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Poly(p-phenylene)-based materials for light-emitting diodes : Electroluminescence and photo-oxidation
ELSEVIER
Synthetic Metals 102 (1999) 106&1062
Poly(p-phenylene)-based Materials for Light-emitting Electroluminescence and Photo-oxidation Suck-Hoon
Diodes :
Shin, Jin-Sung Park, Jong-Wook Park”, and Hwan Kyu Kim*
Dept. of Macromolecular Science, Hannam University, Taejon, Korea 306-791 “Dept. of Polymer Engineering, Chungju National University, Chungbuk Korea 380-702
Abstract A series of alkoxy-substituted poly(p-phenylene) derivatives were synthesized using the well-known Suzuki reaction. The present polymers have strong UV-visible absorption bands around 340 - 450 nm and their PL spectra give a highest peak around 430 - 550 nm in the blue-green region. In this paper, r -conjugated chain lengths were molecularly regulated, leading to the color tune from PPP-based materials. Their EL spectra in a single-layered device show a highest peak around 430-550 nm in the blue-green region with the operating voltage of lower than 7V. Also, the photo-oxidation for poly(p-phenylene) derivatives in air was studied. 1. Introduction Electroluminescence (EL) devices based on polymeric thin layers have attracted much attention because of their academic interests and potential utility of this technology in a wide variety of applications such as large area flat-panel displays and lightemitting diodes.‘.’ The luminescence usually arises at energys just below the oneset of the absorption, and can therefore be tuned by varying the semiconductor HOMO-LUMO gap of the polymer. The HOMO-LUMO gap is related to the conjugation length in the polymer which is a measure of the extent of the delocalization of the in -electrons. EL polymeric materials offer advantages such as low operating voltages, three primary R/G/B colors, fast response time, high quality of display and ease of device processability with semiconductor technologies compared to inorganic EL materials. Since the first report of the polymer lightemitting diodes based on polyb-phenylenevinylene), a number of different polymers have been synthesized and extended efforts have been made to obtain high performance devices from polymeric materials. Very recently, the main materials efforts have been focussed on developing blue light-emitting diodes capable of operating at ambient temperature, low voltages and easy processability with low price.‘” Very recently, we prepared processable alkoxy-substituted PPPbased copolymers as a new type of the blue EL materials using the well-known Suzuki reaction of alkoxy-substituted aromatic boronic esters with various aryl halides. The functionalized side chains allow the PPP-based materials to remain soluble during polymerization as well as processing, thereby offering new opportunities for potential application as electroluminescent material?). Also, these present polymers are easy tune of chemical and optical properties through various structural design. We have been studied them as candidate for active material in blue light emitting diodes. But, when these materials are fabricated for the EL device, we observed a short operating lifetime of a LED device, due to the photodegradation of PPP derivatives during the EL operation’0“2, in spite of its superior properties in term of durability, mechanical strength, 0379.6779/99/$ see front PII: SO379-6779(98)00076-d
matter
environmental stability and thermal stability as one of the potentially most useful engineering materials’0.‘2. In this paper, synthesis, electroluminescence and photo-oxidation of alkoxysubstituted poly(p-phenylene)-based materials are presented. 2. Results and Discussion Alkoxy-substituted PPP-based materials were prepared by the Suzuki reaction of alkoxy-substituted aromatic boronic esters with various aryl halides, as shown in Scheme 1. The polymerization results, optical and luminescent properties of alkoxy- substituted PPP-based materials were summarized in Table 1, The well-defined alternative PPP-based copolymers were highly soluble in common organic solvents such as chloroform, methylene chloride, toluene. etc. and they retained thermal stability.
Scheme 1. Alkoxy-substituted Suzuki reaction.
\
\ PPP-based
copolymers
by the
Their chemical structures were identified by IR and ‘H-NMR spectroscopies. The incorporation of various aryl halides into alkoxy-substituted PPP-based copolymers would tune the linear optical absorption in the ragne of 340 nm to 476 nm. Typical UVvis spectra of thin films using copolymers showed that a maximum absorption wavelength( /1 max) of AOCMPPP, AOCBPPP and AOCPyPPP has a strong absorption band of in -
0 1999 Elsevier Science S.A. All rights reserved.
