Novel photoluminescent polymers containing oligothiophene and m-phenylene-1,3,4-oxadiazole moieties: Synthesis and spectroscopic and electrochemical studies

Article abstract of DOI:10.1021/jo991359c

Three conjugated polymers containing oligothiophene units (from one to three thiophene rings) and aromatic 1,3,4-oxadiazole moieties have been successfully synthesized. The polymer structures were characterized and confirmed by ¹H and ¹³C NMR, FT-IR, and elemental analysis. Thermogravimetric analysis demonstrated that the polymers are highly thermal stable. Tunable absorption (from 342 to 428 nm) and fluorescence (from 411 to 558 nm) properties of polymers were observed: The electrochemical investigation indicated that the LUMO and HOMO energy levels of the new polymers could be adjusted. It was also revealed by the electrochemical analysis that the polymers have good charge injection properties for both p-type and n-type charge carriers, as well as good color tunable luminescence and film-forming properties, which makes them potentially useful for fabricating efficient light-emitting devices.

Full text of DOI:10.1021/jo991359c

3894
J . Org. Chem. 2000, 65, 3894-3901
Novel P h otolu m in escen t P olym er s Con ta in in g Oligoth iop h en e a n d
m -P h en ylen e-1,3,4-oxa d ia zole Moieties: Syn th esis a n d
Sp ectr oscop ic a n d Electr och em ica l Stu d ies
Hong Meng and Wei Huang*
Institute of Materials Research and Engineering, National University of Singapore, 3 Research Link,
Singapore 117602, Republic of Singapore
Received August 25, 1999
Three conjugated polymers containing oligothiophene units (from one to three thiophene rings)
and aromatic 1,3,4-oxadiazole moieties have been successfully synthesized. The polymer structures
1
were characterized and confirmed by H and 13C NMR, FT-IR, and elemental analysis. Thermo-
gravimetric analysis demonstrated that the polymers are highly thermal stable. Tunable absorption
(from 342 to 428 nm) and fluorescence (from 411 to 558 nm) properties of polymers were observed.
The electrochemical investigation indicated that the LUMO and HOMO energy levels of the new
polymers could be adjusted. It was also revealed by the electrochemical analysis that the polymers
have good charge injection properties for both p-type and n-type charge carriers, as well as good
color tunable luminescence and film-forming properties, which makes them potentially useful for
fabricating efficient light-emitting devices.
In tr od u ction
transporting holes) and mainly conduct holes.13 Poor
electron injection has been recognized to limit device
efficiency.14,15 There are few known polymers with good
electron-injection capability because of the difficulty in
synthesis approaches. In the past few years, a number
of polymers with oxadiazole units as side groups or as
part of the polymer main chain have fit this position as
efficient materials for electron transport and emission
layers.16-18 Aromatic oxadiazole compounds such as 1,3,4-
oxadiazoles have been investigated as electron-transport-
ing materials within multilayered devices.19 Our initial
work has also proven that good results could be achieved
if substituted thiophene and 1,4-benzene-1,3,4-oxadiazole
moiety (PRTOBO) (Figure 1) are combined as function
materials used in the fabrication of blue LEDs.20 There-
fore, further development and exploitation of this kind
of new materials with a wide range of color emission as
well as a better charge balance for polymer LEDs is very
interesting.
Polythiophenes (PTs) occupy a prominent position in
polymer electronics because of their unique electronic and
optical properties.1,2 Their amenability to chemical modi-
fications and reversible redox properties offer additional
reasons for further investigation of this class of materi-
als.3,4 Recent research of conjugated polymers has been
devoted to their application of light-emitting diodes
(LEDs) and other optoelectronic devices.5-7 Among con-
jugated polymers used as electroluminescent materials,
PTs and poly(p-phenylene vinylene)s (PPVs) have been
most widely investigated.8-10 Substituted PTs permit
color tuning by means of controlling the conjugation
length of thiophene rings, which has been widely ex-
ploited.11 However, because of their relatively low fluo-
rescent quantum yields, their application toward this goal
is limited.12 Also, conjugated polymers such as PPV, PT,
and their derivatives tend to be p-type (that is, prefer
It is well established that various conjugation lengths
can be obtained in the polythiophene system by judicious
design of the polymer geometry.21 The oligothiophene
blocks exhibit high π-electron density, while the aromatic
oxadiazole blocks show high electron affinity. The length
of the oligothiophene block leaves a possibility of tuning
the luminescence of the polymers, whereas the aromatic
oxadiazole block facilitates the electron-transfer proper-
* Tel: (65) 874 8593/8592. Fax: (65) 874 8592. E-mail: wei-
huang@imre.org.sg.
(1) (a) Roncali, J . Chem. Rev. 1992, 92, 711. (b) Patil, A. O.; Heeger,
A. J .; Wudl, F. Chem. Rev. 1988, 88, 183.
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J ., Marcel, D.,Eds.; New York, 1986; Vol. I.
(3) Granstrom, M. Polym. Adv. Technol. 1997, 8, 424.
(4) Li, J .; Pang, Y. Macromolecules 1997, 30, 7487.
(5) (a) Kraft, A. K.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem.,
Int. Ed. 1998, 37, 402. (b) Tamato, K.; Kodama, S.; Nakajima, I.;
Kumada, M. Tetrahedron 1982, 38, 3347.
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Phys. 1992, 72, 564.
(7) (a) Chen, F.; Mehta, P. G.; Takiff, L.; McCullough, R. D. J . Mater.
Chem. 1996, 6, 1763. (b) Andersson, M. R.; Berggren, M.; Ingana¨s, O.;
Gustafsson, G.; Gustafsson-Carlberg, J . C.; Selse, D.; Hiertberg, T.;
Wennerstro¨m, O. Macromolecules 1995, 28, 7525.
(13) Strukeli, M.; Papadimitrakopoulos, F.; Miller, T. M.; Rothberg,
L. J . Science 1995, 267, 1969.
(14) Yang, Y.; Pei, Q. J . Appl. Phys. 1995, 77, 4807.
(15) Li, X.; Cacialli, F.; Giles, M.; Gruner, J .; Friend, R. H.; Holmes,
A. B.; Moratti, S. C.; Yong, T. M. Adv. Mater. 1995, 7, 898.
(16) Tamoto, N.; Adachi, C.; Nagai, K. Chem. Mater. 1997, 9, 1077.
(17) Pei, Q.; Yang, Y. Chem. Mater. 1995, 7, 1568.
(18) Peng, Z.; Bao, Z.; Galvin, M. E. Adv. Mater. 1998, 10, 680.
(19) (a) Brown, A. R.; Bradley, D. D. C.; Burroughes, J . H.; Friend,
R. H.; Greenham, N. C. Appl. Phys. Lett. 1992, 61, 2793. (b) Adachi,
C.; Tokito, S.; Tsutsui, T.; Saito, S. J pn. J . Appl. Phys. 1988, 27, L713.
(20) (a) Huang, W.; Meng, H.; Yu, W.-L.; Gao, J .; Heeger, A. J . Adv.
