-H).
Diethyl 2,3-bis(40-bromophenyl)terephthalate (7) A mixture of 6 (5 g, 9.1 mmol) and norbornadiene (4.4 g, 36.67 mmol) were dissolved in toluene (100 mL). The solution was refluxed at 1208C for 12 h. After cooling to room temperature, the solution was concentrated in vacuo to remove the solvent and crude product was purified by recrystalliza-tion from methanol to give 3.9 g (78%) of white crystals; mp 1278C. 1 H NMR (CDCl3, d, ppm): 1.28–1.32 (t, J ¼ 7Hz, 6H, CO2CH2CH3), 4.21–4.27 (q, J ¼ 7Hz, 4H, CO2CH2), 7.11–7.15 (m, 4H, aromatic-H), 7.28–7.31 (m, 4H, aromatic-H), 8.07 (s, 2H, aromatic-H).
Diethyl 2,3-bis(40 -(9,9-dihexylfluoren-2-yl)phenyl)-terephthalate (8)
To a mixture of 7 (1.9 g, 3.57 mmol), 2-(4,4,5,5-tet- ramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexyl-fluorene (3.4 g, 7.16 mmol),20 tetrakis(triphenyl-phosphine)palladium (6.0 mg, 5.3 103 mmol), K2CO3 (2.2 g, 15.92 mmol), and Aliquat 336 (0.4 g, 0.99 mmol) was added 15 mL of anhydrous tol-uene and 4 mL of de-ionized water. The resulting
mixture was refluxed at 85 8C for 2 days under
nitrogen atmosphere. After cooling to room tem-perature, the crude product was filtered, washed with a large amount of n-hexane, and dried. It was purified by gel chromatography (silica gel, n-hexane:ethyl acetate¼ 10:1 as the eluent) to give 1.1 g (53%) of pale-yellow liquid. 1 H NMR (CDCl3, d, ppm): 0.91–0.94 (t, J ¼ 7 Hz, 12H, fluorene-(CH2)5CH3), 1.28–1.32 (t, J ¼ 7 Hz, 6H, CO2CH2CH3), 1.33–1.44 (m, 32H, fluorene-CH2(CH2)4), 2.56–2.84 (m, 8H, fluorene-CH2), 4.21–4.27 (q, J ¼ 7 Hz, 4H, CO2CH2), 6.78–6.8 (m, 4H, aromatic-H), 6.99–7.64 (m, 16H, aromatic-H), 7.91–7.92 (m, 2H, aromatic-H), 8.07 (s, 2H, aromatic-H). 1,4-Bis(hydroxymethyl)-2,3-bis(40 -(9,9-dihexylfluo-ren-2-yl)phenyl)benzene (9)
By following the synthetic procedure for 4a and using 8 as starting material, the compound 9 was obtained as white viscous liquid (90% yield).
1 H NMR (CDCl3, d, ppm): 0.91–0.94 (t, J ¼ 7 Hz, 12 H, fluorene-(CH2)5CH3), 1.23–1.44 (m, 32H, fluorene-CH2(CH2)4), 2.56–2.84 (m, 8H, fluorene-CH2), 4.95 (d, J ¼ 7.4 Hz, 4H, CH2OH), 6.816.84 (m, 4H, aromatic-H), 6.99–7.64 (m, 18H, aromatic-H), 7.97.91 (m, 2H, aromatic-H). 1,4-Bis(chloromethyl)-2,3-bis(40 -(9,9-dihexylfluo-ren-2-yl)phenyl)benzene (M4)
By following the synthetic procedure for M1 and using 9 as starting material, the compound M4 was obtained as white viscous liquid (92% yield).