S.-H. Shin et al. I Synthetic
?r * transition of the conjugated segment at 350 nm, 360 nm and 375 nm, respectively. Their maximium absorption wavelengths ( /1 max) of the rr -conjugated polymers depends on both the degree of conjugation and the conjugation length. As a number of consecutive phenylene rings increases and the torsional angle reduces, both the degree of conjugation and the conjugation length increase, exhibiting red shifts. The absorption wavelength( /1 max) of AOCTPPP and AOCBTPPP has a strong absorption band of K-K * transition of the conjugated segment at 440 nm and 450 nm in chloroform solution, due to the strong delocalization of the K -conjugated thiophene units. Table 1. The polymerization results, optical properties of PPP-based materials.
Metals
102 (1999)
IO@-1062
1061
exposed to an UV light source of an unfiltered Hg lamp in the atmosphere, the intensity of their Uv-vis absorption spectra decreases with an exposure time, resulting in a blue-shift from 350 nm to 280 nm for the AOCBPPP as well as from 440 nm to 400 nm for the AOCTPPP (see Fig. 2). However, in the dark, no spectral change was observed, due to no photodegradation of PPP derivatives.
r
and luminescent
300 AOCTPPP AOCBTPPP AOCPyPPP
80 75 75
0.15 0.1. 0.21
I , 1
AAnlA7h
Wavelength (nm) Figure 2. The UV-vis spectra of AOCMPPP exposure time with a Hg-lamp.
I TIS/TTn
The photoluminescence (PL) spectra showed similar shifts to those observed in the absorption spectra for the solution and thin film. With an excitation wavelength of 350 nm, the PL spectra of AOCMPPP and AOCBPPP exhibited around 415 nm, 420 nm and 450 nm in deep blue emission region. The PL spectra of AOCTPPP and AOCBTPPP give a peak in emission spectra at 5 12 nm and 520 nm in the green region, respectively. The single layer light-emitting diode of an AI/PPP derivative/IT0 glass was fabricated. The forward bias current is obtained when the IT0 electrode is positively biased and the Al electrode negatively. The current increases with forward bias voltage and the reverse bias current remains small, which is the typical rectifying characteristics. The threshold voltage is about 47 V for the PPP series (see Table l), which is considerably low compared with the PPV-based block copolymers.‘3
I,.,.,
i 3w
4x3
Wavelength
MO
(nm)
I em
0
2
400
4
B
as a function of an
After the exposure time of 48 hrs in the atmosphere, IR spectra shows that two new peaks of carbonyl groups appeared at 1774 and 1724 cm-‘. The two new bands could result from quinone isomers, aldehyde derivatives, or ester derivatives obtained from the photo-oxidation of alkoxy substituted PPP derivatives. The possible formation of quinone isomers, aldehyde derivatives, or ester derivatives was proved by ‘H-NMR spectroscopy. lH-NMR spectra showed the growing peaks of a quinone type around 6.6 ppm, the typical aldehyde around 9.8 ppm, and nonaromatic esters peak around 4.3 ppm, etc. The initial oxidation occurs at the aryl alkyl ether oxygen-carbon bond in the pedant side chain and the homolytic cleavage of the oxygen-carbon bond could occurs, yielding radicals.
.I 8
Voltage (V)
Figure 1. UV, PL, EL and I-V spectra of The single layer lightemitting diode of Al/AOCMPPP/ITO. As shown in Table 1 and Fig. l., the AOCMPPP, the AOCBPPP, and AOCTPPP at the operating voltage of lower than 7 V give a peak in the EL emissive band at 415, 420 nm and 530 nm, respectively, indicating a blue emission or a green emission. But, we observed a short operating lifetime of a LED device, due to the photodegradation of PPP derivatives during the EL operation. Therefore, the chemically structural change of alkoxy-substituted PPP-based materials as a function of exposure time in the dark or in the the atmosphere (air/h I/ ) was investigated by Uv-vis, IR, and ‘H-NMR spectroscopies. When PPP-based materials were
Scheme 2. Proposed based copolymers.