Mater. 1998, 10, 593. (b) Yu, W.-L.; Meng, H.; Pei, J .; Lai, Y. H.; Chua,
S. J .; Huang, W. Chem. Commun. 1998, 18, 1957. (c) Yu, W.-L.; Meng,
H.; Pei, J .; Huang, W. J . Am. Chem. Soc. 1998, 120, 11808.
(21) Kang, B. S.; Seo, M.-L.; J un, Y. S.; Lee, C. K.; Shin, S. C. Chem.
Commun. 1996, 1167.
(8) Cornil, J .; Beljonne, D.; dos Santos, D. A.; Bre´das, J . L. Synth.
Met. 1996, 76, 101.
(9) Baingent, D. R.; Greenham, N. C.; Gru¨ner, J .; Marks, R. N.;
Friend, R. H.; Moratti, S. C.; Holmes, A. B. Synth. Met. 1994, 67, 3.
(10) Hilberer, A.; Brouwer, H. J .; van der Scheer, B. J .; Wildeman,
J .; Hadziioannou, G. Macromolecules 1995, 28, 4525.
(11) (a) Berggren, M.; Gustafsson, G.; Rasmusson, J .; Andersson,
M. R.; Hjertberg, T.; Wennerstrom, O. Nature 1994, 372, 444. (b)
Berggren, M.; Ingana¨s, O.; Gustafsson, G. Synth. Met. 1995, 71, 2185.
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10.1021/jo991359c CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/31/2000

Novel Photoluminescent Polymers
J . Org. Chem., Vol. 65, No. 13, 2000 3895
F igu r e 1. Novel polymers containing oligothiophenes and aryl-1,3,4-oxadiazole.
diamine (TMEDA), and pyridine (purchased from Aldrich, A.R.
grade) were redistilled prior to use. 2,2′-Bithiophene, 1-bromo-
decane, dichloro-[1,3-bis(diphenylphosphino)propane]nickel(II)
(Ni(dppp)Cl2), magnesium turnings, thionyl chloride, and
phosphorus oxychloride were used as received (Fluka or
Aldrich chemicals).
ties of the polymers when used in LEDs. Materials
composed of a hole-conducting block and an electron-
transporting segment may combine properties of both
components to produce new properties that are not
possible with either one separately. PLED devices based
on such materials can be expected to have high quantum
efficiencies because the limitation of low electron affinity
of normal emissive polymeric materials used could be
improved by introducing the oxadiazole segment into the
polymer backbone. In addition, it is expected that single
layer devices can be fabricated with a satisfactory ef-
ficiency performance, and such an approach can simplify
the device configuration and diminish problems in mul-
tilayer devices such as the space-charge and tunneling
of accumulated holes.
Herein we are further extending our effort toward
these novel materials. We have successfully obtained
three new polymers (PmTOMBO) (see Figure 1) with
different emission colors. Such a design strategy provides
a feasible way for the wavelength tuning of new emissive
polymers. With the use of m-phenylene in the polymer
main chain, the repulsive interaction between the adja-
cent aromatic rings results in a distortion of the π-con-
jugated backbone.22 The emitting colors of such polymers,
from blue to orange, were achieved by varying the
conjugation length of oligothiophene moiety.
Syn th esis of Mon om er s. 3-Octylth iop h en e.23 Into a
three-necked flask containing Ni(dppp)Cl2 (193 mg, 0.36 mmol)
was injected through a syringe 3-bromothiophene (20.0 g, 0.122
mol) in 100 mL of diethyl ether. The reaction mixture cooled
to 0 °C and was placed under N2 and 1-octylmagnesium
bromide prepared from 0.158 mol of 1-bromooctane and 4.10
g (0.174 mol) of Mg in 28 mL of dry ether was added dropwise
into the mixture over 1.5 h. After refluxing for an additional
24 h, the reaction mixture was quenched with 0.2 mol/dm3 HCl
and extracted with diethyl ether. The organic phase was
washed three times with H2O and once with saturated NaCl
and then dried over anhydrous Na2SO4. After removal of the
solvent, the crude product was distilled (80-81 °C/0.2 mmHg)
to give 17.1 g (73%) of the product. MS m/z: 196. 1H NMR
(CDCl3, ppm): δ 7.23 (d, J ) 4.5 Hz, 1H), 6.93 (m, 2H), 2.62-
2.58 (t, J ) 7.3 Hz, 2H), 1.64-1.56 (m, 2H), 1.38-1.20 (m,
10H), 0.88-0.85 (t, J ) 5.7 Hz, 3H).
3,3′,5,5′-Tetr a br om o-2,2′-bith iop h en e.24 Bromine (190 g,
0.59 mol) was added dropwise over 1.5 h to a solution of 2,2′-
bithiophene (46.2 g, 0.278 mol) in the mixed solvent of glacial
acetic acid (200 mL) and chloroform (450 mL) at a temperature
of 5-15 °C. The mixture was stirred at room temperature for
5 h and then under reflux for 24 h. After the mixture was
cooled to room temperature, 500 mL of 10% KOH aqueous
solution was added. The mixture was extracted with CHCl3
twice; the combined chloroform layer was washed with water,
dried over anhydrous MgSO4, and filtered; and the solvent was
removed by rotary evaporation. Recrystallization from ethanol
afforded a light yellow crystal in a yield of 89%. Mp: 139-
141 °C (lit. 139-140 °C). MS m/z: 482. 1H NMR (CDCl3,
ppm): δ 7.05 (s, 2H).
Exp er im en ta l Section
The synthetic procedures for the polymers are depicted in
Schemes 1 and 2.
Mea su r em en ts. NMR spectra were collected on a Bruker
ACF 300 spectrometer with chloroform-d as solvent and
tetramethylsilane as internal standard. FT-IR spectra were
recorded on a Bio-Rad FTS 165 spectrometer by dispersing
samples in KBr disks. UV-visible and fluorescence spectra
were obtained on a Shimadzu UV 3101PC UV-Vis-NIR
spectrophotometer and Perkin-Elmer LS 50B luminescence
spectrometer with a Xenon lamp as light source, respectively.
Thermogravimetric analysis (TGA) was conducted on a DuPont
Thermal Analyst 2100 system with a TGA 2950 thermogravi-
metric analyzer under a heating rate of 20 °C/min and an air
or nitrogen flow rate of 75 cm3/min. Differential scanning
calorimetry (DSC) was run on a DuPont DSC 2910 module in
conjunction with the DuPont Thermal Analyst system. Ele-
mental analysis was performed on a Perkin-Elmer 240C
elemental analyzer for C, H, N, and S determinations. Cyclic
voltammograms were obtained using an EG&G model 273A
potentiostat/galvanostat under argon atmosphere. All poten-
tials were measured against a Ag/AgNO3 0.10 mol/dm3 (aceto-
nitrile) electrode (0.34 V vs SCE), and all experimental values
in this report were corrected with respect to SCE. GPC
analysis was conducted with a Perkin-Elmer model 200 HPLC
system equipped with Phenogel MXL and MXM columns,
using polystyrene as standard and THF as eluant.
