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Design and synthesis of poly(p-phenylenevinylene) derivative with triphenylamine segments on polymer backbone
Synthetic Metals 110 Ž2000. 203–205 www.elsevier.comrlocatersynmet
Design and synthesis of poly žp-phenylenevinylene/ derivative with triphenylamine segments on polymer backbone Minzhao Xue ) , Deyin Huang, Yangang Liu School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniÕersity, 800 Dongchuan Road, Shanghai 200240, China Received 28 June 1999; received in revised form 13 September 1999; accepted 27 October 1999
Abstract A new approach to the synthesis of conjugated polymer with triphenylamine ŽTPA. segments was presented. Electrochemical reversibility and thermal stability were found for this polymer. The introduction of TPA segments to the polymer backbone has modified its optical and electrochemical properties. The estimated highest occupied molecular orbital ŽHOMO. of 5.4 eV from electrochemical measurement suggested that it could be a low hole injection barrier when used as the active layer in the polymer LED. High brightness emission and improved efficiency were obtained in a double-layer device. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Synthesis; Triphenylamine derivatives; PolyŽ p-phenylenevinylene.; Electroluminescent; Electron energy level
1. Introduction Having been reported that conjugated polymers might be the active, light emitting layer in LEDs w1x, a growing interest has developed in this field. Although a great deal of polymers have been synthesized and investigated w2–5x, the search for new polymers with high performance remains to be a big challenge in this area. While conjugated organic polymers or polymers containing fluorescent chromophores, such as polyŽalkylthiophene., polyphenylenes ŽPPP. and polyŽphenylenevinylene. ŽPPV., have been found to possess interesting luminescence, PPV and its derivatives remain the most studied luminescent polymer system, owing to its high luminescence and various properties available by changing substituents and effective conjugation lengths. In order to increase the elctroluminescent quantum efficiency for luminescent polymers, enhancement of charge injection efficiency and luminescent efficiency by the introduction of functional chromophores onto polymer backbone has proved to be an effective way w6,7x. N, N X-diphenyl-N, N X-bisŽ3-methylphenyl.-1,1Xbiphenyl-4,4X-diamine ŽTPD. was reported to be an excellent hole-transport material for use in organic electroluminescence ŽEL. devices w8x. Furthermore, a series of bis-Ždiphenylamino.diphenylpolyenes with up to eight double bonds has been found to possess interesting photonic and )
Corresponding author. Tel.: q86-21-54743270; fax: q86-2154743270; e-mail: [email protected]
electronic properties w9x. More recently, the enhanced properties of light-emitting diodes by using a polyŽarylene vinylene. derivative comprising triphenylamine ŽTPA. as the arylene unite ŽTPA–PPV. as the active layer was reported w10x. In an aim to further fine-tune the photonic and electronic properties of luminescent polymer, report herein is our preliminary approach toward the synthesis and properties investigation of polymer with TPA derivative as the conjugated polymer backbone.
2. Experimental 2.1. Synthesis All reagents were purified by redistillation or recrystallization before use. The synthetic routes are illustrated in Scheme 1. Each of the intermediates was identified by MS, 1 H-NMR and FT-IR spectrum. 2.1.1. Triphenylamine (1a) and 4-methyltriphenylamine (1b) The TPAs were synthesized by Ullmann condensation of the corresponding diphenylamine derivatives and iodobenzene. 1a: yield 85%, m.p.: 125–1268C. 1b: yield 80%, m.p.: 69–708C, MS: 258 ŽMq 100., 243Ž54., 180Ž47., 1 H-NMR: 2.28Žs, 3H., 6.9–7.2Žm, 14H., FT-IR Žcmy1 .: 3000, 2850, 1600, 1500, 1290, 820, 770.
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 8 1 - 7
M. Xue et al.r Synthetic Metals 110 (2000) 203–205
204
between emission layer and cathode. The thickness of polymer and PBD was 100–200 nm.
3. Results and discussion
Scheme 1. Illustration of synthetic routes.
