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Poly(1,4-bis[2-(4-hexylthiophene)]-2,5-dimethylphenylene): a new conjugated electroluminescent polymer
Synthetic Metals 105 Ž1999. 43–47
Poly ž1,4-bisw2- ž4-hexylthiophene/x-2,5-dimethylphenylene/: a new conjugated electroluminescent polymer Jian Pei a
a,b,)
, Wang-Lin Yu
a,b
, Wei Huang a , Alan J. Heeger
b
Institute of Materials Research and Engineering, National UniÕersity of Singapore, Singapore 119260, Singapore b Institute for Polymers and Organic Solids, UniÕersity of California, Santa Barbara, CA 93106-5090, USA Received 4 January 1999; received in revised form 8 March 1999; accepted 9 March 1999
Abstract We report the synthesis of the conjugated polymer, polyŽ1,4-bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene., by FeCl 3 oxidation in high yield. The polymer has perfect head-to-head ŽHH. linkages between the thiophene rings, exhibits good solubility in conventional organic solvents, and exhibits high thermal stability. The optical band gap is ; 2.76 eV. The polymer emits greenish-blue light under UV irradiation with the emission maximum at 507 nm; the photoluminescence ŽPL. efficiency Žsolid thin film. is ; 10%. The external electroluminescence efficiency was measured to be 0.0012% for single layer devices using Al as the cathode material and 0.0035% for single layer devices using Ca as the cathode material. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Conjugated polymers; Light-emitting polymers; Photoluminescence efficiency; Electroluminescence; Polymer light-emitting diodes ŽPLEDs.; Poly1,4-bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene4
1. Introduction Polythiophene and its soluble derivatives ŽPTs. occupy a special position in the development of conjugated polymers as electrically conductive and optoelectronic materials w1–3x because of their stability in both neutral and doped states, their high electrical conductivity, and their interesting electronic and optical properties w4–7x. One of the advantages of PTs is the ease of tailoring electronic and optical properties by attaching different functional groups at the 3- orrand 4-positions of the thiophene ring orrand by controlling the regularity of backbone through synthetic methodology w4–7x. Electroluminescent ŽEL. emission from blue to near infrared has been achieved from PTs by adding different sterically hindering substituents at different positions of the chain w8,9x. However, the application of PTs as EL materials in polymer lightemitting diodes ŽPLEDs. has been limited by the low photoluminescence ŽPL. efficiency. Although a PT with a
)
Corresponding author. Institute for Polymers and Organic Solids, University of California Santa Barbara, Broida Hall, Santa Barbara, CA 93106-5090, USA. Tel.: q1-805-893-3638; fax: q1-805-893-4755; e-mail:[email protected]
2,5-dioctyl-phenyl group at every 3-position ŽPDOPT. was recently reported with 24% PL efficiency w10x, almost all other PTs have PL efficiencies of a few percent as solid thin films. For example, the PL efficiency of polyw3-Ž4-octylphenyl.-2,2X-bithiophenex ŽPTOPT. Ža red to near infrared EL polymer. is 5% w10x, the PL efficiency of a water soluble polythiophene, POWT, as 3% w10,11x, and the PL efficiency of polyŽ3-cyclohexylthiophene. ŽPCHT. is about 2r3 of that of PTOPT w12x. In contrast with the low PL efficiency of the PTs, polymers based on six-membered conjugated rings such as the polyŽ para-phenylenes., PPPs, the polyŽfluorenes., PFs, and the polyŽphenylene vinylenes., PPVs, often have quite high PL efficiencies. Thus, by inserting suitable six-member conjugated rings into the PT backbone, one might expect to improve the PL efficiency. On the other hand, one of the advantages of PTs is the relative ease with which they can be synthesized by a variety of methods, including simple FeCl 3-oxidative polymerization w13x. Reynolds et al. w14x and Ruiz et al. w15x have synthesized a series of substituted polyw1,4-bisŽ2thienyl.phenylenesx as electrically conductive polymers, thereby demonstrating the possibility of synthesizing phenylene-modified PTs by the FeCl 3-oxidative method. However, the electronic and optical properties of these
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 0 7 5 - 2
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J. Pei et al.r Synthetic Metals 105 (1999) 43–47
polymers were not thoroughly investigated. Moreover, since there were no substituents on the thiophene rings, the processibility of the resultant polymers was limited. We report here the initial results of an on-going investigation of phenylene-modified PTs as EL materials; the synthesis and application in PLEDs of poly 1,4-bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene4 ŽPBHTDMP..