3,3′-Dib r om o-2,2′-b it h iop h en e.25 3,3′,5,5′-Tetrabromo-
2,2′-bithiophene (48.2 g, 0.10 mol) is added in portions within
0.5 h to a refluxing zinc powder (25.2 g, 0.40 mol) dispersion
in 500 mL of ethanol containing 50 mL of water, 120 mL of
acetic acid, and 10 mL of 2 mol/dm3 HCl. After refluxing for
additional 2 h and then cooling to room temperature, the
mixture was filtered and washed three times with ethanol,
and the filtrate was collected. The solvent was removed by
evaporation, and then 250 mL of H2O was added. The mixture
was extracted with ether, and the combined ether was washed
with water, dried over anhydrous MgSO4, and filtered. Re-
moval of the solvent and recrystallization from hexane gave a
light green crystal (yield 79%). Mp: 98-99 °C. MS m/z: 324.
1H NMR (CDCl3, ppm): δ 7.41 (d, J ) 5.3 Hz, 2H), 7.08 (d, J
) 5.3 Hz, 2H).
3,3′-Did ecyl-2,2′-bith iop h en e.26 A Grignard reagent pre-
pared from 1-bromodecane (44.2 g, 0.20 mol) and Mg turnings
(5.0 g, 0.21 mol) in 60 mL of diethyl ether was added dropwise
to a solution of 3,3′-dibromo-2,2′-bithiophene (22.7 g, 0.070 mol)
and Ni(dppp)Cl2 (622 mg, 1.19 mmol) in 100 mL of diethyl
ether at 0 °C. After the addition was complete, stirring was
Ma ter ia ls. Diethyl ether was distilled over sodium/ben-
zophenone, and hexane was distilled over calcium hydride.
N-Methylpyrrolidone (NMP), N,N,N′,N′-tetramethylethylene-
(23) McCullough, R. D.; Lowe, R. D.; J ayaraman, M.; Anderson, D.
L. J . Org. Chem. 1993, 58, 904.
(24) Yue, J .; Epstein, A. J . J . Am. Chem. Soc. 1990, 112, 2800.
(25) Gronowitz, S.; Raznikiewicz, T. Org. Synth. 1964, 44, 9.
(26) Tamao, K.; Kodama, S.; Naajima, I.; Kumada, M.; Minato, A.;
Suzuki, K. Tetrahedron 1982, 38, 3347.
(22) Bre´das, J . L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J .
Am. Chem. Soc. 1983, 105, 6555.

3896 J . Org. Chem., Vol. 65, No. 13, 2000
Sch em e 1. Syn th etic Rou te to Mon om er sa
Meng and Huang
aReagents and conditions: (i) 1-bromooctane, Mg, Ni(dppp)Cl2, diethyl ether; (ii) n-BuLi, TMEDA, dry ice, -70 °C to rt; (iii) SOCl2; (iv)
bromine, AcOH/CHCl3; (v) Zn, EtOH/AcOH/HCl; (vi) 1-bromodecane, Mg, Ni(dppp)Cl2, diethyl ether; (vii) NBS, CHCl3/AcOH; (viii) Mg/
THF, 2,5-dibromothiophene, Ni(dppp)Cl2, diethyl ether; (ix) CH3OH/H2SO4; (x) NH2NH2‚H2O, MeOH.
continued for 24 h under refluxing, and then the reaction was
quenched with 0.2 mol/dm3 of HCl and extracted with diethyl
ether. The combined diethyl ether solution was dried over
anhydrous Na2SO4 and filtered, and the solvent was removed
by rotary evaporation. Vacuum distillation (130-132 °C/0.05
mbar) provided 22.2 g (71%) pure product. 1H NMR (CDCl3,
ppm): δ 7.29 (d, J ) 5.4 Hz, 2H), 6.98 (d, J ) 5.4 Hz, 2H),
2.65-2.53 (t, J ) 7.5 Hz, 4H), 1.65-1.57 (m, 4H), 1.46-1.26
(m, 28H), 0.93-0.88 (t, J ) 6.2 Hz, 6H). Anal. Calcd for
anhydrous MgSO4. The solvent was removed by rotary evapo-
ration. The residue was reduced-distilled (90-92 °C/0.5 mmHg)
to yield 21.5 g (82%) of product. 1H NMR (CDCl3, ppm): δ 7.17
(d, J ) 5.6 Hz, 1H), 6.79 (d, J ) 5.6 Hz, 1H), 2.58 (t, J ) 7.5
Hz, 2H), 1.59-1.54 (m, 2H), 1.31-1.26 (m, 10H), 0.89 (t, J )
4.5 Hz, 3H). MS m/z: 275. Anal. Calcd for C12H19BrS: C, 52.36;
H, 6.96; S, 11.64; Br, 29.09. Found: C, 52.43; H, 7.29; S, 12.00;
Br, 28.80.
3,3′′-Dioctyl-2,2′;5′,2′′-ter th iop h en e.28 The Grignard re-
agent of 3-octylthienyl-2-magnesium bromide, obtained from
the reaction of 15.1 g (0.055 mol) 2-bromo-3-octylthiophene in
200 mL of ether with 1.5 g (0.062 mol) of Mg, was added
dropwise into a mixture of 2,5-dibromothiophene 6.05 g (0.025
mol) and 216 mg of Ni(dppp)Cl2 (0.8 mol %) catalyst in ether
over 1.5 h. After refluxing for additional 20 h, the reaction
mixture was quenched with 0.2 mol/dm3 HCl and extracted
with ether. The organic phase was washed with H2O three
times and once with NaCl and then was dried over Na2SO4.
After removal of solvent, a dark brown liquid was obtained. It
was diluted with hexane, decolorized with active charcoal, and
then chromatographied on silica gel using hexane as eluant.
The yield of the final compound (8.0 g, yellow liquid) after flash
chromatography is 68%. MS m/z: 472; HRMS C28H40S3: calcd
C
28H46S2: C, 75.34; H, 10.31; S, 14.35. Found: C, 75.42; H,
9.76; S, 14.09.
2-Br om o-3-octylth iop h en e.27 Into a dry round-bottom
flask was placed 100 mL of acetic acid and 100 mL of CHCl3,
and the flask was then purged with nitrogen for 10 min. Then
19.6 g (0.10 mol) of freshly distilled 3-octylthiophene was
added. The mixture was cooled to 5 °C, whereupon 17.0 g
(0.095 mol) of NBS was added over a period of 2 h while the
temperature was maintained at 5-10 °C. The mixture was
stirred overnight and cooled in an ice bath, followed by the
addition of 100 mL of 0.1 mol/dm3 HCl. The mixture was
extracted with CHCl3. The CHCl3 layer was washed with water
several times until it reached a pH of ∼6 and dried over
(27) Consiglio, G.; Gronowitz, S.; Hornfeldt, A.-B.; Maltesson, B.;
Noto, R.; Spinelli, D. Chem. Scr. 1977, 11, 175.
