Green-emitting poly[(2-alkoxy-5-methyl-1,3-phenylenevinylene)-alt-(1,4-phenylenevinylene)s: Effect of substitution patterns on the optical properties

Article abstract of DOI:10.1021/ma012178r

Green-emitting poly[(2-alkoxy-5-methyl-1,3-phenylenevinylene)-alt-(1,4-phenylenevinyene)]s (2) were synthesized via the Wittig-Horner reaction. The polymers were yellow resins with the molecular weights as high as 54 600. On the basis of FT-IR and ^1<

Full text of DOI:10.1021/ma012178r

Macromolecules 2002, 35, 3819-3824
3819
Green-Emitting
Poly[(2-alkoxy-5-methyl-1,3-phenylenevinylene)-alt-(1,4-phenylenevinylene)s:
Effect of Substitution Patterns on the Optical Properties
Lia n g Lia o 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 December 17, 2001; Revised Manuscript Received February 21, 2002
ABSTRACT: Green-emitting poly[(2-alkoxy-5-methyl-1,3-phenylenevinylene)-alt-(1,4-phenylenevinylene)]s
(2) were synthesized via the Wittig-Horner reaction. The polymers were yellow resins with the molecular
1
weights as high as 54 600. On the basis of FT-IR and H NMR spectra, the olefins in the polymers were
confirmed to be predominantly in the trans configuration. UV-vis absorption of 2 (λmax ≈ 374 - 377 nm)
was about 20 nm blue-shifted from that of poly[(1,3-phenylenevinylene)-alt-(2,5-dialkoxy-1,4-phen-
ylenevinylene)]s (1), although the chromophores in the former had the same number of alkoxy substituents
as in the latter. Comparison of 2 with the model compound 7 showed that the shorter conjugation length
observed from 2 (relative to 1) was the intrinsic property of the chromophore. Polymer 2 also exhibited
a higher photoluminescence (PL) than 1 in both solution and film states, indicating the effect of the
substitution pattern on the optical properties of the polymers. The vibronic structures were assigned
with the aids of the low-temperature UV-vis and fluorescence spectroscopy. LED devices using 2 gave
green EL output (emission λmax at 496 and 520 nm) with the external quantum efficiency of 0.082%.
In tr od u ction
derivative 2, in which the substituents are only present
on the m-phenylene unit.
Since the discovery of polymeric light-emitting diodes
(LEDs)1 in 1990, π-conjugated polymers have attracted
increasing attention over the past decade2 because of
their potential applications in display technologies. The
unique combination of their structural, mechanical,
photonic, and electronic properties also renders them
attractive candidates for use as plastic lasers3 and
chemical sensors4 and for use in other fields.5 Recent
studies6 have shown that poly[(m-phenylenevinylene)-
alt-(p-phenylenevinylene)] (PmPVpPV) derivatives (1)
The molecular fragment A represents a section of
chain structure in PmPVpPV. As demonstrated in our
previous report,6a the chromophore of the polymer can
be described as p-phenylenedivynylene sandwiched
between the two adjacent m-phenylene units. A unique
feature in PmPVpPV is that the m-phenylene is shared
between the two adjacent chromophores. A substituent
“Y” placed on an m-phenylene is, therefore, expected to
influence the two adjacent chromophores simulta-
neously, while that on a p-phenylene is expected to
influence only one chromophore. In other words, sub-
stitution on a different type of the phenyl rings (m-
phenylene or p-phenylene) along the PmPVpPV back-
bone would perturb the electronic band structure of the
polymer in different fashion. The fact that the m-
phenylene occurs at the end of the chromophore may
also contribute to a different substituent effect in
comparison with the p-phenylene at the center of the
chromophore. Although previous studies6 have been
focused on attachment of the substituent to the p-
phenylene unit as shown in 1, no report has been found
in the literature to put substituent on the m-phenylene
of PmPVpPV. Availability of m-phenylene-substituted
PmPVpPVs will allow one to evaluate the substitution
effect, and to establish the valuable structure-property
correlations, which are desirable for future material
development. Here we report the synthesis and char-
acterization of 2, along with its optical properties.
