Chester Clinic

 

No job name

Chem. Mater. 2001, 13, 565-574
Luminescence Properties of Structurally Modified PPVs:
PPV Derivatives Bearing
Dong Won Lee,† Ki-Young Kwon,† Jung-Il Jin,*,† Yongsup Park,‡ Yong-Rok Kim,§ and In-Wook Hwang§ Division of Chemistry and Molecular Engineering and the Center for Electro- and Photo-Responsive Molecules, Korea University, Seoul 136-701, Korea, Surface Analysis Group, Korea Research Institute of Standards and Science, Yusong 305-600, Taejon, Korea, and Department of Chemistry, Yonsei University, Seoul 120-749, Korea Received October 3, 2000. Revised Manuscript Received November 16, 2000 Two new poly(p-phenylenevinylene) (PPV) derivatives bearing 2-phenyl-5-(4-tert-butylphe- nyl)-1,3,4-oxadiazole pendants were prepared, and their photo- and electroluminescence
properties were studied. The first polymer (P-1) is poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-
oxadiazolyl]phenyl}-1,4-phenylenevinylene], which is a PPV derivative having diphenyl-
substituted 1,3,4-oxadiazole pendant that is known to be an excellent electron-transporting
structure. The second polymer (P-2) is poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-
phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]. The only structural difference between
P-1 and P-2 is the presence of additional 2-ethylhexyloxy pendant groups in P-2. Both
polymers were prepared by direct polymerization of the R,R′-dibromo-p-xylene monomers
having the pendant group(s) in the presence of excess potassium tert-butoxide. Both polymers
reveal much improved electroluminescence (EL) properties when compared with PPV. They
emit luminescence light over the wavelength range from about 500 to 600 nm. The external
quantum efficiencies of P-1 and P-2 were respectively 16 and 56 times the value for PPV
when LED devices were fabricated using an indium-tin oxide (ITO) coated glass anode and
the aluminum cathode. In particular, the EL device ITO/poly(3,4-ethylenedioxy-2,4-thie-
nylene)/P-2/Al:Li geometry revealed a maximum luminance of 1090 cd/m2 at the electric
field of 2.36 MV/cm with the external quantum efficiency of 0.045%. The maximum brightness
of the ITO/P-2/Ca/Al was 7570 cd/m2 at the electric field of 2.80 MV/cm.
It is understood that in LED devices electrons and Luminescence properties of poly(p-phenylenevinylene), holes are separately injected from an anode and a PPV, and other conjugated polymers have been attract- cathode, respectively, under a bias voltage into the light- ing a great deal of interests since the first light-emitting emitting polymer layer where the injected negative and diodes (LEDs) based on PPV were reported a decade ago positive carriers form excitons.6 The excitons can disap- by the Cambridge group.1 Electroluminescence (EL) pear via various mechanisms; one of them is lumines- efficiency of the devices, however, was far from satisfac- cence decay or radiative decay. To improve devices' tory. And it soon was found that chemical modifications efficiency of LEDs there have been many attempts7 to of PPV2 and use of different electrodes3 together with balance the injection of carriers from electrodes and also utilization of electron-4 and/or hole-transporting5 layers their mobility in the emitting polymer layer. Unfortu- can improve the device efficiency to impressive extents.
nately, the hole mobility in PPV and its simple deriva-tives is typically higher than that of electron mobility.8 * To whom correspondence should be addressed. E-mail: jijin@ This is one of the reasons why the LED devices † Korea University.
(4) (a) Yu, G.; Nishino, H.; Heeger, A. J.; Chen, T.; Rieke, R. D.
‡ Korea Research Institute of Standards and Science.
Synth. Met. 1995, 72, 249. (b) Pommerehne, J.; Vestweber, H.; Guss,
§ Yonsei University.
W.; Mahrt, R. F.; Bassler, H.; Porsch, M.; Daub. J. Adv. Mater. 1995,
(1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R.
7, 551.
N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990,
(5) (a) Hosokawa, C.; Tokailin, H.; Higashi, H.; Kusumoto, T. Appl. 347, 539.
Phy. Lett. 1993, 63, 1322. (b) Cacialli, F.; Friend, R. H.; Haylett, N.;
(2) (a) Gustafsson, G.; Gao, Y.; Treacy, G. M.; Klavetter, F.; Daik, R.; Feast, W. J.; dos Santos, D. A.; Bredas, J. L. Appl. Phys. Lett. Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477. (b) Greenhan, N.
1996, 69, 3794.
C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature (6) (a) Davis, P. S.; Kogan, S. M.; Parker, I. D.; Smith, D. L. Appl. 1993, 365, 628. (c) Chung, S.-J.; Jin, J.-I.; Kim, K.-K. Adv. Mater. 1997,
Phys. Lett. 1996, 69, 2270. (b) Conewll, E. M.; Wu, M. W. Appl. Phys.
9, 551. (d) Zyung, T.; Hwang, D. H.; Kang, I. N.; Shim, H. K.; Hwang, Lett. 1997, 70, 1867. (c) Brown, A. R.; Bradley, D. D. C.; Burroughes,
W. Y.; Kim, J. J. Chem. Mater. 1995, 7, 1499.
J. H.; Friend, R. H.; Greenham, N. C.; Burn, P. L.; Holmes, A. B.; Kraft, (3) (a) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (b)
A. Appl. Phys. Lett. 1992, 61, 2793.
Parker, I. D. J. Appl. Phys. 1992, 75, 1656. (c) Egusa, S.; Miura, A.;
(7) (a) Swanson, L. S.; Shinar, J.; Brown, A. R.; Bradley, D. D. C.; Gemma, N.; Azuma, M. Appl. Phys. Lett. 1994, 65, 1272. (d) Peng, J.;
Friend, R. H.; Burn, P. L.; Kraft, A.; Holmes, A. B. Phys. Rev. B 1992,
Yu, B.-Y.; Pyun, C.-H.; Kim, C.-H.; Kim, K.-Y.; Jin, J.-I. Jpn. J. Appl. 46, 15072. (b) Greenhan, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, Phys. 1996, 35, L317.
R. H.; Holmes, A. B. Nature 1993, 365, 628.
10.1021/cm000794g CCC: $20.00 2001 American Chemical Society Published on Web 01/16/2001 Chem. Mater., Vol. 13, No. 2, 2001 Lee et al. fabricated using these polymers exhibit rather unsat- structures both in the backbone and in the side chain.
isfactory efficiencies.9 Utilization of low work function They reported the external quantum efficiency of 0.07% metals as cathodes makes the electron injection easier.3 for the ITO/polymer/Al device and 0.15% for the device Calcium and lithium are representative examples.
where the calcium cathode was used instead of the Another approach for improving electron transporting aluminum cathode. Peng et al.10b previously reported is to use of additives of electron-deficient compounds that polymer III showed an external efficiency 40 times
higher than that of PPV.
diazole (PBD).4 But their efficacy can be limited due totheir crystallization and aggregation. This problem ismitigated by incorporating the electron-transportingunits into the main chain10 or as pendants attached tothe backbone of a polymer.11 Or they can be includedboth in the main chain and in the side group.12 Chen et al.11a recently prepared the polymer I that
All the polymers described above are claimed to show bears the oxadiazole moiety as a pendant, but it was enhanced electron injection and improved balance in not soluble in organic solvents. They had to include carrier mobility. We11c also reported briefly the lumi- dialkoxyphenylenevinylene comonomer units to make nescence properties of the polymer P-1 that bears the
the 1:1 alternating copolymer, which became soluble in organic solvents.
pendants directly attached to the PPV backbone. Such
a simple structural modification resulted in a much
higher EL efficiency, 16 times the efficiency of PPV. The
polymer is soluble at room temperature in organic
solvents such as 1,1,2,2-tetrachloroethane and toluene.