(28) Mao, H.; Xu, B.; Holdcroft, S. Macromolecules 1993, 26, 1163.

Novel Photoluminescent Polymers
J . Org. Chem., Vol. 65, No. 13, 2000 3897
Sch em e 2. Syn th etic Rou te to P r ep olym er s a n d F in a l P olym er sa
a
Reagents and conditions: (i) LiCl, pyridine, NMP, 80 °C; (ii) POCl3, reflux.
472.22922; found mass 472.22910; formula C28H40S3. 1H NMR
(CDCl3, ppm): δ 7.17 (d, J ) 5.6 Hz, 2H), 7.04 (s, 2H), 6.93 (d,
J ) 5.6 Hz, 2H), 2.58 (t, J ) 7.5 Hz, 4H), 1.70-1.59 (m, 4H),
1.31-1.26 (m, 20H), 0.89-0.85 (t, J ) 4.5 Hz, 6H). 13CNMR
(CDCl3, ppm): δ 139.98, 136.22, 130.48, 130.06, 126.09, 123.73,
31.89, 30.73, 29.65, 29.55, 29.34, 29.22, 22.79, 14.20. Anal.
Calcd for C28H40S3: C, 71.19; H, 8.47; S, 20.34. Found: C,
71.22; H, 8.11; S, 20.87.
saturated NaCl and dried using MgSO4. Evaporation of the
ether and recrystallization of the white powder from ethanol-
water afforded 14.0 g (78%) of product. MS m/z: 284. 1H NMR
(CDCl3, ppm): δ 7.69 (s, 1H), 2.62-2.57 (t, J ) 7.5 Hz, 2H),
1.60-1.28 (m, 12H), 0.87-0.82 (t, J ) 6.5 Hz, 3H). Anal. Calcd
for C14H20O4S: C, 59.15; H, 7.04; S, 11.27. Found: C, 58.53;
H, 7.18; S, 11.49.
3,3′-Didecyl-2,2′-bith ioph en e-5,5′-dicar boxylic Acid. The
compound was prepared as described above under the same
condition using TMEDA (2.9 g, 0.025 mol) and a solution of
n-BuLi (1.5 mol/dm3 solution in hexane, 17 mL, 0.025 mol) in
30 mL of hexane. 3,3′-Didecyl-2,2′-bithiophene (4.46 g, 0.010
mol) was added under the protection of nitrogen at room
temperature. Recrystallization from ethanol-water yielded 4.1
g (76%) of a pure light yellow powder product. 1H NMR (CDCl3,
ppm): δ 7.76 (s, 2H), 2.55-2.50 (t, J ) 7.7 Hz, 4H), 1.64-1.56
(m, 4H), 1.38-1.23 (m, 28H), 0.89-0.85 (t, J ) 5.5 Hz, 6H).
Anal. Calcd for C30H46O4S2: C, 67.42; H, 8.61; S, 11.98.
Found: C, 67.70; H, 8.75; S, 12.14.
3-Oct ylt h iop h en e-2,5-d ica r b oxylic Acid .29 To a N2-
flushed flask initially containing TMEDA (18.3 g, 0.158 mol)
in 30 mL of hexane and a solution of n-BuLi (1.6 mol/dm3
solution in hexane, 98 mL, 0.158 mol) under a nitrogen
atmosphere was added 12.4 g (0.0633 mol) of 3-octylthiophene
at room temperature. After stirring at room temperature for
1 h, the mixture was refluxed for a further 30 min, cooled to
-70 °C, and slowly poured under N2 into a 500 mL flask half-
filled with crushed dry ice. The mixture was warmed to room
temperature overnight and then poured into 10 mol/dm3 HCl
and ice. The aqueous layer was extracted with ether (two 100
mL portions). The ether extract was extracted with 10% NaOH
(100 mL), and the aqueous layer was acidified with 10 mol/
dm3 HCl. The mixture was extracted with ether (two 50 mL
portions), and the ether extract was washed with water and
3,3′′-Dioct yl-2,2′;5′,2′′-t er t h iop h en e-5,5′′-d ica r b oxylic
Acid . The compound was prepared as described above under
the same condition using 3.5 g (7.4 mmol) 3,3′′-dioctyl-
2,2′;5′,2′′-terthiophene, TMEDA, hexane, and n-BuLi. A yellow
powder was obtained upon recrystallization from ethanol.
Yield: 83%. MS m/z: 560. HRMS C30H40O4S3: calcd 560.20886;
(29) MacDowell, D. W. H.; Ballas, F. L. J . Org. Chem. 1977, 42, 3717.

3898 J . Org. Chem., Vol. 65, No. 13, 2000
Meng and Huang
found mass 560.21109; formula C30H40O4S3. 1H NMR (acetone-
d6, ppm): δ 7.68 (s, 2H), 7.35 (s, 2H), 2.89-2.84 (t, J ) 7.6
Hz, 4H), 1.76-1.74 (m, 4H), 1.42-1.27 (m, 20H), 0.89-0.84
(t, J ) 6.5 Hz, 6H). 13C NMR (acetone-d6, ppm): δ 166.97,
145.85, 141.67, 141.29, 141.04, 136.82, 132.86, 37.18, 35.90,
35.55, 35.28, 35.14, 35.05, 28.40, 18.79. Anal. Calcd for
C30H40O4S3: C, 64.43; H, 7.14; S, 17.14. Found: C, 64.53; H,
7.29; S, 17.54.
P r ep olym er of P DTTOMBO. P oly(3,3′-d id ecyl-2,2′-bi-
t h io p h e n e -5,5′-d ih y d r a zid e -co-t o lu e n e -2,6-d ih y d r a -
zid e). A 0.242 g (0.424 mmol) portion of 3,3′-didecyl-2,2′-
bithiophene-5,5′-dicarbonyl chloride (I-2) was added into a
stirred solution of 2,6-dicarbonylhydrazide-toluene (II) (88.2
mg, 0.424 mmol) in 20 mL of NMP containing 0.10 g of LiCl
and 0.01 mol of pyridine at room temperature. After heating
at 80 °C for 3 h, the brown viscous solution was cooled to 50
°C and then poured into 100 mL of deionized water. The
precipitated polymer was filtered, washed with water and then
ethanol, and finally was dried under vacuum at 60 °C to give
0.288 g of a light yellow powder. Yield: 95%. Anal. Calcd for
C39H54N4O4S2: C, 66.25; H, 7.70; N, 7.92; S, 9.07. Found: C,
64.46; H, 7.84; N, 8.18; S, 9.02.