are green-emitting with high PL efficiency. While the
p-phenylene and m-phenylene are alternately placed
along the polymer chain, the side chain substitutes in
1 are only present on the p-phenylene unit. It is known
that different substitution patterns7 on the π-conjugated
polymers could significantly influence their luminescent
properties. It would be interesting to examine whether
the substitution on a different type of phenylene unit
will affect the luminescent properties of the PmPVpPV
Resu lts a n d Discu ssion
Polymer 2 was synthesized as shown below by using
the Wittig-Horner reaction,8 which is known to produce
trans-alkenes. In the polymer 2, the p-phenylene and
10.1021/ma012178r CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/27/2002

3820 Liao et al.
Macromolecules, Vol. 35, No. 10, 2002
Ta ble 1. Sp ectr oscop ic Da ta for P olym er s 2 a n d th e Mod el Com p ou n d 7
contenta of
trans-CHdCH (%)
emissionb
max (nm)
UV-vis absb
λmax (nm)
polymer
Mw (PDI)
DP
λ
φfl
2a
2b
2c
7
85
86
88
93
54600 (1.8)
22000 (1.8)
6820 (1.2)
104
38
16
412, 436, 464
412, 436, 465
412, 436
377
377
374
371
0.72
0.71
0.72
0.88
413, 436, 464 (sh)
a
b
The content of trans-CHdCH was estimated from the 1H NMR spectra in CDCl3 solvent. The absorption and emission spectra were
acquired from their respective THF solutions at 25 °C.
m-phenylene units were alternated along the backbone,
thereby producing a chromophore with a uniformly
defined structure. The polymers were yellow resins with
the molecular weight as high as 54 600 (Table 1). The
number-average degrees of polymerization of 2 were
determined to be n ≈ 104, 38, and 16 for 2a , 2b, and
2c, respectively. Polymer 2 exhibited good solubility in
common organic solvents such as THF, chloroform, and
toluene. It appeared that only a short alkyl side chain
on the substituted phenyl rings was necessary to keep
the polymer soluble. Uniform thin films could be cast
from the polymer solutions. To aid the polymer structure
characterization, the model compound 7 was synthe-
sized similarly by reacting 6 with benzene-1,4-dicarb-
aldehyde 5.
F igu r e 2. 1H NMR of 2b (top) and 7 (bottom) (only aromatic
and olefinic region is shown for clarity). The starred signal at
7.25 ppm is attributed to trace CHCl3.
The high intensity of absorption at ∼965 cm-1 in the
IR spectrum of 2 (Figure 1) indicated that the olefin
groups in the polymer were predominantly in the trans
configuration.9 The olefinic and aromatic region of the
1H NMR spectrum of 2 is shown in Figure 2, where the
resonance signals were assigned by using the same
labels as in the structures 2 to indicate the correspond-
ing aromatic and olefinic protons. The presence of the
doublet signals at about 7.13 and 7.47 ppm (marked “b”
and “c” in Figure 2) with a large coupling constant (J )
16.4 Hz) confirmed the trans stereochemistry of
F igu r e 3. UV-vis (solid line) and PL (dotted line) spectra of
polymer 2b in THF solution at +25 and -108 °C. The spectra
at -108 °C are offset from those at 25 °C for clarity.
CH-CH in the polymer.10 The assignment of the
resonance signals in the 1H NMR spectrum of the
polymer was further supported by the observed same
coupling pattern (trans-CHdCH) from the model com-
pound 7. The relative signal intensity between the
singlet at ∼7.39 ppm (proton “d”) and the doublet at
∼7.13 and 7.49 ppm concluded that the majority of
olefins in the polymer was in the trans configuration.
The content of trans-CHdCH bond linkage (Table 1)
was estimated by integrating the proton resonance
signals of -OCH2- unit in the alkoxy side chains, which
were shown to be sensitive to the nearby cis-CHdCH
or trans-CHdCH configuration6a in m-phenylene-con-
taining PPVs.
P h otoa bsor p tion a n d P h otolu m in escen ce (P L).
The UV-vis spectra of the polymer 2 in dilute THF
solution (Figure 3) showed one major band with absorp-
tion λmax at about 377 nm, which is quite different from
that of 1 exhibiting two absorption bands6a at about 326
and 397 nm. It appears that the substitution patterns
do have some effect on the electronic band structure of
F igu r e 1. Infrared spectrum of 2b film on NaCl plate. The
vertical scale is transmittance in arbitrary units. The arrow
marks the absorption for olefinic C-H out-of-plane deforma-
tion (trans-CHdCH at ∼965 cm-1).

