Synthesis, photophysics, and electroluminescence of conjugated poly(p-phenylenevinylene) derivatives with 1,3,4-oxadiazoles in the backbone

Article abstract of DOI:10.1021/ma034793w

Starting from 4-bromobenzaldehyde or 1,4-benzenedicarboxaldehyde, two new poly(p-phenylenevinylene) derivatives P1 and P2 were synthesized by a five-step synthetic route. These fully conjugated polymers contain 1 or 2 oxadiazole rings and 3 or 4 vinylene bonds per repeat unit for P1 and P2, respectively, and were amorphous and soluble in common organic solvents. The T_g values were 28°C for P1 and 57°C for P2. The polymers emitted greenish-blue light in solution with photoluminescence (PL) emission maximum at 487-511 nm and yellowish-green light with PL emission maximum at 515-558 nm in thin films. Electroluminescence (EL) was achieved from single-layer LEDs of polymer P1 with the configuration ITO/PEDOT/P1/Al with voltage-tunable EL colors from 558 nm (9 V) to 527 nm (16 V). The observed EL spectral blue shift with increasing voltage is a result of conformational changes of the polymer backbone with increase in temperature, as evidenced from the absorption and PL spectra changes upon annealing of P1 thin films at different temperatures.

Full text of DOI:10.1021/ma034793w

Macromolecules 2003, 36, 9295-9302
9295
Articles
Synthesis, Photophysics, and Electroluminescence of Conjugated
Poly(p-phenylenevinylene) Derivatives with 1,3,4-Oxadiazoles in the
Backbone
J oh n A. Mik r oya n n id is,* Ioa k im K. Sp iliop ou los, a n d Th eod or os S. Ka sim is
Chemical Technology Laboratory, Department of Chemistry, University of Patras,
GR-26500 Patras, Greece
Abh ish ek P . Ku lk a r n i a n d Sa m son A. J en ek h e*
Department of Chemical Engineering and Department of Chemistry, University of Washington,
Seattle, Washington 98195-1750
Received J une 10, 2003; Revised Manuscript Received October 10, 2003
ABSTRACT: Starting from 4-bromobenzaldehyde or 1,4-benzenedicarboxaldehyde, two new poly(p-phen-
ylenevinylene) derivatives P 1 and P 2 were synthesized by a five-step synthetic route. These fully
conjugated polymers contain 1 or 2 oxadiazole rings and 3 or 4 vinylene bonds per repeat unit for P 1 and
P 2, respectively, and were amorphous and soluble in common organic solvents. The T
for P 1 and 57 °C for P 2. The polymers emitted greenish-blue light in solution with photoluminescence
PL) emission maximum at 487-511 nm and yellowish-green light with PL emission maximum at 515-
58 nm in thin films. Electroluminescence (EL) was achieved from single-layer LEDs of polymer P 1 with
g
values were 28 °C
(
5
the configuration ITO/PEDOT/P 1/Al with voltage-tunable EL colors from 558 nm (9 V) to 527 nm (16 V).
The observed EL spectral blue shift with increasing voltage is a result of conformational changes of the
polymer backbone with increase in temperature, as evidenced from the absorption and PL spectra changes
upon annealing of P 1 thin films at different temperatures.
4-17
In tr od u ction
to be promising candidates for light-emitting devices1
due to their high electron affinity and hole-blocking
ability as well as their thermal and hydrolytic stability
Polymer light-emitting diodes (LEDs), based on con-
jugated polymers or polymers with conjugated segments
or pendant groups, have recently attracted a lot of
interest for their potential use in display technology.
Poly(p-phenylenevinylene)s (PPVs) and stilbenoid oli-
gomers are one of the most intensively investigated
classes of emissive materials for fabrication of LEDs.
PPV-type materials are predominantly hole-conducting
materials with high-lying LUMO levels. The unbalanced
charge carrier transport properties and the relative high
barrier for electron injection from electrode metals such
18-23
and photostability.
The present paper describes the synthesis and char-
acterization of two new PPV-type conjugated polymers
with 1,3,4-oxadiazoles in the backbone. They were
successfully synthesized from widely used and inexpen-
sive starting materials by a five-step synthetic route,
the last step of which was Heck coupling. The polymers
are related, but their molecular structures are dif-
ferentiated in order to provide a means for studying
structure-property relationships. The electrolumines-
cence and the associated photophysical properties of
these polymers were also investigated.
1
-10
1
b
1
1
as aluminum are disadvantages of these compounds.
Several strategies have been followed to improve the
efficiency of LEDs with PPV as emitting layer. Ad-
ditional layers of aryl-substituted 1,3,4-oxadiazole with
high electron affinity between the luminescent material
and the metal reduce the barrier for electron injection.
Other electron transport layers that have been used to
improve EL emission efficiency of PPV-based LEDs
Exp er im en ta l Section
Ch a r a cter iza tion Meth od s. IR spectra were recorded on
a Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets.