1.1 g Žyield 84%. of light yellow powder. 1 H NMR ŽCDCl 3 , ppm. 7.38 Ž2H, s, Ar–H., 6.99 Ž2H, s, Th–H., 2.60–2.70 Ž4H, m, CH 2 ., 2.17 Ž6H, s, Ar–CH 3 ., 1.52–1.65 Ž4H, m, CH 2 ., 1.23–1.36 Ž12H, m, CH 2 ., 0.82–0.96 Ž6H, m, CH 3 .; 13 C NMR ŽCDCl 3 , ppm. 142.34, 142.22, 133.32, 133.14, 132.42, 128.63, 128.17, 31.66, 30.80, 29.68, 29.12, 22.92, 20.84, 14.10. Anal. Calcd. for C 28 H 36 S 2 : C, 77.01; H, 8.31; S, 14.68. Found: C, 76.44; H, 8.82; S, 14.95.
2. Experimental
2.3. Characterization
2.1. Synthesis of 1,4-bis(4-hexyl-2-thienyl)-2,5-dimethylbenzene A solution of 3-hexylthiophene Ž0.03 mol. in dry tetrahydronfuran ŽTHF. was added dropwise into a solution of LDA Ž0.032 mol. in THF at y788C. After being stirred for 1 h at this temperature, the mixture was added dropwise into a solution of anhydrous zinc chloride Ž0.032 mol. in dry THF at y788C. The mixture was allowed to stir for an additional 1 h and then was added slowly to a solution of 1,4-dibromo-2,5-dimethylbenzene Ž0.01 mol. with PdŽPPh 3 .4 Ž100 mg. in THF. The reaction mixture was warmed to room temperature and stirred overnight, then heated to reflex for 10 h. The reaction was quenched by cooling down and then pouring the reaction mixture into ice-water. The aqueous solution was extracted with hexane and the combined extract was washed with water and brine. This hexane solution was dried over anhydrous MgSO4 , and evaporated to produce a yellow residue. The yellow residue was recrystallized from ethanol to give 1,4-bisŽ4-hexyl-2-thienyl.-2,5-dimethylbenzene Ž3.81 g, yield 87%. as light yellow needle crystal. 1 H NMR ŽCDCl 3 , ppm. 7.31 Ž2H, s, Ar–H., 6.93 Ž4H, s, Th–H., 2.61–2.66 Ž4H, t, J s 7.4 Hz, CH 2 ., 2.43 Ž6H, s, Ar–CH 3 ., 1.61–1.71 Ž4H, m, CH 2 ., 1.27–1.43 Ž12H, m, CH 2 ., 0.88–0.92 Ž6H, m, CH 3 .; 13 C NMR ŽCDCl 3 , ppm. 143.36, 141.54, 135.03, 134.97, 133.57, 127.86, 119.80, 31.58, 30.49, 30.35, 28.92, 22.50, 20.51, 13.96. MS ŽEI. mre: 438. 2.2. Polymerization A solution of ferric chloride Ž0.009 mol. in 150 ml of dried chloroform was added dropwise to a stirred solution of 1,4-bisŽ4-hexyl-2-thienyl.-2,5-dimethylbenzene Ž0.003 mol. in 50 ml of dried chloroform at 08C. The mixture was stirred for 20 h at this temperature with a slow dynamic flow of nitrogen. Pouring the reaction mixture into methanol precipitated a dark blue solid. After being collected by filtration, the precipitate was dedoped by stirring at concentrated ammonia solution overnight to give a yellow solid. The yellow solid was isolated by filtration and washed thoroughly with methanol and water. The obtained yellow solid was subjected to Soxhlet extraction with acetone and then extracted with chloroform. Removing the chloroform and drying under vacuum afforded
1
H NMR and 13 C NMR spectra were recorded on a Bruker AMX300 spectrometer. Samples were dissolved in CDCl 3 , and the chemical shifts were measured relative to that of tetramethylsilane ŽTMS.. Gel permeation chromatography ŽGPC. was performed on a Perkin Elmer Model 200 HPLC system using polystyrene standards and THF as eluent. Thermogravimetric analysis ŽTGA. was carried out using a DuPont Thermal Analyst 2100 system with an air or nitrogen flow at a heating rate of 208Crmin. DSC measurements were carried out on a DuPont DSC 2910 module in conjunction with above DuPont TGA system. Elemental analysis was performed by the Microanalysis Lab at the National University of Singapore on a Perkin Elmer 240C elemental analyzer. Absorption and fluorescence spectra were taken on a Shimadzu UV-1601 spectrophotometer and a Perkin Elmer LS 50B luminescence spectrometer, respectively.