Macromolecules, Vol. 35, No. 10, 2002
Green-Emitting PmPVpPV 3821
the chromophores. Resulting from the m-phenylene
interruption, the conjugation length of the chromophore
in 1 and 2 would be determined by the fragments 8 and
9, respectively. While both fragments 8 and 9 contain
F igu r e 4. UV-vis (solid line) and PL (dotted line) spectra of
7 at +25 and -108 °C in THF, and at -198 °C in a solvent
mixture (diethyl ether, ethanol, and 2-methylbutane in a ratio
of 1:1:1). The spectra at different temperature are offset for
clarity.
the same number of alkoxy groups, the presence of two
methyl groups in the latter are expected to cause further
increase in conjugation length (additional bathochro-
matic shift).11 On the contrary, the observed conjugation
length for 2 (λmax ) 377 nm) is about ∼20 nm blue-
shifted from the absorption λmax of polymer 1 (∼397
nm).6a This notable difference in conjugation length
between 1 and 2 was also observed in their emission
spectra (Figure 3), where the emission λmax of the
highest energy band was at 412 and 444 nm for 2 and
1, respectively. The fluorescence quantum efficiency12
of 2 was estimated to be φfl ≈ 0.71, in comparison with
that of 1 (φfl ≈ 0.66).
2 suggests that some minor tune in the electronic band-
gap of PmPVpPV can be achieved by simply placing
substitution on m- or p-phenylene units.
Low -Tem p er a tu r e UV-Vis a n d P L. The solution
UV-vis spectrum of 2 at 25 °C (Figure 3) showed a
major peak at 377 nm, and a shoulder at ∼395 nm. As
the temperature was lowered to -108 °C (still in the
liquid state), the shoulder was resolved into a pro-
nounced peak due to the reduced rotation and increased
viscosity at the low temperature. In addition, the
spectrum was slightly red-shifted (∼6 nm) with the
absorption λmax values at 383 and 406 nm at the low
temperature. The fluorescence spectrum at -108 °C also
became more resolved, showing more transitions with
the emission λmax at 416, 441, and 473 nm (correspond-
ing to the wavenumber of 24 038, 22 676, and 21 142
cm-1, respectively). The vibrational energy levels of 2
in the ground state (as shown from the emission peaks)
appears to be about equally spaced with a wavenumber
separation of about 1362 cm-1, or a wavenumber
separation of about 25 nm. The energy difference
between the overlapping low-energy absorption band
(λmax ) 406 nm) and the high-energy emission band
To gain further understanding on the nature of the
electronic structure of the material, spectroscopic com-
parison was made between polymer 2 and its model
compound 7. As shown from Figures 3 and 4, solution
of 7 in THF exhibits absorption and emission profiles
very similar to those of 2. The small difference between
the λmax values of 2 and 7 (∼6 nm in UV-vis absorption,
and ∼1 nm in emission) (Table 1) is within the range of
the anticipated substituent effect, indicating an effective
conjugation interruption at m-phenylene. Very similar
conjugation lengths displayed from 2 and 7 strongly
suggests that the shorter conjugation length observed
from 2 (relative to 1) is the intrinsic property of the
chromophore fragment 9. Because of the different
substitution pattern, the steric interaction between the
side chains and π-conjugated polymer backbone in
polymer 2 is expected to be different from that in
polymer 1. This difference in steric interaction, however,
may not be sufficient to cause a significant change in
the conjugation length of the polymer chromophores.