H NMR (400 MHz) spectra were obtained using a Bruker
1
2
1
spectrometer. Chemical shifts (δ values) are given in parts
per million with tetramethylsilane as an internal standard.
UV-vis absorption spectra were recorded on a Beckman
DU-640 spectrometer with spectrograde THF or a Perkin-
Elmer model Lambda 900 UV/vis/near-IR spectrophotometer.
The PL spectra were obtained with a Perkin-Elmer LS45 lumi-
nescence spectrometer or Photon Technology International
PTI) Inc. model QM-2001-4 spectrofluorimeter. GPC analysis
was conducted with a Waters Breeze 1515 series liquid chro-
matograph equipped with a differential refractometer (Waters
6
-8
9
include polyquinolines,
polybenzobisazoles, and poly-
1
0
pyridines. In addition, the substitution of PPV with
electron acceptors like cyano groups enhances its elec-
1
3
tron affinity. Regarding the improvement of the
electron affinity of the lumophore, stilbenoid chro-
mophores with oxadiazoles as electron-withdrawing side
chains or in the main chain are of current interest.
Oxadiazole-based conjugated polymers have been shown
(
1
0.1021/ma034793w CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/15/2003

9
296 Mikroyannidis et al.
Macromolecules, Vol. 36, No. 25, 2003
(PPh , plus a few crystals of 2,6-di-tert-
2
410) as detector using polystyrene as standard and THF as
the catalyst PdCl
2
3 2
)
eluent. DSC and TGA were performed on a DuPont 990 ther-
mal analyzer system. Ground polymer samples of about 10 mg
each were examined by TGA, and the weight loss comparisons
were made between comparable specimens. The DSC thermo-
butylphenol using toluene as solvent.
4-Bromobenzaldehyde and 4-bromobenzoic hydrazide were
recrystallized from methanol and ethanol, respectively. 1,4-
Benzenedicarboxaldehyde and malonic acid were recrystallized
from distilled water. Dimethylformamide (DMF) and dimethyl-
grams were obtained at a heating rate of 10 °C/min in a N
2
3
atmosphere at a flow rate of 60 cm /min. Dynamic TGA mea-
2
acetamide (DMAc) were dried by distillation over CaH .
surements were made at a heating rate of 20 °C/min in atmos-
Triethylamine was dried by distillation over KOH. All other
solvents and reagents were analytical-grade quality, purchased
commercially, and used without further purification.
3
pheres of N
2
or air at a flow rate of 60 cm /min. Thermome-
chanical analysis (TMA) was recorded on a DuPont 943 TMA
using a loaded penetration probe at a scan rate of 10 °C/min
P r ep a r a tion of Mon om er s a n d P olym er s. 4-Br om o-
cin n a m ic Acid (1a ). Compound 1a was prepared by a
reported method26 that was modified as follows. A mixture of
4-bromobenzaldehyde (0.47 g, 2.54 mmol), malonic acid (0.40
g, 3.81 mmol), pyridine (15 mL), and piperidine (three drops)
was refluxed for 20 h. It was subsequently concentrated under
reduced pressure, and water was added to the concentrate.
The pale brown precipitate was filtered, washed with
water, and dried to afford 1a . It was recrystallized from
acetonitrile (0.36 g, yield 63%, mp 264-266 °C). IR (KBr,
3
in N
2
with a flow rate of 60 cm /min. The TMA experiments
were conducted at least in duplicate to ensure the accuracy of
the results. The TMA specimens were pellets of 8 mm diameter
and 2 mm thickness prepared by pressing powder of polymer
for 3 min under 5-7 kpsi at ambient temperature. Elemental
analyses were carried out with a Carlo Erba model EA1108
analyzer.
f
To measure the PL quantum yields (Φ ), a degassed solution
of polymer in THF was prepared. The concentration was
adjusted so that the absorbance of the solution would be lower
than 0.1, and the excitation was performed at the correspond-
-
1
cm ): 3436, 1684, 1626, 1584, 1486, 1424, 1306, 1282, 1226,
1
1070, 1008, 980, 816, 706, 546, 486. H NMR (DMSO-d , δ):
6
ing λex,max. A solution of quinine sulfate in 1 N H
2
SO
4
, which
12.51 (broad, 1H, COOH); 7.66-7.59, 6.59 (m, 4H, aromatic
has a Φ
f
of 0.546 (λex ) 365 nm), was used as a standard. Cyclic
and 2H, HCdCH).
voltammetry measurements of the polymers were done in
acetonitrile with 0.1 M tetrabutylammonium hexafluorophos-
3,3′-(1,4-P h en ylen e)bis[2-p r op en oic a cid ] (1b). Com-
2
7
pound 1b was prepared by a reported method that was
modified as follows. A mixture of 1,4-benzenedicarboxaldehyde
(3.51 g, 26.20 mmol), malonic acid (8.18 g, 78.60 mmol),
pyridine (30 mL), and a catalytic amount of piperidine was
stirred and heated at about 50 °C for approximately 0.5 h.