It is known that attachment of long alkoxy substituents
on the phenylene rings increases the interchain distance
giving rise to improved LED device efficiencies resulting
from diminished formation of interchain polaron pairs.14
Therefore, we became interested in a further modifica-
tion of P-1 with an additional alkoxy substituent (P-2)
to compare its luminescence properties with P-1 and
In comparison, Bao et al.11b prepared an organic also with PPV. This article describes the synthesis, soluble PPV derivative (II) bearing an electron-trans-
structural analysis, and spectral and luminescence porting pendant and an alkoxy group on the phenylene properties of P-1 and P-2. LED devices were fabricated
ring and also placed an oxymethylene spacer between using the polymers, and the devices' characteristics also the pendant and the backbone.
are discussed. For the purpose of comparison, dataobtained for PPV in our laboratory also are includedwhenever necessary.
This polymer emits light at 580 nm, and the ITO/ polymer/Al device exhibited an external quantum ef-ficiency of 0.02% at a current density of about 8 mA/cm2. PPV itself shows the external quantum efficiencyof about 2.0 × 10-4 to 5.0 × 10-4 % for the ITO/PPV/Aldevice.13 Peng and Zhang12 recently reported a polyconjugated polymer containing the oxadiazole electron-transporting (8) (a) Antomiadis, H.; Abkowitz, M. A.; Hsieh, B. R. Appl. Phys. Lett. 1994, 65, 2030. (b) Blom, D. W. M.; deJong, M. J. M.; Vleggaar,
J. J. M. Appl. Phys. Lett. 1996, 68, 3308.
Results and Discussion
(9) Wang, Y. Z.; Gebler, D. D.; Spry, D. J.; Fu, D. K.; Swager, T.
M.; MacDiarmid, A. G.; Epstein, A. J. IEEE Trans. Electron Devices
1997, 44, 1263.
Synthesis of Monomers, M-1 and M-2. P-1 and P-2
(10) (a) Song, S.-Y.; Jang, M. S.; Shim, H.-K.; Song, I.-S.; Kim, W.- were synthesized by polymerizing bis(bromomethyl) H. Synth. Met. 1999, 102, 1116. (b) Peng, Z.; Bao, Z.; Galvin, M. E.
Adv. Mater. 1998, 10, 680.
(11) (a) Chen, Z.-K.; Meng, H.; Lai, Y.-H.; Huang, W. Macromol- (13) (a) Herold, M.; Gmeiner, J.; Riess, W.; Schwoerer, M. Synth. ecules 1999, 32, 4351. (b) Bao, Z.; Peng, Z.; Galvin, M. E.; Chandross,
Met. 1996, 76, 109. (b) Riess, W. Single- and heterolayer polymeric
E. A. Chem. Mater. 1998, 10, 1201. (c) Chung, S.-J.; Kwon, K.-Y.; Lee,
light emitting diodes based on poly(p-phenylene vinylene) and oxa- S.-W.; Jin, J.-I.; Lee, C. H.; Lee, C. E.; Park, Y. Adv. Mater. 1998, 10,
diazole polymers. In Organic Electroluminescent Materials and Devices; Miyata, S., Nalwa, H. S., Eds.; Gordon and Breach Publishers: (12) Peng, Z.; Zhang, J. Synth. Met. 1999, 105, 73.
Singapore, 1997.
Structurally Modified PPVs Chem. Mater., Vol. 13, No. 2, 2001 Scheme 1. Synthetic Route to M-1 and P-1a
aReagents and conditions: (i) NH 2NH2 H2O, ethanol, reflux, 6 h, 83%; (ii) 4-bromobenzoyl chloride, THF, pyridine, 0 °C, 1 h, 79%; (iii) POCl3, reflux, 12 h, 84%; (iv) (PPh3)4Pd (cat.), toluene/Na2CO3 (2 M in H2O), 2,5-dimethylphenylboronic acid, 24 h, 73%; (v)N-bromosuccinimide (NBS), CCl4, reflux, 2 h, 42%; (vi) KOBut, THF, 4 h, 43%.
Scheme 2. Synthetic Route to M-2 and P-2a
a Reagents and conditions: (i) (PPh3)4Pd (cat.), toluene/Na2CO3 (2 M in H2O), 2,5-dimethyl-4-methoxyphenylboronic acid, 24 h, 59%; (ii) BBr3, methylene chloride, 0 °C, 3 h, 97%; (iii) 2-ethylhexyl bromide, K2CO3, tetrabutylammonium bromide, acetonitrile, 12 h, 92%;(iv) N-bromosuccinimide (NBS), CCl4, reflux, 2 h, 40%; (v) KOBut, THF, 4 h, 40%.
acceptor, pyridine. Compound 2 was reacted by a Suzuki
reaction16 with 2,5-dimethylphenylboronic acid in the presence of the tetrakis(triphenylphosphine)palladium- yl)]-5-(4-tert-butylphenyl)-1,3,4-oxadiazole), both of which (0) catalyst to produce 2-(4-tert-butylphenyl)-5-{4-(2′,5′- prepared via multistep routes shown in Schemes 1 and dimethyl)biphenylyl}-1,3,4-oxadiazole, 3. Finally, com-
2, respectively. For the synthesis of M-1, we had to first
pound 3 was brominated benzylically17 using N-bromo-
succinimide (NBS) and the free-radical generator, ben- 1,3,4-oxadiazole, 2, by dehydrative cyclization of the
zoyl peroxide. All of the synthetic steps produced products in 60-80% yield, with the exception of the last zoyl) hydrazine (1), using POCl3. Cyclization of hy-
step that yielded the final product, M-1, only in the yield
drazides by POCl 15 is a very well-known method widely of 42%. Since several byproducts are formed in the final used in the synthesis of oxadiazoles. Compound 1 was
step which are not easily separable from one another, obtained by condensing 4-tert-butylbenzoylhydrazide the crude product was purified using a silica gel column.
with 4-bromobenzoyl chloride in the presence of the HCl The structures of intermediates and M-1 were confirmed
by elemental analysis and IR and 1H NMR spectroscopy.
The data are given in the Experimental Section.
(14) (a) Friend, R. H.; Bradley, D. D. C.; Townsend, P. D. J. Phys. D 1987, 20, 1367. (b) Leng, J. M.; Jeglinski, S.; Wei, X.; Benner, R. E.;
Vardeny, Z. V. Phys. Rev. Lett. 1994, 72, 156.
(16) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11,
(15) Hayes, F. N.; Rogers, B. S.; Ott, D. G. J. Am. Chem. Soc. 1955,
77, 1850.
(17) Anzalone, L.; Hirsch, J. A. J. Org. Chem. 1985, 50, 2128.