Simple comparison between polymers 2 and 1 shows
that the number of alkoxy group per repeating unit in
2 is only half of that in 1. This might not be the major
reason to account for the short conjugation length of 2,
as it is in contrary to the observation that both 2b and
7 exhibit very similar conjugation length. The different
arrangement of the substituent (or substitution pat-
tern), therefore, might play an important role in the
observed shorter conjugation length from 2. As seen
from their respective chromophores, the alkoxy substit-
uents in 9 appears at the end of the chromophore in
comparison with that in 8 at the center of the chro-
mophore. Comparison of the UV-vis absorption λmax
values between the chromophore model compounds for
2 and 1 offers further support to the substitution effect,
as 7 is blue-shifted (∆λmax ≈ 17 nm) from 1,4-bis(2′-
phenylethenyl)-2,5-dihexyloxybenzene13 to a very simi-
lar degree. The observed blue-shift from polymers 1 to
(λmax ) 416 nm) is estimated to be about 593 cm-1
,
which is significantly smaller than the required adjacent
vibrational energy gap of at least 1362 cm-1 for a lower
energy level in an anharmonic oscillator model.14 There-
fore, it is likely that the emission bands at 416, 441,
and 473 nm in the low-temperature fluorescence spec-
trum of 2 are corresponding to 0-0, 0-1, and 0-2
transitions, respectively. The fluorescence spectra of the
polymers (Figure 3) also revealed a characteristic pat-
tern, in which the short-wavelength vibrational band
(0-0 transition) was the most intense emission, and the
intensity of the emission bands gradually decreased
with increasing wavelength. This characteristics sug-
gested that the geometry of the chromophore in the
excited state was very similar to that of its ground state,
based on the Franck-Condon principle.15
The solution UV-vis spectrum of the model com-
pound 7 at 25 °C showed a major absorption band (λmax
) 371 nm). As the temperature was lowered to -108
°C, a new band at 400 nm became partially resolved in
addition to the major absorption band at 380 nm. To
further confirm the presence of the new band at 400
nm, the sample 7 in a solvent mixture16 of diethyl ether,
ethanol, and 2-methylbutane was cooled to -198 °C in
liquid nitrogen to form a clear transparent glass. The
absorption spectrum at -198 °C (shown in Figure 4)

3822 Liao et al.
Macromolecules, Vol. 35, No. 10, 2002
F igu r e 6. Curves A and B are excitation spectra for films 2b
and 1b while monitoring at 500 nm. Curve A′ and B′ are
respective emission spectra while the films were excited at 397
nm. The emission peaks for 2b (in curve A′) are at 490 and
518 nm and that for 1b is at 530 nm.
F igu r e 5. UV-vis absorbance of films 1b and 2b. The arrows
mark the wavelength at 332 and 397 nm, respectively.
exhibited clearly resolved bands with absorption λmax
) 366, 384, and 406 nm. The fluorescence spectrum of
7 at the low temperature also became more resolved.
By using the same reasoning for assigning the vibronic
structure of polymer 2, the emission bands at 414, 439,
and 466 nm in the spectrum of 7 at -198 °C were
assigned to 0-0, 0-1, and 0-2 transitions, respectively.
By using the 0-0 and 0-1 bands at 417 nm (23 981
cm-1) and 442 nm (22 624 cm-1) from the fluorescence
spectrum of 7 at -108 °C, the vibrational energy gap of
excited at 397 nm, where both films under study had
the same absorbance (Figure 5). The same emission
profiles were also observed when the films were excited
at 332 nm. The relative emission intensity, as deter-
mined by the ratio of integration peak area of 2 to 1,
was estimated to be about 1.8 and 1.9 when excited at
397 and 332 nm, respectively. Therefore, the substitu-
tion pattern on the polymer backbone appears to have
significant influence on the optical properties of the
materials, including the electronic band structure and
luminescent efficiency. Emission of film 2 showed two
distinctive peaks at 490 nm (20 408 cm-1) and 518 nm
(19 305 cm-1).
EL P r op er ties. LED devices ITO/PEDOT/polymer/
Ca were fabricated to examine the EL properties. The
device from polymer 2b gave green emission at 496 and
520 nm (Figure 7) in comparison with that from 1 at
513 and 539 nm.6a The EL spectrum of 2b matched very
well with its PL spectrum, indicating that both PL and
EL originated from the same radiative decay process of
the singlet exciton.18 Turn-on voltage for the device of
2b was ∼4.5 V (Figure 7). The external quantum
efficiency of the device was estimated to be 0.082% for
2b.
7 in the ground state was estimated to be ∼1357 cm-1
,
which is comparable to that of ∼1362 cm-1 for 2. The
vibronic structures of 7 revealed from the low temper-
ature absorption and emission spectra are remarkably
similar to that of 2, further confirming the assumption
that the electronic band structure of the polymer is
determined by the fragment 9. It should be pointed out
that the vibrational energy gap of 2 (∼1362 cm-1) is
noticeably larger than that of 1 (∼1300 cm-1),17 which
seems in agreement with the slightly higher PL ef-
ficiency of the former.
