New phenyl-substituted PPV derivatives for polymer light-emitting diodes-synthesis, characterization and structure-property relationship study

Article abstract of DOI:10.1021/ma021221n

Three new PPV derivatives with dialkoxyphenyl substituents, BEH2P-PPV, BEH3P-PPV, and BEH4P-PPV have been synthesized. The polymers were characterized by FT-IR, ¹H NMR, and elemental analysis. The polymers possess excellent solubility, high molecular weights, high photoluminescence efficiencies and good thermal stability. The influence of substitution pattern on the formation of structural defects has been investigated by measuring the signal due to tolane-bisbenzyl moieties (TBB) in the proton NMR spectra. BEH2P-PPV with a steric phenyl group at the ortho-position on the side phenyl ring shows the lowest amount of TBB, which indicates suitable steric hindrance can be applied to suppress the formation of irregular head-to-head and tail-to-tail linkage in the polymer chains. In addition, the polarity of solvents used for the Gilch polymerization will also affect the amount of irregular structure in the polymers. Polar solvents such as THF will result in polymers with low TBB content. Energy level measurement from cyclic voltammetry revealed that the influences of the substitution pattern on the HOMOs and LUMOs are different. The three polymers possess similar HOMO energy levels while the LUMO of BEH4P-PPV is much higher than that of the other two polymers. Polymer light-emitting diodes fabricated from BEH2P-PPV, BEH3P-PPV, and BEH4P-PPV with the configuration of ITO/PEDOT/polymer/Ba/Al, emitted bright blue-green to green light with the maximum peaks at 496, 488, and 525 nm, respectively. The turn-on electric field and maximum external quantum efficiencies of the diodes are 0.30, 0.50, and 0.42 MV/cm and 0.37%, 0.66%, and 0.25% respectively. The quantum efficiency is mainly determined by the electron injection from the cathode. With the highest luminance, lowest turn-on electric field, and good quantum efficiency as well as negligible structural defects, BEH2P-PPV is the most promising material among the three polymers for polymer light-emitting diodes.

Full text of DOI:10.1021/ma021221n

Macromolecules 2003, 36, 1009-1020
1009
New Phenyl-Substituted PPV Derivatives for Polymer Light-emitting
Diodes-Synthesis, Characterization and Structure-Property
Relationship Study
Zh i-Ku a n Ch en ,* Na n cy Hoi Sim Lee, a n d Wei Hu a n g
Institute of Materials Research and Engineering (IMRE), 3 Research Link,
Singapore 117602, Republic of Singapore
Yi-Sh e Xu a n d Yon g Ca o
Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, People’s Republic of China
Received J uly 29, 2002; Revised Manuscript Received J anuary 3, 2003
ABSTRACT: Three new PPV derivatives with dialkoxyphenyl substituents, BEH2P -P P V, BEH3P -
1
P P V, and BEH4P -P P V have been synthesized. The polymers were characterized by FT-IR, H NMR,
and elemental analysis. The polymers possess excellent solubility, high molecular weights, high
photoluminescence efficiencies and good thermal stability. The influence of substitution pattern on the
formation of structural defects has been investigated by measuring the signal due to tolane-bisbenzyl
moieties (TBB) in the proton NMR spectra. BEH2P -P P V with a steric phenyl group at the ortho-position
on the side phenyl ring shows the lowest amount of TBB, which indicates suitable steric hindrance can
be applied to suppress the formation of irregular head-to-head and tail-to-tail linkage in the polymer
chains. In addition, the polarity of solvents used for the Gilch polymerization will also affect the amount
of irregular structure in the polymers. Polar solvents such as THF will result in polymers with low TBB
content. Energy level measurement from cyclic voltammetry revealed that the influences of the substitution
pattern on the HOMOs and LUMOs are different. The three polymers possess similar HOMO energy
levels while the LUMO of BEH4P -P P V is much higher than that of the other two polymers. Polymer
light-emitting diodes fabricated from BEH2P -P P V, BEH3P -P P V, and BEH4P -P P V with the
configuration of ITO/PEDOT/polymer/Ba/Al, emitted bright blue-green to green light with the maximum
peaks at 496, 488, and 525 nm, respectively. The turn-on electric field and maximum external quantum
efficiencies of the diodes are 0.30, 0.50, and 0.42 MV/cm and 0.37%, 0.66%, and 0.25% respectively. The
quantum efficiency is mainly determined by the electron injection from the cathode. With the highest
luminance, lowest turn-on electric field, and good quantum efficiency as well as negligible structural
defects, BEH2P -P P V is the most promising material among the three polymers for polymer light-emitting
diodes.
In tr od u ction
injection/transporting layers between the light-emitting
polymer film and electrodes to balance the charge
carriers.10,11 The second reason for low quantum ef-
ficiencies in the solid state is because of interchain
interactions such as aggregation and excimer formation,
which will lead to a self-quenching process of ex-
citons.12-15 The effective strategy to suppress this
drawback is to introduce asymmetric structure or bulky
substitution to prevent close packing of the polymer
chains with each other.16-22 The third main factor that
limits the quantum efficiency is the presence of defects
in the polymer where excitons will be trapped and
nonradiative decay will occur. In addition to reducing
quantum efficiency, it is believed that polymer struc-
tural defects are also responsible for the short opera-
tional lifetime of LED devices23 which may be attributed
to accelerated degradation of polymers initiated by the
defects. Compared to the first two issues, to overcome
this obstacle is a bigger challenge. It requires a deeper
understanding of the polymerization mechanism and
careful control of polymerization conditions and also the
need for systematic investigation of the structure-
property relationship of light-emitting polymers.
Polymeric light-emitting diodes (PLEDs) have at-
tracted significant attention from both the academic
community and industry since the first report of the
PLED device fabricated from poly(p-phenylenevinylene)
(PPV) in 1990,1 because of their potential application
for large-area flat panel displays with the advantages
of low cost, wide viewing angle, fast switching time, high
efficiency, and low driving voltage.2-6 Ever since, re-
markable progress has been made on the performance
of PLEDs, which has triggered a strong motivation to
commercialize this technology in displays. However,
there are a number of issues that need to be resolved,
for example, low electroluminescent efficiencies, short
operational lifetime, and luminescent stability. The low
quantum yields are attributed to three main reasons.
The first reason is the imbalance of hole/electron injec-
tion and transport in the emission layer in devices
because most of the emissive polymers conduct holes
in preference to electrons. This situation can largely be
improved by introducing electron-deficient moieties into
the polymer backbone or as the side chains7-9 or by
optimizing device configurations through adding charge
PPV and its derivatives have been intensively inves-
tigated since 1990. Today, they are still one of the most
valuable and frequently used light-emitting materials
* Corresponding author: Telephone: +65-6874 4331. Fax
+65-6872 0785. E-mail: zk-chen@imre.a-star.edu.sg.
10.1021/ma021221n CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/29/2003

1010 Chen et al.
Macromolecules, Vol. 36, No. 4, 2003
chains in the side chain in order to improve solubility
in common organic solvents, as well as to minimize
interchain interactions and thus achieve high photolu-
minescence quantum efficiencies. Additional phenyl
rings incorporated into the polymer side chains were
designed to allow us to investigate the steric effect on
the formation of structural defects as well as to improve
the thermal stability of the polymers. In addition, model
compounds 2EH-P P V and 4EH-P P V have also been
synthesized to study (i) solvent effects and (ii) substitu-
tion effects on polymerization and the number of
structural defects in the polymers. To the best of our
knowledge, although a few solvents have been exploited
for the Gilch route polymerization, no effort on this issue
has been reported.
F igu r e 1. Chemical structures of the dialkoxy-substituted
PPVs and model polymers.
Exp er im en ta l Section
Nuclear magnetic resonance (NMR) spectra were collected
on a Bruker ACF 300 spectrometer using chloroform-d as a
solvent and tetramethylsilane (TMS) as an internal standard.
Electron-impact mass spectra were obtained from a Micromass
VG7035F mass spectrometer using an ion current of 70 eV.
Fourier transform infrared (FT-IR) spectra were recorded on
a Bio-Rad FTS 165 spectrometer by dispersing samples in KBr
disks. Ultraviolet-visible (UV-vis) and fluorescence spectra
were obtained using a Shimadzu UV 3101PC UV-vis-NIR
spectrophotometer and a Perkin-Elmer LS 50B luminescence
spectrometer with a xenon lamp as light source, respectively.
Elemental analyses were performed on a Perkin-Elmer 2400
elemental analyzer for C, H, Cl, and Br determination.
Thermogravimetric analyses (TGA) were conducted on a Du
Pont Thermal Analyst 2100 system with a TGA 2950 thermo-
gravimetric analyzer under a heating rate of 20 °C/min and a
nitrogen flow rate of 70 cm3/min. Cyclic voltammetry measure-
ments were taken on an EG&G Parc model 273A potentiostat/
galvanostat under argon atmosphere. All potentials were
measured against a Ag/Ag+ (0.10 M AgNO3 in acetonitrile)
electrode (0.34 V vs SCE), and all experimental values were
corrected with respect to SCE. GPC analysis was conducted
on Waters 2690 Separation Module equipped with a Waters
410 differential refractometer HPLC system and Waters
Styragel HR3, HR4, and HR5 columns in series using poly-
styrene as a standard and THF as eluent.
LED Devices. Polymers were dissolved in chloroform and
filtered through a 0.45 µm PTFE filter. Patterned indium tin
oxide (ITO) coated glass substrates were cleaned with acetone,
detergent, and distilled water sequentially in an ultrasonic
bath. After treatment with oxygen plasma, 1500 Å of poly-
(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(sty-
renesulfonic acid) (PSS) (Batron-P 4083, Bayer AG) was spin-
coated onto the substrate followed by drying in a vacuum oven
at 80 °C for 8 h. A thin film of polymer was coated onto the
anode by spin casting inside a drybox. The film thickness of
the active layers was around 80 nm, as measured with a
Tencor Alfa step 500 surface profiler (Oriel). Ba and Al layers
were vacuum-evaporated on top of an EL polymer under a
vacuum of 1 × 10-4 Pa. Device performances were measured
inside a drybox. Current-voltage (I-V) characteristics were
recorded with a Keithley 236 source meter. EL spectra were
recorded by Oriel INTASPEC IV CCD Spectrograph. Lumi-
nance and external quantum efficiencies were determined by
a calibrated photodiode.
because of their good proccessability, structural diver-
sity, thermal stability, good mechanical property and
high luminescence. PPVs are normally synthesized by
two methods: precursor route and direct solution po-
lymerization for organic soluble polymers. Several syn-
thetic approaches for soluble polymers have been es-
tablished including, dehydrohalogenation (Gilch route),
Wittig-Horner reaction, Heck and Suzuki reaction, and
so on. Among the methods investigated, the Gilch
procedure is very simple and can normally result in
polymers with high molecular weights, narrow polydis-
persity indices and high structural regularity. A few
PPV derivatives such as MEH-PPV, OC1C10-PPV, and
phenyl-substituted PPVs (Ph-PPVs, reported by COV-
ION) with such promising properties have been suc-
cessfully prepared through the Gilch route.23-25 How-
ever, through COVION’s research, it was found that
there are a large amount of defects in both alkoxy-
substituted PPVs and phenyl-substituted PPVs, espe-
cially in the latter ones, which are not related to the
primary structure.23,26 The main irregularity in the
PPVs is identified to be tolane-bisbenzyl moieties
(TBB), which are formed due to head-to-head and tail-
to-tail linkage during polymerization. They have also
demonstrated that there is an obvious correlation
between the number of structural defects and the
luminance efficiency and lifetime of the corresponding
devices. To address this problem, a research group in
COVION found that introducing an electron-donating
methoxy group into the phenyl-substituted monomer
can result in a largely improved regular structure. On
the basis of this monomer, a few copolymers have been
prepared with very good device performance with
regards to the efficiency and device lifetime. However,
this approach has only partially solved the limitation
caused by structural defects in PPVs. Green-light-
emitting PPVs (without the strong electron-donating
groups in the monomer) with satisfactory operational
stability are still not available. Therefore, further effort
devoted to PPVs to seek effective ways to control or even
completely suppress the formation of structural defects
is essential for development of commercially valuable
light-emitting polymers, especially green-light-emitting
polymers.
Syn th esis of m od el p olym er s 2EH-P P V a n d 4EH-
P P V. The monomers for 2EH-P P V and 4EH-P P V were
synthesized according to the procedure described for poly[2-
(4′-decyloxyphenyl)-1,4-phenylenevinylene].27
In this paper,we report the synthesis, characteriza-
tion, and electroluminescent properties of a series of
dialkoxy phenyl-substituted PPV derivatives. The struc-
tures of the polymers are displayed in Figure 1. The
polymers in this paper, BEH2P -P P V, BEH3P -P P V,
and BEH4P -P P V, possess dialkoxy-branched long
Syn th esis of P oly[2-(2′-(2′′-eth ylh exyloxy)p h en yl)-1,4-
p h en ylen evin ylen e] (2EH-P P V). A solution of 1,4-bis-
(chloromethyl)-2-(2′-(2′′-ethylhexyloxy)phenyl)benzene (0.20 g,
0.53 mmol) in 10 mL of anhydrous THF was added a solution
of 1 M potassium tert-butoxide (3.2 mL) in 25 mL of anhydrous
THF at room temperature with stirring for 24 h. An orange

