Green-emitting PPE-PPV hybrid polymers: Efficient energy transfer across the m-phenylene bridge

Article abstract of DOI:10.1021/ma034028h

Novel π-conjugated polymers (7) consisting of both poly(phenylenevinylene) (PPV) and poly(phenyleneethynylene) (PPE) blocks are synthesized by using a combination of Heck-type coupling and Horner-Wadsworth-Emmons condensation reactions. These polymers hav

Full text of DOI:10.1021/ma034028h

3848
Macromolecules 2003, 36, 3848-3853
Green-Emitting PPE-PPV Hybrid Polymers: Efficient Energy Transfer
across the m-Phenylene Bridge
Qin gh u i Ch u a n d Yi P a n g*
Department of Chemistry & Center for High Performance Polymers and Composites,
Clark Atlanta University, Atlanta, Georgia 30314
Lim in g Din g a n d F r a n k E. Ka r a sz
Department of Polymer Science and Engineering, University of Massachusetts,
Amherst, Massachusetts 01003
Received J anuary 8, 2003; Revised Manuscript Received March 26, 2003
ABSTRACT: Novel π-conjugated polymers (7) consisting of both poly(phenylenevinylene) (PPV) and poly-
(phenyleneethynylene) (PPE) blocks are synthesized by using a combination of Heck-type coupling and
Horner-Wadsworth-Emmons condensation reactions. These polymers have well-defined chemical
structure (-Ph-CtC-Ph-CtC-Ph-CdC-Ph-CdC-)n, in which phenyl rings are linked alternately
at meta and para positions. 13C NMR spectroscopy further confirms the structural regularity of the
polymers. The degree of polymerization (DP) is in the range 27-76. Resulting from efficient energy transfer
from PPE block to PPV block, the PPV/PPE hybrid polymer emits light with the same color as poly[(m-
phenylenevinylene)-alt-(p-phenylenevinylene)], 1. LED devices of ITO/PPV/7/Ca and ITO/PEDOT/7/Ca
configurations have been fabricated, which give green EL emission with external quantum efficiency of
0.15% for the former and 0.32% for the latter.
In tr od u ction
π-Conjugated polymers of high electroluminescence
(EL) have been a subject of great interest due to their
potential application in new display technology.1 Al-
though many green-emitting materials have been re-
ported, efficient ones with high brightness are still rare.
One of the challenges in the field is to improve the
luminescence efficiency of the existing materials without
altering their emissive color. Both poly(phenylenevi-
nylene) (PPV)1,2 and poly(phenyleneethynylene) (PPE)3,4
derivatives have been shown to be useful as active
layers in light-emitting diodes (LEDs). Synthesis of their
hybrid PPV/PPE copolymers has also been indepen-
dently reported by the group of Bunz5 and Egbe6,7 to
give poly[(p-phenylenevinylene)-co-(p-phenyleneethyn-
ylene)] derivatives, in which all phenyl groups are linked
at the para position. In the hybrid, both PPE and PPV
blocks form an integrated chromophore of no clear
boundary. Although the hybrid copolymers exhibit
relatively high fluorescence quantum efficiency in solu-
tion, little is known about their EL properties.7
F igu r e 1. 13C NMR spectrum of copolymer 7a a in CDCl3. The
alkyl region is not shown for clarity. The 13C peaks are
assigned as labeled in the polymer structure 7.
Our recent studies have shown that both polymers
18 and 29 are highly fluorescent materials. Resulting
from the effective π-conjugation interruption at m-
phenylene, the chromophores in 1 and 2 can be repre-
sented by molecular fragments 3 and 4, respectively.
Comparison between the optical properties of 1 and 2
shows that the emission of the high-energy chromophore
4 (emission maximum at ∼406 nm) overlaps very well
with the absorption of the low-energy chromophore 3
between 350 and 450 nm (Figure 3), thereby forming
an ideal donor-acceptor pair for energy transfer.10 The
clear chromophore boundary at m-phenylene linkages,
which cannot be achieved by using only p-phenylene,
may play a positive role in the exciton confinement. In
addition, an isolated emitter will have less tendency to
form aggregate, which leads to formation of weakly
emissive interchain species in thin films and signifi-
F igu r e 2. 1H NMR spectrum of 7a a in CDCl3 solvent.