3. Results and discussion The synthetic approach to the new polymer, poly 1,4bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene4 ŽPBHTDMP., is outlined in Scheme 1. The key step in this approach is the synthesis of the monomer, 1,4-bisŽ4-hexyl2-thienyl.-2,5-dimethylbenzene. The monomer must ensure that the 3-hexylthiophene will react with 2,5-dibromop-xylene at the 5-position. The 1 H NMR spectrum of the monomer shows there are only two singlet peaks at f 7.30 and 6.92 ppm, which are assigned to the protons on benzene ring and thiophene rings, respectively Žthe signals from the two protons on thiophene appear at the same field, perhaps because of the effect of 2,5-dimethylbenzene.. The triplet peak at f 2.63 ppm and the singlet peak at f 2.42 ppm arise from the methylene groups adjacent to the thiophene rings and the methyl groups on the benzene ring, respectively. All the results indicate that there is a high degree of selectivity at the 5-position of 3-hexylthiophene in the coupling reaction with 2,5-dibromobenzene, which is essential for the regioregular control in the next step of the polymerization. As synthesized, the polymer as a yellow powder that readily dissolves in conventional solvents such as chloroform, THF, dichloromethane, and xylene to give clear
J. Pei et al.r Synthetic Metals 105 (1999) 43–47
45
Scheme 1.
yellow solutions. The chemical structure of the polymer was confirmed by FTIR, 1 H NMR, 13 C NMR, and elemental analysis. The 1 H NMR spectrum is given in Fig. 1. In the 1 H NMR spectrum, only two singlet peaks Ž d 7.39 and 7.00 ppm. are observed in the aromatic region. In the 13 C NMR spectrum, there are seven clearly resolved signals in the aromatic region, which correspond to the seven different carbons in the backbone. Together with the NMR
Fig. 1. 1 H NMR spectra of PBHTDMP in CDCl 3 .
analyses of the monomer, these data indicate that the polymer has high regioregularity of the H–H linkage between thiophene rings, similar to that obtained in the polymerization of 4,4X-dialkyl-2,2X-bithiophenes by FeCl 3oxidation, which results in the defined TT–HH polyŽalkylthiophenes. w16,17x. The number averaged molecular weight Ž Mn . was measured to be 20,400 with a polydispersity of 2.9 by gel permeation chromatography ŽGPC. against polystyrene standards using THF as the eluting solvent. TGA indicates that the polymer is stable up to 3008C in air and 3508C in nitrogen. The TGA–DTA chart of the polymer, measured in air, is shown in Fig. 2. There are two obvious thermal decomposition steps. The first step, with a weight loss of 40.8%, corresponds to the degradation of the side chains; the second step signals the breakdown of the backbone. The glass transition temperature ŽTg . was determined to be 15.68C by differential scanning calorimetry ŽDSC.. Thin films on glass substrates were prepared by conventional spin-casting from solution in chloroform; the spincast film is colorless. The absorption and fluorescence spectra of the film on a quartz substrate are shown in Fig. 3. The absorption spectrum shows a structureless peak at 340 nm Ž3.65 eV. with an onset at around 450 nm Ž2.76 eV., which corresponds to the p – p U band gap. The fluorescence spectrum exhibits a peak centered at 507 nm. Upon exposing to UV light, the polymer emits greenishblue light. The maximum absorption of the polymer is blue-shifted by 40–50 nm in comparison with those of the polyŽhexylthiophene.s with regiospecific HH–TT linkages polymerized from 4,4X-dialkyl-2,2X-bithiophenes or 3,3X-dialkyl-2,2X-bithiophenes using the FeCl 3-oxidative method w16,18x. This can be explained by the large torsion angle
46
J. Pei et al.r Synthetic Metals 105 (1999) 43–47
Fig. 2. TGA–DTA chart of PBHTDMP in air. Heating rate: 208Crmin.