Macromolecules, Vol. 36, No. 4, 2003
New Phenyl-Substituted PPV Derivatives 1011
polymer (75 mg) was obtained in 46% yield. FT-IR (cm-1):
3057, 3024, 2923, 2856, 1598, 1445, 1380, 1235, 1108, 1028,
960, 895, 820, 749, 631, 471. 1H NMR (ppm) (300 MHz,
CDCl3): δ 7.70-7.00 (aromatic, 9H), 3.75 (2H), 1.53 (1H),
1.26-1.08 (8H), 0.70 (6H). Anal. Calcd for C22H26O: C, 86.27;
H, 8.50. Found: C, 84.09; H, 8.41.
Syn th esis of P oly[2-(4′-(2′′-eth ylh exyloxy)p h en yl)-1,4-
p h en ylen evin ylen e] (4EH-P P V). 4EH-P P V was synthe-
sized according to the procedure described for 2EH-P P V
using 1,4-bis(chloromethyl)-2-(4′-(2′′-ethylhexyloxy)phenyl)ben-
zene (0.22 g, 0.59 mmol) in 10 mL of anhydrous THF and 1 M
potassium tert-butoxide (3.5 mL) in 25 mL of anhydrous THF.
An orange polymer (84 mg) was obtained in 47% yield. FT-IR
(cm-1): 3038, 2956, 2925, 2850, 1607, 1511, 1492, 1464, 1240,
1174, 1110, 1027, 961, 830, 617. 1H NMR (ppm) (300 MHz,
CDCl3): δ 7.70-7.00 (aromatic, 9H), 3.90 (2H), 1.76 (1H),
1.50-1.35 (8H), 0.92 (6H). Anal. Calcd for C22H26O: C, 86.27;
H, 8.50. Found: C, 84.29; H, 8.25.
927, 760, 698, 637, 576. 1H NMR (ppm) (300 MHz, acetone-
d6): δ 7.88-7.85 (m, 2H), 7.41-7.31 (m, 3H), 7.10 (s, 2H). 13C
NMR (ppm) (75.5 MHz, acetone-d6): δ 135.62, 133.94, 131.72,
128.97. Anal. Calcd for C6H7BO2: C, 59.10; H, 5.79. Found:
C, 58.86; H, 5.53.
Syn th esis of 4,5-Bis(2′-eth ylh exyloxy)-1-br om o-2-p h en -
ylben zen e (1c). Under nitrogen atmosphere, phenylboronic
acid (1bi) (1.22 g, 10 mmol), 4,5-bis(2′-ethylhexyloxy)-1,2-
dibromobenzene (1b) (4.48 g, 9.1 mmol), 200 mL of THF, 200
mL of 2 M Na2CO3, and tetrakis(triphenylphosphine)palladium
were refluxed at 70 °C for 12 h. Partitioning between dichlo-
romethane and water, followed by column chromatography
using hexane as the eluent, gave a colorless oil (1.05 g) in 24%
yield. MS: m/z 488, 490 (53%, 54%). FT-IR (cm-1): 3058, 3028,
2958, 2928, 2869, 1599, 1560, 1511, 1485, 1466, 1381, 1329,
1307, 1253, 1233, 1197, 1029, 909, 857, 764, 734, 701, 647. 1H
NMR (ppm) (300 MHz, CDCl3): δ 7.44-7.34 (m, 5H), 7.15 (s,
1H), 6.87 (s, 1H), 3.92-3.91 (d, J ) 5.64 Hz, 2H), 3.88-3.86
(d, J ) 5.61 Hz, 2H), 1.83-1.76 (m, J ) 5.62 Hz, 2H), 1.61-
1.31 (m, 16H), 1.01-0.90 (m, 12H). 13C NMR (ppm) (75.5 MHz,
CDCl3): δ 149.29, 148.64, 141.23, 134.52, 129.48, 127.83,
127.17, 117.51, 115.99, 112.23, 71.78, 71.72, 39.49, 39.46,
30.51, 29.03, 23.86, 23.00, 13.98, 11.11, 11.08. Anal. Calcd for
The influence of solvent on the structural defects was
studied using 1,4-dioxane and p-xylene as a polymerization
solvent for 4EH-P P V while keeping all other conditions the
same.
Syn th esis of Dia lk oxyp h en ylSu bstitu ted P P Vs. Syn -
th esis of 1,2-Bis(2′-eth ylh exyloxy)ben zen e (1a ). A solution
of KOH (35.07 g, 0.63 mol) in 50 mL of ethanol was slowly
added to a stirred solution of catechol (27.53 g, 0.25 mol) in
150 mL of ethanol. The reaction mixture was stirred at room
temperature for 1 h. A solution of 2-ethylhexyl bromide (119.87
g, 0.75 mol) in 50 mL of ethanol was added dropwise. The
reaction mixture was refluxed overnight. Ethanol was removed
by rotary evaporation and the reaction mixture was partitioned
between ethyl acetate and sodium carbonate solution. After
drying over sodium sulfate, the product was obtained by
column chromatography using hexane as the eluent. A color-
less oil (44.30 g) was obtained in 53% yield. MS: m/z 334. FT-
IR (cm-1): 3064, 3037, 2959, 2929, 2873, 2860, 1592, 1506,
C
28H41BrO2: C, 68.71; H, 8.38; Br, 16.36. Found: C, 68.46; H,
8.15; Br, 16.50.
Syn th esis of p-Xylen ebor on ic Acid (1ci). 1ci was syn-
thesized according to the procedure described for 1bi using
magnesium turnings (5.76 g, 0.24 mol), 2-bromo-p-xylene
(37.00 g, 0.20 mol) and trimethyl borate (113.5 mL, 1.0 mol)
to give white crystals (19.27 g) in 64% yield. Mp: 191.1-192.5
°C. FT-IR (cm-1): 3293, 3104, 3014, 2968, 2921, 2862, 1906,
1860, 1809, 1767, 1609, 1573, 1493, 1418, 1064, 815, 745, 712,
1
686, 607, 507. H NMR (ppm) (300 MHz, acetone-d6): δ 7.38
(br s, 1H), 7.06-7.01 (m, 2H), 6.98 (s, 2H), 2.43 (s, 3H), 2.25
(s, 3H). 13C NMR (ppm) (75.5 MHz, acetone-d6): δ 140.52,
135.99, 135.93, 131.32, 131.29, 130.95, 22.81, 21.66. Anal.
Calcd for C8H11BO2: C, 64.07; H, 7.39. Found: C, 63.46; H,
7.15.
1
1465, 1254, 1222, 1123, 1032, 737. H NMR (ppm) (300 MHz,
CDCl3): δ 6.88 (s, 4H), 3.86-3.84 (d, J ) 5.62 Hz, 4H), 1.79-
1.71 (m, J ) 6.02 Hz, 2H), 1.54-1.23 (m, 16H), 0.95-0.83 (m,
12H). 13C NMR (ppm) (75.5 MHz, CDCl3): δ 149.54, 120.76,
113.77, 71.51, 39.53, 30.55, 29.08, 23.85, 23.04, 14.04, 11.11.
Anal. Calcd for C22H38O2: C, 79.04; H, 11.38. Found: C, 78.56;
H, 11.58.
Syn th esis of 2-(4′,5′-Bis(2′′-eth ylh exyloxy)-2′-p h en yl)-
p h en yl-p-xylen e (1d ). 1d was synthesized according to the
procedure described for 1c using p-xyleneboronic acid (1ci)
(0.96 g, 6 mmol) and 4,5-bis(2′-ethylhexyloxy)-1-bromo-2-
phenylbenzene (1c) (1.05 g, 2 mmol) to give a colorless oil (0.95
g) in 86% yield. MS: m/z 514. FT-IR (cm-1): 3027, 2958, 2927,
2867, 1602, 1515, 1490, 1465, 1380, 1346, 1243, 1194, 1164,
1133, 1035, 866, 810, 772, 699. 1H NMR (ppm) (300 MHz,
CDCl3): δ 7.16-7.08 (m, 5H), 6.99 (br s, 1H), 6.95 (s, 1H),
6.94-6.93 (m, 2H), 6.78 (s, 1H), 3.95-3.93 (d, J ) 5.61 Hz,
2H), 3.89-3.88 (d, J ) 5.61 Hz, 2H), 2.28 (s, 3H), 1.83 (s, 3H),
1.83-1.76 (m, 2H), 1.58-1.27 (m, 16H), 0.99-0.89 (m, 12H).
13C NMR (ppm) (75.5 MHz, CDCl3): δ 148.44, 148.24, 141.55,
141.13, 134.42, 133.25, 132.92, 132.67, 131.38, 129.48, 129.25,
127.50, 127.44, 125.88, 115.84, 115.14, 71.71, 71.68, 39.63,
39.56, 30.56, 29.06, 23.89, 23.86, 23.00, 20.83, 20.79, 19.46,
19.43, 13.98, 11.15, 11.12. Anal. Calcd for C36H50O2: C, 84.05;
H, 9.73. Found: C, 83.67; H, 9.27.
Syn th esis of 4,5-bis(2′-eth ylh exyloxy)-1,2-d ibr om oben -
zen e (1b). A solution of bromine (17.20 g, 0.11 mol) in 200
mL of glacial acetic acid was added to a solution of 1,2-bis(2′-
ethylhexyloxy)benzene (1a ) (18.00 g, 50 mmol) in 600 mL of a
mixture of methanol and chloroform at 0 °C. The reaction
mixture was stirred for 2 h, after which it was basified by
addition to sodium carbonate (10%, 1000 mL) and extracted
with dichloromethane (2 × 500 mL). The combined organic
layers were washed with water (200 mL) and dried over
anhydrous sodium sulfate. Column chromatography using
hexane as the eluent yielded a colorless oil (24.60 g) in 93%
yield. MS: m/z 492. FT-IR (cm-1): 3083, 2959, 2928, 2872,
1
2859, 1497, 1464, 1381, 1351, 1249, 1202, 1029, 846, 652. H
NMR (ppm) (300 MHz, CDCl3): δ 7.05 (s, 2H), 3.82-3.80 (d,
J ) 5.62 Hz, 4H), 1.75-1.69 (m, J ) 5.89 Hz, 2H), 1.53 -1.28
(m, 16H), 0.94-0.83 (m, 12H). 13C NMR (ppm) (75.5 MHz,
CDCl3): δ 149.30, 117.60, 114.33, 71.79, 39.34, 30.40, 28.96,
23.79, 22.91, 13.94, 11.02. Anal. Calcd for C22H36Br2O2: C,
53.66; H, 7.32; Br, 32.52. Found: C, 53.66; H, 7.17; Br, 31.26.
Syn th esis of P h en ylbor on ic Acid (1bi). To a suspension
of magnesium turnings (5.76 g, 0.24 mol) in anhydrous THF
was added a solution of bromobenzene (31.40 g, 0.20 mol) in
anhydrous THF. After refluxing for 4 h, the mixture was cooled
to -78 °C and transferred dropwise into a solution of trimethyl
borate (113.5 mL, 1 mol) in anhydrous THF. The reaction
mixture was stirred at room temperature for 24 h, followed
by the addition of 500 mL of 2 M HCl and stirring for 24 h.
The reaction mixture was then partitioned between ether and
water. Recrystallization from acetone gave white crystals
(12.91 g) in 53% yield. Mp: 217.5-218.5 °C (lit. mp: 216 °C).28
FT-IR (cm-1): 3273, 3077, 3051, 3026, 1965, 1915, 1829, 1780,
1699, 1604, 1573, 1560, 1496, 1444, 1395, 1181, 1088, 1023,
Syn th esis of 1,4-Bis(h yd r oxym eth yl)-2-(2′-p h en yl-4′,5′-
bis(2′′-eth ylh exyloxy) p h en yl)ben zen e (1e). A mixture of
2-(4′,5′-bis(2′′-ethylhexyloxy)-2′-phenyl)-p-xylene (0.95 g, 2
mmol) (1d ), N-bromosuccinimide (0.72 g, 4 mmol) and 2,2′-
azobis(2-methylpropionitrile) (AIBN) (20 mg) in benzene (5
mL) was heated at reflux for 6 h. After cooling, the solvent
was removed and the mixture was partitioned between ethyl
acetate and water. The organic layer was dried over Na2SO4,
and the solvent was removed to give a brown oil (1.48 g)
containing a mixture of brominated products. A mixture of the
crude residue and anhydrous potassium acetate (4 g) in glacial
acetic acid (8 mL) was heated at reflux for 16 h. After cooling,
the mixture was partitioned between dichloromethane and
water. The organic layer was dried over Na2SO4 and the
solvent was removed to give a brown oil (1.11 g) containing a
mixture of acetylated products. A mixture of the crude residue
was added dropwise to a suspension of LiAlH4 (0.35 g) in 5
mL of THF and stirred at room temperature overnight.
Saturated Na2SO4 was added carefully, the reaction mixture