cantly reduces the luminescence quantum efficiency of
LED devices.11,12 Reasoning that proper combination of
both chromophores 3 and 4 in a single polymer chain
could produce new materials with improved lumines-
cence, we decide to explore the synthesis of their hybrid,
i.e., PPV/PPE copolymer 7, in which chromophores 3
and 4 are bridged via sharing a common m-phenylene
for intimate contact (Scheme 1). The present molecular
architecture partially overlaps molecular orbitals of two
different chromophores, which any other intermolecular
interaction cannot match, further facilitating the desir-
able energy migration. Study of 7 with distinguishable
10.1021/ma034028h CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/06/2003

Macromolecules, Vol. 36, No. 11, 2003
PPE-PPV Hybrid Polymers 3849
Sch em e 1. Syn th esis of Hybr id Cop olym er
F igu r e 3. UV-vis absorption (solid line) and emission (dotted
line) spectra of homopolymers 1 and 2 and PPV-PPE copoly-
mer 7 in THF solution at room temperature. The fluorescence
spectra were acquired while the polymer solutions were excited
at 375 nm.
Ch a r t 1
Ta ble 1. P olym er iza tion a n d Molecu la r Weigh ts of
P P E/P P V Hybr id P olym er s
contenta of
polymer trans-CHdCH (%)
yield
(%)
Mh w
Mh w/Mh n DP
PPE and PPV blocks will provide useful information to
understand the optical behavior of the hybrid material.
7a a
7bb
7a b
7ba
92
93
96
95
80 500
84 700
46 000
117 200
1.36
1.40
1.72
1.59
73
53
27
76
82
85
80
88
Resu lts a n d Discu ssion
P olym er Syn th esis a n d Ch a r a cter iza tion . The
desired dialdehyde monomer 5 was obtained by reacting
3-ethynylbenzaldehyde with 1,4-diiodo-2,5-dialkoxyben-
zene in the presence of the palladium catalyst. Con-
struction of ethynyl bond in the monomer synthesis step
would minimize or eliminate possible contamination of
polymer from butadiyne unit (-CtC-CtC-), which is
often formed as a byproduct during the coupling reac-
tion. Horner-Wadsworth-Emmons condensation13 be-
tween dialdehyde 5 and bisphosphonate 6 provided
copolymer 7 with vinylene in the trans configuration.
The polymers exhibited good solubility in common
organic solvents such as tetrahydrofuran (THF), tolu-
ene, and chloroform. Size exclusion chromatography
(SEC) analysis, which was equipped with refractive
index, viscosity, and light-scattering detectors, showed
that the polymer had moderately high molecular weights
with the degree of polymerization n ≈ 27-76 (Table 1).
A uniform thin film could be conveniently cast from
their solutions.
a
The content of trans-CHdCH was estimated from 1H NMR
spectra.
cm-1 in medium intensity (trans-CHdCH) and a very
weak band at 892 cm-1 (cis-CHdCH) suggested that the
olefin was predominately present in the trans configu-
ration. The quantitative 13C NMR spectrum of 7 further
showed that the polymer had a regular structure. The
13C NMR resonance signals were assigned as labeled
in Figure 1 on the basis of polymers 18,14 and 2.9 Only
two acetylenic carbons were observed in a ratio of 1:1,
further confirming the absence of butadiyne -CtC-
CtC- impurity. Two major signals at 153.4 and 150.9
ppm were attributed to the characteristic aromatic
carbons attached to the alkoxy substituents. The minor
13C NMR signal at 150.03 ppm was not due to the end
group, but indicative of the small amount of cis-CHd
CH linkage along the polymer chain. The result was in
1
agreement with its H NMR spectrum (Figure 2), which
detected a major (at ∼4.01 ppm) and a minor (at ∼3.55
ppm) -OCH2- signal. These -OCH2- signals are
related to cis- and trans-olefins8 and used to determine
the relative content of cis-CHdCH (∼3-8%) in the
polymer.
Spectroscopic studies of polymer 7 confirmed the
proposed chemical structure. Infrared spectroscopy of
7 detected no absorption at about 2700 and 1700 cm-1
for terminal aldehyde. Observation of a band at 963

3850 Chu et al.
Macromolecules, Vol. 36, No. 11, 2003
F igu r e 5. Fluorescence spectra of 7a a at 25 and -108 °C.
F igu r e 4. Excitation spectra of of 2, 1, and 7a a in THF
solution.