2.1.2. 4,4X-Diformyl-triphenylamine (2a) and 4,4X-diformyl4Y-methyl-triphenylamine (2b) 2a and 2b were synthesized by means of the Vilsmeier reaction with their corresponding aromatic amines. 2b: yield 80%, m.p.: 1468C, FAB-MS: 315, 1 H-NMR: 2.28Žs, 3H., 6.9–7.8Žm, 12H., 9.80Ž2H.. FT-IR Žcmy1 .: 2850, 1690, 1600, 1500, 1325, 1290, 1220, 1180, 845. 2.1.3. Polymerization Polymer was obtained by the polycondensation of the dialdehyde 2b and the 1,4-xylylene-bisŽtriphenylphosphonium bromide.: 2b, 1,4-xylylene-bisŽtriphenylphosphonium bromide. and NaH were stirred in THF at 5–108C for 4 h. The polymer was purified by redissolving in chloroform and precipitation in methanol three times, washed with methanol and dried in vacuum. 2.2. Characterization and measurements Polymer was characterized with a Varian WM-300 H-NMR instrument and a Perkin-Elmer PRA-1000 FT-IR spectrophotometer. Gel permeation chromatography ŽGPC. measurement was performed on a Perkin-Elmer instrument by using THF as solvent and a PL column, calibrated with polystyrene standards. UV–VIS spectra were recorded on a HP-7530 spectrophotometer. Differential scanning calorimetry ŽDSC. and thermal gravimetric analysis ŽTGA. for the polymer were carried out under nitrogen using the Perkin-Elmer instruments ŽPYRIS1 and TGA-7.. Cyclic voltammograms of the polymer film were recorded in a typical three-electrode cell with a working electrode Žindium–tin oxide coated on glass., a AgrAgq reference electrode, and a counter electrode ŽPt gauze. under a nitrogen atmosphere at a sweeping rate of 50 mVrs. A solution of tetrabutylammonium perchlorate ŽTBAP. in acetonitrile Ž1 M. was used as an electrolyte. Single-layer device was fabricated with an ITO glass anode and a MgrAg Ž9:1. alloy cathode. For the doublelayer devices, an oxadiazole derivative PBD, which served as an electron injectionrtransport layer, was sandwiched 1
On an earlier attempt, we have prepared 2a by Wilson’s procedure w11x and found that this procedure provided a mixture of mono-, bis-, tri-formyl-substituted products and low molecular weight polymers; the yields of 2a were quite low Ž30–50%.. In order to synthesize bis-formyl-triphenylamine derivatives in high yield, we firstly used 1b, which has two functionalities, as starting material. Then modified Wilson’s procedure by varying the molecule ratio of TPA derivatives and DMF. These efforts have prevented the condensation reaction of TPA derivatives with carbonyl compounds w12x and the generation of triformyl-triphenylamine derivatives, thus, dramatically increased the yield of 2b to 80%. The procedure explored by us was similar to that reported recently w13x. Polycondensation of the corresponding comonomers gave polymer 3 ŽScheme 1.. The polymer Ž3. was an orange red powder and was soluble in conventional solvents, such as THF, toluene and chloroform. FT-IR spectrum showed a drastic decrease in the intensity of the aldehyde carbonyl stretching band and the appearance of olefinic C–H stretch at 3050 and 974 cmy1 as compared with the dialdehyde comonomer. The molecule structure of polymer was also confirmed from the 1 H-NMR spectra. We stress that the molecule structure of the novel polymers has been successfully designed and synthesized. The molecule weight of polymer 3, as determined by GPC, was Mw s 31248, MwrMns 2.1. Measurement of DSC and TGA showed that the polymer was stable up to 2508C, and then started to degrade after 2608C due to thermal oxidation. DSC measurement revealed a glass transition temperature of 2208C. Fig. 1 showed the UV–VIS and photoluminescence spectrum of the polymer; this polymer has a maximum absorption at 450 nm. This implies a lower emission
Fig. 1. UV–Vis ŽP P P P P P., PL Ž- - - - - -. spectra of polymer and the EL Ž . of a device.