Chem. Mater., Vol. 13, No. 2, 2001 Lee et al. The second monomer, M-2, was synthesized in a
similar fashion, but its synthesis is a little bit more
complicated because the monomer bears the 2-ethyl-
hexyloxy group on the phenylene ring. Compound 2 was
coupled with the 2,5-dimethyl-4-methoxyphenylboronic
acid via the Suzuki coupling reaction16 as shown in
Scheme 2. The methoxy group in the resulting product
was deprotected by BBr 18
at 0 °C to give 2-(4-tert- butylphenyl)-5-{4-(2′,5′-dimethyl-4′-hydroxy)biphenylyl}-
1,3,4-oxadiazole, 5. After being purified by column
chromatography and dried in a vacuum oven, compound
5 was reacted with 2-ethylhexylbromide using K2CO3
as base in acetonitrile to yield product 6, i.e., 2-(4-tert-
butylphenyl)-5-{4′-(4′-(2-ethylhexyloxy)-2′,5′-dimethyl)-
Figure 1. Comparison of UV-vis absorption spectra of P-1,
biphenylyl}-1,3,4-oxadiazole. Finally, monomer M-2 was
P-2, and PPV. The numbers in the parentheses indicate the
synthesized via benzylic bromination17 of 6 using N-
time (hours) of thermal treatment at 150 °C, 0.1 Torr.
bromosuccinimide (NBS) and benzoyl peroxide (BPO)
as utilized in synthesis of M-1.
P-1, P-2, and PPV films. PPV was prepared via the
Synthesis of Polymers, P-1 and P-2. Both polymers
Wessling-Zimmerman20 water-soluble precursor route were prepared by polymerization of respective mono- and thermally treated at 270 °C in vacuo for 12 h. P-1
mers at room temperature in THF in the presence of a and P-2(3) have a common feature in their spectra, a
strong base, potassium tert-butoxide. This polymeriza- strong absorption centered around 300 nm (λmax tion was first reported by Gilch and Wheelwright.19 This nm for P-1 and 302 nm for P-2(3)) and a broader peak
method requires the use of excess strong alkali to ensure at about 330-510 nm for P-1 and at about 370-530
the formation of the fully eliminated structure as shown nm for P-2(3). In contrast, PPV absorbs broadly over
in Schemes 1 and 2. Therefore, the base acts not only the wavelength of 290-570 nm. The absorption by P-1
as a condensing agent but also as a dehydrobrominating and P-2(3) in the shorter wavelength region is at-
agent. The polymers obtained were purified by Soxhlet tributed to the oxadiazole pendant,21 and those in the extraction for 3 days using methanol and acetone longer wavelength region are attributed to the π-π* transitions of the main chains. The absorption position
for the backbone π-π* transition of P-1 is slightly blue-
The polymers are soluble in organic solvents such as shifted as compared with PPV. This is ascribed to a tetrahydrofuran and 1,1,2,2-tetrachloroethane. In par- partial destruction of the π-delocalization along the ticular, P-2 shows a better solubility in these solvents.
backbone by the bulky substituents that disrupt the Molecular weights of P-1 and P-2 determined by gel-
coplanarity of the π-conjugated backbone. However, the permeation chromatography against polystyrene stan- same blue-shift is not observed for P-2(3) because of
dards are M ) 24 000 and 12 500, respectively. Their the presence of the electron-donating alkoxy substituent polydispersity indices are 1.21 and 1.32. Relatively that usually causes a bathochromic shift.22 narrow molecular weight distribution must be brought The UV-vis spectrum of P-2 is strongly dependent
about by removal of the low molar mass portion by the on thermal history as shown in Figure 1. It is evident extraction process. Both polymers readily form free- that additional double bonds are formed when the standing films when cast from 1,1,2,2-tetrachloroethane polymer was thermally treated at 150 °C under a solutions. We were not able to detect the glass transi- pressure of 0.1 Torr. Not only the absorption intensity tions for these polymers up to 300 °C by differential of the π-π* transition grows but also the absorption scanning calorimetry. In TGA analyses, both polymers position moves red as the period of thermal treatment did not lose any weight up to 300 °C and started to lengthens up to 3 h. The virgin sample P-2(0) showed
undergo fast weight loss at 350 °C and a major decom- λmax at 436 nm, which moved to 450 nm when it was position at about 450 °C for P-1 and at about 400 °C
heat-treated at 150 °C for 3 h under a vacuum. On the for P-2. The presence of the alkoxy substituent appears
contrary, the spectrum of P-1 exhibited very little
to cause a little diminished thermal stability for P-2
dependence on thermal history, which suggests that when compared with P-1.
conjugative backbone was more readily formed in P-1
The wide-angle X-ray diffractograms (WAXD) of P-1
during polymerization than in P-2. The presence of the
and P-2 obtained at room temperature tell us that both
bulky electron-donating group on the phenylene rings polymers are amorphous. The virgin sample of P-2(0)
of P-2 appears to hinder the dehydrobromination reac-
exhibits the same WAXD as the one obtained for the tion mentioned above. The λmax value for the π-π* sample (P-2(3)) thermally treated for 3 h at 150 °C in
transition of the P-1 backbone is 428 nm.
vacuo. In other words, additional thermal elimination Photoluminescence (PL) spectra of P-1, P-2, and PPV
and annealing did not change the amorphous nature of are compared in Figure 2. The wavelength ranges of the the polymer.
UV-Vis Absorption and Photoluminescence
(20) (a) Wessling, R. A.; Zimmerman, R. G. U.S. Pat. 3401152, 1968.
Properties. Figure 1 compares the UV-vis spectra of
(b) Wessling, R. A. J. Polym. Sci. Polym., Symp. 1986, 72, 55.
(21) Krasovitskii, B. M.; Bolotin, B. M. Organic Luminescent Materials; VCH: Weinheim, Germany, 1988; p 284.
(18) McOmie, J. F. W.; West, D. E. Organic Syntheses; Wiley: New (22) (a) Berlman, I. B. Handbook of Fluorescence Spectra of York, 1973; Collect. Vol. V, p 412.
Aromatic Molecules, 2nd ed.; Academic Press: New York and London, (19) Gilch, H. G.; Wheelwright, W. L. J. Polym. Sci., Part A-1 1966,
1971; p 71. (b) Breadas, J. L.; Heeger, A. J. Chem. Phys. Lett. 1994,
4, 1337.
217, 507.
Structurally Modified PPVs Chem. Mater., Vol. 13, No. 2, 2001 Figure 2. Comparison of PL spectra of P-1, P-2, and PPV.