between the phenylene ring and thiophene ring, as has been demonstrated in polyŽ2,5-diheptyl-1,4-phenylene-alt2,5-thiophene. w19x. Since there are no TT linkages between thiophene rings in this new polymer, there will be reduced steric hindrance. Therefore, inserting phenyl rings into the backbone of polythiophene provides a useful way to increase the band gap of thiophene-based polymers. The absorption and fluorescence spectra of the polymer in CHCl 3 solutions were also measured; they exhibit maxim at 324 and 472 nm, respectively. The red shift of spectra of the polymer as films with reference to those in solutions is attributed to the larger conjugation length as a result of chain extension in the solid state. The absolute PL efficiency of the polymer as films was measured to be ; 10% Žat room temperature. using an integrating sphere and a calibrated silicon diode, about twice that measured for PTOPT w10x, indicating an improvement in PL efficiency by modifying the PT backbone with the insertion of the phenylene ring. This value is still
Fig. 3. UV-visible absorption, fluorescence spectra of PBHTDMP on quartz plate and electroluminescence spectra obtained from the LED of ITOrPBHTDMPrCa films at room temperature.
in the lower rank of PL efficiencies in conjugated EL polymers. Attempts to further increase the EL efficiency of conjugated polymers based on this backbone structure are in progress in our laboratory. Single-layer light-emitting diodes ŽLEDs. were fabricated using the new polymer as the emissive material. Thin films Ž; 100 nm. of the emissive layer were spin-cast from solution in xylene Ž2 wt.%. onto indiumrtin-oxide ŽITO.-coated glass substrates. The cathode layer Žeither aluminum 100 nm, or calcium 200 nm,. was thermally evaporated onto the polymer film. The cathode area defined the size of the emitting area as ; 0.1 cm2 . Upon the application of a forward bias, visible greenish-blue light could be seen above 17–18 V both for the Ca-devices and for the Al devices. Fig. 4 shows the I–V characteristic and light output obtained from a device with the ITOrPolymerrAl configuration. Current and light intensity all increase rapidly with bias above 18 V. Bright greenish-blue light could be seen in room light above 22 V. The EL spectrum obtained from a ITOrPolymerrCa device is shown in Fig. 3; the maximum is at 528 nm with a
Fig. 4. Current versus voltage Ž I – V . and light output of a PLED with the configuration of ITOrPBHTDMPrAl.
J. Pei et al.r Synthetic Metals 105 (1999) 43–47
shoulder at 570 nm. The peak is shifted to longer wavelengths by 21 nm with respect to the PL spectrum. A similar shift was observed recently with a copolymer of di-n-hexylfluorene and anthracene w20x. Usually the peak values of the PL and EL spectra of conjugated polymers are nearly identical. The origin of the shift is not clear. The external EL quantum efficiency for the devices with Al as the cathode material was measured to be 0.0012%, while for the devices with Ca as the cathode material, the external EL efficiency was 0.0035%. References w1x N.C. Greenham, R.H. Friend, in: H. Ehrenreich, F. Spaepen ŽEds.., Solid State Physics, Vol. 49, Academic Press, London, 1995, p. 2. w2x A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 Ž1997. 402. w3x J.L. Segura, Acta Polym. 49 Ž1998. 319. w4x G. Tourillon, in: T.J. Skotheim ŽEd.., Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986, p. 293. w5x J. Roncali, Chem. Rev. 92 Ž1992. 711. w6x J. Roncali, Chem. Rev. 97 Ž1997. 173. w7x R.D. McCullough, Adv. Mater. 10 Ž1998. 93.