P r epolym er of P OTTTOMBO. P oly(3,3′′-dioctyl-2,2′;5′,2′′-
t er t h iop h en e-5,5′′-d ih yd r a zid e-co-t olu en e-2,6-d ih yd r a -
zid e). A 0.500 g (0.837 mmol) portion of 3,3′′-dioctyl-2,2′;5′,2′′-
terthiophene-5,5′′-dicarbonyl chloride was added with stirring
into a solution containing 0.174 g (0.837 mmol) of II, 15 mL
of NMP, 0.05 g of LiCl, and 1 drop of pyridine at room
temperature for 5 h. The viscous solution was then poured into
100 mL of deionized water. The polymer was filtered, washed
with water and ethanol, and then dried in a vacuum at 60 °C
to give a light yellow powder (0.54 g, 90%). Anal. Calcd for
3-Octylth iop h en e-2,5-d ica r bon yl Ch lor id e (I-1). A 25
mL portion of distilled thionyl chloride was added to 5 g of
3-octylthiophene-2,5-dicarboxylic acid. The mixture was heated
at reflux for 5 h, and the excess thionyl chloride was removed
under reduced pressure. The residue was distilled under
vacuum to give a light yellow oil at 132-134 °C/0.1 mbar, 5.0
1
g (89%). MS m/z: 321. H NMR (CDCl3, ppm): δ 7.78 (s, 1H),
2.95-2.90 (t, J ) 7.6 Hz, 2H), 1.66-1.56 (m, 2H), 1.38-1.27
(m, 10H), 0.90-0.85 (t, J ) 7.0 Hz, 3H). 13C NMR (CDCl3,
ppm): δ 159.70, 158.61, 154.35, 142.90, 139.79, 138.84, 31.70,
30.28, 29.51, 29.24, 29.13, 29.02, 22.53, 13.95. Anal. Calcd for
C
14H18Cl2O2S: C, 52.34; H, 5.65; S, 9.98; Cl, 22.07. Found: C,
52.47; H, 5.78; S, 10.03; Cl, 22.18.
3,3′-Did ecyl-2,2′-bith iop h en e-5,5′-d ica r bon yl Ch lor id e
(I-2). A 15 mL portion of thionyl chloride was added to 3.0 g
(5.62 mmol) of 3,3′-didecyl-2,2′-bithiophene-5,5′-dicarboxylic
acid. The mixture was heated at reflux for 5 h, and then the
excess thionyl chloride was removed under reduced pressure.
The residue was recrystallized from hexane to give 2.9 g (92%)
C
39H48N4O4S3: C, 63.90; H, 6.60; N, 7.64; S, 13.12. Found: C,
62.97; H, 6.89; N, 7.56; S, 13.31.
P oly(3-octylth ioph en e-2,5-diyl-1,3,4-oxadiazole-2,5-diyl-
tolu en e-2,6-d iyl-1,3,4-oxa d ia zole-2,5-d iyl) (P OTOMBO).31
A 0.457 g portion of the prepolymer of POTOMBO and 15 mL
of POCl3 were refluxed for about 10 h and then poured into
water. The filtered precipitate was washed with water, etha-
nol, and ether and then dried under vacuum to afford a light
yellow powder polymer (0.368 g, 87%). 1H NMR (TFA-d/CDCl3,
ppm): δ 8.21 (d, br, 2H), 7.97 (s, br, 1H), 7.69 (m, br, 1H),
3.18-3.15 (t, br, 2H), 3.01 (s, 3H), 1.81-1.77 (br, 2H), 1.44-
1
of a yellow powder. Mp: 68-69 °C. H NMR (CDCl3, ppm): δ
7.85 (s, 2H), 2.56-2.51 (t, J ) 7.6 Hz, 4H), 1.57-1.54 (m, 4H),
1.23 (m, 28H), 0.89-0.85 (t, J ) 6.6 Hz, 6H). 13C NMR (CDCl3,
ppm): δ 159.32, 145.06, 138.96, 138.68, 137.18, 31.77, 30.32,
29.45, 29.36, 29.18, 29.16, 29.10, 28.81, 22.56, 13.98. Anal.
Calcd for C30H44Cl2O2S2: C, 63.03; H, 7.76; S, 11.22. Found:
C, 62.38; H, 8.01; S, 11.54.
3,3′′-Dioctyl-2,2′;5′,2′′-ter th ioph en e-5,5′′-dicar bon yl Ch lo-
r id e. (I-3). The compound was prepared as described above
under the same condition using 2.6 g (4.64 mmol) of 3,3′′-
dioctyl-2,2′;5′,2′′-terthiophene-5,5′′-dicarboxylic acid reacted
with 25 mL of thionyl chloride for 8 h. MS m/z: 596. HRMS
1.27 (br, 10H), 0.88-0.84 (t, br, 3H). Anal. Calcd for C23H24
-
N4O2S: C, 65.69; H, 5.75; N, 13.32; S, 7.62. Found: C, 64.34;
H, 5.48; N, 12.54; S, 7.46.
P oly(3,3′-d id ecyl-2,2′-b it h iop h en e-5,5′-d iyl-1,3,4-oxa -
d ia zole -2,5-d iyl-t olu e n e -2,6-d iyl-1,3,4-oxa d ia zole -2,5-
d iyl) (P DTTOMBO). A mixture of 200 mg of the prepolymer
of PDTTOMBO and 25 mL of POCl3 was refluxed for 6 h. After
cooling, the reaction mixture was poured into water. The
precipitate was collected by filtration, washed with water,
ethanol, and then ether, and finally dried under vacuum. A
C
30H38Cl2O2S3: calcd 596.14111; found mass 596.14220; for-
mula C30H38Cl2O2S3. 1H NMR (CDCl3, ppm): δ 7.80 (s, 2H),
7.29 (s, 2H), 2.83-2.78 (t, J ) 7.9 Hz, 4H), 1.71-1.64 (m, 4H),
1.46-1.20 (m, 20H), 0.88-0.86 (t, J ) 7.0 Hz, 6H). 13C NMR
(CDCl3, ppm): δ 159.13, 142.40, 141.48, 140.17, 136.28, 133.93,
128.28, 31.73, 30.15, 29.39, 29.33, 29.23, 29.10, 22.54, 13.98.
Anal. Calcd for C30H38Cl2O2S3: C, 60.28; H, 6.41; S, 16.09.
Found: C, 59.98; H, 7.19; S, 17.21.
1
brown powder was obtained (180 mg, 93%). H NMR (CDCl3,
ppm): δ 8.17 (br, 2H), 7.77 (s, br, 2H), 7.57 (br, 1H), 3.12 (s,
br, 3H), 2.64 (s, br, 4H), 1.84-1.47 (s, br, 4H), 1.47-1.23 (br,
28H), 0.90-0.84 (br, 6H). 13C NMR (CDCl3), δ 163.6, 160.5,
144.6, 138.9, 132.4, 131.4, 126.5, 126.4, 125.2, 124.9, 31.8, 30.5,
29.5, 29.4, 29.2, 29.1, 29.0, 28.9, 22.6, 19.3, 14.0 ppm. Anal.
Calcd for C39H50N4O2S2: C, 69.81; H, 7.51; N, 8.35; S, 9.56.
Found: C, 68.34; H, 7.25; N, 7.98; S, 9.30.