Carbon dioxide was evolved, and a white solid precipitated.
The mixture was subsequently heated at about 100 °C for 3
h. It was poured into water containing 5% (v/v) hydrochloric
acid. The whitish solid was filtered, dried, and extracted with
chloroform in a Soxhlet apparatus for 100 h to afford 1b. It
6
phate (TBAPF ) as the supporting electrolyte at a scan rate
of 80 mV/s. Platinum wire electrodes were used as both counter
and working electrodes, and silver/silver ion (Ag in 0.1 M
3
AgNO solution, from Bioanalytical Systems, Inc.) was used
as a reference electrode. Using ferrocene as an internal
standard, the potential values obtained were converted to vs
SCE (saturated calomel electrode), and the corresponding
highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) energy levels were estimated
from the onset redox potentials.
was recrystallized from DMF/CH
3
CN [7.00 g, yield 70%, mp
-
1
F a br ica tion a n d Ch a r a cter iza tion of LEDs. The single-
layer LEDs were fabricated as sandwich structures between
aluminum (Al) cathodes and indium-tin oxide (ITO) anodes.
ITO-coated glass substrates (Delta Technologies Ltd., Still-
water, MN) were cleaned sequentially in ultrasonic baths of
detergent, 2-propanol/deionized water (1:1 volume) mixture,
toluene, deionized water, and acetone. A 50 nm thick hole
injection layer of poly(ethylenedioxythiophene) doped with
poly(styrenesulfonate) (PEDOT) was spin-coated on top of ITO
from a 0.7 wt % dispersion in water and dried at 150 °C for 1
h under vacuum. Thin films of polymer P 1 and its 6 wt %
blends with 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC)
were spin-coated from their chloroform solutions onto the
PEDOT layer and dried at 50 °C in a vacuum overnight. The
film thicknesses obtained were ca. 50 nm, which were mea-
sured by an Alpha-Step 500 surface profiler (KLA Tencor,
Mountain View, CA). Finally, 100-130 nm Al electrodes were
thermally evaporated through a shadow mask onto the poly-
mer films using an AUTO 306 vacuum coater (BOC Edwards,
Wilmington, MA), typical evaporations being carried out at
345 °C dec (by DTA)]. IR (KBr, cm ): 3454, 1676, 1622, 1424,
1
1312, 1282, 1218, 1114, 980, 940, 878, 824, 680, 544, 506. H
NMR (DMSO-d , δ): 8.38-8.13 (m, 4H, aromatic); 7.17 (s, 2H,
6
olefinic of position 1 of vinyl groups); 6.83 (s, 2H, olefinic of
position 2 of vinyl groups). The carboxylic protons were
unobserved.
4-Br om ocin n a m oyl Ch lor id e (2a ). Compound 2a was
prepared by a reported method28 that was modified as follows.
A flask equipped with a magnetic stirrer, condenser, and gas
trap was charged with a mixture of 1a (0.24 g, 1.06 mmol),
thionyl chloride (5 mL), and a few drops of DMF. The mixture
was stirred and refluxed for 5 h. Thionyl chloride was stripped
off under vacuum, and the residue was triturated with
petroleum ether. The white solid was filtered and dried to
afford 2a . It was recrystallized from diethyl ether/n-hexane
-
1
(0.12 g, yield 47%, mp 104-106 °C). IR (KBr, cm ): 1748,
1725, 1622, 1606, 1588, 1562, 1488, 1402, 1324, 1292, 1262,
1
1184, 1114, 1070, 1026, 976, 810, 750, 638, 616, 484. H NMR
(CDCl , δ): 7.79-7.25, 6.66-6.62 (m, aromatic and HCdCH).
3
3,3′-(1,4-P h en ylen e)bis[2-pr open oyl ch lor ide] (2b). Com-
pound 2b was synthesized according to the procedure described
for 2a as a pale yellow solid in 85% yield (2.23 g) by reacting
1a (2.00 g, 9.20 mmol) with thionyl chloride (15 mL) in the
presence of a few drops of DMF. The residue was triturated
with diethyl ether, and the reaction product was recrystallized
-
6
base pressures lower than 2 × 10 Torr. The active area of
2
each EL device was 0.2 cm . Electroluminescence (EL) spectra
were obtained using a PTI QM-2001-4 spectrophotometer.
Current-voltage characteristics of the LEDs were measured
using a HP4155A semiconductor parameter analyzer (Yokoga-
wa Hewlett-Packard, Tokyo). The luminance was simulta-
neously measured using a model 370 optometer (UDT Instru-
ments, Baltimore, MD) equipped with a calibrated luminance
sensor head (model 211). The device external quantum ef-
ficiencies were calculated using a standard expression involv-
ing the measured ratio of forward directed luminance and
device current density, taking into account the EL spectral
distribution and the photopic spectrum.24 All the device
fabrication and characterization steps were done under ambi-
ent laboratory conditions.