M. Xue et al.r Synthetic Metals 110 (2000) 203–205
Fig. 2. A typical CV curve of polymer 3 Žanodically swept, 50 mVrs..
wavelength of EL device. The optical Eg, calculated from the onset of its UV–VIS spectrum, was 2.75 eV. In order to investigate the charge injection properties, cyclic voltammetry of spin coated film were recorded. When swept anodically, the polymer was found to present reversible oxidation peaks at relatively low potential Žonset of 0.6 V. compared with that of PPV Žonset of 0.85 V.. This suggested enhancement of a hole injection for the polymer. The reduction potential turned out to be beyond the instrument limit Žy2.0 V.. The oxidation process corresponds to the removal of charge from the highest occupied molecular orbital ŽHOMO. band, whereas the reduction cycle corresponds to the filling of the energy state by electrons to the lowest unoccupied molecular orbital ŽLUMO. band. Therefore, the onset oxidation and reduction potentials were closely related to the energies of the HOMO and LUMO levels of an organic molecule and thus can provide important information regarding the magnitude of the energy gap. The energy values were calculated using the ferrocence ŽFOC. value of y4.8 eV with respect to the vacuum level which has been defined as zero w14x. According to the redox peak position and referenced to the ITO energy level Žy4.8 eV.. The energy levels of LUMO, HOMO and Eg were estimated to be y5.4 eV, y2.85 eV and y2.55 eV, respectively. A typical anodical sweep curve of polymer film was presented in Fig. 2. Blue light emitting was obtain in single-layer device, current density increased as the voltage increased and a maximum current density of 140 mArcmy2 was obtained at 20 V. The efficiency of single-layer device was lower than that of double-layer device. This result would be explained by the existence of hole injection barrier at anoderpolymer interface and the high hole drift mobility
205
of polymer w10x. Double-layer device was fabricated with the introduction of PBD electron injectionrtransport layer, bright orange light-emission was observed under forward bias voltage of 7 V, the durability, brightness and the efficiency were improved. Fig. 1 also showed the EL spectra of double-layer device. The shape of the EL spectrum was almost the same as that of PL spectrum; it is clear that charge recombination and emission occurred on the designed polymer. Studies on optimizing the fabrication processes and devices performances based the polymer will be reported in a different publication.
4. Conclusion The conjugated polymer with triphenylamine ŽTPA. segment was successfully synthesized. Electrochemical reversibility and thermal stability were found in this polymer. The introduction of TPA segment to the polymer backbone has modified the optical and electrochemical properties, which implied it would be a new emission and lower hole injection barrier material when used as the active layer in the polymer LED.
References w1x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 Ž1990. 539. w2x D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 Ž1991. 1982. w3x G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colanen, A.J. Heeger, Nature 357 Ž1992. 477. w4x M. Berggren, O. Inganas, ¨ G. Gustafsson, J. Rasmusson, M.R. Anderson, T. Hjertberg, O. Wennerstrom, Nature 372 Ž1994. 444. w5x G.M.V. Germ, F. Meghdadi, C. Paa, J. Stampfl, S. Tasch, G. Leising, Synth. Met. 71 Ž1995. 2193. w6x S.C. Moratti, R. Cervini, A.B. Holmes, D.R. Baigent, R.H. Friend, N.C. Greenham, J. Gruner, P.J. Hamer, Synth. Met. 71 Ž1995. 2117. ¨ w7x A. Lux, A.B. Holmes, R. Cervini, J.E. Davies, S.C. Moratti, J. Gruner, F. Cacilli, R.H. Friend, Synth. Met. 84 Ž1997. 293. ¨ w8x C. Adachi, K. Nagai, N. Tamoto, Appl. Phys. Lett. 66 Ž1995. 2679. w9x E.H. Elandaloussi, C.W. Spangler, Polym. Prepr. 39 Ž1998. 1055. w10x G. Yu, Y. Liu, X. Wu, M. Zhang, F. Bai, D. Zhu, Appl. Phys. Lett. 74 Ž1999. 2295. w11x C.D. Wilson, U.S. Patent 2558285, 1951. w12x J.-M. Son, K. Ogino, N. Yonezawa, H. Sato, Synth. Met. 98 Ž1998. 71.