1 H NMR (CDCl3, d, ppm): 0.91–0.94 (t, J ¼ 7 Hz, 12 H, fluorene-(CH2)5CH3), 1.23–1.44 (m, 32H, fluorene-CH2(CH2)4), 2.56–2.84 (m, 8H, fluo-rene-CH2), 5.07 (d, J ¼ 7.2 Hz, 4H, CH2Cl), 6.81–6.84 (m, 4H, aromatic-H), 6.99–7.64 (m, 18H, aromatic-H), 7.9–7.91 (m, 2H, aromatic-H). 1,4-Bis(chloromethyl)-2,5-dimethoxybenzene (M5) and 1,4-Bis(chloromethyl) -2-[40 -(3,7-dimethyloc-toxy)phenyl]-3-phenylbenzene (M6)
Monomers M5 and M6 were synthesized as described previously in the literatures.16,21 Synthesis of Polymers: General Procedure
Schemes 1 and 3 outline the syntheses of polymers P1–P6. An experimental procedure for the poly-mer P1 is given below. To a solution of the mono-mer M1 (2.0 g, 4.68 mmol) in THF (50 mL), a solu-tion of potassium tert-butoxide (tert-BuOK, 12 equiv) in THF (15 mL) was added. 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 (20 mL) was then added and stirred for 6 h. The polymer was obtained by pouring the mixture into methanol and filtered. It was purified by dis-solving in THF and reprecipitated from methanol twice. After drying under vacuum for 24 h, the polymer was obtained as brown solid (0.55 g, 41%). Device Fabrication and Measurements
Double-layer devices were fabricated as sandwich structures between calcium (Ca) cathodes and in-dium-tin oxide (ITO) anodes. ITO-coated glass sub-strates were cleaned sequentially in ultrasonic baths of detergent, 2-propanol/deionized water (1:1 volume) mixture, toluene, de-ionized water, and
ace-tone. A 50 nm thick hole injection layer of poly(ethy-lenedioxythiophene) (PEDOT) doped with poly(styr-enesulfonate) (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 synthe-sized polymers were spin-coated from toluene solu-tions onto the PEDOT layer and dried at 508C over-night in a vacuum. The thickness of the active layer was50 nm. Finally, 35 nm Ca and 100 nm Al elec-trodes were made through a shadow mask onto the polymer films by thermal evaporation using an AUTO 306 vacuum coater (BOC Edwards, Wilming-ton, MA). Evaporations were carried out typically at base pressures lower than 2 106torr The active area of each EL device was 4 mm2 and the device was characterized following a published protocol.22
RESULTS AND DISCUSSION
Synthesis of Polymers
Scheme 1 outlines the synthetic route for mono-mers M1–M3. Different substituents of R1and R2
were incorporated on the C5 and C6 positions of the phenylene moiety via the Diels–Alder reac-tion. The compound 4 with dihydroxy groups was obtained by the reduction of 3, following the chlorination to give monomers M1–M3. Scheme 2 outlines the synthesis of monomer M4. The syn-thetic route is similar to the Scheme 1, except the incorporation of two bulky fluorenyl groups via the Suzuki coupling reaction. Schemes 1 and 3 outline the syntheses of polymers P1–P6. The po-lymerization was carried out via a modified Gilch route to obtain soluble DP-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 often incorporated on the aromatic rings to improve the solubility of the resulting polymers. The polymer-ization condition is mild and the molecular weights of obtained polymers are relatively large. In our experiments P1–P4 are homopolymers, while P5 and P6 are copolymers obtained from copolymerizing M4 with two different monomers M5 and M6. It has been reported that incorpora-tion of M5 increased carrier mobility inside
mer layer.23 M6 was first synthesized and reported by our group. The syntheses, characteri-zation, and electro-optical properties of several DP-PPV derivatives based on M6 have been pub-lished previously.16We expect that incorporation of M5 and M6 can adjust the optical and electrical properties of the homopolymer P4.
Table 1 summarizes the feed ratio of monomers, molecular weights, and polydispersity index (PDI) of resulting polymers. The number-average
molec-ular weight Mn and weight-average molecular
weights Mw of P1 and P3 are below 104,
regard-ing as oligomers. The Mn and Mw of P2 are 8882
and 41,025, respectively; however, its PDI value is large (4.61). The polymerization results for P1– P3 are not quite satisfactory. For polymers P4– P6, the Mnvalues are in the range from 3.3 105
to 5.8 105, while the Mwvalues are in the range
from 3.6 105 to 6 105. The molecular weight distribution is relatively narrow (PDI< 1.2). The obtained polymers P4–P6 are soluble in common organic solvents, such as chloroform, toluene, and chlorobenzene. Transparent and self-standing films can be cast from their solutions. Although two fluorenyl substituents are bulky, polymers with high molecular weights can still be obtained. However, the molecular weights of polymers P1–
P3 are much smaller. We explain this as a result of multiple substituents on the phenylene moiety. Monomers M1–M3 belong to tri- or tetra-substi-tuted bis(chloromethyl)benzene, resulting in large steric hindrance. They are hindered to react with each other and hence polymers with low molecular weights and wide distribution were obtained. These materials show insufficient film-forming property that limits their potential use in thin-film devices. In contrast, monomers M4–M6 belong to disubstituted ones and the steric hindrance is smaller.
Thermal Properties
Table 2 summarizes the thermal properties of poly-mers P1–P6. Most polypoly-mers show good thermal
Figure 1. UV–vis absorption spectra of polymers P1– P6 in thin film state.