P h otoa bsor p tion a n d P h otolu m in escen ce. UV-
vis absorption of 7 in THF (Figure 3) showed two peaks
(λmax ≈ 317 and 391 nm), which are located between
polymers 1 (λmax ≈ 329 and 404 nm) and 2 (λmax ≈ 313
and 376 nm). This was because the absorption of 7
resulted from a combination of two chromophores 3 and
4, which were individually present in the corresponding
homopolymers 1 and 2. The fluorescence spectrum of 7
in THF solution, however, exhibited emission peaks
with λmax ≈ 446 and 473 nm, which were essentially
the same as that of 1 (emission λmax ≈ 445 and 471 nm).
A very similar emission profile and energy between 1
and 7 suggested that the emission occurred only from
chromophore 3, indicating an efficient energy transfer
from chromophore 4 to 3. To further confirm the
presence of energy transfer, the polymer solution was
excited at ∼375 nm, which is near the absorption peak
of chromophore 4. The same emission spectra were
obtained from 7 while tuning the excitation wavelength
at 375 and 405 nm, supporting the assumption of
efficient energy transfer.
Excitation spectra of 1, 2, and 7 revealed more
evidence for energy transfer. As shown in Figure 4, the
excitation band of copolymer 7 was significantly broader
than that of homopolymers 1 and 2. The excitation band
of 7 exhibited a shoulder at about 380 nm, which is very
similar in energy to that of 2 and indicates the presence
of a high band-gap chromophore (PPE fragment 4). The
excitation spectrum of 7 also showed a peak at 397 ppm,
which is nearly identical to that of 1 and corresponds
to the second chromophore (low band-gap, PPV frag-
ment 3). In the excitation spectrum of 7, similar
intensities at 380 and 397 nm indicate that both
chromophores 3 and 4 can be excited equally well under
the photon irradiation condition used. Irradiation of the
polymer solution at ∼380 and 397 nm gave the same
fluorescence spectrum as shown in Figure 3. In addition
to the identical emission profile observed from 7 and 1,
the result further supports the assumption that the
emission occurred only from the low-band-gap chro-
mophore as a result of efficient energy transfer from
PPE to PPV fragments.
F igu r e 6. UV-vis absorption and emission spectra of films
7 on quartz substrates are normalized to same scale. The
spectra of different polymer films are slightly offset for clarity.
and 19 084 cm-1, respectively). The observed vibronic
structure appears to fit the model of an anharmonic
oscillator with a wavenumber separation of ∼1716 cm-1
,
which is comparable to that of ∼1659 cm-1 observed
from 1.15 The result shows that the vibronic structure
of 7 in the ground-state retains the same vibrational
characteristics as that of 1.
Figure 6 shows the absorption and emission spectra
of films 7 spin-cast on quartz plates. The emission λmax
of film 7 is located near 500 nm, which is about the same
as that of film 1. The absorption spectra of film 7 exhibit
two bands with λmax at about 321 and 395 nm, which
are notably blue-shifted (by about 20 nm) from that of
film 1 (Table 2). The blue-shifted absorption helps to
reduce the overlapping area between the absorption and
emission spectra of film 7, which is consistent with what
observed in the solution spectra (Figure 3). Emission of
film 7bb is red-shifted by ∼30 nm from that of film 7a a ,
7a b, and 7ba , indicating the influence of side chain
length and arrangement to the optical properties. The
copolymer 7 emitted strongly from both solution and
film states. The fluorescence quantum efficiency of 7 in
THF was estimated to be about 0.5, which was lower
than that of 1 but higher than that of 2. Direct
comparison between films 7 and 1 showed that the
former emitted about 1.4 times as intense as the latter.
EL P r op er ties. The observed high fluorescence ef-
ficiency in the film state indicated potential use of
Figure 5 shows the fluorescence spectra of 7 at room
and low temperature. As temperature was lowered to
-108 °C, the vibronic bands became more resolved,
showing emission peaks at 452, 490, and 524 nm
(corresponding to the wavenumber of 22 124, 20 408,

Macromolecules, Vol. 36, No. 11, 2003
PPE-PPV Hybrid Polymers 3851
Ta ble 2. UV-Vis Absor p tion a n d P h otolu m in escen ce of Cop olym er 7
UV, λmax (nm)a PL, λmax (nm)a
solution
polymer
film
solution
film
φfl
7a a
7a b
7ba
7bb
1
317, 391
317, 391
317, 391
317, 391
329, 404
313, 376
320, 394
320, 394
321, 401
320, 393
330, 415
316, 383
446, 476 (sh)
446, 475 (sh)
446, 473 (sh)
446, 473 (sh)
444, 475 (sh)
405, 428 (sh)
464 (sh), 500, 536 (sh)
501, 535 (sh)
500, 530 (sh)
528
505, 535 (sh)
471, 495 (sh)
0.55
0.56
0.54
0.55
0.60
0.44
2
a
The bold number indicates the most intense peak. The solution data were obtained by dissolving polymers in THF solvent.