Design and synthesis of poly žp-phenylenevinylene/ derivative with triphenylamine segments on polymer backbone Minzhao Xue ) , Deyin Huang, Yangang Liu School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniÕersity, 800 Dongchuan Road, Shanghai 200240, China Received 28 June 1999; received in revised form 13 September 1999; accepted 27 October 1999
Abstract A new approach to the synthesis of conjugated polymer with triphenylamine ŽTPA. segments was presented. Electrochemical reversibility and thermal stability were found for this polymer. The introduction of TPA segments to the polymer backbone has modified its optical and electrochemical properties. The estimated highest occupied molecular orbital ŽHOMO. of 5.4 eV from electrochemical measurement suggested that it could be a low hole injection barrier when used as the active layer in the polymer LED. High brightness emission and improved efficiency were obtained in a double-layer device. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Synthesis; Triphenylamine derivatives; PolyŽ p-phenylenevinylene.; Electroluminescent; Electron energy level
1. Introduction Having been reported that conjugated polymers might be the active, light emitting layer in LEDs w1x, a growing interest has developed in this field. Although a great deal of polymers have been synthesized and investigated w2–5x, the search for new polymers with high performance remains to be a big challenge in this area. While conjugated organic polymers or polymers containing fluorescent chromophores, such as polyŽalkylthiophene., polyphenylenes ŽPPP. and polyŽphenylenevinylene. ŽPPV., have been found to possess interesting luminescence, PPV and its derivatives remain the most studied luminescent polymer system, owing to its high luminescence and various properties available by changing substituents and effective conjugation lengths. In order to increase the elctroluminescent quantum efficiency for luminescent polymers, enhancement of charge injection efficiency and luminescent efficiency by the introduction of functional chromophores onto polymer backbone has proved to be an effective way w6,7x. N, N X-diphenyl-N, N X-bisŽ3-methylphenyl.-1,1Xbiphenyl-4,4X-diamine ŽTPD. was reported to be an excellent hole-transport material for use in organic electroluminescence ŽEL. devices w8x. Furthermore, a series of bis-Ždiphenylamino.diphenylpolyenes with up to eight double bonds has been found to possess interesting photonic and )
Corresponding author. Tel.: q86-21-54743270; fax: q86-2154743270; e-mail: [email protected]
electronic properties w9x. More recently, the enhanced properties of light-emitting diodes by using a polyŽarylene vinylene. derivative comprising triphenylamine ŽTPA. as the arylene unite ŽTPA–PPV. as the active layer was reported w10x. In an aim to further fine-tune the photonic and electronic properties of luminescent polymer, report herein is our preliminary approach toward the synthesis and properties investigation of polymer with TPA derivative as the conjugated polymer backbone.
2. Experimental 2.1. Synthesis All reagents were purified by redistillation or recrystallization before use. The synthetic routes are illustrated in Scheme 1. Each of the intermediates was identified by MS, 1 H-NMR and FT-IR spectrum. 2.1.1. Triphenylamine (1a) and 4-methyltriphenylamine (1b) The TPAs were synthesized by Ullmann condensation of the corresponding diphenylamine derivatives and iodobenzene. 1a: yield 85%, m.p.: 125–1268C. 1b: yield 80%, m.p.: 69–708C, MS: 258 ŽMq 100., 243Ž54., 180Ž47., 1 H-NMR: 2.28Žs, 3H., 6.9–7.2Žm, 14H., FT-IR Žcmy1 .: 3000, 2850, 1600, 1500, 1290, 820, 770.
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 8 1 - 7
M. Xue et al.r Synthetic Metals 110 (2000) 203–205
204
between emission layer and cathode. The thickness of polymer and PBD was 100–200 nm.
3. Results and discussion
Scheme 1. Illustration of synthetic routes.