2
7
from dioxane (mp 171-172 °C, lit. 171-172 °C). IR (KBr,
-
1
cm ): 1678, 1622, 1512, 1424, 1312, 1282, 1220, 1112, 978,
1
940, 824, 678, 544, 506. H NMR (acetone-d , δ): 7.60-7.35
6
m, 4H, aromatic); 6.48 (s, 2H, olefinic of position 1 of vinyl
groups); 6.32 (s, 2H, olefinic of position 2 of vinyl groups).
{2-[1-Oxo-3-(4-br om op h en yl)-2-p r op en yl]h yd r a zid e}-4-
br om oben zoic Acid (3a ). Compound 2a (0.12 g, 0.47 mmol)
dissolved in DMAc (3 mL) was added dropwise at 0 °C to a
stirred mixture of 4-bromobenzoic hydrazide (0.10 g, 0.47
mmol), DMAc (5 mL), and triethylamine (0.05 g, 0.49 mmol).
Stirring of the mixture was continued for 4 h at ambient
Rea gen ts a n d Solven ts. The Stille coupling reaction was
used to prepare 1,4-didodecyloxy-2,5-divinylbenzene.25 In par-
ticular, the latter was prepared by reacting 1,4-bis(dodecyloxy)-
2
temperature under N . The mixture was subsequently poured
into water. The white precipitate was filtered, washed with
2
,5-dibromobenzene with tributylvinyltin in the presence of
water, and dried to afford 3a . It was recrystallized from

Macromolecules, Vol. 36, No. 25, 2003
Conjugated Poly(p-phenylenevinylene) Derivatives 9297
Sch em e 1
1
DMF/water (0.16 g, yield 80%, mp 189-191 °C). IR (KBr,
For the H NMR spectrum of P 1, see Results and Discussion.
Anal. Calcd for (C50 : C, 80.82; H, 8.95; N, 3.77.
-
1
cm ): 3192, 1638, 1596, 1562, 1492, 1468, 1372, 1224, 1180,
66 2 3 n
H N O )
1
5
7
154, 1072, 1010, 970, 900, 844, 818, 756, 710, 658, 620, 548,
Found: C, 79.65; H, 9.12; N, 3.64.
1
16. H NMR (DMSO-d
.98-7.54, 6.80-6.76, 6.60-6.55 (m, 8H, aromatic and 2H,
6
, δ): 10.66, 10.30 (m, 2H, CONH);
P olym er P 2. Polymer P 2 was prepared according to the
procedure described for P 1 as a yellow-brown solid in 68% yield
(0.17 g) from the reaction of 4b (0.1580 g, 0.274 mmol) with
1,4-didodecyloxy-2,5-divinylbenzene (0.1365 g, 0.274 mmol),
HCdCH).
{2,2′-[1,4-P h en ylen ebis(1-oxo-2-p r op en e-3,1-d iyl)]d ih y-
d r a zid e}-p-br om oben zoic Acid (3b). Compound 3b was
synthesized according to the procedure described for 3a as a
white solid in 70% yield (0.39 g) by reacting 2b (0.23 g, 0.90
mmol) with 4-bromobenzoic hydrazide (0.39 g, 1.80 mmol) in
DMAc (6 mL) in the presence of triethylamine (0.19 g, 1.9
mmol). It was recrystallized from DMF (mp > 300 °C). IR (KBr,
Pd(OAc)
mmol), DMAc (30 mL), and triethylamine (3 mL). The number-
average molecular weight (M ) was 4300, and the polydisper-
2 3
(0.0026 g, 0.12 mmol), P(o-tolyl) (0.0192 g, 0.063
n
-1
sity index was 2.1 (by GPC). IR (KBr, cm ): 2922, 2850, 1633,
1604, 1548, 1530, 1493, 1464, 1417, 1202, 1071, 1032, 967, 672.
1
H NMR (CDCl
3
, δ): 8.12-6.90 (m, 14H, aromatic and 8H,
); 1.86, 1.26, 0.86 (m, 46H, other
aliphatic). Anal. Calcd for (C60 : C, 78.91; H, 7.95;
-
1
cm ): 3182, 1634, 1592, 1562, 1466, 1224, 1156, 1070, 1010,
HCdCH); 4.02 (m, 4H, OCH
2
1
9
1
2
64, 898, 836, 750, 710, 658, 546, 524. H NMR (DMSO-d
6
, δ):
72 4 4 n
H N O )
0.66, 10.31 (broad, 4H, CONH); 7.86-7.58 (m, 12H, aromatic,
H, olefinic â to carbonyl); 6.83-6.79 (m, 2H, olefinic R to
N, 6.13. Found: C, 78.03; H, 7.82; N, 6.20.