1012 Chen et al.
Macromolecules, Vol. 36, No. 4, 2003
was filtered and dried over Na2SO4, and the solvent was
removed to give a brown oil. The residue was chromatographed
on silica gel using hexane:ethyl acetate (10:1) as the eluent to
yield colorless crystals (0.70 g) in 70% yield. Mp: 111.4-112.6
°C. MS: m/z 546. FT-IR (cm-1): 3382, 3309, 3052, 3026, 2958,
2925, 2869, 1518, 1492, 1465, 1416, 1243, 1195, 1024, 822, 702.
1H NMR (ppm) (300 MHz, CDCl3): δ 7.35-7.33 (d, J ) 8.04
Hz, 1H), 7.29-7.25 (dd, J ) 8.04, 1.59 Hz, 1H), 7.23 (br s, 1H),
7.20-7.08 (m, 5H), 6.97 (s, 1H), 6.83 (s, 1H), 4.67 (s, 2H), 4.31-
4.27 (d, J ) 13.23 Hz, 1H), 4.16-4.12 (d, J ) 12.84 Hz, 1H),
3.96-3.94 (d, J ) 5.22 Hz, 2H), 3.90-3.88 (d, J ) 5.61 Hz,
2H), 1.82-1.74 (m, J ) 6.02 Hz, 2H), 1.63-1.35 (m, 16H),
0.99-0.91 (m, 12H). 13C NMR (ppm) (75.5 MHz, CDCl3): δ
148.95, 148.58, 141.00, 140.26, 139.71, 138.05, 133.04, 130.90,
129.56, 129.48, 128.23, 127.84, 126.31, 125.86, 115.84, 115.07,
71.76, 71.71, 64.94, 62.69, 39.61, 39.58, 30.54, 29.08, 29.02,
23.91, 23.86, 22.97, 13.96, 11.10. Anal. Calcd for C36H50O4: C,
79.12; H, 9.16. Found: C, 79.30; H, 9.19.
Syn th esis of 1,5-Bis(2′-eth ylh exyloxy)-2,4-d ibr om oben -
zen e (2b). 2b was synthesized according to the procedure
described for 1b using bromine (17.20 g, 0.11 mol) and 1,3-
bis(2′-ethylhexyloxy)benzene (2a ) (18.00 g, 50 mmol) to give a
colorless oil (24.62 g) in 93% yield. MS: m/z 492. FT-IR (cm-1):
3074, 2959, 2929, 2873, 2860, 1715, 1580, 1570, 1488, 1458,
1402, 1369, 1287, 1196, 1061, 924, 875, 808, 772, 728, 704,
681. 1H NMR (ppm) (300 MHz, CDCl3): δ 7.63 (s, 1H), 6.45 (s,
1H), 3.89-3.88 (d, J ) 5.22 Hz, 4H), 1.81-1.73 (m, J ) 5.92
Hz, 2H), 1.58-1.26 (m, 16H), 0.97-0.85 (m, 12H). 13C NMR
(ppm) (75.5 MHz, CDCl3): δ 155.69, 135.23, 102.70, 99.26,
71.74, 39.31, 30.39, 28.99, 23.80, 22.89, 13.95, 11.09. Anal.
Calcd for C22H36Br2O2: C, 53.66; H, 7.32; Br, 32.52. Found:
C, 54.30; H, 6.91; Br, 31.68.
Syn th esis of 1,5-Bis(2′-eth ylh exyloxy)-4-br om o-2-p h en -
ylben zen e (2c). 2c was synthesized according to the proce-
dure described for 1c using phenylboronic acid (1.22 g, 10
mmol) and 1,5-bis(2′-ethylhexyloxy)-2,4-dibromobenzene (2b)
(4.48 g, 9.1 mmol) to give a colorless oil (1.88 g) in 42% yield.
MS: m/z 488, 490 (44%, 45%). FT-IR (cm-1): 3058, 3027, 2958,
2927, 2872, 2859, 1598, 1509, 1463, 1402, 1377, 1306, 1239,
Syn th esis of 1,4-Bis(ch lor om eth yl)-2-(2′-p h en yl-4′,5′-
bis(2′′-eth ylh exyloxy) p h en yl)ben zen e (1f). A mixture of
1,4-bis(hydroxymethyl)-2-(2′-phenyl-4′,5′-bis(2′′-ethylhexyloxy)-
phenyl)benzene (1e) (0.70 g, 1.3 mmol) and thionyl chloride
(1.4 mL, 19 mmol) was refluxed at 79 °C overnight. On
completion of the reaction, the mixture poured into ice-cold
water and partitioned between ethyl acetate and water. The
organic layer was dried over Na2SO4 and the solvent was
removed to give a brown oil. The residue was chromatographed
on silica gel using hexane:ethyl acetate (100:1) as the eluent
to yield a colorless oil (0.56 g) in 75% yield. MS: m/z 582, 584
(12%, 8%). FT-IR (cm-1): 3057, 3014, 2958, 2927, 2873, 1601,
1518, 1464, 1383, 1249, 1199, 1033, 868, 781, 703. 1H NMR
(ppm) (300 MHz, CDCl3): δ 7.42-7.40 (d, J ) 8.04 Hz, 1H),
7.30-7.27 (dd, J ) 8.01, 2.01 Hz, 1H), 7.21-7.14 (m, 4H),
7.09-7.06 (m, 2H), 7.00 (s, 1H), 6.95 (s, 1H), 4.53-4.49 (d, J
) 11.64 Hz, 1H), 4.48-4.44 (d, J ) 11.25 Hz, 1H), 4.41-4.37
(d, J ) 11.64 Hz, 1H), 4.26-4.22 (d, J ) 11.64 Hz, 1H), 3.99-
3.95 (overlapping d, 4H), 1.84-1.79 (m, J ) 5.35 Hz, 2H),
1.65-1.31 (m, 16H), 1.03-0.87 (m, 12H). 13C NMR (ppm) (75.5
MHz, CDCl3): δ 149.16, 148.46, 141.43, 140.75, 136.98, 135.64,
133.53, 131.59, 130.13, 129.76, 129.46, 127.79, 127.42, 126.26,
115.74, 115.20, 71.67, 63.65, 45.48, 43.77, 39.65, 39.58, 34.59,
34.44, 31.50, 30.61, 29.11, 26.84, 25.21, 23.94, 23.00, 22.56,
13.98, 11.15. Anal. Calcd for C36H48Cl2O2: C, 74.10; H, 8.23;
Cl, 12.18. Found: C, 73.35; H, 8.25; Cl, 11.63.
1
1189, 1041, 698. H NMR (ppm) (300 MHz, CDCl3): δ 7.48-
7.46 (d, J ) 7.23 Hz, 3H), 7.39-7.34 (m, J ) 7.26 Hz, 2H),
7.30-7.26 (m, J ) 6.81 Hz, 1H), 6.55 (s, 1H), 3.97-3.95 (d, J
) 5.22 Hz, 2H), 3.84-3.82 (d, J ) 5.22 Hz, 2H), 1.84-1.78
(m, J ) 5.89 Hz, 1H), 1.66-1.25 (m, 17H), 1.01-0.83 (m, 12H).
13C NMR (ppm) (75.5 MHz, CDCl3): δ 156.33, 155.52, 134.06,
129.37, 127.71, 126.55, 124.60, 98.84, 71.51, 71.03, 39.37,
39.33, 30.43, 29.05, 28.90, 23.82, 23.79, 22.97, 22.91, 14.03,
13.96, 11.15, 11.02. Anal. Calcd for C28H41BrO2: C, 68.71; H,
8.38; Br, 16.36. Found: C, 69.47; H, 8.10; Br, 15.41.
Syn th esis of 2-(2′,4′-Bis(2′′-eth ylh exyloxy)-5′-p h en yl)-
p h en yl-p-xylen e (2d ). 2d was synthesized according to the
procedure described for 1c using p-xyleneboronic acid (0.74 g,
5.0 mmol) and 1,5-bis(2′-ethylhexyloxy)-4-bromo-2-phenylben-
zene (2c) (1.86 g, 3.8 mmol) to give a colorless oil (1.17 g) in
60% yield. MS: m/z 514. FT-IR (cm-1): 3029, 2958, 2927, 2872,
2859, 1610, 1580, 1569, 1515, 1487, 1464, 1447, 1405, 1378,
1314, 1286, 1239, 1186, 1164, 1133, 1040, 903, 809, 770, 698.
1H NMR (ppm) (300 MHz, CDCl3): δ 7.58-7.54 (dd, 2H), 7.38-
7.33 (m, 2H), 7.28-7.23 (m, 1H), 7.17 (s, 1H), 7.13-7.10 (m,
1H), 7.04-7.02 (m, 2H), 6.62 (s, 1H), 3.92-3.90 (d, J ) 5.22
Hz, 2H), 3.84-3.82 (d, J ) 5.22 Hz, 2H), 2.32 (s, 3H), 2.16 (s,
3H), 1.71-1.65 (m, 1H), 1.59-1.20 (m, 17H), 0.99-0.79 (m,
12H). 13C NMR (ppm) (75.5 MHz, CDCl3): δ 156.52, 156.18,
138.38, 138.18, 134.22, 133.76, 132.66, 131.15, 129.50, 129.18,
127.58, 127.42, 126.05, 123.95, 122.83, 98.16, 98.09, 71.05,
70.82, 39.53, 39.40, 30.58, 30.43, 28.99, 28.90, 23.92, 23.78,
22.91, 20.80, 20.76, 19.65, 19.62, 13.94, 11.10, 10.99. Anal.
Calcd for C36H50O2: C, 84.05; H, 9.73. Found: C, 83.40; H,
9.86.
Syn th esis of 1,4-Bis(h yd r oxym eth yl)-2-(2′,4′-bis(2′′-eth -
ylh exyloxy)-5′-p h en yl)p h en yl)ben zen e (2e). 2e was syn-
thesized according to the procedure described for 1e using
2-(2′,4′-bis(2′′-ethylhexyloxy)-5′-phenyl)-p-xylene (2d ) (1.17 g,
2 mmol), N-bromosuccinimide (0.89 g, 5 mmol), and 2,2′-azobis-
(2-methylpropionitrile) (AIBN) (20 mg) to yield colorless
crystals (0.62 g) in 50% yield. Mp: 60.6-62.1 °C. MS: m/z 546.
FT-IR (cm-1): 3346, 3053, 3027, 2957, 2928, 1608, 1569, 1509,
1486, 1464, 1402, 1380, 1313, 1282, 1236, 1185, 1162, 1124,
1037, 902, 825, 769, 699. 1H NMR (ppm) (300 MHz, CDCl3):
δ 7.51-7.45 (m, 3H), 7.34-7.29 (m, 3H), 7.25-7.22 (m, 1H),
7.18 (s, 1H), 7.15 (s, 1H), 6.62 (s, 1H), 4.64 (s, 2H), 4.46-4.44
(d, J ) 5.61 Hz, 2H), 3.89-3.87 (d, J ) 5.22 Hz, 2H), 3.72 (br
s, 2H), 1.71-1.63 (m, 1H), 1.60-1.12 (m, 17H), 0.97-0.72 (m,
12H). 13C NMR (ppm) (75.5 MHz, CDCl3): δ 156.71, 156.30,
140.06, 138.99, 137.95, 137.64, 132.65, 129.47, 129.09, 128.65,
127.65, 126.34, 126.00, 123.91, 122.68, 98.95, 98.89, 72.19,
70.89, 64.91, 63.61, 39.48, 39.35, 30.56, 30.30, 28.98, 28.82,
23.91, 23.68, 22.94, 22.84, 22.58, 13.93, 11.08, 10.89. Anal.
Calcd for C36H50O4: C, 79.12; H, 9.16. Found: C, 78.69; H,
9.53.
Syn th esis of P oly[2-(2′-p h en yl-4′,5′-bis(2′′-eth ylh exyl-
oxy)p h en yl)-1,4-p h en ylen evin ylen e] (BEH2P -P P V). A
solution of 1,4-bis(chloromethyl)-2-(2′-phenyl-4′,5′-bis(2′′-eth-
ylhexyloxy) phenyl)benzene (1f) (0.32 g, 0.55 mmol) in 7 mL
of anhydrous THF was added to a solution of 1 M potassium
tert-butoxide (3.3 mL) in 14 mL of anhydrous THF at room
temperature with stirring for 20 h. The mixture was poured
into methanol, and the resulting yellow precipitate was
collected by filtration and reprecipitated from methanol.
Soxhlet extraction using methanol followed by acetone gave a
bright yellow polymer (220 mg) in 79% yield. FT-IR (cm-1):
3052, 3026, 2956, 2925, 2864, 1600, 1514, 1488, 1465, 1380,
1344, 1239, 1178, 1031, 963, 863, 768, 698. 1H NMR (ppm)
(300 MHz, CDCl3): δ 7.46, 7.36, 7.04, 6.85, 6.67 (aromatic,
12H), 3.92 (4H), 1.78 (2H), 1.49-1.33 (16H), 0.91 (12H). Anal.
Calcd for C36H46O2: C, 84.71; H, 9.02. Found: C, 82.92; H,
8.85.
Syn th esis of 1,3-Bis(2′-eth ylh exyloxy)ben zen e (2a ). 2a
was synthesized according to the procedure described for 1a
using KOH (35.0 g, 0.63 mol), resorcinol (27.5 g, 0.25 mol),
and 2-ethylhexyl bromide (119.9 g, 0.75 mol) to give a colorless
oil (43.95 g) in 53% yield. MS: m/z 334. FT-IR (cm-1): 3064,
3037, 2959, 2929, 2873, 2860, 1592, 1506, 1465, 1254, 1222,
1
1123, 1032, 737. H NMR (ppm) (300 MHz, CDCl3): δ 6.88 (s,
4H), 3.86-3.84 (d, J ) 5.62 Hz, 4H), 1.79-1.71 (m, J ) 6.02
Hz, 2H), 1.54-1.23 (m, 16H), 0.95-0.83 (m, 12H). 13C NMR
(ppm) (75.5 MHz, CDCl3): δ 160.58, 129.56, 106.48, 101.41,
70.36, 39.33, 30.48, 29.01, 23.81, 22.97, 13.98, 11.01. Anal.
Calcd for C22H38O2: C, 79.04; H, 11.38. Found: C, 79.10; H,
11.34.
Syn th esis of 1,4-Bis(ch lor om eth yl)-2-(2′,4′-bis(2′′-eth -
ylh exyloxy)-5′-p h en yl)p h en yl)ben zen e (2f). 2f was syn-
thesized according to the procedure as for 1f using 1,4-