2.1.2. 4,4X-Diformyl-triphenylamine (2a) and 4,4X-diformyl4Y-methyl-triphenylamine (2b) 2a and 2b were synthesized by means of the Vilsmeier reaction with their corresponding aromatic amines. 2b: yield 80%, m.p.: 1468C, FAB-MS: 315, 1 H-NMR: 2.28Žs, 3H., 6.9–7.8Žm, 12H., 9.80Ž2H.. FT-IR Žcmy1 .: 2850, 1690, 1600, 1500, 1325, 1290, 1220, 1180, 845. 2.1.3. Polymerization Polymer was obtained by the polycondensation of the dialdehyde 2b and the 1,4-xylylene-bisŽtriphenylphosphonium bromide.: 2b, 1,4-xylylene-bisŽtriphenylphosphonium bromide. and NaH were stirred in THF at 5–108C for 4 h. The polymer was purified by redissolving in chloroform and precipitation in methanol three times, washed with methanol and dried in vacuum. 2.2. Characterization and measurements Polymer was characterized with a Varian WM-300 H-NMR instrument and a Perkin-Elmer PRA-1000 FT-IR spectrophotometer. Gel permeation chromatography ŽGPC. measurement was performed on a Perkin-Elmer instrument by using THF as solvent and a PL column, calibrated with polystyrene standards. UV–VIS spectra were recorded on a HP-7530 spectrophotometer. Differential scanning calorimetry ŽDSC. and thermal gravimetric analysis ŽTGA. for the polymer were carried out under nitrogen using the Perkin-Elmer instruments ŽPYRIS1 and TGA-7.. Cyclic voltammograms of the polymer film were recorded in a typical three-electrode cell with a working electrode Žindium–tin oxide coated on glass., a AgrAgq reference electrode, and a counter electrode ŽPt gauze. under a nitrogen atmosphere at a sweeping rate of 50 mVrs. A solution of tetrabutylammonium perchlorate ŽTBAP. in acetonitrile Ž1 M. was used as an electrolyte. Single-layer device was fabricated with an ITO glass anode and a MgrAg Ž9:1. alloy cathode. For the doublelayer devices, an oxadiazole derivative PBD, which served as an electron injectionrtransport layer, was sandwiched 1
On an earlier attempt, we have prepared 2a by Wilson’s procedure w11x and found that this procedure provided a mixture of mono-, bis-, tri-formyl-substituted products and low molecular weight polymers; the yields of 2a were quite low Ž30–50%.. In order to synthesize bis-formyl-triphenylamine derivatives in high yield, we firstly used 1b, which has two functionalities, as starting material. Then modified Wilson’s procedure by varying the molecule ratio of TPA derivatives and DMF. These efforts have prevented the condensation reaction of TPA derivatives with carbonyl compounds w12x and the generation of triformyl-triphenylamine derivatives, thus, dramatically increased the yield of 2b to 80%. The procedure explored by us was similar to that reported recently w13x. Polycondensation of the corresponding comonomers gave polymer 3 ŽScheme 1.. The polymer Ž3. was an orange red powder and was soluble in conventional solvents, such as THF, toluene and chloroform. FT-IR spectrum showed a drastic decrease in the intensity of the aldehyde carbonyl stretching band and the appearance of olefinic C–H stretch at 3050 and 974 cmy1 as compared with the dialdehyde comonomer. The molecule structure of polymer was also confirmed from the 1 H-NMR spectra. We stress that the molecule structure of the novel polymers has been successfully designed and synthesized. The molecule weight of polymer 3, as determined by GPC, was Mw s 31248, MwrMns 2.1. Measurement of DSC and TGA showed that the polymer was stable up to 2508C, and then started to degrade after 2608C due to thermal oxidation. DSC measurement revealed a glass transition temperature of 2208C. Fig. 1 showed the UV–VIS and photoluminescence spectrum of the polymer; this polymer has a maximum absorption at 450 nm. This implies a lower emission
Fig. 1. UV–Vis ŽP P P P P P., PL Ž- - - - - -. spectra of polymer and the EL Ž . of a device.