2,6-Dica r bon ylh yd r a zid e-tolu en e (II). A 7.0 g (0.034
mol) portion of dimethyl 2-methylisophthalate was added into
a solution of 10 mL of monohydrazine hydrate (99%) in 70 mL
of CH3OH. The reaction mixture was refluxed for 24 h. After
cooling and filtering, a white precipitate was obtained. The
precipitate was washed with chloroform and dried under
vacuum to give the product as a white powder. Mp: 108.0-
P oly(3,3′′-d ioctyl-2,2′;5′,2′′-ter th iop h en e-5,5′′-d iyl-1,3,4-
oxa d ia zole-2,5-d iyl-tolu en e-2,6-d iyl-1,3,4-oxa d ia zole-2,5-
d iyl) (P OTTTOMBO). A mixture of 400 mg of prepolymer
POTTTOMBO and 20 mL of POCl3 was refluxed for 12 h. After
cooling, the reaction mixture was poured into water. The
precipitate was collected by filtration, washed with water,
ethanol, and then ether, and finally dried under vacuum. An
1
109.5 °C. MS m/z: 208. H NMR (DMSO-d6, ppm): δ 9.45 (s,
2H), 7.31-7.23 (d, J ) 8.8 Hz, 3H), 4.46 (s, 4H), 2.28 (s, 3H).13C
NMR (CDCl3, ppm): δ 168.17, 136.95, 133.02, 127.86, 125.11,
16.19. Anal. Calcd for C9H12N4O2: C, 51.92; H, 5.77; N, 26.92.
Found: C, 52.19; H, 5.68; N, 26.52.
Syn t h esis of P olym er s. P r ep olym er of P OTOMBO.
P oly(3-octylth iop h en e-2,5-d ih yd r a zid e-co-tolu en e-2,6-d i-
h yd r a zid e).30 A 3.2 g (0.010 mol) portion of 3-octylthiophene-
2,5-dicarbonyl chloride (I-1) was added with stirring into a
solution containing 2.08 g (0.010 mol) of II, 20 mL of NMP,
0.10 g of LiCl, and 0.02 mol of pyridine at room temperature
for 3 h. The viscous solution was then poured into 200 mL of
water, and the precipitated polymer was collected, washed
with water and ethanol, and then dried in a vacuum at 60 °C
1
orange powder was obtained (320 mg, 84%). H NMR (CDCl3/
TFA-d, ppm): δ 8.19-8.17 (d, J ) 6.2 Hz, 2H), 7.85 (s, 2H),
7.73-7.66 (m, 1H), 7.33 (s, 2H), 2.97 (s, 3H), 2.92-2.87 (t, J
) 7.5 Hz, 4H), 1.74-1.72 (m, br, 4H), 1.43-1.28 (m, br, 20H),
0.87-0.85 (t, br, J ) 6.0 Hz, 6H). Anal. Calcd for C39H44
-
N4O2S3: C, 67.21; H, 6.36; N, 8.04; S, 13.80. Found: C, 65.91;
H, 6.59; N, 7.86; S, 13.90.
Resu lts a n d Discu ssion
to give a white powder (4.0 g, 90%). Anal. Calcd for C23H28
-
The polymers were synthesized through precursor
polymers of polyhydrazides that were prepared by poly-
condensation of R,ω-dicarbonyl chlorides of alkyl-substi-
N4O4S: C, 60.51; H, 6.18; N, 12.27; S, 7.02. Found: C, 58.89;
H, 6.07; N, 11.44; S, 6.82.
(30) Xie, M.-L.; Oishi, Y.; Kakimoto, M.-A.; Umai, Y. J . Polym. Sci.
Polym. Chem. 1991, 29, 55.
(31) Schulz, B.; Bruma, M.; Brehmer, L. Adv. Mater. 1997, 9, 601.

Novel Photoluminescent Polymers
J . Org. Chem., Vol. 65, No. 13, 2000 3899
tuted oligothiophenes with 2,6-dihydrazide-toluene in
NMP in the presence of LiCl and pyridine.32 The dicar-
bonyl chlorides of alkyl-substituted oligothiophenes and
2,6-dihydrazide-toluene were synthesized following the
routes depicted in Scheme 1. For the synthesis of the
dicarbonyl chlorides, the oligomers of alkyl-substituted
thiophene were synthesized first, and then the carboxy-
lation at the R-positions of the thiophene rings through
n-BuLi and dry ice and a following reaction with SOCl2
were performed.33 The 2,6-dihydrazide-toluene (II) was
obtained by the reaction of dimethyl-2-methylisophtha-
late with excess hydrazine monohydrate. The prepoly-
mers-polyhydrazides were obtained as white or light
yellow powders. They are not soluble in common organic
solvents but dissolve completely in NMP. As shown in
Scheme 2, the polyhydrazides were converted to the final
polymers via a cyclodehydration reaction by refluxing in
POCl3, which was used as both dehydrating reagent and
solvent. Polymers P OTOMBO, P DTTOMBO, and
P OTTTOMBO were obtained as light yellow, yellow, and
orange powders, respectively. P OTOMBO and P OTT-
TOMBO are partially soluble in chloroform and THF.
Polymer P DTTOMBO is completely soluble in these two
solvents. However, all three polymers readily dissolve in
chloroform with a little amount of trifluoroacetic acid
(TFA) to give clear solutions. Further purification of the
polymers was carried out by reprecipitation in which the
polymer was dissolved in CHCl3 or CHCl3 with a little
amount of TFA and then precipitated in methanol.
Str u ctu r a l Elu cid a tion . The successful cyclodehy-
dration could be confirmed by the FT-IR spectra. Two
intensive absorption bands at 1660 and 3246 cm-1
assigned to the stretching vibration of the carbonyl
groups and N-H in the polyhydrazides, respectively,
disappeared after the treatment with POCl3. In the
meantime, two new peaks at around 1586 and 1081 cm-1
attributed to the 1,3,4-oxadiazole ring stretching ap-
peared in the final polymers.34 The weak peaks at 1650
cm-1 and the negligible absorption in the range of 3200-
3400 cm-1 may be caused by the end groups of the
polymers’ chains. This result also agrees well with the
report by Pei et al.17
rings and benzene rings. The chemical shift at δ 7.85 ppm
was assigned to the substituted thiophene ring, and that
at δ 7.33 ppm was assigned to the unsubstituted
bithiophene rings. The other two protons at δ 8.19 ppm
(d, J ) 6.6, 1H) and 7.66 ppm (m, 2H) are assigned to
the benzene rings. The peaks at around δ 3.00 ppm
corresponding to the aliphatic chains are assigned to the
methyl group on benzene ring and the methylene groups
adjacent to the thiophene rings. Comparing the asym-
metrically substituted polymer P OTOMBO to the sym-
metrically substituted polymers P DTTOMBO and P OTT-
TOMBO, it is interesting to note that the 1H NMR
spectra are not more complicated. The phenomena are
observed in the random polymerization of asymmetric
monomers of thiophenes, which leads to both head-to-
head and head-to-tail linkages. Since the adjacent
thiophene rings are separated by two oxadiazole rings
and one benzene ring in P OTOMBO, the interactions
between the side chains on the thiophene rings with
head-to-head and/or head-to-tail linkages diminish dra-
matically. Therefore, the effects of such structural defects
on the optoelectronic properties are negligible. In the 13C
NMR spectrum of P DTTOMBO, there are 10 well-
resolved signals in the aromatic region that correspond
to the 10 aromatic carbons in the polymer. The 10
aliphatic signals are assigned to the alkyl long chains
on the thiophene rings. The 13C NMR spectra of the other
two polymers have not been obtained because of the low
signals of the samples. The NMR spectra supported the
conclusion that the polymers have well-defined structures
and the conversion from prepolymers to final polymers
is almost complete. The elemental compositions of the
polymers also confirmed the structures of the polymers.