Macromolecules, Vol. 36, No. 4, 2003
New Phenyl-Substituted PPV Derivatives 1013
bis(hydroxymethyl)-2-(2′,4′-bis(2′′-ethylhexyloxy)-5′-phenyl)-
benzene (2e) (0.52 g, 0.94 mmol) and thionyl chloride (1.0 mL,
14 mmol) to yield a colorless oil (0.45 g) in 63% yield. MS: m/z
582, 584 (100%, 50%). FT-IR (cm-1): 3027, 2958, 2928, 2868,
1608, 1579, 1510, 1463, 1401, 1379, 1315, 1262, 1239, 1188,
1168, 1116, 1043, 903, 818, 768, 699. 1H NMR (ppm) (300 MHz,
CDCl3): δ 7.58-7.54 (m, 3H), 7.40-7.34 (m, 3H), 7.30-7.24
(m, 3H), 6.62 (s, 1H), 4.59 (s, 2H), 4.55 (s, 1H), 4.52 (s, 1H),
3.93-3.91 (d, J ) 5.22 Hz, 2H), 3.87-3.83 (d, J ) 13.65 Hz,
2H), 1.73-1.69 (m, J ) 5.82 Hz, 1H), 1.57-1.54 (m, J ) 4.82
Hz, 2H), 1.48-1.12 (m, 15H), 1.00-0.74 (m, 12H). 13C NMR
(ppm) (75.5 MHz, CDCl3): δ 156.95, 156.28, 138.48, 137.96,
136.89, 136.46, 132.69, 131.19, 129.58, 129.50, 127.65, 127.51,
126.25, 123.11, 120.82, 97.74, 97.67, 70.93, 70.82, 45.70, 44.27,
39.46, 39.29, 30.55, 30.43, 28.98, 28.90, 28.86, 23.89, 23.79,
22.92, 22.85, 13.93, 11.07, 10.98. Anal. Calcd for C36H48Cl2O2:
C, 74.10; H, 8.23, Cl, 12.18. Found: C, 74.47; H, 7.90, Cl, 11.91.
Syn th esis of P oly[2-(2′,4′-bis(2′′-eth ylh exyloxy)-5′-ph en -
nesio-p-xylene, prepared from the reaction of 2-bromo-p-xylene
(1.80 g, 10 mmol) with magnesium (0.28 g, 12 mmol) in 10
mL of THF, was added dropwise into a solution of 2,5-bis(2′-
ethylhexyloxy)-1-bromo-4-phenylbenzene (3c) (1.58 g, 3 mmol)
in 20 mL of THF containing Ni(dppp)2Cl2 (18 mg, 0.03 mmol)
as a catalyst. After refluxing overnight, the reaction mixture
was quenched with 0.2 M HCl and extracted with ethyl
acetate. The extract was washed with water and brine,
followed by drying over anhydrous sodium sulfate. The reac-
tion mixture was subjected to column chromatography using
hexane as the eluent. A light yellow oil (1.17 g) was obtained
in 40% yield. MS: m/z 514. FT-IR (cm-1): 3056, 3032, 2959,
2927, 2872, 1508, 1487, 1408, 1379, 1205, 1057, 872, 808, 767,
1
698. H NMR (ppm) (300 MHz, CDCl3): δ 7.63-7.59 (m, 2H),
7.44-7.37 (m, 2H), 7.32-7.31 (m, 1H), 7.15-7.13 (m, 1H),
7.07-7.05 (m, 2H), 6.96 (s, 1H), 6.81 (s, 1H), 3.76-3.74 (d, J
) 5.61 Hz, 2H), 3.73-3.71 (d, J ) 5.22 Hz, 2H), 2.35 (s, 3H),
2.18 (s, 3H), 1.80-1.16 (m, 18H), 0.96-0.73 (m, 12H)). 13C
NMR (ppm) (75.5 MHz, CDCl3): δ 149.82, 149.74, 138.41,
138.35, 134.34, 133.53, 130.91, 130.80, 130.02, 129.55, 129.33,
128.21, 127.64, 127.59, 127.42, 126.47, 115.92, 115.54, 71.75,
71.72, 39.48, 39.40, 30.47, 30.41, 29.03, 28.92, 23.85, 23.81,
22.86, 20.82, 20.78, 19.54, 14.05, 13.95, 11.06, 10.98. Anal.
Calcd for C36H50O2: C, 84.05; H, 9.73. Found: C, 83.93; H,
9.84.
yl)p h en yl)-1,4-p h en ylen evin ylen e]
(BE H 3P -P P V).
BEH3P -P P V was synthesized according to the procedure
described for BEH2P -P P V using 1,4-bis(chloromethyl)-2-(2′-
phenyl-3′,5′-bis(2′′-ethylhexyloxy) phenyl)benzene (2f) (0.35 g,
0.60 mmol) in 8 mL of anhydrous THF and 1 M potassium
tert-butoxide (3.6 mL) in 16 mL of anhydrous THF. An orange
polymer (125 mg) was obtained in 41% yield. FT-IR (cm-1):
3021, 2956, 2924, 2862, 1604, 1461, 1377, 1309, 1232, 1171,
1115, 1041, 960, 898, 817, 766, 697. 1H NMR (ppm) (300 MHz,
CDCl3): δ 7.68, 7.67, 7.50, 7.23, 7.08, 6.61 (aromatic, 12H),
3.91-3.82 (2H), 3.53 (2H), 1.68 (1H), 1.43-1.09 (17H), 0.99-
0.71 (12H). Anal. Calcd for C36H46O2: C, 84.71; H, 9.02.
Found: C, 82.21; H, 8.31.
Syn th esis of 1,4-Bis(h yd r oxym eth yl)-2-(2′,5′-bis(2′′-eth -
ylh exyloxy)-4′-p h en yl)p h en yl)ben zen e (3e). A mixture of
2-(2′,5′-bis(2′′-ethylhexyloxy)-4′-phenyl)-p-xylene (3d ) (1.17 g,
2 mmol), N-bromosuccinimide (0.89 g, 5 mmol) and 2,2′-azobis-
(2-methylpropionitrile) (20 mg) in benzene (2 mL) was heated
at reflux for 6 h. After cooling, the solvent was removed, and
the mixture was partitioned between ethyl acetate and water.
The organic layer was dried over Na2SO4, and the solvent was
removed to give a brown oil (2.33 g) containing a mixture of
brominated products. A mixture of the crude residue and
anhydrous potassium acetate (4 g) in glacial acetic acid (8 mL)
was heated at reflux for 16 h. After cooling, the mixture was
partitioned between dichloromethane and water. The organic
layer was dried over Na2SO4, and the solvent was removed to
give a brown oil (3.20 g) containing a mixture of acetylated
products. A mixture of the crude residue, methanol (15 mL),
and 10% sodium hydroxide (15 mL) was heated at reflux for 2
h. After cooling, the mixture was partitioned between dichlo-
romethane and water. The organic layer was dried over Na2-
SO4 and the solvent was removed to give a brown oil. The
residue was chromatographed on silica gel using hexane:ethyl
acetate (10:1) as the eluent to yield a pale yellow oil (0.48 g)
in 40% yield. MS: m/z 546. FT-IR (cm-1): 3403, 3056, 3027,
2958, 2928, 2871, 1513, 1486, 1463, 1419, 1379, 1205, 1032,
Syn th esis of 1,4-Bis(2′-eth ylh exyloxy)ben zen e (3a ). 3a
was synthesized according to the procedure described for 1a
using KOH (35.00 g, 0.63 mol), hydroquinone (27.50 g, 0.25
mol), and 2-ethylhexyl bromide (145 g, 0.75 mol) to give a
colorless oil (57.60 g) in 69% yield. MS: m/z 334. FT-IR (cm-1):
3047, 2960, 2930, 2874, 2860, 1508, 1467, 1380, 1228, 1040,
823, 786. 1H NMR (ppm) (300 MHz, CDCl3): δ 6.83 (s, 4H),
3.80-3.78 (d, J ) 6.02 Hz, 4H), 1.75-1.67 (m, J ) 6.02 Hz,
2H), 1.58-1.29 (m, 16H), 0.96-0.72 (m, 12H). 13C NMR (ppm)
(75.5 MHz, CDCl3): δ 153.37, 115.29, 71.12, 39.38, 30.45,
29.01, 23.76, 22.97, 14.00, 11.01. Anal. Calcd for C22H38O2: C,
79.04; H, 11.38. Found: C, 78.50; H, 11.77.
Syn th esis of 2,5-Bis(2′-eth ylh exyloxy)-1,4-d ibr om oben -
zen e (3b). 3b was synthesized according to the procedure
described for 1b using bromine (19.20 g, 0.12 mol) and 1,4-
bis(2′-ethylhexyloxy)benzene (3a ) (20.00 g, 60 mmol) to give a
colorless oil (25.00 g) in 85% yield. MS: m/z 492. FT-IR (cm-1):
2959, 2928, 2872, 2859, 1494, 1460, 1362, 1212, 1064, 1028,
1
1
1007, 850, 807. H NMR (ppm) (300 MHz, CDCl3): δ 7.08 (s,
768, 698. H NMR (ppm) (300 MHz, CDCl3): δ 7.60-7.57 (m,
2H), 3.84-3.82 (d, J ) 5.62 Hz, 4H), 1.76-1.70 (m, J ) 6.02
Hz, 2H), 1.57-1.26 (m, 16H), 0.96-0.83 (m, 12H). 13C NMR
(ppm) (75.5 MHz, CDCl3): δ 150.10, 118.12, 110.98, 72.46,
39.34, 30.35, 28.93, 23.78, 22.91, 13.94, 11.04. Anal. Calcd for
2H), 7.56-7.53 (d, J ) 8.04 Hz, 1H), 7.44-7.28 (m, 5H), 7.01
(s, 1H), 6.82 (s, 1H), 4.74 (s, 2H), 4.53-4.49 (d, J ) 11.64 Hz,
1H), 4.45-4.41 (d, J ) 12.45 Hz, 1H), 3.84 (br s, OH), 3.77-
3.75 (d, J ) 5.22 Hz, 2H), 3.71-3.70 (d, J ) 5.22 Hz, 2H),
3.60 (br s, OH), 1.63-1.53 (m, J ) 6.02 Hz, 1H), 1.45-1.09
(m, 17H), 0.86-0.68 (m, 12H). 13C NMR (ppm) (75.5 MHz,
CDCl3): δ 150.76, 150.02, 140.11, 138.86, 138.09, 137.93,
131.28, 130.60, 129.48, 129.21, 128.53, 127.74, 126.89, 126.37,
117.35, 115.55, 71.58, 65.00, 62.28, 39.45, 39.39, 30.43, 30.22,
28.86, 28.79, 23.78, 23.60, 22.88, 22.82, 22.79, 13.91, 10.97.
Anal. Calcd for C36H50O4: C, 79.12; H, 9.16. Found: C, 79.59;
H, 9.23.
Syn th esis of 1,4-Bis(ch lor om eth yl)-2-(2′,5′-bis(2′′-eth -
ylh exyloxy)-4′-p h en yl)p h en yl)ben zen e (3f). 3f was syn-
thesized according to the procedure as for 1f using 1,4-
bis(hydroxymethyl)-2-(4′-phenyl-2′,5′-bis(2′′-ethylhexyloxy)-
phenyl)benzene (3e) (0.39 g, 0.72 mmol) and thionyl chloride
(0.8 mL, 10 mmol) to yield a colorless oil (0.31 g) in 75% yield.
MS: m/z 582, 584 (100%, 50%). FT-IR (cm-1): 3055, 3028,
2958, 2928, 2871, 1513, 1486, 1464, 1420, 1379, 1262, 1206,
1033, 768, 699. 1H NMR (ppm) (300 MHz, CDCl3): δ 7.63-
7.57 (m, 3H), 7.46-7.32 (m, 5H), 6.98 (s, 1H), 6.95 (s, 1H),
4.63-4.60 (m, 2H), 4.57-4.55 (d, J ) 6.84 Hz, 2H), 3.82-3.80
(d, J ) 5.64 Hz, 2H), 3.75-3.73 (d, J ) 5.22 Hz, 2H), 1.64-
1.60 (m, 1H), 1.58-1.14 (m, 17H), 0.90-0.62 (m, 12H). 13C
C
22H36Br2O2: C, 53.66; H, 7.32; Br, 32.52. Found: C, 53.96;
H, 7.57; Br, 31.45.
Syn th esis of 2,5-Bis(2′-eth ylh exyloxy)-1-br om o-4-p h en -
ylben zen e (3c). 3c was synthesized according to the proce-
dure described for 1c using phenylboronic acid (0.12 g, 1 mmol)
and 2,5-bis(2′-ethylhexyloxy)-1,4-dibromobenzene (3b) (0.50 g,
1 mmol) to give a colorless oil (0.25 g) in 51% yield. MS: m/z
488, 490 (23%, 24%). FT-IR (cm-1): 3058, 3028, 2959, 2928,
2872, 2859, 1485, 1465, 1378, 1270, 1209, 1059, 1033, 764, 698.
1H NMR (ppm) (300 MHz, CDCl3): δ 7.53-7.51 (dd, J ) 8.24,
1.40 Hz, 2H), 7.43-7.31 (m, 3H), 7.18 (s, 1H), 6.91 (s, 1H),
3.91-3.89 (d, J ) 5.61 Hz, 2H), 3.78-3.76 (d, J ) 5.61 Hz,
2H), 1.82-1.74 (m, J ) 6.02 Hz, 2H), 1.63-1.24 (m, 16H),
0.99-0.82 (m, 12H). 13C NMR (ppm) (75.5 MHz, CDCl3): δ
150.45, 149.90, 137.87, 130.99, 129.34, 127.76, 126.99, 118.08,
116.04, 111.10, 72.42, 71.80, 39.46, 39.36, 30.41, 28.99, 28.85,
23.82, 23.76, 22.95, 22.88, 13.97, 13.91, 12.12, 11.08, 10.95.
Anal. Calcd for C28H41BrO2: C, 68.71; H, 8.38; Br, 16.36.
Found: C, 68.26; H, 8.88; Br, 15.98.
Syn th esis of 2-(2′,5′-Bis(2′′-eth ylh exyloxy)-4′-p h en yl)-
p h en yl-p-xylen e (3d ). A Grignard reagent of 2-bromo-mag-