Table 1. Feed Ratio and Polymerization Results of Polymers P1–P6 Polymer n m p Yield (%) Mn Mw PDI P1 100 – 33 1728 4888 2.59 P2 100 – 41 8882 41,025 4.61 P3 100 – 36 3695 6596 1.78 P4 100 – 55 580,800 557,000 1.15 P5 90 10 – 58 556,200 600,300 1.08 P6 80 10 10 54 332,900 360,000 1.08
Table 2. Thermal and Optical Properties of Polymers P1–P6
Polymer Tg
(8C) (8C)Td
UV–vis (nm) PL (nm) Solution Film Solution Film
P1 166 330 297 303 407 384
P2 168 310 334 332 463 483
P3 160 433 389 390 500 506
P4 125 423 438 441 472 498
P5 144 421 449 452 505 535
P6 138 398 448 451 519 541 Figure 2. PL emission spectra of polymers P1–P6 in thin film state.
stabilities with high glass-transition temperatures (Tg) over 1308C and high decomposition tempera-tures (Td) over 3108C. Polymers P3–P6 show even higher Td than the others, which is attributed to high molecular weights and rigid fluorenyl group on the phenyl ring. On the other hand, P1 and P2
show lower Td owing to low molecular weights;
however, they show higher Tgas a result of tetra-substitution on the phenylene moiety, which hin-ders the segmental motion of the main chain.
Optical Properties
Figure 1 shows the UV–vis absorption spectra of polymers P1–P6 in thin film state. Table 2 summa-rizes the UV–vis absorption maxima of all poly-mers in different states. The absorption maxima of synthesized polymers in toluene are located in the range from 297 to 449 nm, which is attributed to the p–p* transition along the conjugated backbone. A small red-shift of the absorption band in thin film state is observed, which is due to the effect of
inter-chain p-stacking. P1 shows the most blue-shifted absorption band because of lowest molecular weight, referring to the shortest conjugation length among six polymers. P3 shows more red-shifted absorption band than P1 and P2, implying a longer conjugation length. This is attributable to the steric effect of less substituents on the phenylene moiety, and a smaller torsional angle between two repeat-ing units is formed. In contrast, copolymers P5 and P6 show similar absorption bands and are red-shifted than P4, owing to two electron-donating alkoxy group on M5. It is also noted that incorpora-tion of M6 does not affect the conjugaincorpora-tion length significantly.
Figure 2 reveals the PL emission spectra of poly-mers P1–P6 in thin film state. The excitation wavelengths were based on their UV–vis absorp-tion maxima in the same state. The PL emission maxima in different states are also summarized in Table 2. The maximum emission bands are located from 407 to 519 nm in solution state and from 384 to 541 nm in thin film state. Similar tendency of red-shift from solution to thin film state is observed, exclusive of P1. The emission maximum of P1 is located at 407 nm in toluene, while it is blue-shifted to 384 nm in film state. It is generally thought that red-shift in one state means some close packing of the polymer chains to extend the conjugation length. It seems that P1 in toluene has stronger p–p interaction than in film state, may be due to similar structure of solvent and polymer structure. On the other hand, the shapes of PL spectra show some fea-tures. For P1–P3, no significant vibronic band is observed, indicating no or weak chain–chain interactions. The homopolymer P4 shows a clear emission shoulder band at 530 nm, while the shoulder bands of copolymers P5 and P6 become less significant. This may be due to random struc-tures of polymer main chains, which prevent close packing of neighboring polymers.
Figure 3. Cyclic voltammograms of homopolymers P1 and P4.
Table 3. Electrochemical Properties of Polymers P1–P6 in Solid Films
Polymer Eox(V) HOMO (eV) UV edge (nm) EG (eV) LUMO (eV)
P1 1.5 5.9 376 3.3 2.6 P2 1.1 5.5 443 2.8 2.7 P3 1.2 5.6 461 2.7 3.0 P4 1.5 5.9 483 2.6 3.3 P5 1.4 5.8 517 2.4 3.4 P6 1.2 5.6 510 2.4 3.2
Electrochemical Analysis
CV was employed to investigate the electrochemical behaviors of polymers synthesized and to estimate their energy levels. The oxidation process is clear and directly associated with the conjugation struc-ture of the polymer. Figure 3 shows cyclic voltam-mograms of P1 and P4 in the oxidation process. The HOMO energy level is determined from the onset of the oxidation curve (Eox), which is given by
HOMOðeVÞ ¼ jEoxþ 4:4j
which is in the range from 5.9 to 5.5 eV. The energy gaps (EG) of materials are determined from the edge of their UV–vis absorption spectra (konset), which is given by
EGðeVÞ ¼ 1240=konset
which is in the range from 2.4 to 3.3 eV. Combin-ing the electrochemical data and UV–vis
charac-teristics give an estimate of the LUMO energy levels. Table 3 summarizes the HOMO, LUMO, and EG values of polymers P1–P6. The energy level diagram of these materials is illustrated in Figure 4. The relatively small Eoxof P2, P3, and P6 reveals that they are favored for p-doping pro-cess when compared with others. The smaller energy barrier between polymer P2 (and P3, P6) and PEDOT layer also shows the benefit for hole injection. Turning to EA value, P3 and P6 show smaller energy barriers of 0.2 eV to the cathode Ca. This also implies that electron injection is favored for these two polymers.