F igu r e 8. Current density (b)-voltage-luminance (O) rela-
tionship for device ITO/PPV/7a a /Ca (a) and ITO/PEDOT/7a a /
Ca (b).
F igu r e 7. EL spectra for device ITO/PPV/7a a /Ca (top) and
ITO/PEDOT/7a a /Ca (bottom) at different voltages.
from band-gap distribution in the polymer.17 At higher
applied voltage, the high-band-gap segments in the
polymer appeared to contribute slightly more to the
emission. The EL spectra from both device configura-
tions matched well with the film PL spectrum of 7a a ,
indicating that both PL and EL originated from the
same radiative decay process of the singlet exciton.18
The device of ITO/PPV/7/Ca had an external quantum
efficiency of 0.15%, which is much higher than that of
2 and comparable to that of 1.15 Current density-
voltage-luminance characteristics (Figure 8) showed
that the injection of electrons and holes in the device
remained to be balanced as observed in 1, suggesting
that presence of PPE block in the PPV/PPE copolymer
had little influence on the charge carrier injection. With
the device configuration of ITO/PEDOT/7/Ca, the EL
efficiency increased to 0.32% as a direct result of quite
polymer 7 as an emissive layer in LED devices. To test
this hypothesis, a device of ITO/PPV/7/Ca configuration
was fabricated. The device gave pure green electro-
luminescence (EL) with a turn-on voltage of about 8 V.
While an external voltage of 8-10 V was applied to the
device, the EL emission maxima were centered near λmax
≈ 513 and 529 nm (Figure 7a). As the applied voltage
was raised to 12 V, the emission was slightly blue-
shifted to about 504 and 522 nm, showing that the EL
spectrum was slightly dependent on the voltage used.
The same trend was also observed from the device of
ITO/PEDOT/7a a /Ca (Figure 7b), with the EL emission
λmax ≈ 501 and 529 nm at 11 V and λmax ≈ 498 and 523
nm at 15 V. The blue shift in EL at higher voltage could
be due to J oule heating in the LED, which results in
thermochromism.16 This phenomenon might also result

3852 Chu et al.
Macromolecules, Vol. 36, No. 11, 2003
described for 5a , using 2,5-diiodo-1,4-didodecyloxybenzene as
a starting material. The product 5b, which was needlelike
crystals (mp 78.5-79.5 °C), had the following spectral proper-
lower current density. The higher PL efficiency of film
7 than that of film 1 suggests that the EL efficiency of
the former could be further improved.
1
ties. H NMR (CDCl3, 400 MHz, δ): 0.86 (t, J ) 7.2 Hz, 6H,
-CH3), 1.22-1.55 (m, 36H, -CH2-), 1.86 (m, 4H, -CH2-),
4.04 (t, J ) 7.2 Hz, 4H, -OCH2-), 7.03 (s, 2H, Ar′-H), 7.52
(t, J ) 7.8 Hz, 2H, Ar-H), 7.76 (d, J ) 7.8 Hz, 2H, Ar-H),
7.84 (d, J ) 7.8 Hz, 2H, Ar-H), 8.02 (s, 2H, Ar-H), 10.01 (s,
2H, -CHO). 13C NMR (CDCl3, 100 MHz, δ): 13.85, 22.41,
25.83, 29.07, 29.15, 29.36, 31.63, 69.36, 87.23, 93.14, 113.53,
116.63, 124.40, 128.62, 128.82, 132.66, 136.24, 136.77, 153.47,
191.19. Anal. Calcd for C48H62O4: C, 82.01; H, 8.89. Found:
C, 82.00; H, 8.84.