1014 Chen et al.
Macromolecules, Vol. 36, No. 4, 2003
Sch em e 1a
a
Reagents and conditions: i, KOH/EtOH, 2-ethylhexyl bromide; ii, Br2/AcOH; iii, phenylboronic acid, 2 M Na2CO3, Pd(PPh3)4;
iv, 2-boronic acid-p-xylene, 2 M Na2CO3, Pd(PPh3)4; v, N-bromosuccinimide (NBS)/AIBN/benzene; vi, KOAc/AcOH; vii, LiAlH4/
THF; viii, SOCl2; ix, tert-BuOK/THF.
NMR (ppm) (75.5 MHz, CDCl3): δ 150.10, 150.04, 138.49,
138.29, 136.99, 136.23, 131.50, 130.85, 130.07, 129.82, 129.52,
128.32, 127.80, 127.74, 127.60, 126.87, 116.01, 115.61, 71.72,
45.64, 44.27, 39.45, 39.36, 30.45, 30.39, 28.85, 23.79, 23.73,
22.91, 22.85, 22.59, 14.01, 13.91, 10.98, 10.91. Anal. Calcd for
monomer 1,4-bis(chloromethyl)-2-(2′-(2′′-ethylhexyloxy)-
phenyl)benzene or 1,4-bis(chloromethyl)-2-(4′-(2′′-eth-
ylhexyloxy)phenyl)benzene. Polymerization at room tem-
perature according to the Gilch route using tert-BuOK
as the base and THF as the solvent afforded the light
yellow (2EH-P P V) and orange polymers (4EH-P P V),
respectively.
C
36H48Cl2O2: C, 74.10; H, 8.23, Cl, 12.18. Found: C, 73.28; H,
8.62, Cl, 11.25.
Syn th esis of P oly[2-(2′,5′-bis(2′′-eth ylh exyloxy)-4′-ph en -
yl)p h en yl)-1,4-p h en ylen evin ylen e]
(BE H 4P -P P V).
In the synthesis of BE H 2P -P P V, BE H 3P -P P V,
and BEH4P -P P V, catechol, resorcinol, and hydro-
quinone were used as the starting materials, respec-
tively. The synthetic procedure for BEH2P -P P V as an
example is depicted in Scheme 1. After Williamson ether
reaction, nuclear bromination of 1a was effected using
bromine in glacial acetic acid to give 4,5-bis(2′-ethyl-
hexyloxy)-1,2-dibromobenzene (1b). The Suzuki coupling
reaction of phenylboronic acid (1bi) with 1b yielded 4,5-
bis(2′-ethylhexyloxy)-1-bromo-2-phenylbenzene (1c) which
was subsequently reacted with p-xyleneboronic acid or
the Grignard reagent of 2-bromo-p-xylene to give 2-(2′-
phenyl-4′,5′-bis(2′′-ethylhexyloxy)phenyl-p-xylene (1d ).
The reason for employing the Suzuki reaction for the
synthesis of 1c, 2c, and 3c was because it was found
that a higher yield resulted, which is probably due to
the higher selectivity of the Suzuki coupling reaction
as compared to that of the Grignard coupling reaction.
Both Suzuki and Grignard coupling reactions can be
exploited for the following synthesis of 1d , 2d , and 3d
with satisfactory yields. The monomer 1,4-bis(chloro-
methyl)-2-(2′-phenyl-4′,5′-bis(2′′-ethylhexyloxy)phenyl)-
benzene (1f) was obtained through a multistep proce-
dure starting with NBS bromination of 1d , esterification
of the brominated products, followed by reduction of the
acetylated products to 1,4-bis(hydroxymethyl)-2-(2′-
phenyl-4′,5′-bis(2′′-ethylhexyloxy)phenyl)benzene (1e),
BEH4P -P P V was synthesized according to the procedure
described for BEH2P -P P V using 1,4-bis(chloromethyl)-2-[4′-
phenyl-2′,5′-bis(2′′-ethylhexyloxy) phenyl]benzene (3f) (0.17 g,
0.30 mmol) in 4 mL of anhydrous THF and 1 M potassium
tert-butoxide (1.78 mL) in 8 mL of anhydrous THF. A bright
yellow polymer (83 mg) was obtained in 55% yield. FT-IR
(cm-1): 3058, 3027, 2956, 2862, 1510, 1463, 1376, 1203, 1031,
961, 767, 696. 1H NMR (ppm) (300 MHz, CDCl3): δ 7.60, 7.38,
7.34, 6.99, 6.85 (aromatic, 12H), 3.71 (4H), 1.23-1.08 (18H),
0.80-0.72 (12H). Anal. Calcd for C36H46O2: C, 84.71; H, 9.02.
Found: C, 83.41; H, 8.84.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . The synthetic
route to 2EH-P P V and 4EH-P P V is similar to the
procedure described in ref 27. 2-Bromophenol or 4-bro-
mophenol was used as a starting material for the
synthesis of 1-bromo-2-(2′-ethylhexyloxy)benzene or
1-bromo-4-(2′-ethylhexyloxy)benzene through the Wil-
liamson ether synthesis. The ether products were
coupled with the Grignard reagent of 2-bromo-p-xylene
to yield 2-(2′-(2′′-ethylhexyloxy)phenyl)-p-xylene or 2-(4′-
(2′′-ethylhexyloxy)phenyl)-p-xylene. A multistep proce-
dure starting with NBS bromination, esterification of
the brominated products, followed by reduction of the
acetylated products to yield dihydroxylated compounds,
and finally reaction with thionyl chloride afforded the