Figure 3. Comparison of excitation spectra of P-1, P-2(0),
and P-2(3). All spectra were obtained at their maximum
emitted light by the three polymers are not much emission wavelengths indicated in the figure.
different from one another with P-2 emitting light in
the slightly longer wavelength. In contrast with PPV,
P-1 and P-2 show structureless emission, although their
PL spectra reveal shoulders in the longer wavelength
side. As expected, thermally treated sample of P-2(3)
luminesces in a very slightly longer wavelength region
than the virgin sample, P-2(0). PPV's spectrum is
composed of well-resolved vibronic bands as reported
earlier by many other groups.23 This suggests that the
pendants in P-1 and P-2 are involved in electronic
interactions with the main chain, e.g., charge transfer
interaction as reported recently by us.24 Since the
oxadiazole ring contains three electronegative atoms, it
can act as a π-electron acceptor. The general feature of
PL spectra of P-1 and P-2(3) remains the same regard-
Figure 4. Time-resolved PL spectra of P-1, P-2, and PPV.
less the wavelength of excitation light, when the excita-tion wavelength is varied from 270 to 420 nm. As was all the same, which is as expected since all of them explained above, P-1 and P-2(3) absorb strongly at λ
are for emission from the π-electron systems of polymer ) 300 nm by the oxadiazole pendant and also at λ 428 or 450 nm by the backbone π-system. The PL Figure 4 compares the time-resolved PL spectra of spectra, for example, obtained at the excitation wave- P-1, P-2, and PPV obtained by a pico-second laser at
length of 300 and 420 nm are exactly the same in the the excitation wavelength of 300 nm. As one can see spectral feature. This is another indication for a facile from the figure, PL decays of P-1 and P-2(3) are much
electronic interactions between the pendant and the slower than that of PPV. But the difference between P-1
backbone. Another point to be noted is that, as one and P-2(3) is not significant. The presence of bulky
increases the wavelength of excitation beam, the PL substituents in P-1 and P-2(3) appears to slow PL decay
intensity increases. Exceptions are for the wavelengths by stabilizing the intrachain excitons formed and also (360 and 390 nm) where P-2(3) absorbs only weakly.
by reducing the formation of interchain excitons due to Figure 3 supports this supposition very clearly. The increased interchain distances.14 excitation spectra shown in Figure 3 were obtained for emitting wavelengths of maximum fluorescence intensi- diode (LED) devices were prepared from P-1 and P-2
ties of P-1 and P-2(3). In other words, they are for π-π*
using the indium-tin oxide (ITO) coated glass anode transitions of the backbones only. Comparison of these and the aluminum or aluminum:lithium alloy or calcium excitation spectra with the corresponding absorption cathode. Polymers (ca. 15 mg/mL) were dissolved in spectra given in Figure 1 strongly suggests that absorp- 1,1,2,2-tetrachloroethane and spin coated on an ITO- tion by BPD pendants makes a significant contribution coated glass, and the metal cathode was vapor depos- to the emission by the backbone. This will be possible ited. The surfaces of the spin-coated films were very only when the singlet excitons formed by pendants smooth, and the average surface roughness measured migrate to the backbones and undergo radiative decay.
by the AFM analysis is only 12.9 and 12.1 Å respectively In other words, the excited energy transfer from the for P-2(0) and P-2(3). Thermal treatment of P-2(0) for
pendants to the backbone occurs readily. The spectral 3 h at 150 °C did not change the surface roughness to feature of the excitation spectra of P-1 and P-2(3) for
any significant extent.
different wavelengths of emitted light (530-620 nm) The EL spectra given in Figure 5 were obtained for ITO/polymer/Al devices at the operating electric field (23) (a) Colaner, N. F.; Bradley, D. D. C.; Friend, R. H.; Burn, P.
L.; Holmes, A. B.; Spangler, C. W. Phys. Rev. B 1990, 42, 11670. (b)
of 1.4, 3.8, and 3.6 MV/cm respectively for PPV, P-1,
Pichler, K.; Halliday, D. A.; Bradley, D. D. C.; Burn, P. L.; Friend, R.
P-2(0), and P-2(3). The EL spectra are much the same
H.; Holmes, A. B. J. Phys.: Condens. Matter 1993, 7155.
as their corresponding PL spectra shown above in (24) Kim, Y. H.; Jeoung, S. C.; Kim, D.; Chung, S.-J.; Jin, J.-I. Chem. Mater. 2000, 12, 1067.
Figure 2. This fact indicates that light-emitting mech- Chem. Mater., Vol. 13, No. 2, 2001 Lee et al. Figure 7. Comparison of current density-external quantum
Figure 5. Comparison of EL spectra of P-1, P-2, and PPV.
efficiency curves for P-1, P-2, and PPV.
As reported earlier by us,11c data obtained from ultraviolet photoelectron spectroscopy (UPS) and UV-
vis spectrum of P-1 indicate that the ionization potential
(IP) or the HOMO level and electron affinity (EA) or
the LUMO level are 6.42 and 3.93 eV, respectively. The
EA values were evaluated from the optical band gaps
estimated from UV-vis spectra and the IP values
obtained from the UPS data. The corresponding values
for P-2(3) are 6.32 and 3.98 eV, respectively. Therefore,
it can be conjectured that hole and electron injections
are slightly more favored in P-2(3) than in P-1.
External quantum efficiencies (number of photons Figure 6. Comparison of I-V and light intensity-V curves
emitted per electron injected) of the devices constructed for P-1, P-2, and PPV.
are compared in Figure 7. The highest efficiency was
attained from the ITO/P-2(3)/Al:Li device and is about
anisms and the nature of so-called excitons are the same 0.04%, which is greater by more than 2 orders of for both cases.
magnitude when compared with the ITO/PPV/Al device.
The current/emitted light intensity-electric field curves With aluminum as the cathode, the device prepared of the LED devices are shown in Figure 6. According to from P-2(3) exhibits the external quantum efficiency
the figure, the turn-on electric field for the ITO/P-2(3)/
of about 0.015%. The efficiency (3 × 10-3%) of the ITO/ Al:Li device is the lowest (1.93 MV/cm for the current P-1/Al device lies inbetween P-2(3) and PPV devices.
density of 0.1 mA/mm2) and the value for the ITO/P-1/
The presence of the 2-ethylhexyloxy substituent in P-2
Al device is the highest (2.52 MV/cm for the current definitely improves the EL efficiency when compared density of 0.1 mA/mm2). Evidently, the cathode prepared with P-1 that does not bear the substituent. It is very
from the Al:Li alloy facilitates the electron injection well documented25 that the presence of long or bulky when compared with the aluminum electrode. It is well- substituents enhances the device efficiency by reducing known that the work function of lithium is much lower the possibility for the formation of interchain polaron than that of aluminum, 2.9 vs 4.2 eV. Another important pairs. The bulky substituents are expected to increase point to note for the device of P-2 for which the Al:Li
the interchain distance. And the efficiency for P-2(3) is
alloy was utilized as the cathode is that not only the slightly higher than for P-2(0).
turn-on electric field for current is the same as that for According to the present results, attachment of the light emission but also the dependence of light-emission electron-withdrawing and electron-transporting BPD on the electric field closely follows the current-electric pendant on the PPV backbone definitely improves the field curve (see the inset in Figure 6). This strongly device efficiency to about the same extent reported by indicates an equally efficient injection and transport of Bao et al.,11b who prepared PPV derivatives carrying the both carriers, i.e., holes and electrons. On the other similar pendants. These polymers, however, exhibit hand, for the devices where the aluminum electrode was lower EL efficiencies than the polymer12 containing the used, current increases much more rapidly than the oxadiazole moieties both in the backbone and side chain.
emitted light intensity does as the electric field in- Having the additional oxadiazole structure along the creases. This may due to an unbalanced injection of the main chain appears further help balancing the charge carriers. Most probably, despite the presence of the injection and transport by its ability to block hole oxadiazole pendants, electron injection is less efficient transport. Song et al.10a recently reported improved EL than hole injection. Moreover, hole mobility through the efficiencies for the devices prepared from blends consist- polymers (P-1 and P-2) may still be much faster than
ing of a polymer containing the oxadiazole unit in the electron mobility. But this point still has to be experi- main chain and a dialkoxy-substituted PPV.
mentally verified. The maximum brightness observed
for the ITO/P-2(3)/Al:Li device was 415 cd/m2 at the
(25) (a) Wudl, F.; Allemand, P. M.; Srdanov, G.; Ni, Z.; Mcbranch, electric field of 2.93 MV/cm for the current density of D. ACS Symp. Ser. 1991, 455, 683. (b) Ohmori, Y.; Uchida, M.; Muro,
K.; Yoshino, K. Solid State Commun. 1991, 80, 605. (c) Andersson, M.