Exp er im en ta l Section
Ma ter ia ls a n d Mea su r em en ts. 3-Bromobenzaldehyde and
trimethylsilylacetylene were purchased from Aldrich Chemical
Co. and GFS Chemical Co., respectively. 2,5-Diiodo-1,4-di-
alkoxybenzene19 and 2,5-dialkoxy-p-xylylenebis(diethylphos-
phonate)20 were prepared according to literature procedures.
NMR spectra were collected on a 400 MHz Bruker ARX-400
spectrometer. Infrared spectra were acquired on a Nicolet
Impact 400 FT-IR spectrometer. UV-vis spectra were acquired
on a Hewlett-Packard 8453 diode array spectrophotometer.
Fluorescence spectra were obtained on a PTI steady-state
fluorometer. The quantum yields were measured in THF
solvent by using quinine sulfate as a standard (φfl ) 0.577).21
The sample solutions were excited at 390 nm. Size exclusion
chromatography (SEC) was carried out on a Viscotek SEC
assembly consisting of a model P1000 pump, a model T60 dual
detector, a model LR40 laser refractometer, and three mixed
bed columns (10 µm). Polymer concentrations for SEC experi-
ments were prepared in a concentration of about 3 mg/mL.
The SEC system was calibrated prior to use by using narrow
and broad polystyrene standards that were purchased from
American Polymer Standards Corp.
3-Eth yn ylben za ld eh yd e. 3-Bromobenzaldehyde (9.25 g,
50 mmol) and trimethylsilacetylene (4.91 g, 50 mmol) were
dissolved in a mixture of solvents consisting of toluene (100
mL) and triethylamine (25 mL) in a 250 mL round-bottomed
flask. After replacing the air with argon in the flask, PdCl2-
(PPh3)2 (350 mg, 1 mol %) and Cu(OAc)2 (90 mg, 2 mol %) were
added. The reaction mixture was then heated at 70 °C for 20
h under a positive argon atmosphere. After cooling to room
temperature, the mixture was passed through a thin layer of
Celite to remove salts and catalysts, and the solvents were
P r ep a r a tion of P olym er 7a a . Monomers 5a (0.2893 g, 0.5
mmol) and 2,5-dihexoxy-p-xylylenebis(diethylphosphonate)
(0.2673 g, 0.5 mmol) were dissolved in 10 mL of dry THF.
While the solution was cooled by an ice/water bath, potassium
tert-butoxide (1.0 M in THF, 1.2 mL, 1.2 mmol) was added
dropwise via a syringe over a period of 1 h under an argon
atmosphere. During the initial reaction period, a pink-red color
was formed and quickly disappeared upon addition of potas-
sium tert-butoxide. A steady orange color was formed at the
end of addition. The reaction mixture was then further stirred
overnight at room temperature. The polymer was precipitated
out by dropwise addition of its THF solution into methanol
(150 mL) for three times. After drying at 40 °C under vacuum
for 24 h, the polymer 7a a (0.33 g, 82%) was obtained as orange
powder and had the following spectral properties. 1H NMR
(CDCl3, 400 MHz, δ): 0.89 (br, 12H, -CH3), 1.35 (br, 12H,
-CH2-), 1.56 (br, 8H, -CH2-), 1.86 (br, 8H, -CH2-), 3.57
(br, 0.32H, cis-OCH2-); 4.05 (br, 7.68H, trans-OCH2-),
7.01-7.14 (m, 6H, Ar-H), 7.34-7.51 (m, 8H, Ar-H), 7.69 (br,
2H, Ar-H). 13C NMR (CDCl3, 100 MHz, δ): 13.86, 22.44, 25.60,
29.16, 31.39, 69.47, 85.71, 94.55, 110.62, 113.76, 116.85,
123.55, 124.17, 126.17, 126.58, 128.00, 128.56, 129.45, 130.31,
137.88, 150.88, 153.40. IR (film): 2930 (s), 2862 (s), 2010 (w),
1591 (m), 1504 (s), 1468 (m), 1421 (m), 1387 (m), 1203 (s), 1029
(m), 963 (m), 892 (w), 859 (w), 787 (m). Anal. Calcd for
evaporated on
a rotatory evaporator. Purification of the
resulting residues on a silica gel column (eluent: ethyl acetate/
hexane ) 1:10) offered 3-trimethylsilylethynylbenzaldehyde
(7.0 g, 70%) which was further treated for 2 h with KOH (2.0
g) in 50 mL of solvent mixture of methanol and THF (1:1) at
room temperature. After recrystallization from hexanes, the
desired product was obtained as crystals (4.0 g, 90%; mp )
75-76 °C), which had the following spectral properties. 1H
NMR (CDCl3, 400 MHz, δ): 3.15 (s, 1H, -C≡CH), 7.50 (t, J )
7.8 Hz, 1H, Ar-H), 7.72 (d, J ) 7.8 Hz, 1H, Ar-H), 7.85 (d, J
) 7.8 Hz, 1H, Ar-H), 7.98 (s, 1H, Ar-H), 9.99 (s, 1H, -CHO).