Macromolecules, Vol. 36, No. 4, 2003
New Phenyl-Substituted PPV Derivatives 1015
Ta ble 1. Effect of Solven t P ola r ity on th e Am ou n t of
TBB in 4EH-P P V
solvent
THF
1,4-dioxane
p-xylene
solvent polarity
amount of TBB (%)
21
16.4
7.4
2.55
3.18
4.64
and finally reaction with thionyl chloride. Polymeriza-
tion at room temperature according to the Gilch route
using t-BuOK as the base and THF as the solvent
afforded the bright yellow polymer. The synthetic routes
to BEH3P -P P V and BEH4P -P P V are essentially the
same as that of BEH2P -P P V, except that the starting
materials used were resorcinol and hydroquinone, re-
spectively. BEH3P -P P V was obtained as an orange
polymer while BEH4P -P P V was a bright yellow
polymer. The polymers were fully soluble at room
temperature in common organic solvents such as CHCl3,
THF, toluene, and p-xylene.
One of the main efforts of this work is to seek an
effective way to suppress the structural defects in the
polymers. Therefore, the amount of tolane-bisbenzyl
(TBB) defects in this series of polymers was estimated
by the integration of a small signal around 2.4-2.8 ppm
in the proton NMR spectra, relative to that of the alkoxy
protons. For 2EH-P P V and 4EH-P P V, 2.46% and
2.55% of TBB were observed, respectively. This is
significantly lower as compared to DOP-PPV27 or Ph-
PPVs.23,25 DOP-PPV and Ph-PPVs, results of polym-
erization in which o-xylene and 1,4-dioxane were used
as the solvent, possessed 5.99% and 5-6% TBB defects,
respectively. Hence, we postulate that the solvent in
which polymerization takes place plays a very important
role in the amount of TBB defects observed.
The effect of polymerization solvent on the amount
of TBB defects was thus investigated. The monomer of
4EH-P P V was polymerized under identical conditions
with the exception of the polymerization solvent used.
Three different solvents, THF, 1,4-dioxane, and p-
xylene, were used, and the amounts of TBB defects
observed in the polymers were 2.55%, 3.18% and 4.64%
respectively. Hence, a very obvious trend can be identi-
fied for the relationship between the polarity of the
solvent and the amount of TBB observed. The relation-
ship is an inverse one where the higher the polarity of
the solvent, the lower is the amount of TBB observed.
The solvent polarity quoted as follows is referenced
against water, which has a value of 100.29 The results
are summarized in Table 1. As a solvent for the Gilch
polymerization, THF would be the most ideal solvent
for minimizing the amount of TBB defects observed.
From the phenomenon of solvent polarity affecting the
formation of structural defects in polymers, it may be
interpreted that solvents can influence the polymeriza-
tion process. The mechanism of the Gilch route to
prepare PPVs is still arguable. Both radical and anionic
chain reactions have been proposed.30-33 However, a
dual reaction mechanism, which involves both pro-
cesses, is more reasonable.34,35 During chain propaga-
tion, both radical and anionic species may form head-
to-head or tail-to-tail irregular linkage. However, the
possibility of irregularity from the anionic reaction is
lower than that from the radical reaction due to
electronic effects. It is well-known that polar solvents
can accelerate the anionic chain propagation due to the
stronger coordination effect of polar solvents to anionic
species.36 On the other hand, the polarity has no
significant effect on the radical reaction. Therefore, it
F igu r e 2. 1H NMR showing the TBB signal at 2.4-2.8 ppm.
is rational to conclude that polymerization in more polar
solvents such as THF will result in a low amount of
head-to-head and tail-to-tail linkage due to anionic
polymerization dominating the chain propagation.
Besides the effect of solvents, the monomer structure,
which is associated with electronic property and steric
effect of the substituents, will also influence the polym-
erization process. The effect of electronic property of
substituents on the monomer has been demonstrated
by COVION.23,26 The steric effect will be discussed here.
From the model polymers 2EH-P P V and 4EH-P P V,
we can find that the introduction of the alkoxy side
chain at the ortho-position of the side phenyl ring
(2EH-P P V) is not as efficient in suppressing the
irregular structure formation, as compared to 4E H -
P P V. Therefore, a structure with more hindrance is
required to achieve this target. In the new series of
polymers, BEH2P -P P V, BEH3P -P P V, and BEH4P -
P P V, the amount of TBB observed was found to be
0.36%, 3.79%, and 3.72%, respectively. Among the three
polymers, BEH2P -P P V contains the lowest amount of
TBB, which is in accordance with our expectation. Their
1H NMR spectra, in which the signals of TBB can be
found around 2.4-2.8 ppm, are shown in Figure 2.
Because of the highly rigid structure of BEH2P -P P V,
the phenyl substituent in position-2 of the side chain
imposes great steric hindrance during the polymeriza-
tion process, hence minimizing abnormal head-to-head
linkages and thus dramatically reducing the amount of
TBB defects observed. The effect of steric hindrance on
the amount of TBB defects can be explained by the
mechanism, which is illustrated in Scheme 2. When the
monomer reacts with 1 mol of the base, both of the
chloromethyl groups can be deprotonated. If the mono-
mer is asymmetric, attack of either of the two chloro-
methyl groups gives rise to two different xylylene “real”
monomers, leading to a certain amount of TBB defects.
In the case of BEH2P -P P V, the bulky substituent on
position-2 on the side group allows the base to attack
only the sterically less hindered chloromethyl group.
Hence, only one xylylene intermediate results and on
propagation of the regular head-to-tail linkage, the
formation of TBB defects is suppressed. As a result,
negligible TBB defects were detected in BEH2P -P P V