mechanism
for photo-oxidation
of PPP-
Phenoxy radicals formed at the backbones could combine together to give crosslinked polymers, providing insoluble polymers. Phenoxy radicals isomerize to form quinone isomers. In another way, CT-methylne to oxygen atoms was oxidized to generate peroxy radicals and then rearranged to give nonaromatic esters, etc. In a such way, the mechanisms was much more complicated, due to the isomerization of radicals and photo-fries rearrangement, etc. ‘O-i* These observations lead us to suggest the
1062
S.-H. Shin et al. I Synthetic Metals 102 (1999) 1060-1062
mechanism of the photo-oxidation of alkoxy-substituted PPPbased materials through a model compound of dihexyloxysubstituted terphenyl derivative, as shown in Scheme 2. In a near future, the mechanistic study on the photo-oxidation for alkoxysubstituted poly(p-phenylene) derivatives will be described in the more detail at the new other paper. I4 Acknowledgement This research was financially supported from the Korean Research Foundation made in the program year of 1997. References I. G. Leising, et al, Adv. Mater. 1992, 4, 36, 2. (a) J. H. Lee, S. J. Kang, H. K. Kim, T. Zyung, I. Cho, S. K. Choi, Mol.Gyst. Liq. Crysr 1996, 280, 391. (b) H. K. Kim, et. al, Mol. Cryst. Liq. Cryst., 1997, 295, 27 3. (a) H. K. Kim, M. K. Ryu, et al, Macromolecules 1998, 3 1, 1114.,(b) H. K. Kim, K. T. No, et al, Macromolecules 1998, 3 1,7267. 4. A. Suzuki, N. Miyaura, T. Yano, Tetrahedron Lett, 1980, 21, 2865, 5. M. Rehahn, A. -D. Schluter, G. Wegner, Macromol. Chem., 1990,191,1991. 6. T. Vahlenkamp, G. Wegner, Macromol. Chem. Phys., 1994, 195, 1933. 7. M. Remmers, M. Schulze, G. Wegner, Macromol. Rapid. Commun., 1996, 17,239.. 8. K. C. Park, L. R. Dodd, K. Levon, T. K. Kwei, Macromolecules, 1996, 29, 7149. 9. W.-X. Jing, A. Kraft, S. C. Moratti, et al, Synthethic Metals, 1994, 67, 161. 10. J. F. Rabek, Polymer Photodegradation Mechanism and Experimental Methods, 1995, 1 st Ed, Chapman & Hall Press Il. B. H. Cumpston, K. F. Jensen, Synthetic Metals, 1995, 73, 195. 12. B. H. Cumpston, K. F. Jensen, TRIP. Vol. 4 & 5, May 1996. 13. Z. Yang, I. Sokolik, F. E. Karasz, Macromolecules, 1993, 26, 1188. 14. J. S. Park, S. H. Shin, H. K. Kim, Synthetic Metals, to be submitted.
Synthetic Metals 102 (1999) 106&1062
Poly(p-phenylene)-based Materials for Light-emitting Electroluminescence and Photo-oxidation Suck-Hoon
Diodes :
Shin, Jin-Sung Park, Jong-Wook Park”, and Hwan Kyu Kim*
Dept. of Macromolecular Science, Hannam University, Taejon, Korea 306-791 “Dept. of Polymer Engineering, Chungju National University, Chungbuk Korea 380-702
Abstract A series of alkoxy-substituted poly(p-phenylene) derivatives were synthesized using the well-known Suzuki reaction. The present polymers have strong UV-visible absorption bands around 340 - 450 nm and their PL spectra give a highest peak around 430 - 550 nm in the blue-green region. In this paper, r -conjugated chain lengths were molecularly regulated, leading to the color tune from PPP-based materials. Their EL spectra in a single-layered device show a highest peak around 430-550 nm in the blue-green region with the operating voltage of lower than 7V. Also, the photo-oxidation for poly(p-phenylene) derivatives in air was studied. 1. Introduction Electroluminescence (EL) devices based on polymeric thin layers have attracted much attention because of their academic interests and potential utility of this technology in a wide variety of applications such as large area flat-panel displays and lightemitting diodes.‘.’ The luminescence usually arises at energys just below the oneset of the absorption, and can therefore be tuned by varying the semiconductor HOMO-LUMO gap of the polymer. The HOMO-LUMO gap is related to the conjugation length in the polymer which is a measure of the extent of the delocalization of the in -electrons. EL polymeric materials offer advantages such as low operating voltages, three primary R/G/B colors, fast response time, high quality of display and ease of device processability with semiconductor technologies compared to inorganic EL materials. Since the first report of the polymer lightemitting diodes based on polyb-phenylenevinylene), a number of different polymers have been synthesized and extended efforts have been made to obtain high performance devices from polymeric materials. Very recently, the main materials efforts have been focussed on developing blue light-emitting diodes capable of operating at ambient temperature, low voltages and easy processability with low price.‘” Very recently, we prepared processable alkoxy-substituted PPPbased copolymers as a new type of the blue EL materials using the well-known Suzuki reaction of alkoxy-substituted aromatic boronic esters with various aryl halides. The functionalized side chains allow the PPP-based materials to remain soluble during polymerization as well as processing, thereby offering new opportunities for potential application as electroluminescent material?). Also, these present polymers are easy tune of chemical and optical properties through various structural design. We have been studied them as candidate for active material in blue light emitting diodes. But, when these materials are fabricated for the EL device, we observed a short operating lifetime of a LED device, due to the photodegradation of PPP derivatives during the EL operation’0“2, in spite of its superior properties in term of durability, mechanical strength, 0379.6779/99/$ see front PII: SO379-6779(98)00076-d