3.62 mA/mm2.
R.; Yu, G.; Heeger, A. J. Synth. Met. 1997, 85, 1275.
Structurally Modified PPVs Chem. Mater., Vol. 13, No. 2, 2001 thirds of the value for the ITO/P-2(3)/Al:Li device
described above, and the maximum brightness (Figure
8 inset) attainable was about 1090 cd/m2 at the electric
field of 2.36 MV/cm. As described above, the device
without the PEDOT layer revealed the maximum
brightness of 415 cd/m2 at the field of 2.93 MV/cm.
Finally, we constructed a device having the ITO/P-
2(3)/Ca/Al geometry and the device performance was
studied. The current vs electric field and intensity of
emitted light vs electric field characteristics are shown
in Figure 8. Calcium was deposited onto the light-
emitting layer first, and then an aluminum layer was
deposited onto it in order to protect the calcium cathode.
Even without the PEDOT layer, the turn-on electric
field, arbitrarily taken as the field for the current
density of 0.1 mA/mm2, is 1.46 MV/cm. Moreover,
comparison of the I-V curves given in Figure 8 implies
that use of the calcium cathode leads to more efficient
carrier injection and flow than the Al:Li alloy cathode.
The two metals (Li and Ca) are known to have almost
Figure 8. Comparison of I-V curves and brightness of P-2(3)
the same value of work function (2.90 and 2.87 eV, repectively).29 It is conjectured that the calcium cathode Highly Light-Emitting Devices. Although P-2
gives rise to a better contact between the electrode and exhibits a much enhanced EL efficiency when compared the organic polymer emitting layer. Moreover, when the with unsubstituted PPV, there are many aspects that Al:Li alloy was utilized for deposition of the metal still require much further improvement to serve in cathode, the lithium layer might have not been ef- practical applications. Too high a turn-on and operating fectively protected by aluminum. The maximum bright- electric fields, relatively low brightness of emitted light, ness (Figure 8 inset) attainable was 7570 cd/m2 at the and poor long-term stability are some of the most current density of 9.50 mA/mm2 and the electric field important drawbacks that need much more studies for of 2.80 MV/cm. The efficiency of the device was 0.13 improvement. Especially, contact barrier or poor contact lm/W at the current density of 2-9 mA/mm2.
between the inorganic electrodes and organic emitting Since, in our initial studies, P-2(3) was found to be
layer appears to be one of the most difficult technical much superior to P-1 in the external quantum efficiency
problems to overcome when one is to design efficient for the ITO/polymer/Al device, we did not study the EL devices based on organic light-emitting polymers.
performance of P-1 in other devices. It, however, is our
One of the approaches being employed is to apply an belief that a similar improvement in device performance organic conducting layer between the ITO-coated glass can be achieved even for P-1 if we used Al:Li or Ca
anode and the organic light-emitting layer.26-28 Polya- cathode together with the PEDOT layer.
nilines26 and polythiophenes27 are some of the organicconducting polymers frequently utilized for this purpose.
Copper phthalocyanine28 is another conducting organo- New chemically modified PPV derivatives bearing the metallic compounds employed for the same purpose.
We have fabricated a device using a poly(3,4-ethyl- substituents directly attached to the phenylene rings enedioxythiophene) (PEDOT) doped with a sulfonated of the PPV main chain were synthesized and character- polystyrene in order to construct an ITO-coated glass ized for photo- and electroluminescence properties. The anode/PEDOT/P-2(3)/Al:Li cathode structure. Figure 8
two polymers described in this work are simple ho- shows the characteristics of the device thus constructed.
mopolymers and are readily soluble in various organic For comparison, the results obtained from the device of solvents. They emit yellowish-green color light in PL ITO/P-2(3)/Al:Li structure are also shown. As one can
and EL. Especially, attachment of the electron-with- see from Figure 8, the turn-on electric field has been drawing BPD pendant and a bulky, branched alkoxy significantly reduced to 1.35 MV/cm that is about two- group onto the PPV backbone results in much improvedEL performance of the LED devices. Due to electronic (26) (a) Yu, G. Synth. Met. 1996, 80, 143. (b) Lima, J. R.; Schreiner,
interactions between the BPD pendant and the back- C.; Hummelgen, C.; Fornarl, C. M., Jr.; Ferreira, C. A.; Nart, F. C. J. bone, fluorescence lifetime is lengthened significantly Appl. Phys. 1998, 84, 1445. (c) Yang, Y.; Heeger, A. J. J. Appl. Phys.
when compared with PPV, which is related to enhanced Lett. 1994, 64, 1245. (d) Yang, Y.; Westerweele, E.; Zhang, C.; Simth,
P.; Heeger, A. J. J. Appl. Phys. 1995, 77, 694. (e) Scott, J. C.; Carter,
EL devices' external quantum efficiencies of P-1 and
S. A.; Karg, S.; Angelopoulos, M. Synth. Met. 1997, 85, 1197. (f) Yang,
P-2. The presence of the BPD pendant is expected to
Y.; Heeger, A. J. Mol. Cryst. Liq. Cryst. 1994, 256, 537.
(27) (a) Gao, Y.; Yu, G.; Zhang, C.; Menon, R.; Heeger, A. J. Synth. increase its electron-transport ability resulting in the Met. 1997, 87, 171. (b) Aleshin, A. N.; Williams, S. R.; Heeger, A. J.
more favorable balance in injection and transport of Synth. Met. 1998, 94, 173. (c) Carter, S. A.; Angelopoluos, M.; Karg,
holes and electrons, as shown by us in the recent report S.; Brock, P. J.; Scott, J. C. Appl. Phys. Lett. 1997, 70, 2067. (d) Carter,
J. C.; Grizzi, I.; Heeks, S. K.; Lacey, D. J.; Latham, S. G.; May, P. G.;
on the transient EL30 behavior of P-1 and also by the
De los Panos, O. R.; Pichler, K.; Towns, C. R.; Wittmann, H. F. Appl.
Phys. Lett. 1997, 71, 34.
(28) Lee, S. T.; Wang, Y. M.; Hou, X. Y.; Tang, C. W. Appl. Phys. (29) (a) Gaudart, L.; Riviora, R. Appl. Opt. 1971, 10, 2336. (b)
Lett. 1999, 74, 670.
Ovchinnikov, A. P.; Tsarev, B. M. Sov. Phys. Solid State 1968, 9, 2766.