13C NMR (CDCl3, 100 MHz, δ): 78.65, 81.85, 123.03, 128.82,
129.22, 133.13, 136.12, 137.33, 191.08. Anal. Calcd for
C9H6O: C, 83.06; H, 4.65. Found: C, 83.15; H, 4.54.
C
56H68O4: C, 83.54; H, 8.51. Found: C, 83.05; H, 8.10.
P r ep a r a tion of P olym er 7a b. By using the same proce-
dure as for 7a a , polymer 7a b (yield 85%) was obtained as an
orange resin and had the following spectral properties. 1H
NMR (CDCl3, 400 MHz, δ): 0.85 (br, 12H, -CH3), 1.23 (br,
36H, -CH2-), 1.54 (br, 8H, -CH2-), 1.87 (br, 8H, -CH2-),
3.57 (br, 0.52H, cis-OCH2-), 4.05 (s, 7.48H, trans-OCH2-),
7.01-7.14 (m, 6H, Ar-H), 7.34-7.51 (m, 8H, Ar-H), 7.69 (br,
2H, Ar-H). IR (film): 2922 (s), 2853 (s), 2210 (vw), 1591 (m),
1503 (m), 1466 (m), 1420 (m), 1384 (m), 1203 (m), 1302 (m),
963 (m), 891 (w), 862 (w), 786 (m). Anal. Calcd for C68H92O4:
C, 83.90; H, 9.53. Found: C, 83.15; H, 8.97.
P r ep a r a tion of P olym er 7ba . Polymer 7ba (yield 80%)
was obtained as yellow powder and had the following spectral
properties. 1H NMR (CDCl3, 400 MHz, δ): 0.88 (br, 12H,
-CH3), 1.24 (br, 36H, -CH2-), 1.56 (br, 8H, -CH2-), 1.88 (br,
8H, -CH2-), 3.60 (br, 0.33H, cis-OCH2-), 4.06 (s, 7.67H, trans-
OCH2-), 7.01-7.14 (m, 6H, Ar-H), 7.34-7.51 (m, 8H, Ar-
H), 7.69 (br, 2H, Ar-H). IR (film): 2924 (s), 2855 (s), 2209
(vw), 1592 (m), 1505 (m), 1470 (m), 1423 (m), 1385 (m), 1202
(m), 1302 (m), 963 (m), 894 (w), 862 (w), 787 (m). Anal. Calcd
for C68H92O4: C, 83.90; H, 9.53. Found: C, 83.27; H, 9.07.
P r ep a r a tion of P olym er 7bb. Polymer 7bb (yield 88%)
was obtained as a yellow resin and had the following spectral
properties. 1H NMR (CDCl3, 400 MHz, δ): 0.85 (br, 12H,
-CH3), 1.23 (br, 64H, -CH2-), 1.54 (br, 8H, -CH2-), 1.87 (br,
8H, -CH2-), 3.57 (br, 0.52H, cis-OCH2-), 4.05 (s, 7.48H, trans-
OCH2-), 7.01-7.14 (m, 6H, Ar-H), 7.34-7.51 (m, 8H, Ar-
H), 7.69 (br, 2H, Ar-H). IR (film): 2924 (s), 2855 (s), 2207
(vw), 1593 (m), 1503 (m), 1466 (m), 1420 (m), 1378 (m), 1202
(m), 1032 (m), 963 (m), 891 (w), 859 (w), 787 (m). Anal. Calcd
for C80H116O4: C, 84.15; H, 10.24. Found: C, 83.87; H, 9.65.