matter
environmental stability and thermal stability as one of the potentially most useful engineering materials’0.‘2. In this paper, synthesis, electroluminescence and photo-oxidation of alkoxysubstituted poly(p-phenylene)-based materials are presented. 2. Results and Discussion Alkoxy-substituted PPP-based materials were prepared by the Suzuki reaction of alkoxy-substituted aromatic boronic esters with various aryl halides, as shown in Scheme 1. The polymerization results, optical and luminescent properties of alkoxy- substituted PPP-based materials were summarized in Table 1, The well-defined alternative PPP-based copolymers were highly soluble in common organic solvents such as chloroform, methylene chloride, toluene. etc. and they retained thermal stability.
Scheme 1. Alkoxy-substituted Suzuki reaction.
\
\ PPP-based
copolymers
by the
Their chemical structures were identified by IR and ‘H-NMR spectroscopies. The incorporation of various aryl halides into alkoxy-substituted PPP-based copolymers would tune the linear optical absorption in the ragne of 340 nm to 476 nm. Typical UVvis spectra of thin films using copolymers showed that a maximum absorption wavelength( /1 max) of AOCMPPP, AOCBPPP and AOCPyPPP has a strong absorption band of in -
0 1999 Elsevier Science S.A. All rights reserved.
S.-H. Shin et al. I Synthetic
?r * transition of the conjugated segment at 350 nm, 360 nm and 375 nm, respectively. Their maximium absorption wavelengths ( /1 max) of the rr -conjugated polymers depends on both the degree of conjugation and the conjugation length. As a number of consecutive phenylene rings increases and the torsional angle reduces, both the degree of conjugation and the conjugation length increase, exhibiting red shifts. The absorption wavelength( /1 max) of AOCTPPP and AOCBTPPP has a strong absorption band of K-K * transition of the conjugated segment at 440 nm and 450 nm in chloroform solution, due to the strong delocalization of the K -conjugated thiophene units. Table 1. The polymerization results, optical properties of PPP-based materials.
Metals
102 (1999)
IO@-1062
1061
exposed to an UV light source of an unfiltered Hg lamp in the atmosphere, the intensity of their Uv-vis absorption spectra decreases with an exposure time, resulting in a blue-shift from 350 nm to 280 nm for the AOCBPPP as well as from 440 nm to 400 nm for the AOCTPPP (see Fig. 2). However, in the dark, no spectral change was observed, due to no photodegradation of PPP derivatives.
r
and luminescent
300 AOCTPPP AOCBTPPP AOCPyPPP
80 75 75
0.15 0.1. 0.21
I , 1
AAnlA7h
Wavelength (nm) Figure 2. The UV-vis spectra of AOCMPPP exposure time with a Hg-lamp.
I TIS/TTn
The photoluminescence (PL) spectra showed similar shifts to those observed in the absorption spectra for the solution and thin film. With an excitation wavelength of 350 nm, the PL spectra of AOCMPPP and AOCBPPP exhibited around 415 nm, 420 nm and 450 nm in deep blue emission region. The PL spectra of AOCTPPP and AOCBTPPP give a peak in emission spectra at 5 12 nm and 520 nm in the green region, respectively. The single layer light-emitting diode of an AI/PPP derivative/IT0 glass was fabricated. The forward bias current is obtained when the IT0 electrode is positively biased and the Al electrode negatively. The current increases with forward bias voltage and the reverse bias current remains small, which is the typical rectifying characteristics. The threshold voltage is about 47 V for the PPP series (see Table l), which is considerably low compared with the PPV-based block copolymers.‘3
I,.,.,
i 3w
4x3
Wavelength
MO
(nm)
I em
0
2
400
4
B
as a function of an
After the exposure time of 48 hrs in the atmosphere, IR spectra shows that two new peaks of carbonyl groups appeared at 1774 and 1724 cm-‘. The two new bands could result from quinone isomers, aldehyde derivatives, or ester derivatives obtained from the photo-oxidation of alkoxy substituted PPP derivatives. The possible formation of quinone isomers, aldehyde derivatives, or ester derivatives was proved by ‘H-NMR spectroscopy. lH-NMR spectra showed the growing peaks of a quinone type around 6.6 ppm, the typical aldehyde around 9.8 ppm, and nonaromatic esters peak around 4.3 ppm, etc. The initial oxidation occurs at the aryl alkyl ether oxygen-carbon bond in the pedant side chain and the homolytic cleavage of the oxygen-carbon bond could occurs, yielding radicals.