1016 Chen et al.
Macromolecules, Vol. 36, No. 4, 2003
Sch em e 2. Mech a n ism of th e Gilch P olym er iza tion Rou te for BEH2P -P P V w ith Low TBB Con ten ta
a
EH represents 2-ethylhexyl.

Macromolecules, Vol. 36, No. 4, 2003
New Phenyl-Substituted PPV Derivatives 1017
as compared to BEH3P -P P V and BEH4P -P P V. In
Scheme 2, two possible routes based on the anionic
chain reaction were illustrated to explain the polymer-
ization process. In fact, the mechanism is also applicable
to the radical polymerization process.
The influence of tolane-bisbenzyl structural defects
on the device performance is still under investigation.
It is suggested that the tolane-bisbenzyl moieties will
lower the charge carrier mobility due to an interruption
of the π-conjugation along the polymer chains, which
may lead to charge carrier build up, resulting in
additional breaks in conjugation.23,37 This is a self-
accelerating process of degradation. In addition, the
active benzyl groups of the structural defects may also
be converted to carbonyl groups through photooxidation
or electro-oxidation during device operation. The newly
generated exciton trapping sites will reduce the device
quantum efficiency. In fact, a similar phenomenon has
recently been reported in poly(dialkylfluorene)-based
LED devices.38 Both channels will lead to a higher
driving voltage to operate the device at a constant
current. Therefore, the tolane-bisbenzyl structural
defects may not directly influence the efficiency of the
as-prepared LED devices but more of the performance
during the aging test. Since the amount of TBB defects
adversely affects the device efficiency and lifetime of
PLEDs, BEH2P -P P V would be expected to demon-
strate the best performance as compared to the other
polymers when used as the emissive layer in PLEDs.
F igu r e 3. Absorption and fluorescence spectra of BEH2P -
P P V, BEH3P -P P V, and BEH4P -P P V polymer solutions.
Besides investigation of TBB formation in the poly-
mers, their physical, optical, and electrochemical prop-
erties have also been studied. GPC analysis revealed
that the number-average molecular weights (Mn) of the
polymers BEH2P -P P V, BEH3P -P P V, and BEH4P -
P P V were 3.6 × 105, 1.3 × 105, and 5.7 × 105 respec-
tively while the weight-average molecular weights (Mw)
were 7.7 × 105, 3.1 × 105 and 9.2 × 105 with a
polydispersity indexes of 2.11, 2.45, and 1.61, respec-
tively. The onsets of degradation of the polymers
BEH2P -P P V, BEH3P -P P V, and BEH4P -P P V were
observed through thermogravimetric analysis (TGA) in
nitrogen to be 314, 339, and 300 °C, respectively,
indicating good thermal stability, an essential property
for application of the polymers in PLEDs. No obvious
Tg was observed from this series of polymers.
F igu r e 4. Absorption and fluorescence spectra of BEH2P -
P P V, BEH3P -P P V, and BEH4P -P P V polymer films.
a result of the highly rigid structure of the polymers,
leading to a loss in planarity between the phenyl rings.
As such, a decrease in the conjugation length results
and hence the blue shift in UV λmax is observed as
compared to the other polymers. On the other hand, the
red shift in the PL λmax of BEH2P -P P V in the film
state, which is not observed when in the solution state,
may be caused by the difference in geometries of the
ground and excited states, which may be associated with
the bulky phenyl ring in position-2 on the side group.
In conjunction with the relaxation of vibronic energy
bands, the difference in the minimum points of the
potential curves of the ground and excited states on the
configuration coordinates leads to further bathochromic
shift between absorption and emission maxima. Gener-
ally, the geometric change in excited states will reduce
the quantum efficiency due to nonradiative transition,
which will be further discussed later. All the polymers
exhibit well-defined side peaks in the PL spectra which
is a result of vibronic coupling of excitons. The PL λmax
of the film samples show a slight red shift as compared
to the solution samples. As seen from the values of the
π-π* band gap, the emission from the polymers falls
in the region of green. The π-π* band gaps were
estimated from the edge absorptions of the film samples
to be 2.47, 2.51, and 2.52 for BEH2P -P P V, BEH3P -
P P V, and BEH4P -P P V respectively. The relative
fluorescence quantum efficiency (ηPL) of the polymers
was determined, in a dilute chloroform solution (1 ×
10-6 M) using quinine sulfate (1 × 10-5 M) dissolved in
0.1 M H2SO4 as a standard, to be 0.66, 0.69, and 0.78
for BEH2P -P P V, BEH3P -P P V, and BEH4P -P P V
respectively. There is a clear correlation between the
Op tica l a n d P h otolu m in escen e P r op er ties. The
absorption and photoluminescence (PL) spectra of the
polymer solutions in CHCl3 and that of the polymer
films spin-cast from xylene solutions were measured at
room temperature. On comparison of the spectra of
polymers (Figures 3 and 4), it can be seen that the
polymers show structureless absorption spectra except
for BEH4P -P P V, where an additional peak was ob-
served at 316 nm for the solution sample and at 317
nm for the film sample. These peaks are a result of
absorption due to the full conjugation of the three
phenyl rings in the side chain. The UV λmax of BEH2P -
P P V is slightly blue shifted as compared to those of
BEH3P -P P V and BEH4P -P P V. This should be due
to the steric hindrance caused by the bulky phenyl ring
in position-2 on the side group, resulting in a loss of
planarity and hence shortening of the effective conjuga-
tion length. As for the polymers in the film state, both
the UV and PL spectra of BEH4P -P P V and BEH3P -
P P V are almost identical, whereas BE H2P -P P V
displays a slight blue shift in its UV λmax and a slight
red shift in its PL λmax. Again, this can be explained as