LED Device F a br ica tion a n d Mea su r em en t. PPV/ITO
substrates were obtained by spin-casting sulfonium polyelec-
trolyte precursor polymer2 methanol solution onto ITO glass
(OFC Co.) and then doing thermal conversion to PPV under
flowing argon at 250 °C for 2.5 h. PEDOT/PSS (Bayer Co.) was
1,4-Bis(3-for m ylp h en yl-1-eth yn yl)-2,5-d ih exyloxyben -
zen e (5a ). 2,5-Diiodo-1,4-dihexyloxybenzene (2.651 g, 5 mmol)
and 3-ethynylbenzaldehyde (1.302 g, 10 mmol) were dissolved
in a solvent mixture consisting of 5 mL of triethylamine and
50 mL of toluene. The solution was degassed under vacuum
and subsequently filled with argon to remove oxygen. After
addition of PdCl2(PPh3)2 (35 mg, 1 mol %) and CuI (20 mg, 2
mol %), the reaction mixture was stirred overnight at room
temperature under a positive argon atmosphere. After remov-
ing salt precipitate via filtration and solvents on a rotatory
evaporator, further purification on a silica gel column (hex-
anes/ethyl acetate ) 10:1) afforded 5a (2.20 g, 82%) as
1
yellowish crystals (mp ) 102-103 °C). H NMR (CDCl3, 400
MHz, δ): 0.87 (t, J ) 6.8 Hz, 6H, -CH3), 1.32-1.55 (m, 12H,
-CH2-), 1.85 (m, 4H, -CH2-), 4.04 (t, J ) 6.8 Hz, 4H,
-OCH2-), 7.02 (s, 2H, Ar′-H), 7.52 (t, J ) 7.8 Hz, 2H, Ar-
H), 7.76 (d, J ) 7.8 Hz, 2H, Ar-H), 7.83 (d, J ) 7.8 Hz, 2H,
Ar-H), 8.02 (s, 2H, Ar-H), 10.01 (s, 2H, -CHO). 13C NMR
(CDCl3, 100 MHz, δ): 13.76, 22.38, 25.49, 29.00, 31.31, 69.36,
87.21, 93.14, 113.51, 116.62, 124.39, 128.65, 128.83, 132.65,
136.23, 136.78, 153.47, 191.24. Anal. Calcd for C36H38O4: C,
80.87; H, 7.16. Found: C, 80.78; H, 7.14.
1,4-Bis(3-for m ylp h en yl-1-et h yn yl)-2,5-d id od ecyloxy-
ben zen e (5b). The synthesis of 5b was similar to that

Macromolecules, Vol. 36, No. 11, 2003
PPE-PPV Hybrid Polymers 3853
(2) Burroughes, J . H.; Bradley, D. D. C.; Brown, A. R.; Marks,
R. N.; MacKay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B.
Nature (London) 1990, 347, 539-541.
spin-cast onto ITO glass. The polymer solutions (20 mg/mL in
chloroform) were filtered through 0.2 µm Millex-FGS Filters
(Millipore Co.) and were then spin-cast onto dried PPV/ITO
or PEDOT/ITO substrates under a nitrogen atmosphere. The
polymer films were typically 75 nm thick. Calcium electrodes
of 400 nm thickness were evaporated onto the polymer films
at about 10-7 Torr, followed by a protective coating of alumi-
num. The devices were characterized using a system con-
structed in our laboratory described elsewhere.22
(3) Recent reports on electroluminescence of PPEs include: (a)
Pschirer, N. G.; Miteva, T.; Evans, U.; Roberts, R. S.;
Marshall, A. R.; Neher, D.; Myrick, M. L.; Bunz, U. H. F.
Chem. Mater. 2001, 13, 2691-2696. (b) Schmitz, C.; Posch,
P.; Thelakkat, M.; Schmidt, H.-W.; Montali, A.; Feldman, K.;
Smith, P.; Weder, C. Adv. Funct. Mater. 2001, 11, 41-46.
(4) m-Phenylene-containing PPEs has also shown some attrac-
tive EL properties. Chu, Q.; Pang, Y.; Ding, L.; Karasz, F. E.
Macromolecules 2002, 35, 7569-7574.
Con clu sion s
(5) Brizius, G.; Pschirer, N. G.; Steffen, W.; Stitzer, K.; Loye, H.-
C.; Bunz, U. H. F. J . Am. Chem. Soc. 2000, 122, 12435-
12440.
(6) Egbe, D. A. M.; Tillmann, H.; Birckner, E.; Klemm, E.