Th in -F ilm Op tica l P r op er ties. To further examine
the optical properties in the solid state, polymer films
were prepared by spin-casting the polymer solution (2
mg/mL in THF) on a quartz plate at a spin rate of about
1700-1800 rpm. The films 1 and 2 thus prepared were
uniform and had very similar thickness as shown by
their similar absorbance at absorption λmax (Figure 5).
Polymer 2 exhibited one major absorption band at 383
nm, in comparison with two absorption bands at about
336 and 425 nm observed from the film 1. In addition,
the absorption profile of 2 in the solid state is quite
similar to that in solution, supporting the assumption
that the chromophore in the polymer is well-defined.
While the UV-vis spectra of both films 2 and 1 were
slightly red-shifted from their respective spectra in THF
solutions (λmax ) 377 for 2, and λmax ) 410 for 1), the
peak absorption difference ∆λmax between the film and
solution spectra is estimated to be ∼6 nm for 2 and ∼15
nm for 1. The observed smaller bathochromic shift from
solution to film states indicates that the chromophore
in 2 seems to be less affected by the molecular environ-
ments than that of 1.
Excitation spectra under the identical conditions
indicated that emission intensity of film 2 in general
was stronger than that of 1 (Figure 6), when the
excitation wavelength is between 275 and 420 nm.
Although the excitation spectra of 1 and 2 exhibited the
similar profile as observed in their respective absorption
spectra, the excitation λmax values are noticeably dif-
ferent from the respective absorption λmax values. The
emission spectra were acquired while the films were
Exp er im en ta l Section
Ma ter ia ls a n d In str u m en ta tion . Terephthalaldehyde
(99%), triethyl phosphite P(OEt)3, 4-hydroxytoluene (Acros
Organics), potassium tert-butoxide (1.0 M in THF), and
hydrogen bromide (30 wt % solution in acetic acid) (Aldrich
Chemical Co.) were used without further purification. Solvents
were dried, distilled, and stored under nitrogen or argon.
Polymer 1b was synthesized by using the Wittig-Horner
reaction,6d and 1-alkoxy-2,6-bis(bromomethyl)-4-methylben-
zene 3 was prepared by using the procedure reported previ-
ously.19 IR spectra were recorded on a Nicolet Impact 400 FT-
IR spectrometer from films on NaCl plates. UV/vis spectra
were recorded either in distilled dry tetrahydrofuran (THF)
or from films spin-cast on quartz plates on a Hewlett-Packard
8543 diode array spectrophotometer. 1H NMR spectra were
acquired on a Bruker ARX400 spectrometer. Fluorescence
spectra were recorded on a PTI steady-state fluorometer at
23 ( 1 °C. Corrected fluorescence spectra of the polymer films
were recorded on quartz plates in air. The solution PL
quantum yields (φfl) were measured relative to quinine sulfate
in 0.5 M H2SO4 at 25 °C, by using the same procedure6a as
described previously. Size exclusion chromatography (SEC)
was carried out on a Viscotek SEC assembly consisting of a
model P1000 pump, a model T60 dual detectors, a model LR40
laser refractometer, and three mixed bed columns (10 µm).
Polymer concentrations for SEC experiments were prepared
in a concentration of about 3 mg/mL. The SEC system was

Macromolecules, Vol. 35, No. 10, 2002
Green-Emitting PmPVpPV 3823
by using the same procedure as for 2b. 1H NMR (400 MHz,
CDCl3), δ: 1.04 (br, 3H, -CH3), 1.68 (br, 2H, -CH2-), 1.89
(m, 2H, -CH2-), 2.24 (s, 0.47H, Ar-CH3 (cis-olefin fragment)),
2.43, (s, 2.53H, Ar-CH3 (trans-olefin fragment)), 3.88 (m,
1.69H, -OCH2- (trans-olefin fragment)), 3.97 (m, 0.31H,
-OCH2- (cis-olefin fragment)), 7.17 (d, J ) 16.4 Hz, 2H, trans-
CHdCH-), 7.43 (s, 2H, Ar-H), 7.51 (d, J ) 16.4 Hz, 2H, trans-
CHdCH-), 7.59 (s, 4H, Ar-H). IR (NaCl, thin film), νmax
(cm-1): 3033 (m), 2956 (s), 2871 (m), 1513 (m), 1459 (s), 1382
(w), 1258 (w), 1212 (s), 965 (s), 849 (m). Anal. Calcd for
C
21H22O: C, 86.85; H, 7.64. Found: C, 85.72; H, 7.48.