1018 Chen et al.
Macromolecules, Vol. 36, No. 4, 2003
Ta ble 2. P h ysica l P r op er ties of th e P olym er s
UV λmax, nm
PL λmax, nm
Mn/Mw
(×105)
3.6/7.7
1.3/3.1
5.7/9.2
Td (°C)
(onset)
band
gap (eV)
ηPL
polymer
solvent
film
solvent
film
solvent
BEH2P -P P V
BEH3P -P P V
BEH4P -P P V
314
339
300
423
437
418
434
2.47
2.51
2.52
487 (516)
486 (517)
485 (518)
502 (532)
496 (525)
494 (523)
0.66
0.69
0.78
(316) 436
(317) 432
Ta ble 3. Electr och em ica l Da ta of th e P olym er sa
p-doping (V)
n-doping (V)
energy levels (eV)
red
polymer
[Eonset]°x
Ea
Ec
[Eonset
]
Ec
Ea
HOMO
LUMO
band gap (eV)
BEH2P -P P V
BEH3P -P P V
BEH4P -P P V
0.90
0.85
0.86
1.34
1.31
1.18
-1.68
-1.60
-2.02
-2.06
-1.96
-2.34
-1.89
-1.86
-2.31
-5.30
-5.25
-5.26
-2.72
-2.80
-2.38
2.58
2.45
2.88
1.17
a
Ea and Ec represent the anodic and cathodic peak potentials, respectively.
η
PL and the polymer structure. The more well-separated
were estimated to be 2.58, 2.45, and 2.88 eV for
BEH2P -P P V, BEH3P -P P V, and BEH4P -P P V re-
spectively. The values for BEH2P -P P V and BEH3P -
P P V are quite close to those obtained by optical method
while BEH4P -P P V shows higher energy gap. Accord-
the polymer chains, the higher the ηPL. As a result, the
PL efficiency for BEH4P -P P V was observed to be the
highest among the series of polymers. This may be due
to the branched 2′-ethylhexyloxy side chains on opposite
sides of the phenyl substituent, which effectively serve
to separate the polymer chains from one another. The
results are in accordance to those obtained for poly-
(2-(2′,5′-bis(2′′-ethylhexyloxy)phenyl)-1,4-phenylenevi-
nylene)17 and poly(3-(2,5-dioctylphenyl)-thiophene)
(PDOPT). These polymers have demonstrated ∼60%
and 16% of PL efficiency in the film state, respectively.
Compared to other PPVs or poly(3-alkylthiophene)s, the
improvement in efficiencies is very significant.39,40 The
physical properties of the polymers are summarized in
Table 2.
ing to the equations,41,42 IP ) -([Eonset
]
ox + 4.4) eV and
red
ox
EA ) -([Eonset
]
+ 4.4) eV, where [Eonset
]
and
red
[Eonset
]
are the onset potentials for the oxidation and
reduction of the polymer vs the reference electrode, the
LUMO and HOMO of the polymers were estimated to
be -2.72, -2.80, and -2.38 eV and -5.30, -5.25, and
-5.26 eV for BE H 2P -P P V, BE H 3P -P P V, and
BEH4P -P P V respectively. The electrochemical data
of the polymers are summarized in Table 3. From Table
3, we find that the HOMO and LUMO energy levels of
BEH2P -P P V and BEH3P -P P V are quite similar. As
for BEH4P -P P V, its HOMO is close to the other two
polymers while the LUMO is much higher. Therefore,
the substitution pattern of the side chains will affect
not only the polymerization process but also the energy
levels of the polymers.
Electr och em ica l P r op er ties. Cyclic voltammetry
(CV) was employed to investigate the electrochemical
behavior of the polymers as well as estimate the HOMO
and LUMO energy levels of the materials. The polymer
films deposited on a Pt electrode were scanned both
positively and negatively separately in 0.10 M tetrabu-
tylammonium perchlorate (Bu4NClO4) in anhydrous
acetonitrile. In Figure 5, all the polymers showed
reversible n-doping processes and except for BEH4P -
P P V, which showed a partially reversible p-doping
process, the other polymers showed irreversible p-
doping processes. The onset potentials for oxidation
were observed to be 0.90, 0.85, and 0.86 V (vs SCE) for
BEH2P -P P V, BEH3P -P P V, and BEH4P -P P V re-
spectively. On the other hand, the onset potentials for
reduction were -1.68, -1.60, and -2.02 V (vs SCE),
respectively. From the onset potentials of the oxidation
and reduction processes, the band gaps of the polymers
E lect r olu m in escen ce P r op er t ies of LE D De-
vices. Double layer LED devices based on the three
polymers with the configuration of ITO/PEDOT/polymer/
Ba/Al were fabricated. Poly(3,4-ethylenedioxythiophene)
(PEDOT) doped with poly(styrenesulfonic acid) (PSS)
(Batron-P 4083) with a thickness of 150 nm was used
as the hole injection/transporting layer. A thin layer of
Ba (5 nm) was employed as the cathode because it
normally results in devices with longer lifetime due to
its larger atomic size and mass as compared with Ca or
Mg.43 The Ba layer was coated with a 180 nm layer of
Al. The thickness of the emissive layer is around 80 nm.
The electroluminescence spectra are displayed in Figure
6, in which BEH2P -P P V and BEH3P -P P V follow the
traces of their PL spectra, with peaks at 496, 488 nm
and clear side peaks and shoulders at 529, 577 nm and
520, 567 nm, respectively. The similarity of the PL and
EL spectra indicates that same excitations are involved
in both cases. In contrast, the EL spectrum of BEH4P -
P P V is different from its PL counterpart, which peaks
at 525 nm with two side peaks at 490 and 566 nm.
Compared with its PL spectrum, the change in relative
intensities in vibronic structure indicates that aggregate
emission is dominant in its EL process.44,45 The differ-
ence between the PL and EL spectra of BEH4P -P P V
may be attributed to the following two reasons. First,
the film for PL measurement was spin coated from
xylene solution while that for LED device fabrication
was prepared from chloroform solution. Different sol-
vents may result in different amount of interchain
F igu r e 5. Cyclic voltammograms of BEH2P -P P V, BEH3P -
P P V, and BEH4P -P P V.

Macromolecules, Vol. 36, No. 4, 2003
New Phenyl-Substituted PPV Derivatives 1019
F igu r e 8. Dependence of quantum efficiency of the LED
devices on current.
F igu r e 6. Electroluminescence spectra of the LEDs.
which is higher by factors of about 1.5 (BEH2P -P P V,
∼0.4%) and 2.5-3.0 (BEH4P -P P V, ∼0.2%). When we
investigate the relationship between energy levels and
devices performance, we find that the external quantum
efficiency is mainly determined by the LUMOs of the
polymers. Because the HOMO energy levels of the three
polymers are much closer to the work function of
PEDOT (5.2 eV), the barrier is very small for hole
injection. On the other hand, the LUMO of the polymers
are different from each other. The higher the LUMO of
the polymer, the lower is the QE of the device. There-
fore, holes are the major charge carriers in the devices
and the rate of electron injection from the cathode
determines the device efficiency among the investigated
polymer diodes. Other possible factors which may affect
the device efficiencies include interchain interactions in
the film states. For example, aggregation emission in
BEH4P -P P V is another key reason responsible for its
lowest QE among the three polymer diodes. As for
BEH2P -P P V, both the HOMO and LUMO energy
levels are comparable to BEH3P -P P V; however, the
quantum efficiency is lower. This might be ascribed to
the geometric change in the excited state, as discussed
before. Such a geometric change will generally lead to
a bathochromic shift in emission spectrum, faster charge
carrier mobility, and reduced quantum efficiency due
to intersystem crossing. This may also be partially
responsible for the lowest turn-on electric field for
BEH2P -P P V diode.
F igu r e 7. Current-voltage-luminance characteristics of ITO/
PEDOT/polymer/Ba/Al devices: current, open markers; lumi-
nance, closed markers.
interactions, which will influence the emission charac-
teristics.44,45 The second reason may be ascribed to the
different emissive processes involved for PL and EL. An
aggregate state in the polymer layer could serve as a
trapping center since aggregates in the polymers are
lower energy sites. For PL process, it only concerns
exciton trapping, while during the EL process, there is
an additional parallel process where the aggregates can
trap the injected electrons and holes. If trapping of
charges is significant compared with trapping of exci-
tons, the difference between the PL and EL spectra will
be notable. Because of concentration quenching in such
sites, the emission efficiency is reduced. This behavior
has also been observed in polyfluorene derivatives.38
Figure 7 compares the I-V-L curves of the double
layer devices. With an increase of forward electric field,
both the current and luminance increase synchronically.
Among the diodes, the EL performance of BEH2P -
P P V shows the lowest turn-on electric field, followed
by BEH4P -P P V and BEH3P -P P V. The turn-on
electric fields are 0.30, 0.42, and 0.50 MV/cm, respec-
tively. The lowest turn-on electric field observed in the
BEH2P -P P V diode can be attributed to the lowest
amount of tolane-bisbenzyl structural defects in
BEH2P -P P V, which renders it the longest π electron
conjugation and highest charge carrier mobility among
the three polymers.37 The required electric fields for 100
Cd/m2 of emissive brightness are 0.36, 0.57, and 0.73
MV/cm. The maximum QEs are 0.37%, 0.66% and 0.25%
for the diodes of BEH2P -P P V, BEH3P -P P V, and
BEH4P -P P V respectively. We can find in Figure 8
that BEH3P -P P V holds the highest QE (0.5-0.7%),
Con clu sion
A new series of dialkoxyphenyl-substituted PPV
derivatives, BE H2P -P P V, BE H 3P -P P V, and
BEH4P -P P V, has been successfully synthesized using
the Gilch method. The polymers possess desirable
properties such as excellent solubility, high molecular
weight, high PL efficiency, and good thermal stability.
1H NMR investigation of the polymers indicates that
substitution on the side chains which imposes strong
steric hindrance is an effective approach to suppress the
formation of structural defects during polymerization.
In addition, solvents used for the Gilch route also play
an important role with regards to the amount of TBB
moieties in the polymers. Polar solvent resulting in a
low amount of TBB may be ascribed to the accelerated
anionic chain reaction during polymerization.
Double-layered LED devices with the configuration
of ITO/PEDOT/polymer/Ba/Al revealed that BEH2P -
P P V posseses the highest luminance, the lowest turn-
on electric field, and a medium external quantum
efficiency and is the most promising material among the

1020 Chen et al.
Macromolecules, Vol. 36, No. 4, 2003
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longer operational lifetime is expected as a result of its
negligible TBB defects along the polymer main chain.
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