Resu lts a n d Discu ssion
carbonyl).
[
2-(4-Br om op h e n yl)-5-(4-b r om ost yr yl)]-1,3,4-oxa d i-
Syn t h esis a n d Ch a r a ct er iza t ion of P olym er s.
Schemes 1 and 2 outline the synthetic route to the
synthesis of the monomers and polymers. In particular,
a zole (4a ). Compound 3a (0.62 g, 1.46 mmol) was dissolved
by heating in POCl (25 mL). The mixture was refluxed for 17
3
h, and it was subsequently concentrated under reduced
pressure. Water was added to the concentrate, and the whitish
solid was filtered, washed thoroughly with water, and dried
to afford 4a . It was recrystallized from acetonitrile (0.36 g,
yield 61%, mp 190-192 °C). IR (KBr, cm ): 1638, 1600,
562, 1520, 1476, 1406, 1072, 1010, 976, 832, 814, 734, 720,
30, 508. H NMR (DMSO-d , δ): 8.05-7.17 (m, aromatic and
HCdCH).
4
-bromobenzaldehyde was condensed with malonic acid
in pyridine in the presence of a catalytic amount of
piperidine to afford 4-bromocynnamic acid (1a ). 4-Bro-
mocynnamoyl chloride (2a ) was obtained from the
reaction of 1a with SOCl2 utilizing DMF as catalyst. The
condensation of 2a with 4-bromobenzoic hydrazide
afforded 3a . Dehydrocyclization of 3a using POCl3 as
both dehydrating agent and reaction solvent resulted
in the oxadiazole derivative 4a . Finally, the Heck
coupling of 4a with 1,4-didodecyloxy-2,5-divinylbenzene
yielded polymer P 1. An analogous reaction sequence of
five steps was applied to prepare P 2 starting from 1,4-
benzenedicarboxaldehyde. In this case, poly(phosphoric
acid) (PPA) was used instead of POCl3 for dehydrocy-
clization of 3b due to its limited solubility.
-
1
1
5
1
6
2
,2′-(1,4-P h en ylen ed i-2,1-et h en ed iyl)b is[5-(4-b r om o-
p h en yl)-1,3,4-oxa d ia zole] (4b). Compound 3b (0.38 g, 0.62
mmol) was dissolved by heating in poly(phosphoric acid) (PPA)
20 g), and the solution was stirred and heated at 130 °C for
0 h. It was subsequently cooled and poured portionwise into
(
4
ice-water. The pale yellow-brown precipitate was filtered,
washed thoroughly with water, and dried to afford 4b. It was
recrystallized from DMF/water (0.18 g, yield 51%). IR (KBr,
-
1
cm ): 1636, 1598, 1528, 1476, 1402, 1264, 1086, 1068, 964,
1
1
The FT-IR and H NMR spectra of the polymers were
8
(
30, 810, 758, 730, 498. H NMR (DMSO-d
aromatic, and HCdCH).
P olym er P 1. A flask was charged with 4a (0.1030 g, 0.254
mmol), 1,4-didodecyloxy-2,5-divinylbenzene (0.1263 g, 0.254
mmol), Pd(OAc) (0.0024 g, 0.011 mmol), and P(o-tolyl) (0.0177
g, 0.058 mmol).The flask was degassed and purged with N
DMAc (5 mL) and triethylamine (2 mL) were added, and the
mixture was stirred and heated at 110 °C for 24 h under N
Then, it was filtered and the filtrate was poured into methanol.
The yellow-brown precipitate was filtered, washed with metha-
nol, and dried to afford P 1 (0.15 g, yield 80%). The number-
average molecular weight (M
sity index was 1.9 (by GPC). IR (KBr, cm-1): 2920, 2850, 1636,
598, 1526, 1492, 1466, 1420, 1202, 1010, 962, 808, 852, 696.
6
, δ): 8.09-7.15
consistent with their chemical structures. In partic-
ular, the FT-IR spectrum of P 2 showed characteristic
absorptions at 2922, 2850 (C-H stretching of aliphatic
segments); 1604, 1494, 1464 (aromatic); 1633, 1548,
2
3
2
.
1
530 (1,3,4-oxadiazole ring); 1202, 1071, 1032 (C-O-C
-1
stretching of ether bond and oxadiazole), and 967 cm
2
.
1
(HCdCH trans). The H NMR spectrum of P 1 in CDCl3
solution displayed peaks at 8.11-7.09, (m, 10H, aro-
matic, and 6H, HCdCH); 4.08 (m, 4H, OCH2); 1.90, 1.26,
0
.86 (m, 46H, other aliphatic).