Chem. Mater., Vol. 13, No. 2, 2001 Lee et al. device characteristics of P-1 and P-2 described in the
tetrachloroethane, and acetonitrile were purifed by the method present investigation. In the light of less than satisfac- in ref 31. All other compounds were used as received.
tory device performance of P-1 and P-2, it is conjectured
that the BPD pendants are not so effective as a hole zine (1). Method i. The mixture of methyl 4-tert-butylbenzoate
(30.0 g, 156 mmol) and hydrazine monohydrate (31.2 g, 624
blocker although it increases electron transport. The mmol) was refluxed for 24 h in 600 mL of ethanol. After the presence of the bulky alkoxy pendant provides a further reaction was completed, the mixture were cooled to room increase in EL performance, which is explained by the temperature and then poured into cold water to precipitate increased interchain distance by their presence result- the white solid, which was collected on a filter and washed ing in less probability for the formation of interchain with hexane and small amount (ca. 100 mL) of cold water to polaron pairs claimed to give rise to a low quantum remove the unreacted starting materials. The product, 4-tert-butylbenzylhydrazide, was dried for 1 h by vacuum filtration efficiency due to their tendency to undergo radiationless followed by drying in a vacuum oven for 1 day. The product decay. Promotion of a better contact between the yield was 83% (24.9 g). Mp: 126 °C.
inorganic electrode and organic light-emitting layer and Method ii. To 18.5 g (104 mmol) of 4-bromobenzoic acid, 50 utilization of a low work function metal cathode vastly mL of SOCl2 was added, and the mixture was refluxed for 6 h improve the LED device efficiency.
with a catalytic amount of purified pyridine to give 4-bro-mobenzoyl chloride. The excess SOCl2 was removed by vacuumdistillation, and the reaction mixture was cooled to room temperature. After 30 min, 20.0 g of 4-tert-butylbenzylhy-drazide and 8.4 mL (104 mmol) of pyridine dissolved in 100 Measurements. 1H NMR (300 MHz) and IR spectra were
mL THF were added to the reaction flask containing 4-bro- recorded on a Varian AM 300 spectrometer and on a Bomem mobenzoyl chloride through a dropping funnel over a period MB FT-IR instrument, respectively. Elemental analyses were of 20 min. White precipitates were generated immediately.
performed by the Center for Organic Reactions, Sogang After being stirred for 2 h, the reaction mixture was poured University, Seoul, Korea, using an Eager 200 elemental into distilled water. The product was collected on a filter and analyzer. The purity of products was also determined by a washed with water and then dried in a vacuum oven. The yield combination of TLC on silica gel plates (MERCK, silica gel 60 was 79% (30.8 g). Mp: 224 °C. 1H NMR (300 MHz, DMSO-d6, F254) with UV lamp (254 or 365 nm) and a visualization ppm): 1.31 (s, 9H, -C(CH3)3), 7.54 (d, 2H, Ar-H), 7.75 (d, 2H, reagent. Thermal properties were studied under a nitrogen Ar-H), 7.86 (d, 2H, Ar-H), 7.87 (d, 2H, Ar-H), 10.48 (s, 1H, atmosphere on a Mettler DSC 821e instrument. Thermogravi- -NHNH-), 10.62 (s, 1H, -NHNH-). Anal. Calcd for metric analysis (TGA) was also performed under a nitrogen C18H19BrN2O2: C, 57.6; H, 5.1; N, 7.5. Found: C, 57.6; H, 5.2; atmosphere at the heating/cooling rate 10 °C/min on a DuPont 9900 thermogravimetric analyzer. GPC analysis was con- ducted with a Waters GPC 410 system equipped with five Compound 1 (30.0 g, 80.0 mmol) was placed in a 500 mL two-
Ultra-µ-stragel columns (2 × 105, 104, 103, 500 Å) in THF at a necked round-bottomed flask. POCl flow rate of 1.0 mL/min at 30 °C using polystyrene as the 3 (250 mL) was added to the flask. The mixture were refluxed for 6 h under a nitrogen calibration standard. The UV-vis spectra of the polymer films atmosphere. After the completion of the reaction was confirmed were obtained on a Hewlett-Packard 8452A diode array by TLC, the reaction mixture was slowly poured into cold water spectrophotometer. The thickness of polymers were determined in a ice bath and 0.5 M NaOH solution was added to neutralize by a TENCOR P-10 surface profiler. The ultraviolet photo- the reaction mixture. Then the precipitate was collected on a electron spectroscopy (UPS) data were acquired at room filter, washed with distilled water, and finally recrystallized temperature with a VG ESCALab 220i spectrometer (Manches- from ethanol/water ) 3:1 (v/v). The yield was 84% (24.0 g).
ter, U.K.) with a VG UV lamp. UPS analysis was performed Mp: 134 °C. 1H NMR (300 MHz, CDCl using He I (21.2 eV) photons. The base pressure of the analysis chamber was lower than 1 × 10-10 Torr, and the combined 3)3), 7.50 (d, 2H, Ar-H), 7.73 (d, 2H, Ar-H), 8.01 (d, 2H, Ar-H), 8.05 (d, 2H, Ar-H). Anal. Calcd for C instrumental resolution was about 0.1 eV. Atomic force 60.5; H, 4.8; N, 7.8. Found: C, 60.2; H, 4.9; N, 7.6.
microscopy (AFM) was conducted on a AutoProbe CP (ParkScientific Instruments) by the Korea University Engineering Laboratory Center, Seoul, Korea.
oxadiazole (3). To compound 2 (17.7 g, 50.0 mmol) dissolved
in 150 mL of toluene was added (PPh3)4Pd (2.89 g, 2.50 mmol)
The luminescence spectra for the polymers were recorded and 50 mL of 2 M Na2CO3 (100 mmol). The mixture was stirred on an AMINCO-Bowman Series 2 luminescence spectrometer for 5 min under a nitrogen atmosphere. 2,5-Dimethylphenyl- at room temperature. The current and luminescence intensity boronic acid (9.0 g, 60 mmol) dissolved in 10 mL of ethanol as a function of applied field were measured using an assembly was added to the reaction pot, and the mixture was refluxed consisting of PC-based dc power supply (HP 6623A) and a for 24 h. And then, the reaction mixture was cooled to room digital multimeter (HP 34401), and also a light power meter temperature. Dark impurities were removed by filtration using (Newport Instruments, model 818-UV) was used to measure Celite and charcoal as a filter aids. Solvents were removed by the device light output in microwatts. Luminance was mea- evaporation under a reduced pressure, and the crude product sured by a Minolta LS-100 luminance meter. A picosecond was purified by column chromatography on silica gel using time-correlated single photon counting (TCSPC) system was n-hexane/ethyl acetate ) 1/6 (v/v) as an eluent. The product employed for the time-resolved fluorescence decay measure- is viscous liquid. The yield was 73% (14.0 g). 1H NMR (300 ments. The system was consisted of cavity dumped dual-jet MHz, CDCl3, ppm): 1.39 (s, 9H, -C(CH3)3,), 2.31 (s, 3H, Ar- dye laser (700 series, coherent) that was synchronously CH3), 2.38 (s, 3H, Ar-CH3), 7.04-7.24 (m, 3H, Ar-H), 7.48 (d, pumped by Nd:YAG laser (Antares 76-YAG, coherent). The full 2H, Ar-H), 7.59 (d, 2H, Ar-H), 8.12 (d, 2H, Ar-H), 8.20 (d, 2H, width at half-maximum of the instrumental response function Ar-H). Anal. Calcd for C26H26N2O: C, 81.6; H, 7.0; N, 7.3.
was 67 ps. The fluorescence decays were measured at magic Found: C, 81.6; H, 6.9; N, 7.3.
angle emission polarization, and their exponential fittings were performed by the least-squares deconvolution fitting method.