Device Performance
Double-layer light-emitting diodes with the con-figuration of ITO/PEDOT/Polymer/Ca/Al were fabricated to evaluate the potential use of synthe-sized DP-PPV derivatives. Among them, P1–P3 has relatively low molecular weights and could
Table 4. Device Performance of Polymers P4–P6 in ITO/PEDOT/ Polymer/Ca/Al Devices Polymer EL (nm) Vturn-on (V) Max. Brightness (cd/m2) Max. Yield (cd/A) CIE01931 x y P4 492 6 492 0.11 0.24 0.5 P5 530 5 1184 0.14 0.37 0.58 P6 524 3 1298 0.22 0.34 0.58
not provide sufficient film-forming properties. This may affect the device fabrication and per-formance evaluation. Hence, we fabricated three EL devices using P4–P6 as the active layer. The maximum EL emission bands of P4–P6 are listed in Table 4, which are located at 492, 530, and 524 nm, respectively. The CIE coordinates of the three polymers are also shown in Table 4, referring to pure green (P4) and yellowish green (P5, P6). Some extent of blue-shift of the EL emission with increasing the operation voltage is also observed. This phenomenon has been observed previously and explained as a result of charge-trapping/ defect site from some head-to-head isomers.16
Figures 5 and 6 show the brightness-voltage and efficiency-current density characteristics, respectively, of the three devices fabricated using P4–P6 as the active layer. The device using homopolymer P4 as active layer showed a maxi-mum brightness of 492 cd/m2at 10 V, and a maxi-mum current efficiency of 0.11 cd/A at 8V. Using copolymer P5 as active layer, the device showed an improved brightness of 1184 cd/m2and current efficiency of 0.14 cd/A at 10 V. The luminescence is roughly improved by incorporating 2,5-dime-thoxy-1,4-phenylene vinylene unit. The device of P6 showed even higher brightness and current ef-ficiency, which was 2-fold higher than the homo-polymer P4. The better performance of devices using P5 and P6 as active layer is explained by the smaller energy barrier between metals and emissive layer. Holes/electrons are easier to be injected into copolymers P5 and P6 than the homopolymer P4, and thus higher opportunity of charge recombination occurs (Fig. 4). Moreover, the charge mobility may be somewhat improved
by incorporating M5 and M6. The device perform-ance of these DP-PPV derivatives is summarized in Table 4. Compared with previous reports, the materials synthesized in this work showed mod-erate EL properties. Better device performance was obtained when compared with previous tri-substituted DP-PPVs, but not as high as those with one fluorenyl ring as pendant group.15,16We consider that two bulky substituents on the main chain may also hinder carrier transport and movement inside polymer layer.
CONCLUSIONS
In this work, six DP-PPV derivatives containing bulky substituents were synthesized. Highly phe-nylated DP-PPV derivatives with blue-shifted emission were obtained. However, the molecular weights are not very high. Bulky fluorenyl sub-stituents with two hexyl chains were introduced on the pendant phenyl ring to increase steric hin-drance and to prevent close packing of the main chains. The molecular weights of difluorenyl sub-stituted DP-PPVs are sufficiently high. Double-layer devices with the configuration of ITO/ PEDOT/polymer/Ca/Al were fabricated and
char-acterized. Pure green light with CIE01931 at
(0.24, 0.5) was observed for P4. Using P6 as the emissive layer, the maximum luminescence and current efficiency were both improved.
The authors thank the National Science Council (NSC) of the Republic of China (NSC 94-2120M-009-009) for financial support of this research.
Figure 5. Brightness–voltage characteristics of poly-mers P4–P6 in ITO/PEDOT/Polymer/Ca/Al devices.
Figure 6. Efficiency–current density characteristics of polymers P4–P6 in ITO/PEDOT/Polymer/Ca/Al devi-ces.
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