Although the polymers contained phenylene or oxa-
n
) was 5600, and the polydisper-
1
diazole rings along the main chain and three or four

9
298 Mikroyannidis et al.
Macromolecules, Vol. 36, No. 25, 2003
Sch em e 2
Ta ble 1. Tg Va lu es a n d Th er m a l Sta bility of P olym er s
in N2
in air
polymer Tga (°C) T1b (°C) T10b (°C) Yc (%) T1b (°C) T10b (°C)
c
P 1
P 2
28
57
313
340
392
411
36
42
286
325
336
404
a
Tg: glass transition temperature determined by the TMA
b
method. T1, T10: temperatures at which weight loss of 1 and 10%,
respectively, was observed by TGA. Yc: char yield at 800 °C by
c
TGA.
vinylene bonds per repeat unit that led to a fully
conjugated backbone, they showed excellent solubility
because of the dodecyloxy side groups. Specifically, they
dissolved at room temperature in THF as well as various
chlorinated solvents such as chloroform, dichloromethane,
1
,2-dichloroethane, 1,1,2,2-tetrachloroethane, chloroben-
zene, and 1,2-dichlorobenzene. P 1 was more soluble in
these solvents than P 2. Transparent and self-standing
films of the polymers can be cast using these solvents.
F igu r e 1. Absorption and PL excitation spectra as well as
PL spectrum of polymer P 1 in THF solution.
No phase transitions of the polymers were recorded
by DSC, even after repeated scans, thus supporting
their amorphous character. The Tgs of the polymers
were determined by thermomechanical analysis (TMA)
when a penetration probe was used (Table 1). The Tg
was obtained from the onset temperature of the drop-
ping step that was recorded during the second heating.
The lower Tg value of P 1 (28 °C) than that of P 2 (57
troscopic data are collected in Table 2. Both polymers
contain distyrylbenzene moieties along the main chain
that are connected with phenylene and/or oxadiazole
rings. P 1 has a repeat unit with one oxadiazole ring,
whereas P 2 possesses a longer repeat unit with two
oxadiazole rings. Therefore, the two polymers are
related but have distinct differences in the molec-
ular structures that can lead to differing optical proper-
ties.
°C) is ascribed to a higher composition of the side alkoxy
groups in its backbone. In addition, for this reason P 1
was thermally less stable than P 2 and afforded lower
anaerobic char yield (Yc) at 800 °C (Table 1). No weight
loss was observed up to approximately 290-340 °C in
N2 or air, and the Yc was about 40%.
Op tica l P r op er ties of P olym er s. The UV-vis
absorption, PL excitation (PLE), and PL emission
spectra of polymers P 1 and P 2 were recorded both in
solution and in thin film (Figures 1-4), and the spec-
The UV-vis absorption spectrum of P 1 in THF
(
3
Figure 1) showed two peaks with maxima (λa,max) at
48 and 428 nm that can be assigned to the phenylene
or oxadiazole rings and the π-π* transition along the
conjugated backbone, respectively. In contrast, the
absorption spectrum of P 2 in THF (Figure 2) showed
only one peak with λa,max at 360 nm. The PLE spectra
of the polymers in THF overlapped significantly with

Macromolecules, Vol. 36, No. 25, 2003
Conjugated Poly(p-phenylenevinylene) Derivatives 9299
Ta ble 2. Op tica l P r op er ties of P olym er s
λa,maxa in
soln (nm)
λex,maxb in
soln (nm)
λf, max in
c
Φfd
in soln
λa,maxa in thin
film (nm)
λex,maxb in thin
film (nm)
λf, max in thin
c
e
polymer
soln (nm)
Eg (eV)
film (nm)
P 1
P 2
348, 428f
360
394
395
511
487
0.34
0.22
349, 428f
366
2.38
2.49
470
472
558
515
a
b
λa,max: the absorption maxima from the UV-vis spectra in THF solution or in thin film. λex,max: the PL excitation maxima in THF
solution or in thin film. λf,max: the PL maxima in THF solution or in thin film. Φf: PL quantum yield. e Eg: the optical energy gap
calculated from the UV-vis spectra in thin film. Italic numerical values denote absolute maxima.
c
d
f
F igu r e 4. Absorption and PL excitation spectra as well as
F igu r e 2. Absorption and PL excitation spectra as well as
PL spectrum of polymer P 2 in thin film.
PL spectrum of polymer P 2 in THF solution.
(
Table 2) were determined30 using quinine sulfate as a
standard. P 1 with a shorter repeat unit was more
fluorescent than P 2 with a higher Φf value of 0.34 vs
0
.22 for P 2.
The absorption spectra of the polymer thin films
Figures 3 and 4) spin-coated on quartz substrate were
(
almost identical with those in solution. This suggests
that the molecular arrangement of polymer chains is
same both in solution and in thin film, and consequently
the polymers lack order even in the solid state. The
optical band gaps (Eg) of the polymers determined from
the absorption edge were 2.38 and 2.49 eV for P 1 and
P 2, respectively. The value of 2.38 eV for P 1 is lower
than those that have been reported for other related
PPV polymers bearing oxadiazole rings along the main
1
4,15
chain (2.44-2.53 eV).