P oly[(1-octyloxy-4-m eth yl-2,5-p h en ylen evin ylen e)-a lt-
(1,4-p h en ylen evin ylen e)] (2c) was synthesized in 30% yield
1
by using the same procedure as for 2b. H NMR (CDCl3, 400
MHz), δ: 0.87 (br, 3H, -CH2CH3), 1.22-1.61 (m, 6H, -CH2-
), 1.59 (br, 4H, -CH2-), 1.85 (m, 2H, -CH2-), 2.10 (s, 0.36H,
Ar-CH3 (cis-olefin fragment)), 2.39 (s, 2.64H, Ar-CH3 (trans-
olefin fragment)), 3.83 (m, 1.76H, -OCH2- (trans-olefin frag-
ment)), 3.91 (m, 0.24 H, -OCH2- (cis-olefin fragment)), 7.13
(d, J ) 16.4 Hz, 2H, trans-CHdCH-), 7.39 (s, 2H, Ar-H), 7.47
(d, J ) 16.4 Hz, 2H, trans-CHdCH-), 7.55 (s, 4H, Ar-H). IR
(NaCl, thin film), νmax (cm-1): 3033 (w), 2925 (s), 2856 (m),
1513 (w), 1382 (w), 1250 (m), 1212 (m), 1027 (s), 965 (s), 849
(w). Anal. Calcd for C25H30O: C, 86.66; H, 8.73. Found: C,
85.97; H, 8.71.
Syn th esis of 3-Br om om eth yl-4-h exyloxytolu en e. To a
suspension of 1-hexyloxy-4-methylbenzene (3.845 g, 20 mmol)
and paraformaldehyde (0.615 g, 20.5 mmol) in acetic acid (10
mL) was added hydrogen bromide (30 wt % in AcOH, 4.09 mL,
20.5 mmol) via syringe. After the mixture was heated at 70-
80 °C for 5 h, it was cooled to room temperature and poured
onto ice/water (100 mL). The aqueous layer was extracted with
four portions of hexanes (100 mL each). The combined organic
layer was washed with water (four times, 20 mL each), and
then dried over anhydrous MgSO4. After removing the solvent
on a rotatory evaporator, purification on column chromatog-
raphy (silica gel, eluant hexanes) afforded 3-bromomethyl-4-
F igu r e 7. EL spectrum (top) and current density-voltage-
brightness relationship (bottom) for the device ITO/PEDOT/
2b/Ca.
1
hexyloxytoluene as a colorless oil (4.9 g, yield 86%). H NMR
(400 MHz, CDCl3), δ: 0.90 (t, J ) 6.5 Hz, 3H, -CH2CH3),
1.30-1.39 (m, 4H, -CH2-), 1.46-1.56 (m, 2H, -CH2-), 1.76-
1.85 (m, 2H, -CH2-), 2.26 (s, 3H, Ar-CH3), 3.99 (t, J ) 6.4
Hz, -OCH2-), 4.54 (s, 2H, -CH2Br), 6.75 (d, J ) 8.3 Hz, 1H,
Ar-H), 7.04 (d, J ) 8.3 Hz, 1H, Ar-H), 7.12 (s, 1H, Ar-H).
Anal. Calcd for C14H21BrO: C, 58.96; H, 7.42. Found: C, 58.68;
H, 7.40.
calibrated by using narrow and broad polystyrene standards
prior to use. The polystyrene standards were purchased from
American Polymer Standards Corp.