The results are summarized in the Experimental Section.
Molecu la r Weigh ts a n d Th er m a l P r op er ties. The
molecular weights of the polymers were measured by
means of GPC using THF as eluant against polystyrene
standards. Because polymers P OTOMBO and P OTT-
TOMBO are partially soluble in THF, only the molecular
weights of the THF-soluble parts of the polymers were
measured. The molecular weights of the THF-soluble
parts of polymers P OTOMBO and P OTTTOMBO were
measured as Mw ) 6890 (Mw/ Mn ) 1.8) and 5734 (Mw/
Mn ) 2.0), respectively. Polymer P DTTOMBO is com-
pletely soluble in THF; thus Mw is 35105 with a polydis-
persity of 1.41. The actual molecular weights of the
partially soluble polymers should be higher than these
measured values because of the insoluble parts with
higher molecular weights.
The chemical structures of the polymers were further
1
confirmed by H and 13C NMR and elemental analysis.
The 1H and 13C NMR spectra were obtained either in
CDCl3 or in CDCl3/TFA-d. All of the chemical shift values
are referenced to tetramethylsilane (TMS, 0 ppm). Since
the resonance of the polymers is broadened, the coupling
constants could not be obtained. In the spectrum of
P OTOMBO, there are three clear single broad peaks in
the aromatic region, which correspond to the â-proton on
the thiophene ring at δ 7.97 ppm (1H) and the protons
on the benzene ring at δ 8.21 (br, 2H) and 7.67 (m, 1H)
ppm, respectively. The aliphatic protons of methylenes
attaching on the thiophene rings are assigned with less
information regarding the resonance constants as a result
of the broad peaks as mentioned above. The NMR spectra
of polymer P DTTOMBO shows a little difference from
that of polymer P OTOMBO. For polymer P OTTTOM-
BO, there are four signals in the aromatic region, which
are assigned to two kinds of protons on the thiophene
The thermal stability of the polymers was determined
by thermogravimetric analysis (TGA) under nitrogen. All
of the polymers exhibited an onset degradation temper-
ature higher than 300 °C under nitrogen. All of the
degradation patterns for the three polymers are quite
similar, with a main weight loss step in the range of 380-
450 °C. The result reveals that the polymers have good
thermal stability. The glass transition temperatures (Tg)
of the polymers were measured by DSC. Tg is 127.8 °C
for P OTOMBO, 60.2 °C for P DTTOMBO, and 122.4 °C
for P OTTTOMBO. The relatively lower Tg for P DT-
TOMBO might be owing to the bithiophene segment in
the polymer main chain with a head-to-head linkage.
Op tica l P r op er ties. The absorption and fluorescence
spectra of spin-coated films of the polymers are shown
in Figure 2. The absorption spectra of the polymers show
a broad main peak with a couple of the minor shoulders
(32) Zapp, J . A. Science 1975, 190, 442.
(33) Chadwick, D. J .; Willbe, C. J . Chem. Soc., Perkin Trans. 1 1997,
887.
(34) Sato, M.; Kamita, T.; Nakadera, K.; Mukaida, K.-I. Eur. Polym.
J . 1995, 31, 395.

3900 J . Org. Chem., Vol. 65, No. 13, 2000
Ta ble 1. Ch a r a cter istics a n d P h ysica l P r op er ties of New P olym er s
Meng and Huang
Tg
(°C)
Mw
(g/mol)
λmax
(nm/abs)
λmax
(nm/abs)
λmax
(nm/PL)
λmax
(nm/PL)
polymer
P OTOMBO
P DTTOMBO
P OTTTOMBO
127.8
60.2
122.4
6890
35105
5734
358 (film)
376 (film)
430 (film)
462 (film)
498/526 (film)
568 (film)
342 (CHCl3)
444/462 (CHCl3)
F igu r e 3. Cyclic voltamograms of P OTOMBO, P DTTOM-
BO, and P OTTTOMBO films on Pt electrodes in acetonitrile
supported by Bu4ClO4 (0.10 mol/dm3) at
50 mV/s.
a scan rate of
F igu r e 2. The UV-vis absorption and fluorescent spectra of
polymer films spin-coated on quartz plates.
Electr och em ica l Stu d ies. Cyclic voltammetry (CV),
a dynamic electrochemical method, is an important
technique for measuring band gaps, electron affinities,
and work functions of various conjugated polymers. The
electrochemical studies of the polymers were carried out
in n-Bu4NClO4 in acetonitrile. The CV spectra are shown
in Figure 3. The electrochemical reduction of the thin
polymer films (P OTOMBO, P DTTOMBO, and P OTT-
TOMBO) exhibit reversible n-doping processes. The
cathodic peaks appear at -1.73 V for P OTOMBO, -1.86
V for P DTTOMBO, and -1.76 V for P OTTTOMBO
with corresponding anodic peaks at -1.53, -1.75, and
-1.67 V, respectively. Generally speaking, the reduction
potential of the polymers is not changed remarkably with
variation of the number of thiophene rings in the oligo-
thiophene blocks. This implies that the reduction proper-
ties of the polymers are dominated by the 2,6-bis(1,3,4-
oxadiazole-2,5-diyl)toluene segments. These reduction
potentials are lower than those of 2-(4-biphenylyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole (PBD) (-1.95 to -1.94
V vs SCE), one of the most widely used electron-
transporting/hole-blocking materials, and other 1,3,4-
oxadiazole-containing polymer materials and are com-
parable to those of some good electron-transporting
materials. The ionization potential (IP) and electron
affinity (EA) of the polymers were estimated according
to the equations37
at the low energy side. As expected, gradual bathochro-
mic shift of absorption spectra is exhibited with increase
in the length of oligothiophenes in the polymers. The
absorption maximum increases from P OTOMBO (λmax
) 358 nm) to P DTTOMBO (λmax ) 376 nm) to P OTT-
TOMBO (λmax ) 430 nm). The emission spectra of the
polymers were obtained under excitation at the main
absorption wavelength of each polymer. Polymer P OTO-
MBO emits intense blue light, while polymers P DT-
TOMBO and P OTTTOMBO emit green and pale orange
light, respectively. The emission spectra of polymers are
shown together with their UV spectra in Figure 2. The
emissive maximums are 462 nm for P OTOMBO, 498 nm
with a subpeak at 526 nm for P DTTOMBO, and 568 nm
for P OTTTOMBO. The physical properties of the three
polymers are summarized in Table 1.