The polymers emitted yellow-
ish-green light in thin film with PL maxima at 558 nm
for P 1 (Figure 3) and 515 nm for P 2 (Figure 4).
However, the PL emission of P 2 thin film was very weak
compared to that of P 1.
F igu r e 3. Absorption and PL excitation spectra as well as
PL spectrum of polymer P 1 in thin film.
the corresponding absorption spectra and displayed
identical absolute maximum (λex,max) at 395 nm.
Electr och em ica l P r op er ties. Parts a and b of
Figure 5 show the cyclic voltammograms of polymers
P 1 and P 2, respectively. Figure 5a shows a reversible
reduction with an onset potential at ca. -1.8 V vs SCE
(saturated calomel electrode) for P 1, whereas the oxida-
tion is irreversible with an onset at ca. 0.65 V vs SCE.
This suggests that the presence of one oxadiazole ring
per repeat unit in the main chain has made the polymer
more suitable for n-doping than p-doping. On the basis
of the onset redox potentials, the electron affinity
(LUMO level) and ionization potential (HOMO level) of
P 1 are estimated to be 2.60 and 5.05 eV, respectively,
by using a value of -4.4 eV as the SCE energy level
The polymers emitted greenish-blue light in solution
with PL maxima (λf,max) at 511 nm for P 1 and 487 nm
for P 2 (Figures 1 and 2). Thus, the emission maximum
of P 1 is red-shifted by 24 nm compared to that of P 2,
suggesting increased electron delocalization along the
P 1 backbone. This could be due to the higher composi-
tion of the electron-donating dodecyloxy groups per
repeat unit of P 1 and/or a possible disruption of the
conjugation along the backbone caused by the two
oxadiazole rings in P 2. Thus, increase in the oxadiazole
composition in the polymer backbone of P 2 did not
extend the effective chromophore. This PL emission of
both the polymers is blue-shifted in comparison to poly-
3
1
relative to vacuum. The corresponding electrochemical
band gap is 2.45 eV, which matches fairly well with the
optical band gap of 2.38 eV. Figure 5b shows an
irreversible oxidation for P 2 with an onset potential of
2
9
(
2,5-dialkoxy-1,4-phenylenevinylene)s (513 nm). The
PL quantum yields (Φf) of the polymers in THF solution

9
300 Mikroyannidis et al.
Macromolecules, Vol. 36, No. 25, 2003
F igu r e 6. (a) Current density-voltage-luminance charac-
teristics of LED from polymer P 1 and (b) EL spectra of
corresponding device. The inset shows the device schematic.
reversible in that, for a device working at 16 V (527 nm),
decreasing the applied bias back down to 9 V did not
shift the EL maximum to 558 nm. This suggests that
the application of higher electric fields and the conse-
quent greater local heating in the polymer film leads to
changes in the conformation of the polymer backbone.
The plausible reasons for the origin of this shift in EL
spectra are discussed later.
F igu r e 5. Cyclic voltammograms of (a) P 1 and (b) P 2 in 0.1
M TBAPF in acetonitrile at 80 mV/s scan rate.
6
ca. 0.75 eV vs SCE. The estimated HOMO level is 5.15
eV, which is higher than that of P 1, probably due to
the presence of an additional oxadiazole ring in the main
chain. A clear reduction wave could not be observed for
P 2, making the estimation of the LUMO level very
approximate.
To improve the device performance and efficiency, a
blend of P 1 was made with a hole transport molecule,
6 wt % 1,1-bis(di-4 -tolylaminophenyl)cyclohexane
(TAPC), to enhance the hole transport through the
device. The current density-voltage-luminance char-
acteristics of the corresponding LED are shown in
Figure 7a. The turn-on voltage of the diode is ca. 10 V.
Electr olu m in escen ce P r op er ties. The current den-
sity-voltage-luminance characteristics of P 1 LED of
the type ITO/PEDOT/P 1/Al are shown in Figure 6a. The
turn-on voltage of the diode is ca. 9 V with a maximum
2
A maximum brightness of 70 cd/m and a maximum
2
brightness of 23 cd/m and maximum external quantum
external quantum efficiency of 0.01% at 17.5 V were
seen. This represents factors of 3-5 enhancement in
performance compared to the P 1 LED. The modest EL
device efficiencies could be further improved by optimiz-
ing the blend composition. A voltage-tunable EL color
is again seen in the corresponding EL spectra shown in
Figure 7b. At the lower voltage of 11 V, the EL
maximum is at 556 nm, and then the EL spectra
gradually blue shift with increase in applied bias,
showing EL maxima at 542 nm at 15 V and finally 513
nm at 17.5 V. As with the neat P 1 device, this trend
was repeatable and irreversible for a given active pixel.