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w8x M. Berggren, O. Inganas, ¨ G. Gustafsson, J. Rasmusson, M.R. Andersson, T. Hjertberg, O. Wennerstrom, ¨ Nature 372 Ž1994. 444. w9x M.R. Andersson, M. Berggren, O. Inganas, ¨ G. Gustafsson, J.C. Gustafsson-Carlberg, D. Selse, T. Hjertberg, O. Wennerstrom, ¨ Macromolecules 28 Ž1995. 7525. w10x O. Inganas, ¨ T. Granlund, M. Theander, M. Berggren, M.R. Andersson, A. Ruseckas, V. Sundstrom, ¨ Opt. Mater. 9 Ž1998. 104. w11x M.R. Andersson, P.O. Ekeblad, T. Hjerberg, O. Wennerstrom, ¨ O. Inganas, ¨ Polym. Commun. 32 Ž1991. 546. w12x M. Berggren, G. Gustafsson, O. Inganas, ¨ M.R. Andersson, O. Wennerstrom, ¨ T. Hjerberg, Adv. Mater. 6 Ž1994. 488. w13x M. Leclerc, F.M. Diaz, G. Wegner, Makromol. Chem. 190 Ž1989. 3105. w14x J.H. Reynolds, J.P. Ruiz, A.D. Child, K. Nayak, D.S. Marynick, Macromolecules 24 Ž1991. 678. w15x J.P. Ruiz, J.R. Dharia, J.R. Reynolds, Macromolecules 25 Ž1992. 849. w16x M. Zagorska, I. Kulszewicz-Bajer, A. Pron, L. Firlcj, P. Berier, M. Galtier, Synth. Met. 45 Ž1991. 385. w17x M. Zagorska, B. Krishe, Polymer 31 Ž1990. 1379. w18x R.M. Souto Maior, K. Hinkelmann, H. Eckert, F. Wudl, Macromolecules 23 Ž1990. 1268. w19x J. Birgerson, K. Kaeriyama, P. Barta, P. Broms, M. Fahlman, T. ¨ Granlund, W.R. Salaneck, Adv. Mater. 8 Ž1996. 982. w20x G. Klarner, M.H. Davey, W.-D. Chen, J.C. Scott, R.D. Miller, Adv. ¨ Mater. 10 Ž1998. 993.
Poly ž1,4-bisw2- ž4-hexylthiophene/x-2,5-dimethylphenylene/: a new conjugated electroluminescent polymer Jian Pei a
a,b,)
, Wang-Lin Yu
a,b
, Wei Huang a , Alan J. Heeger
b
Institute of Materials Research and Engineering, National UniÕersity of Singapore, Singapore 119260, Singapore b Institute for Polymers and Organic Solids, UniÕersity of California, Santa Barbara, CA 93106-5090, USA Received 4 January 1999; received in revised form 8 March 1999; accepted 9 March 1999
Abstract We report the synthesis of the conjugated polymer, polyŽ1,4-bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene., by FeCl 3 oxidation in high yield. The polymer has perfect head-to-head ŽHH. linkages between the thiophene rings, exhibits good solubility in conventional organic solvents, and exhibits high thermal stability. The optical band gap is ; 2.76 eV. The polymer emits greenish-blue light under UV irradiation with the emission maximum at 507 nm; the photoluminescence ŽPL. efficiency Žsolid thin film. is ; 10%. The external electroluminescence efficiency was measured to be 0.0012% for single layer devices using Al as the cathode material and 0.0035% for single layer devices using Ca as the cathode material. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Conjugated polymers; Light-emitting polymers; Photoluminescence efficiency; Electroluminescence; Polymer light-emitting diodes ŽPLEDs.; Poly1,4-bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene4
1. Introduction Polythiophene and its soluble derivatives ŽPTs. occupy a special position in the development of conjugated polymers as electrically conductive and optoelectronic materials w1–3x because of their stability in both neutral and doped states, their high electrical conductivity, and their interesting electronic and optical properties w4–7x. One of the advantages of PTs is the ease of tailoring electronic and optical properties by attaching different functional groups at the 3- orrand 4-positions of the thiophene ring orrand by controlling the regularity of backbone through synthetic methodology w4–7x. Electroluminescent ŽEL. emission from blue to near infrared has been achieved from PTs by adding different sterically hindering substituents at different positions of the chain w8,9x. However, the application of PTs as EL materials in polymer lightemitting diodes ŽPLEDs. has been limited by the low photoluminescence ŽPL. efficiency. Although a PT with a
)
Corresponding author. Institute for Polymers and Organic Solids, University of California Santa Barbara, Broida Hall, Santa Barbara, CA 93106-5090, USA. Tel.: q1-805-893-3638; fax: q1-805-893-4755; e-mail:[email protected]
2,5-dioctyl-phenyl group at every 3-position ŽPDOPT. was recently reported with 24% PL efficiency w10x, almost all other PTs have PL efficiencies of a few percent as solid thin films. For example, the PL efficiency of polyw3-Ž4-octylphenyl.-2,2X-bithiophenex ŽPTOPT. Ža red to near infrared EL polymer. is 5% w10x, the PL efficiency of a water soluble polythiophene, POWT, as 3% w10,11x, and the PL efficiency of polyŽ3-cyclohexylthiophene. ŽPCHT. is about 2r3 of that of PTOPT w12x. In contrast with the low PL efficiency of the PTs, polymers based on six-membered conjugated rings such as the polyŽ para-phenylenes., PPPs, the polyŽfluorenes., PFs, and the polyŽphenylene vinylenes., PPVs, often have quite high PL efficiencies. Thus, by inserting suitable six-member conjugated rings into the PT backbone, one might expect to improve the PL efficiency. On the other hand, one of the advantages of PTs is the relative ease with which they can be synthesized by a variety of methods, including simple FeCl 3-oxidative polymerization w13x. Reynolds et al. w14x and Ruiz et al. w15x have synthesized a series of substituted polyw1,4-bisŽ2thienyl.phenylenesx as electrically conductive polymers, thereby demonstrating the possibility of synthesizing phenylene-modified PTs by the FeCl 3-oxidative method. However, the electronic and optical properties of these
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 0 7 5 - 2
44
J. Pei et al.r Synthetic Metals 105 (1999) 43–47
polymers were not thoroughly investigated. Moreover, since there were no substituents on the thiophene rings, the processibility of the resultant polymers was limited. We report here the initial results of an on-going investigation of phenylene-modified PTs as EL materials; the synthesis and application in PLEDs of poly 1,4-bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene4 ŽPBHTDMP..