The effective tuning of the emissive wavelength of the
new series of polymers could be attributed to the remark-
able difference of band gap between oligothiophenes and
2,6-bis(1,3,4-oxadiazole-2,5-diyl)-toluene, the two blocks
constructing the polymers alternatively. As demonstrated
by head-to-tail regioregular poly(3-alkylthiophenes),35
oligothiophenes may have low band gap, whereas most
of conjugated polymers containing 1,3,4-oxadiazole and
molecular materials exhibit quite large band gaps (larger
than 3.0 eV).36 The new polymers could be regarded as
derivatives of polythiophene in which the 2,6-bis(1,3,4-
oxadiazole-2,5-diyl)-toluene is inserted. The extension of
conjugation along the backbone of polythiophene is
limited or changed as a result of this insertion, and the
total conjugated properties of the resultant polymers will
be varied with the change of the length of the oligo-
thiophene blocks.
IP ) Eox(onset vs SCE) + 4.4 eV
EA ) Ered(onset vs SCE) + 4.4 eV
(37) (a) Li, X.-C.; Kraft, A.; Cervini, R.; Spencer, G. C. W.; Cacialli,
F.; Friend, R. H.; Gruener, J .; Holmes, A. B.; Mello, J . C.; Moratti, S.
C. Mater. Res. Soc. Symp. Proc. 1996, 413, 13. (b) Cervini, R.; Li, X.-
C.; Spencer, W. C.; Holmes, A. B.; Moratti, S. C.; Friend, R. H. Synth.
Met. 1997, 84, 359. (c) J anietz, S.; Wedel, A. Adv. Mater. 1997, 9, 403.
(35) Murray, K. A.; Moratti, S. C.; Baigent, D. R.; Greenham, N. C.;
Pichler, K.; Holmes, A. B.; Friend, R. H. Synth. Met. 1995, 69, 395.
(36) Herrema, J . K.; van Hutten, P. F.; Gill, R. E.; Wildeman, J .;
Wieringa, R. H.; Hadziioannou, G. Macromolecules 1995, 28, 8102.

Novel Photoluminescent Polymers
J . Org. Chem., Vol. 65, No. 13, 2000 3901
Ta ble 2. Electr och em ica l a n d Op tica l Da ta of Va r iou s P olym er s
polymer
IP (eV)
EA (eV)
Eox,pa/Eox,pc (V)
Ered pc
,
/Ered,pa (V)
Eg (eV, Echem)
Eg (eV, optical)
P OTOMBO
P DTTOMBO
P OTTTOMBO
Poly3OcTa
PBDb
6.00
5.74
5.43
4.90
5.90
6.04
5.35
4.87
5.55
2.98
2.90
2.92
2.63
2.60
2.90
2.80
2.60
3.02
1.95/......
1.64/......
1.38/1.24
-1.73/-1.53
-1.86/-1.75
-1.76/-1.67
3.02
2.84
2.51
2.33
3.30
3.14
2.50
2.27
2.53
3.04
2.83
2.54
polyoxadiazolec
PPVc
3.19
2.55
2.29
2.65
MEH-PPVc
CN-PPVc
a
c
Poly3OcT (poly(3-octylthiophene)) data measured under the same conditions as the new polymers. bData from ref 37c. See ref 37a.
where Eox
and Ered
are the onset
(onset vs SCE)
Con clu sion s
(onset vs SCE)
potentials for the oxidation and reduction of polymers
versus the reference electrode, respectively. The onset
potentials were determined from the intersection of the
two tangents drawn at the rising current and baseline
charging current of the CVs. The electrochemical data
and the optical properties of the polymers are sum-
marized in Table 2.
A series of novel p-n diblock conjugated copolymers
containing oligothiophenes and 2,6-di(1,3,4-oxadiazole-
2,5-diyl)-toluene have been prepared by polycondensation
reaction. The structures of the polymers are consistent
1
with the H and 13C NMR spectra, FT-IR spectra, and
elemental analysis results. These polymers have well-
defined structures and exhibit highly thermal stability.
The p-n diblock structure may be a useful molecular
design for tuning the emissive wavelength of conjugated
polymers. The emissive color of the polymers presented
here could be tuned from blue to green to light-orange
just by increasing the number of thiophene rings in the
oligothiophene blocks from one to three. The electro-
chemical behavior of the p-n diblock conjugated copoly-
mers could be adjusted effectively. Comparison of the
electrochemical data to those of the currently used
electroluminescence materials suggests that the synthe-
sis of this new series of polymers provide a promising
approach to balance the barrier between the HOMO of
an electroluminescent material and the work function of
ITO and the barrier between the LUMO of the electrolu-
minescent material and the work functions of stable
metal electrodes (such as Al) in PLEDs.
The electron affinities of the polymers are around 2.95
eV. These values are comparable to those materials which
show good electron transport ability in PLED devices.
Cyclic voltammetric reduction potentials could be used
as a surrogate for the LUMO energy levels. The results
suggest that the LUMO energy levels of the polymers
may be lower than those of conventional p-dope-type
electroluminescent polymers such as poly(3-octylthio-
phene), PPV, MEH-PPV, and PBD and may be compa-
rable to that of poly(cyanoterephthalylidene) (CN-PPV)
(EA, 3.02 eV) and other oxadiazole polymers. Such energy
levels may provide a closer match to the work function
of Al when they are used as active materials in PLEDs.
When we scanned the polymer films anodically, an
interesting electrochemical phenomenon was observed.
Both polymers P OTOMBO and P DTTOMBO showed
irreversible oxidation with the anodic peaks at 1.95 and
1.64 V, respectively, while polymer P OTTTOMBO showed
a partially reversible anodic peak at 1.38 V with a
cathodic peak at 1.24 V. The oxidation potential of the
polymers decreases with increasing length of the oligo-
thiophene blocks in the polymers. For polymer P OTT-
TOMBO, the ionization potential is 5.43 eV. This value
is higher than that of MEH-PPV and is close to those of
other electroluminescent PPVs that are hole-injection
favorable electroluminescent materials. The results in-
dicate that the HOMO energy levels of the p-n diblock
copolymers might be adjusted by changing the length of
the oligothiophene blocks to a suitable level for hole
injection when the polymers are used as the electrolu-
minescent materials in PLEDs.
Ack n ow led gm en t. The authors are grateful to Dr.
Wanglin Yu, Dr. Zhikuan Chen, Dr. J ian Pei, and Dr.
Lianhui Wang for helpful discussions and Ms Yang Xiao
for her support.
Su p p or tin g In for m a tion Ava ila ble: FT-IR spectra of pre-
polymers and final polymers; 1H NMR spectra of POTOMBO,
PDTTOMBO, and POTTTOMBO; 13C NMR spectra of PDT-
TOMBO; thermogravimetric analysis of POTOMBO, PDT-
TOMBO, and POTTTOMBO; and differential scanning calo-
rimetry of POTOMBO, PDTTOMBO, and POTTTOMBO. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O991359C