.I 8
Voltage (V)
Figure 1. UV, PL, EL and I-V spectra of The single layer lightemitting diode of Al/AOCMPPP/ITO. As shown in Table 1 and Fig. l., the AOCMPPP, the AOCBPPP, and AOCTPPP at the operating voltage of lower than 7 V give a peak in the EL emissive band at 415, 420 nm and 530 nm, respectively, indicating a blue emission or a green emission. But, we observed a short operating lifetime of a LED device, due to the photodegradation of PPP derivatives during the EL operation. Therefore, the chemically structural change of alkoxy-substituted PPP-based materials as a function of exposure time in the dark or in the the atmosphere (air/h I/ ) was investigated by Uv-vis, IR, and ‘H-NMR spectroscopies. When PPP-based materials were
Scheme 2. Proposed based copolymers.
mechanism
for photo-oxidation
of PPP-
Phenoxy radicals formed at the backbones could combine together to give crosslinked polymers, providing insoluble polymers. Phenoxy radicals isomerize to form quinone isomers. In another way, CT-methylne to oxygen atoms was oxidized to generate peroxy radicals and then rearranged to give nonaromatic esters, etc. In a such way, the mechanisms was much more complicated, due to the isomerization of radicals and photo-fries rearrangement, etc. ‘O-i* These observations lead us to suggest the
1062
S.-H. Shin et al. I Synthetic Metals 102 (1999) 1060-1062
mechanism of the photo-oxidation of alkoxy-substituted PPPbased materials through a model compound of dihexyloxysubstituted terphenyl derivative, as shown in Scheme 2. In a near future, the mechanistic study on the photo-oxidation for alkoxysubstituted poly(p-phenylene) derivatives will be described in the more detail at the new other paper. I4 Acknowledgement This research was financially supported from the Korean Research Foundation made in the program year of 1997. References I. G. Leising, et al, Adv. Mater. 1992, 4, 36, 2. (a) J. H. Lee, S. J. Kang, H. K. Kim, T. Zyung, I. Cho, S. K. Choi, Mol.Gyst. Liq. Crysr 1996, 280, 391. (b) H. K. Kim, et. al, Mol. Cryst. Liq. Cryst., 1997, 295, 27 3. (a) H. K. Kim, M. K. Ryu, et al, Macromolecules 1998, 3 1, 1114.,(b) H. K. Kim, K. T. No, et al, Macromolecules 1998, 3 1,7267. 4. A. Suzuki, N. Miyaura, T. Yano, Tetrahedron Lett, 1980, 21, 2865, 5. M. Rehahn, A. -D. Schluter, G. Wegner, Macromol. Chem., 1990,191,1991. 6. T. Vahlenkamp, G. Wegner, Macromol. Chem. Phys., 1994, 195, 1933. 7. M. Remmers, M. Schulze, G. Wegner, Macromol. Rapid. Commun., 1996, 17,239.. 8. K. C. Park, L. R. Dodd, K. Levon, T. K. Kwei, Macromolecules, 1996, 29, 7149. 9. W.-X. Jing, A. Kraft, S. C. Moratti, et al, Synthethic Metals, 1994, 67, 161. 10. J. F. Rabek, Polymer Photodegradation Mechanism and Experimental Methods, 1995, 1 st Ed, Chapman & Hall Press Il. B. H. Cumpston, K. F. Jensen, Synthetic Metals, 1995, 73, 195. 12. B. H. Cumpston, K. F. Jensen, TRIP. Vol. 4 & 5, May 1996. 13. Z. Yang, I. Sokolik, F. E. Karasz, Macromolecules, 1993, 26, 1188. 14. J. S. Park, S. H. Shin, H. K. Kim, Synthetic Metals, to be submitted.