1.1 g Žyield 84%. of light yellow powder. 1 H NMR ŽCDCl 3 , ppm. 7.38 Ž2H, s, Ar–H., 6.99 Ž2H, s, Th–H., 2.60–2.70 Ž4H, m, CH 2 ., 2.17 Ž6H, s, Ar–CH 3 ., 1.52–1.65 Ž4H, m, CH 2 ., 1.23–1.36 Ž12H, m, CH 2 ., 0.82–0.96 Ž6H, m, CH 3 .; 13 C NMR ŽCDCl 3 , ppm. 142.34, 142.22, 133.32, 133.14, 132.42, 128.63, 128.17, 31.66, 30.80, 29.68, 29.12, 22.92, 20.84, 14.10. Anal. Calcd. for C 28 H 36 S 2 : C, 77.01; H, 8.31; S, 14.68. Found: C, 76.44; H, 8.82; S, 14.95.
2. Experimental
2.3. Characterization
2.1. Synthesis of 1,4-bis(4-hexyl-2-thienyl)-2,5-dimethylbenzene A solution of 3-hexylthiophene Ž0.03 mol. in dry tetrahydronfuran ŽTHF. was added dropwise into a solution of LDA Ž0.032 mol. in THF at y788C. After being stirred for 1 h at this temperature, the mixture was added dropwise into a solution of anhydrous zinc chloride Ž0.032 mol. in dry THF at y788C. The mixture was allowed to stir for an additional 1 h and then was added slowly to a solution of 1,4-dibromo-2,5-dimethylbenzene Ž0.01 mol. with PdŽPPh 3 .4 Ž100 mg. in THF. The reaction mixture was warmed to room temperature and stirred overnight, then heated to reflex for 10 h. The reaction was quenched by cooling down and then pouring the reaction mixture into ice-water. The aqueous solution was extracted with hexane and the combined extract was washed with water and brine. This hexane solution was dried over anhydrous MgSO4 , and evaporated to produce a yellow residue. The yellow residue was recrystallized from ethanol to give 1,4-bisŽ4-hexyl-2-thienyl.-2,5-dimethylbenzene Ž3.81 g, yield 87%. as light yellow needle crystal. 1 H NMR ŽCDCl 3 , ppm. 7.31 Ž2H, s, Ar–H., 6.93 Ž4H, s, Th–H., 2.61–2.66 Ž4H, t, J s 7.4 Hz, CH 2 ., 2.43 Ž6H, s, Ar–CH 3 ., 1.61–1.71 Ž4H, m, CH 2 ., 1.27–1.43 Ž12H, m, CH 2 ., 0.88–0.92 Ž6H, m, CH 3 .; 13 C NMR ŽCDCl 3 , ppm. 143.36, 141.54, 135.03, 134.97, 133.57, 127.86, 119.80, 31.58, 30.49, 30.35, 28.92, 22.50, 20.51, 13.96. MS ŽEI. mre: 438. 2.2. Polymerization A solution of ferric chloride Ž0.009 mol. in 150 ml of dried chloroform was added dropwise to a stirred solution of 1,4-bisŽ4-hexyl-2-thienyl.-2,5-dimethylbenzene Ž0.003 mol. in 50 ml of dried chloroform at 08C. The mixture was stirred for 20 h at this temperature with a slow dynamic flow of nitrogen. Pouring the reaction mixture into methanol precipitated a dark blue solid. After being collected by filtration, the precipitate was dedoped by stirring at concentrated ammonia solution overnight to give a yellow solid. The yellow solid was isolated by filtration and washed thoroughly with methanol and water. The obtained yellow solid was subjected to Soxhlet extraction with acetone and then extracted with chloroform. Removing the chloroform and drying under vacuum afforded
1
H NMR and 13 C NMR spectra were recorded on a Bruker AMX300 spectrometer. Samples were dissolved in CDCl 3 , and the chemical shifts were measured relative to that of tetramethylsilane ŽTMS.. Gel permeation chromatography ŽGPC. was performed on a Perkin Elmer Model 200 HPLC system using polystyrene standards and THF as eluent. Thermogravimetric analysis ŽTGA. was carried out using a DuPont Thermal Analyst 2100 system with an air or nitrogen flow at a heating rate of 208Crmin. DSC measurements were carried out on a DuPont DSC 2910 module in conjunction with above DuPont TGA system. Elemental analysis was performed by the Microanalysis Lab at the National University of Singapore on a Perkin Elmer 240C elemental analyzer. Absorption and fluorescence spectra were taken on a Shimadzu UV-1601 spectrophotometer and a Perkin Elmer LS 50B luminescence spectrometer, respectively.