Syn t h esis of P oly[(2-h exyloxy-5-m et h yl-1,3-p h en -
ylen evin ylen e)-a lt-(1,4-p h en ylen evin ylen e)] (2b). A mix-
ture of 2,6-bis(bromomethyl)-1-hexyloxy-4-methylbenzene 3a
(0.567 g, 1.50 mmol) and triethyl phosphite (0.548 g, 3.30
mmol) was heated at 155-165 °C under an argon atmosphere
for 2 h. The excess triethyl phosphite and byproducts were
removed under vacuum (0.5 mmHg, 100 °C for 0.5 h). The
obtained 4-alkoxy-1-methyl-3,5-xylene tetraethyldiphospho-
nate (4)18 (a colorless liquid) was dissolved in anhydrous THF
(25 mL) with terephthaldicarboxaldehyde (5) (0.201 g, 1.50
mmol). To this solution was added dropwise potassium tert-
butoxide (1 M in THF, 3.0 mL) at room temperature via a
syringe over a period of 15 min. A red-purple color was formed
and quickly disappeared upon the addition of potassium tert-
butoxide solution. After addition, the solution was stirred for
an additional 3 h at room temperature. The resulting solution
was separated from the salt, and the polymer was precipitated
out via dropwise addition to methanol (125 mL). The yellow
precipitate was collected and washed with methanol on a
Soxhlet extractor for 48 h, and dried by vacuum to afford a
light yellow resin (0.150 g, yield 30.7%), which had the
Syn t h esis of Bis[2-(2-h exyloxy-5-m et h yl-1-p h en yl)-
eth en yl]-1,4-ben zen e (7). A mixture of 3-bromomethyl-4-
hexyloxytoluene (0.570 g, 2 mmol) and triethyl phosphite (0.40
g, 2.4 mmol) in a 25 mL flask was heated at 155-160 °C under
an argon atmosphere for 5 h. The excess triethyl phosphite
and byproducts were removed by vacuum (0.5 mmHg) at 100
°C for 0.5 h to give diethyl 2-hexyloxy-5-methylbenzylphos-
phonate (6) as a colorless liquid, which was then dissolved in
10 mL of anhydrous THF with terephthaldicarboxaldehyde (5)
(0.154 g, 1.15 mmol). To this solution was added potassium
tert-butoxide (1 M in THF, 2.0 mL, 2 mmol) dropwise via a
syringe over a period of 15 min. After addition, the reaction
mixture was stirred for 4 h at room temperature. The clear
solution was separated from precipitates via filtration. Fol-
lowing the removal of solvent, the product was purified by
using column chromatography (silica gel, eluant: hexanes) to
give pale yellow crystals (0.372 g, yield 72.8%, mp 90-92 °C).
1H NMR (400 MHz, CDCl3), δ: 0.95 (t, J ) 6.6 Hz, 6H, -CH3),
1.29-1.45 (m, 8H, -CH2-), 1.52-1.60 (m, 4H, -CH2-), 1.83-
1.92 (m, 4H, -CH2-), 2.35 (s, 6H, Ar-CH3), 3.96 (t, J ) 6.5
Hz, 0.28H, -OCH2- (cis-olefin fragment)), 4.03 (t, J ) 6.5 Hz,
3.72H, -OCH2-, trans-olefin fragment), 6.82 (d, J ) 8.2 Hz,
2H, Ar-H), 7.04 (d, J ) 8.2 Hz, 2H, Ar-H), 7.16 (d, J ) 16.4
Hz, 2H, trans-CHdCH-), 7.43 (s, 2H, Ar-H), 7.50 (d, J ) 16.4
Hz, 2H, trans-CHdCH-), 7.53 (s, 4 H, Ar-H). Anal. Calcd.
C, 84.66; H, 9.08. Found: C, 84.42; H, 9.27.
1
following spectral properties. H NMR (400 MHz, CDCl3), δ:
0.82-0.95 (br, 3H, -CH2CH3), 1.36-1.44 (br, 4H, -CH2-),
1.51-1.64 (br, 2H, -CH2-), 1.81-1.92 (m, 2H, -CH2-), 2.38
(s, 3H, Ar-CH3), 3.83 (br, 1.68H, -OCH2- (in trans-olefin
fragment)), 3.88 (br, 0.32H, -OCH2- (in cis-olefin fragment)),
7.13 (d, J ) 16.4 Hz, 2H, trans-CHdCH-), 7.40 (s, 2H, Ar-
H), 7.47 (d, J ) 16.4 Hz, 2H, trans-CHdCH-), 7.45 (s, 4H,
Ar-H). IR (NaCl, thin film), νmax (cm-1): 3033 (w), 2925 (s),
2863 (s), 1513 (m), 1451 (s), 1382 (w), 1250 (w), 1212 (m), 965
(s), 865 (w). Anal. Calcd for C23H26O: C, 86.75; H, 8.23.