efficiency of 0.002% at 16 V. The corresponding EL
spectra shown in Figure 6b reveal a voltage-tunable EL
colors. At the turn-on voltage of 9 V, the EL maximum
is at 558 nm (yellow), which is the same as the
homopolymer PL maximum. However, the EL spectra
gradually blue-shifted with increase in applied bias,
showing EL maxima at 545 nm at 13.5 V, 540 nm at 15
V, and finally 527 nm at 16 V. This trend was repeatable
in all the fabricated devices and was consistent for a
single active pixel or freshly turned-on pixels at each
individual applied bias. Moreover, the shift was ir-

Macromolecules, Vol. 36, No. 25, 2003
Conjugated Poly(p-phenylenevinylene) Derivatives 9301
F igu r e 7. (a) Current density-voltage-luminance charac-
teristics of LED from 6 wt % blend of TAPC with polymer P 1
and (b) EL spectra of the corresponding device.
F igu r e 8. (a) Optical absorption spectra and (b) PL emission
spectra (370 nm excitation) of thin film of homopolymer P 1
annealed at different temperatures in a vacuum for 1 h.
In the case of polymer P 2, the devices did not show
measurable EL properties as the PL emission was very
weak compared to that of P 1.
Or igin of Volta ge-Tu n a ble EL Sp ectr a . To inves-
tigate the reason for the observed blue shift in the EL
spectra of homopolymer P 1 (Figure 6b) with increasing
bias, we examined the photophysics of P 1 thin films
annealed at different temperatures. After the initial
drying of the films at 40 °C in a vacuum overnight, they
were then annealed at temperatures of 150, 175, and
The normalized PL emission spectra of the annealed
P 1 films are shown in Figure 8b. With excitation at 370
nm, we see a distinct blue shift in the PL maxima with
increasing annealing temperatures. For the films an-
nealed at 40, 150, 175, and 200 °C, the PL maxima are
seen at 558, 553, 545, and 533 nm, respectively. This
suggests that the emission originates from varying
conformations of the polymer backbone. It should also
be noted that the Tg of P 1 is low (28 °C) due to the long
dodecyloxy side groups. Hence, the polymer lends itself
to facile rearrangements, of the cis-trans type for
instance, along the backbone with increasing temper-
ature. A similar thermochromic effect has been previ-
ously reported for cyano-substituted poly(2,5-dialkoxy-
p-phenylenevinylene)s having hexyloxy, octyloxy, and
2
00 °C for 1 h under vacuum and cooled back down to
room temperature. The optical absorption and PL emis-
sion spectra of the annealed films are shown in parts a
and b of Figure 8, respectively. In the normalized
absorption spectra shown in Figure 8a, two peaks are
seen for the film dried at 40 °Csone at 353 nm and the
other at 434 nm. The peak at 353 nm can be assigned
to the oxadiazole or phenyl rings, and the one at 434
nm to the π-π* transition of the conjugated PPV
backbone. For the films annealed at 150, 175, and 200
32
decyloxy side groups. Comparison of the PL spectra
in Figure 8b to the EL spectra of P 1 LEDs in Figures
6
b and 7b suggests that the same conformational
changes occur in the presence of local heat developed
under the application of electric field in the homopoly-
mer LED, leading to the observed blue shift in the EL
maxima with increasing bias. The observed coupling of
thermochromism with electroluminescence of P 1 is an
interesting new phenomenon not previously seen in
conjugated polymers, and thus it warrants a future
detailed study.
°
4
C, the π-π* transition peak gradually blue shifts to
27, 412, and 401 nm, respectively (indicated by the
arrow), whereas the peak at 353 nm is unchanged.
Correspondingly, a slight increase in the absorption
band edge is also observed. This indicates that there is
a conformational change along the backbone with tem-
perature leading to subtle changes in the electronic band
structure of P 1.

9
302 Mikroyannidis et al.
Macromolecules, Vol. 36, No. 25, 2003
Con clu sion s
(10) Dailey, S.; Halim, M.; Rebourt, E.; Horsburgh, L. E.; Samuel,
I. D. W.; Monkman, A. P. J . Phys.: Condens. Matter 1998,
Two new PPV-type conjugated polymers P 1 and P 2
with different chemical structures were synthesized by
Heck coupling. Upon comparing these polymers, P 1 was
found to be more soluble in common organic solvents,
less thermally stable with a lower Tg and more fluores-
cent material with lower optical band gap than P 2. The
PL emission maxima of the polymers were at 487-511
nm in solution and 515-558 nm in thin films. EL was
achieved from single layer LEDs of P 1 with a voltage-
tunable EL color from 558 nm (9 V) to 527 nm (16 V).
The observed blue shift in the EL spectra with increas-
ing bias is a result of changes in structural conforma-
tions of the polymer backbone, leading to subtle varia-
tions in the electronic band structure.
1
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2
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MA034793W