3. Results and discussion The synthetic approach to the new polymer, poly 1,4bisw2-Ž4-hexylthiophene.x-2,5-dimethylphenylene4 ŽPBHTDMP., is outlined in Scheme 1. The key step in this approach is the synthesis of the monomer, 1,4-bisŽ4-hexyl2-thienyl.-2,5-dimethylbenzene. The monomer must ensure that the 3-hexylthiophene will react with 2,5-dibromop-xylene at the 5-position. The 1 H NMR spectrum of the monomer shows there are only two singlet peaks at f 7.30 and 6.92 ppm, which are assigned to the protons on benzene ring and thiophene rings, respectively Žthe signals from the two protons on thiophene appear at the same field, perhaps because of the effect of 2,5-dimethylbenzene.. The triplet peak at f 2.63 ppm and the singlet peak at f 2.42 ppm arise from the methylene groups adjacent to the thiophene rings and the methyl groups on the benzene ring, respectively. All the results indicate that there is a high degree of selectivity at the 5-position of 3-hexylthiophene in the coupling reaction with 2,5-dibromobenzene, which is essential for the regioregular control in the next step of the polymerization. As synthesized, the polymer as a yellow powder that readily dissolves in conventional solvents such as chloroform, THF, dichloromethane, and xylene to give clear
J. Pei et al.r Synthetic Metals 105 (1999) 43–47
45
Scheme 1.
yellow solutions. The chemical structure of the polymer was confirmed by FTIR, 1 H NMR, 13 C NMR, and elemental analysis. The 1 H NMR spectrum is given in Fig. 1. In the 1 H NMR spectrum, only two singlet peaks Ž d 7.39 and 7.00 ppm. are observed in the aromatic region. In the 13 C NMR spectrum, there are seven clearly resolved signals in the aromatic region, which correspond to the seven different carbons in the backbone. Together with the NMR
Fig. 1. 1 H NMR spectra of PBHTDMP in CDCl 3 .
analyses of the monomer, these data indicate that the polymer has high regioregularity of the H–H linkage between thiophene rings, similar to that obtained in the polymerization of 4,4X-dialkyl-2,2X-bithiophenes by FeCl 3oxidation, which results in the defined TT–HH polyŽalkylthiophenes. w16,17x. The number averaged molecular weight Ž Mn . was measured to be 20,400 with a polydispersity of 2.9 by gel permeation chromatography ŽGPC. against polystyrene standards using THF as the eluting solvent. TGA indicates that the polymer is stable up to 3008C in air and 3508C in nitrogen. The TGA–DTA chart of the polymer, measured in air, is shown in Fig. 2. There are two obvious thermal decomposition steps. The first step, with a weight loss of 40.8%, corresponds to the degradation of the side chains; the second step signals the breakdown of the backbone. The glass transition temperature ŽTg . was determined to be 15.68C by differential scanning calorimetry ŽDSC.. Thin films on glass substrates were prepared by conventional spin-casting from solution in chloroform; the spincast film is colorless. The absorption and fluorescence spectra of the film on a quartz substrate are shown in Fig. 3. The absorption spectrum shows a structureless peak at 340 nm Ž3.65 eV. with an onset at around 450 nm Ž2.76 eV., which corresponds to the p – p U band gap. The fluorescence spectrum exhibits a peak centered at 507 nm. Upon exposing to UV light, the polymer emits greenishblue light. The maximum absorption of the polymer is blue-shifted by 40–50 nm in comparison with those of the polyŽhexylthiophene.s with regiospecific HH–TT linkages polymerized from 4,4X-dialkyl-2,2X-bithiophenes or 3,3X-dialkyl-2,2X-bithiophenes using the FeCl 3-oxidative method w16,18x. This can be explained by the large torsion angle
46
J. Pei et al.r Synthetic Metals 105 (1999) 43–47
Fig. 2. TGA–DTA chart of PBHTDMP in air. Heating rate: 208Crmin.