Found: C, 86.08; H, 8.29.
LED Device F a br ica tion a n d Mea su r em en t. PEDOT/
PSS (Bayer Co.) was spin-cast onto ITO glass (OFC Co.) to be
used as an anode. The polymer solutions (20 mg/mL in
chloroform) were filtered through 0.2 µm Millex-FGS Filters
P oly[(1-b u t oxy-4-m et h yl-2,5-p h en ylen evin ylen e)-a lt-
(1,4-p h en ylen evin ylen e)] (2a ) was synthesized in 75% yield

3824 Liao et al.
Macromolecules, Vol. 35, No. 10, 2002
(7) Some examples are as follows: (a) Li, J .; Pang, Y. Macro-
molecules 1998, 31, 5740-5745. (b) Xu, B.; Holdcroft, S.
Macromolecules 1993, 26, 4457-4460.
(8) Lawrence, N. J . The Wittig Reaction and Related Methods.
In Preparation of Alkenes: A Practical Approach; Williams,
J . M. J ., Ed.; Oxford University Press: New York, 1996.
(9) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J . G.
The Handbook of Infrared and Raman Characteristic Fre-
quencies of Organic Molecules; Academic: Boston, MA, 1991;
Chapter 6.
(Millipore Co.), and were spin-cast onto ITO glass or dried
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.20
Ack n ow led gm en t. Support of this work has been
provided by AFOSR (Grant No. F49620-00-1-0090)
and by NIH/NIGMS/MBRS/SCORE (Grant No.
S06GM08247).
(10) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 5th ed.; J ohn Wiley &
Sons: New York, 1991; pp 192, 193, 221.
(11) Reference 10; Chapter 7.
(12) The φfl value for 2 appeared to be dependent on the excitation
wavelength used. When excited at 350 and 366 nm, for
example, the solution of 2b gave the φfl values of 0.85 and
0.71, respectively. Under the same condition, the solution of
1 (R ) n-hexyl) gave the φfl values of 0.66 when excited at
366 nm. The φfl values listed in Table 1 are obtained by
exciting the sample solution at 366 nm.
Refer en ces a n d Notes
(1) 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.
(2) For a recent review on electroluminescent conjugated poly-
mers, see: Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew.
Chem., Int. Ed. 1998, 37, 402-428.
(13) 1,4-Bis(2′-phenylethenyl)-2,5-dihexyloxybene6a is the model
chromophore for polymer 1. Its UV-vis absorption shows λmax
) 325 and 388 nm in THF at room temperature.
(14) Turro, N. J . Modern Molecular Photochemistry; University
Science: Mill Valley, CA, 1991; Chapter 4.
(3) McGehee, M. D.; Heeger, A. J . Adv. Mater. 2000, 12, 1655-
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(4) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev.
2000, 100: 2537.
(5) Skotheim, T. A.; Elsenbaumer, R. A.; Reynolds, J . R., Eds.
Handbook of Conducting Polymers; Marcel Dekker: New
York, 1998.
(6) (a) Pang, Y.; Li, J .; Hu, B.; Karasz, F. E. Macromolecules
1999, 32, 3946-3950. (b) Davey, A. P.; Drury, A.; Maier, S.;
Byrne, H. J .; Blau, W. J . Synth. Met. 1999, 103, 2478-2479.
(c) Ohnishi, T.; Doi, S.; Tsuchida, Y.; Noguchi, T. Photonic
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Washington, DC, 1997; pp 345-357. (d) Liao, L.; Pang, Y.;
Ding, L.; Karasz, F. E. Macromolecules, 2001, 34, 6756-6760.
(15) Perkampus, H.-H. UV-vis Spectroscopy and Its Applications;
Springer-Verlag: Berlin, 1992; Chapter 8.
(16) Scha¨fer, E. P. Dye Lasers, 3rd ed.; Springer-Verlag: Berlin,
1990; p 18.
(17) Liao, L.; Pang, Y. J . Mater. Chem. 2001, 11, 3078-3081.
(18) Baigent, D. R.; Friend, R. H.; Lee, J . K.; Schrock, R. R. Synth.
Met. 1995, 71, 2171-2172.
(19) Liao, L.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules
2001, 34, 7300-7305.
(20) Hu, B.; Karasz, F. E. Chem. Phys. 1998, 227, 263-270.
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