Macromol. Chem. Phys. 2001, 202, 2712-2726.
(7) (a) Egbe, D. A. M.; Roll, C. P.; Birckner, E.; Grummt, U.-W.;
Stockmann, R.; Klemm, E. Macromolecules 2002, 35, 3825-
3837. (b) Egbe, D. A. M.; Roll, C. P.; Klemm, E. Des.
Monomers Polym. 2002, 5, 245-275.
(8) Pang, Y.; Li, J .; Hu, B.; Karasz, F. E. Macromolecules 1999,
32, 3946-3950.
(9) Pang, Y.; Li, J .; Hu, B.; Karasz, F. E. Macromolecules 1998,
31, 6730-6732.
(10) Lakowicz, J . R. Principles of Fluorescence Spectroscopy;
Kluwer Academic: New York, 1999; Chapter 13.
(11) J akubiak, R.; Collison, C. J .; Wan, W. C.; Rothberg, L. J .;
Hsieh, B. R. J . Phys. Chem. A 1999, 103, 3294-3298.
(12) Schouwink, P.; Scha¨fer, A. H.; Fuchs, H. Thin Solid Films
2000, 372, 163-168.
(13) (a) Lawrence, N. J . Preparation of Alkenes: A Practical
Approach; Oxford University Press: Oxford, 1996; pp 19-
58. (b) The synthetic sequence for polymer 7 is similar to that
reported by Egbe,6,7 except that the aldehyde in monomer 5
is located at the meta rather than at para position (relative
to ethynyl group).
(14) The 13C NMR resonance signals of 1 (CDCl3, 100 MHz) are
at 150.6, 137.7, 128.3, 126.4, 124.8, 123.2, 110.3, 69.1, 31.0,
28.9, 25.3, 22.0, and 13.5 ppm.
PPE-PPV hybrid copolymers 7 have been synthe-
sized, in which the phenylenes are alternately linked
at the para and meta position along the chain for
reduced chain rigidity and improved morphology. Al-
though the absorption λmax of 7 falls between PmPVpPV
1 and PmPEpPE 2, the emission wavelength of 7 is
nearly identical to that of 1, attributing to the efficient
intramolecular energy transfer across m-phenylene. In
addition, the vibronic structure of 7 in the ground state
exhibits very similar feature as that of 1 (anharmonic
oscillator), showing that the chromophore in the PPV/
PPE hybrid polymer retains the emissive feature of the
chromophore in 1. In other words, placement of a PPV
chromophore block between two adjacent PPE blocks
has little effect on the emission wavelength of the PPV
block. Blue-shifted absorption reduces the overlapping
between the absorption and emission spectra, thereby
alleviating the self-absorption problem during device
operation. The hybrid copolymer exhibits improved
luminescence in film state, partly attributing to PPV
chromophore isolation. With the device configuration of
ITO/PEDOT/7/Ca, the polymer EL efficiency increased
to 0.32%. The device has a turn-on voltage of ∼8 V,
indicating the relatively higher charge-injection barrier.
Future study will focus on modification of polymer
structure to improve the charge injection characteristics
of the hybrid material.
(15) Liao, L.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules
2002, 35, 6055-6059.
(16) Braun, D.; Moses, D.; Zhang, C.; Heeger, A. J . Appl. Phys.
Lett. 1992, 61, 3092-3094.
(17) Bolognesi, A.; Bajo, G.; Paloheimo, J .; O¨ stergård, T.; Stubb,
H. Adv. Mater. 1997, 9, 121-124.
Ack n ow led gm en t. Support of this work has been
provided by AFOSR (Grant F49620-00-1-0090) and by
NIH/NIGMS/MBRS/SCORE (Grant S06GM08247).
(18) Baigent, D. R.; Friend, R. H.; Lee, J . K.; Schrock, R. R. Synth.
Met. 1995, 71, 2171-2172.
(19) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules
1998, 31, 52-58.
Su p p or tin g In for m a tion Ava ila ble: Infrared spectrum
and SEC chromatogram of polymer 7. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(20) Liao, L.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules
2001, 34, 6756-6760.
(21) Demas, J . N.; Crosby, G. A. J . Phys. Chem. 1971, 75, 991-
1024.
(22) Hu, B.; Karasz, F. E. Chem. Phys. 1998, 227, 263-270.
Refer en ces a n d Notes
(1) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402-428.
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