between the phenylene ring and thiophene ring, as has been demonstrated in polyŽ2,5-diheptyl-1,4-phenylene-alt2,5-thiophene. w19x. Since there are no TT linkages between thiophene rings in this new polymer, there will be reduced steric hindrance. Therefore, inserting phenyl rings into the backbone of polythiophene provides a useful way to increase the band gap of thiophene-based polymers. The absorption and fluorescence spectra of the polymer in CHCl 3 solutions were also measured; they exhibit maxim at 324 and 472 nm, respectively. The red shift of spectra of the polymer as films with reference to those in solutions is attributed to the larger conjugation length as a result of chain extension in the solid state. The absolute PL efficiency of the polymer as films was measured to be ; 10% Žat room temperature. using an integrating sphere and a calibrated silicon diode, about twice that measured for PTOPT w10x, indicating an improvement in PL efficiency by modifying the PT backbone with the insertion of the phenylene ring. This value is still
Fig. 3. UV-visible absorption, fluorescence spectra of PBHTDMP on quartz plate and electroluminescence spectra obtained from the LED of ITOrPBHTDMPrCa films at room temperature.
in the lower rank of PL efficiencies in conjugated EL polymers. Attempts to further increase the EL efficiency of conjugated polymers based on this backbone structure are in progress in our laboratory. Single-layer light-emitting diodes ŽLEDs. were fabricated using the new polymer as the emissive material. Thin films Ž; 100 nm. of the emissive layer were spin-cast from solution in xylene Ž2 wt.%. onto indiumrtin-oxide ŽITO.-coated glass substrates. The cathode layer Žeither aluminum 100 nm, or calcium 200 nm,. was thermally evaporated onto the polymer film. The cathode area defined the size of the emitting area as ; 0.1 cm2 . Upon the application of a forward bias, visible greenish-blue light could be seen above 17–18 V both for the Ca-devices and for the Al devices. Fig. 4 shows the I–V characteristic and light output obtained from a device with the ITOrPolymerrAl configuration. Current and light intensity all increase rapidly with bias above 18 V. Bright greenish-blue light could be seen in room light above 22 V. The EL spectrum obtained from a ITOrPolymerrCa device is shown in Fig. 3; the maximum is at 528 nm with a
Fig. 4. Current versus voltage Ž I – V . and light output of a PLED with the configuration of ITOrPBHTDMPrAl.
J. Pei et al.r Synthetic Metals 105 (1999) 43–47
shoulder at 570 nm. The peak is shifted to longer wavelengths by 21 nm with respect to the PL spectrum. A similar shift was observed recently with a copolymer of di-n-hexylfluorene and anthracene w20x. Usually the peak values of the PL and EL spectra of conjugated polymers are nearly identical. The origin of the shift is not clear. The external EL quantum efficiency for the devices with Al as the cathode material was measured to be 0.0012%, while for the devices with Ca as the cathode material, the external EL efficiency was 0.0035%. References w1x N.C. Greenham, R.H. Friend, in: H. Ehrenreich, F. Spaepen ŽEds.., Solid State Physics, Vol. 49, Academic Press, London, 1995, p. 2. w2x A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 Ž1997. 402. w3x J.L. Segura, Acta Polym. 49 Ž1998. 319. w4x G. Tourillon, in: T.J. Skotheim ŽEd.., Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986, p. 293. w5x J. Roncali, Chem. Rev. 92 Ž1992. 711. w6x J. Roncali, Chem. Rev. 97 Ž1997. 173. w7x R.D. McCullough, Adv. Mater. 10 Ž1998. 93.
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