1,3,4-oxadiazole (M-1). A mixture of compound 3 (8.00 g, 20.9
Materials. All the compounds were purchased from Aldrich,
mmol), N-bromosuccinimide (8.18 g, 46.0 mmol), and benzoyl TCI, or Fluka chemicals, and thionyl chloride, tetrahydrofuran, peroxide (1.00 g, 4.18 mmol) was dissolved in 200 mL of CCl4, pyridine, toluene, N-bromosuccinimide, benzoyl peroxide, 1,1,2,2- and the mixture was refluxed for 2 h under nitrogen atmo- (30) Jang, J. W.; Oh, D. K.; Lee, C. H.; Lee, C. E.; Lee, D. W.; Jin, (31) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory J.-I. J. Appl. Phys. 2000, 87, 3183.
Chemicals, 3rd ed.; Pergamon Press: New York, 1988.
Structurally Modified PPVs Chem. Mater., Vol. 13, No. 2, 2001 sphere. The mixture was cooled to room temperature, and Ar-H), 7.47 (d, 2H, Ar-H), 7.57 (d, 2H, Ar-H), 8.09 (d, 2H, Ar- insolubles were removed by filtration. The solvent was re- H), 8.16 (d, 2H, Ar-H). Anal. Calcd for C26H26N2O2: C, 78.3; moved by evaporation under a reduced pressure to obtain H, 6.6; N, 7.0. Found: C, 78.1; H, 6.6; N, 7.1.
white crystals. The crude product was washed with 200 mL of n-hexane and further purified by column chromatography biphenylyl]-1,3,4-oxadiazole (6). A mixture of 8.70 g of 2-eth-
on a silica gel column using n-hexane/ethyl acetate ) 5/1 (v/ ylhexyl bromide (45.2 mmol), compound 5 (9.00 g, 22.6 mmol),
v) as an eluent. The yield was 42% (4.74 g). Mp: 115 °C. 1H and K2CO3 (6.32 g, 45.2 mmol) in 150 mL of acetonitrile was NMR (300 MHz, CDCl3, ppm): 1.39 (s, 9H, -C(CH3)3,), 4.42 refluxed for 12 h to give compound 6. After completion of
(s, 2H, Ar-CH2Br), 4.53 (s, 2H, Ar-CH2Br), 7.25 (s, 1H, reaction was confirmed by TLC, the reaction mixture was Ar-H), 7.37-7.49 (m, 2H, Ar-H), 7.59 (d, 2H, Ar-H), 7.64 (d, filtered to remove undissolved K2CO3, and the filtrate was 2H, Ar-H), 8.10 (d, 2H, Ar-H), 8.27 (d, 2H, Ar-H). Anal. Calcd evaporated at a reduced pressure. The crude product was for C26H24Br2N2O: C, 57.4; H, 4.5; N, 5.2. Found: C, 57.8; H, purified by column chromatography on a silica gel column 4.6; N, 5.2.
using n-hexane/ethyl acetate ) 4/1 (v/v) as an eluent. The product is viscous liquid. The yield was 92% (10.61 g). 1H NMR 1,4-phenylenevinylene] (P-1). Monomer M-1 (1.00 g, 1.85 mmol)
(300 MHz, CDCl3, ppm): 0.88-1.00 (m, 6H, -CH3), 1.37 (s, was dissolved in 100 mL of THF, and the polymerization flask 9H, -C(CH3)3), 1.35-1.60 (m, 8H, -CH(CH2CH3)CH2CH2CH2- was charged with argon. After the inner atmosphere was fully 3), 1.78-1.84 (m, 1H, -CH2CH(CH2 )CH2 ), 2.24 (s, 3H, refreshed with argon, 11.1 mL of potassium tert-butoxide (1.0 Ar-CH3), 2.29 (s, 3H, Ar-CH3), 3.90 (d, 2H, -OCH2CH-), 6.75 M solution in THF, 11.1 mmol) was added dropwisely over a (s, 1H, Ar-H), 7.05 (s, 1H, Ar-H), 7.47 (d, 2H, Ar-H), 7.55 (d, period of 5 min. After the reaction proceeded 4 h, the reaction 2H, Ar-H), 8.08 (d, 2H, Ar-H), 8.16 (d, 2H, Ar-H). Anal. Calcd mixture was poured into a 5:1 mixture of methanol and for C34H42N2O2: C, 80.0; H, 8.3; N, 5.5. Found: C, 79.9; H, distilled water not only to precipitate the polymer but also to 8.3; N, 5.4.
remove the unreacted base and produced salts. The precipi- tated polymer was collected on a filter. The polymer was 5-(4-tert-butylphenyl)-1,3,4-oxadiazole (M-2). A mixture of
dissolved in 1,1,2,2-tetrachloroethane and reprecipitated in compound 6 (5.11 g, 10.0 mmol), N-bromosuccinimide (3.92 g,
methanol, and this dissolution-precipitation process was 22.0 mmol), and benzoyl peroxide (0.48 g, 2.0 mmol) was repeated twice more. Finally, the polymer solid was ground dissolved in 200 mL of CCl4, and the mixture was heated and low molecular weight materials were removed by Soxhlet slowly to and kept at 76 °C under nitrogen atmosphere for 4 extraction using methanol and acetone for 2 days, respectively.
h. After the completion of the reaction was confirmed by TLC, The product has orange color, and the recovery yield was 43% the mixture was cooled to room temperature. And the in- (0.30 g). 1H NMR (300 MHz, 1,1,2,2-tetrachloroethane-d2, solubles were removed by filtration. The CCl4 solvent was ppm): 1.22 (s, 9H, -C(CH3)3,), 6.90-7.11 (br m, 2H, -CHd removed by evaporation under a reduced pressure to obtain a CH-), 7.25-8.23 (m, 11H, Ar-H). Anal. Calcd for C26H22N2O: crude product. It was first purified by recrystallization from C, 82.5; H, 5.9; N, 7.4. Found: C, 82.3; H, 6.0; N, 7.3.