M. Xue et al.r Synthetic Metals 110 (2000) 203–205
Fig. 2. A typical CV curve of polymer 3 Žanodically swept, 50 mVrs..
wavelength of EL device. The optical Eg, calculated from the onset of its UV–VIS spectrum, was 2.75 eV. In order to investigate the charge injection properties, cyclic voltammetry of spin coated film were recorded. When swept anodically, the polymer was found to present reversible oxidation peaks at relatively low potential Žonset of 0.6 V. compared with that of PPV Žonset of 0.85 V.. This suggested enhancement of a hole injection for the polymer. The reduction potential turned out to be beyond the instrument limit Žy2.0 V.. The oxidation process corresponds to the removal of charge from the highest occupied molecular orbital ŽHOMO. band, whereas the reduction cycle corresponds to the filling of the energy state by electrons to the lowest unoccupied molecular orbital ŽLUMO. band. Therefore, the onset oxidation and reduction potentials were closely related to the energies of the HOMO and LUMO levels of an organic molecule and thus can provide important information regarding the magnitude of the energy gap. The energy values were calculated using the ferrocence ŽFOC. value of y4.8 eV with respect to the vacuum level which has been defined as zero w14x. According to the redox peak position and referenced to the ITO energy level Žy4.8 eV.. The energy levels of LUMO, HOMO and Eg were estimated to be y5.4 eV, y2.85 eV and y2.55 eV, respectively. A typical anodical sweep curve of polymer film was presented in Fig. 2. Blue light emitting was obtain in single-layer device, current density increased as the voltage increased and a maximum current density of 140 mArcmy2 was obtained at 20 V. The efficiency of single-layer device was lower than that of double-layer device. This result would be explained by the existence of hole injection barrier at anoderpolymer interface and the high hole drift mobility
205
of polymer w10x. Double-layer device was fabricated with the introduction of PBD electron injectionrtransport layer, bright orange light-emission was observed under forward bias voltage of 7 V, the durability, brightness and the efficiency were improved. Fig. 1 also showed the EL spectra of double-layer device. The shape of the EL spectrum was almost the same as that of PL spectrum; it is clear that charge recombination and emission occurred on the designed polymer. Studies on optimizing the fabrication processes and devices performances based the polymer will be reported in a different publication.
4. Conclusion The conjugated polymer with triphenylamine ŽTPA. segment was successfully synthesized. Electrochemical reversibility and thermal stability were found in this polymer. The introduction of TPA segment to the polymer backbone has modified the optical and electrochemical properties, which implied it would be a new emission and lower hole injection barrier material when used as the active layer in the polymer LED.
References w1x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 Ž1990. 539. w2x D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 Ž1991. 1982. w3x G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colanen, A.J. Heeger, Nature 357 Ž1992. 477. w4x M. Berggren, O. Inganas, ¨ G. Gustafsson, J. Rasmusson, M.R. Anderson, T. Hjertberg, O. Wennerstrom, Nature 372 Ž1994. 444. w5x G.M.V. Germ, F. Meghdadi, C. Paa, J. Stampfl, S. Tasch, G. Leising, Synth. Met. 71 Ž1995. 2193. w6x S.C. Moratti, R. Cervini, A.B. Holmes, D.R. Baigent, R.H. Friend, N.C. Greenham, J. Gruner, P.J. Hamer, Synth. Met. 71 Ž1995. 2117. ¨ w7x A. Lux, A.B. Holmes, R. Cervini, J.E. Davies, S.C. Moratti, J. Gruner, F. Cacilli, R.H. Friend, Synth. Met. 84 Ž1997. 293. ¨ w8x C. Adachi, K. Nagai, N. Tamoto, Appl. Phys. Lett. 66 Ž1995. 2679. w9x E.H. Elandaloussi, C.W. Spangler, Polym. Prepr. 39 Ž1998. 1055. w10x G. Yu, Y. Liu, X. Wu, M. Zhang, F. Bai, D. Zhu, Appl. Phys. Lett. 74 Ž1999. 2295. w11x C.D. Wilson, U.S. Patent 2558285, 1951. w12x J.-M. Son, K. Ogino, N. Yonezawa, H. Sato, Synth. Met. 98 Ž1998. 71.