methylene chloride/methanol ) 1/5 (v/v) twice. Finally, the product was purified by column chromatography on a silica lyl}-1,3,4-oxadiazole (4). Compounds 4-6 and M-2 were
gel column using n-hexane/ethyl acetate ) 5/1 (v/v) as an synthesized as shown in Scheme 2. Compound 2 (14.2 g, 40.0
eluent. The yield was 40% (2.67 g). Mp: 132 °C. 1H NMR (300 mmol) dissolved in 150 mL of toluene was mixed with MHz, CDCl3, ppm): 0.87-1.04 (m, 6H, -CH2CH3), 1.38 (s, 9H, 3)4Pd (2.89 g, 2.50 mmol) and 40 mL of 2 M Na2CO3 under 3)3), 1.43-1.64 (m, 8H, -CH(CH2CH3)CH2CH2CH2CH3), 1.78-1.89 (m, 1H, -CH )CH2 ), 4.10 (d, 2H, acid (10.8 g, 60.0 mmol) dissolved in 10 mL of ethanol was -OCH2CH-), 4.43 (s, 2H, Ar-CH2Br), 4.56 (s, 2H, charged into the reaction flask. The reaction mixture was Ar-CH2Br), 7.03 (s, 1H, Ar-H), 7.26 (s, 1H, Ar-H), 7.57 (d, 2H, refluxed for 24 h. Dark impurities formed were removed by Ar-H), 7.61 (d, 2H, Ar-H), 8.18 (d, 2H, Ar-H), 8.22 (d, 2H, filtration using Celite and charcoal as a filter aids. Then, Ar-H). Anal. Calcd for C34H40Br2N2O2: C, 61.1; H, 6.0; N, 4.2.
solvents in the filtrate were evaporated out under a reduced Found: C, 61.1; H, 6.0; N, 4.2.
pressure and the crude product was purified by column chromatography on a silica gel column using n-hexane/ethyl acetate ) 1/5 (v/v) as an eluent. The yield was 59% (12.17 g).
was synthesized as the same manner as used in the synthesis Mp: 149 °C. 1H NMR (300 MHz, CDCl3, ppm): 1.38 (s, 9H, of P-1. The product has orange color, and the recovery yield
-C(CH3)3), 2.24 (s, 3H, Ar-CH3), 2.31 (s, 3H, Ar-CH3), 3.88 was 40% (0.30 g). 1H NMR (300 MHz, 1,1,2,2-tetrachlorothane- (s, 3H, -OCH3), 6.76 (s, 1H, Ar-H), 7.06 (s, 1H, Ar-H), 7.48 (d, d2, ppm): 0.69-0.98 (br, 6H, -CH2CH3), 1.08-1.80 (br, 18H, 2H, Ar-H), 7.56 (d, 2H, Ar-H), 8.09 (d, 2H, Ar-H), 8.17 (d, 2H, Ar-H). Anal. Calcd for C27H28N2O2: C, 78.6; H, 6.8; N, 6.8; O, d, 2H, -OCH2CH3), 6.93-7.76 (br m, 6H, -Ar-CH)CH- 7.8. Found: C, 78.4; H, 6.9; N, 7.3.
Ar),7.90-8.19 (br m, 2H, Ar-H). Anal. Calcd for C34H38N2O2: C, 80.1; H, 7.6; N, 5.5. Found: C, 80.7; H, 7.7; N, 5.5.
lyl}-1,3,4-oxadiazole (5). Compound 4 (10.0 g, 24.2 mmol) was
Device Fabrication. The ITO-coated glass (1.1 × 1.2 cm2)
dissolved in 50 mL of methylene chloride, and the solution was with a sheet resistance of 25 Ω cm-1 (LG Co., Korea) was cooled to -20 °C. BBr3 solution (1.0 M in methylene chloride, patterned by immersing into the concentrated HCl solution 45.0 mL, 45.0 mmol) was added dropwise to the reaction flask for 15 min and washed in the stream of water. The glass was over a period of 30 min while the temperature was maintained further cleaned by sequential ultrasonication in acetone, at -20 °C for 1 h. Then the bath was warmed to 0 °C. After methanol, distilled water, acetone, and 2-propanol for 10 min, the mixture was kept at 0 °C for 2 h, when the completion of respectively, and finally dried in a stream of dry nitrogen. The the reaction was confirmed by TLC, the mixture was poured polymer solution in 1,1,2,2-tetrachloroethane (ca. 15 mg/mL) into the distilled water to quench the excess BBr3. The product filtered through a syringe filter (Nalgene, 0.45 µm) was spin- formed was extracted with methylene chloride three times (150 coated onto the cleaned glass substrate under an argon mL × 3). After the extraction solution washed with water and atmosphere using a Laurell spin coater to obtain films about brine sequentially, the solution was dried over 15 g of MgSO4 600-800 nm thick. In the case of P-2(3), the coated films were
for 2 h. And then the drying agent was removed by filtration.
thermally treated at 150 °C for 3 h in a vacuum to the final Removal of the solvent by evaporation at a reduced pressure polyconjugated polymers. For ITO/PEDOT/polymer/Li:Al de- produced a white solid, which was purified by column chro- vice, PEDOT solution doped with polystyrenesulfonate (PSS) matography on a silica gel using methylene chloride as an (Bayer; 10 S cm-1) was spin coated onto the ITO-coated glass eluent. Mp: 237 °C. 1H NMR (300 MHz, CDCl3, ppm): 1.38 substrate preliminarily to obtain 20 nm thick films. Then, the (s, 9H, -C(CH3)3,), 2.24 (s, 3H, Ar-CH3), 2.29 (s, 3H, Ar-CH3), Al or Al:Li electrode 1200 Å thick was vapor deposited on the 5.82-6.09 (br s, 1H, Ar-OH), 6.78 (s, 1H, Ar-H), 7.04 (s, 1H, polymer layer using a Leybold L560 (Ko¨lm, Germany) Chem. Mater., Vol. 13, No. 2, 2001 Lee et al. apparatus at a deposition rate of 5 Å/s at pressure of 1.0 × spectively. And all theses steps were conducted in an argon 10-6 Torr. Deposition of the cathode electrodes were conducted filled glovebox without exposing to air.
by the Korea Basic Science Institute-Seoul Branch, Korea.
The active layer of the device was 4.91 mm2.
Acknowledgment. This research was supported by
In the case of the ITO/polymer/Ca/Al devices, they were the Ministry of Science and Technology, Republic of fabricated with ITO coated glass with a sheet resistance of 15Ω cm-1, and ITO layers were partially etched. After being Korea, and the Korea Science and Engineering Founda- etched, the glass substrates were cleaned by UVO (ultraviolet- tion through the Center for Electro- and Photo-Respon- ozone) cleaner in water first and then by ultrasonication in sive Molecules, Korea University. D.W.L. wishes to isopropyl alcohol and acetone sequentially for 13 min, respec- acknowledge the financial support (Post Graduate Schol- tively. And the P-2 solutions in 1,1,2,2-tetrachloroethane (1.0
arship) by Daewoo Foundation. Y.P. was partially wt %) were spin coated to obtain 70 nm thick films. Finally, supported by the MOST and the KOSEF through the Ca and Al cathodes were vacuum deposited from the tungstenboat at deposition rates of 2 and 4 Å/s, respectively, at the National Research Laboratory Program and the Atomic- pressure of 3.0 × 10-7 Torr. Luminescence properties were Scale Surface Science Research Center, Yonsei Univer- recorded on ISS PC1 (ISS Inc.) photon counting spectrofluo- sity, Seoul. We also are grateful to Byung-Hee Sohn at rometer, and film thickness was determined by a Tencor P-10 the Samsung Advanced Institute of Technology for his surface profilmeter. Current-voltage (I-V) and luminance help in the preparation of some of the LED devices.
curves were obtained by Keithley 238 electrometer and TopconBM-7 luminance colormeter (Topcon Technologies, Inc.), re-

Source: http://spond.khu.ac.kr/file/23.CM2001-13-565.pdf