Amphiphilic phenyl-ethynylene polymers and copolymers. Synthesis, characterization, and optical emission properties

Article abstract of DOI:10.1021/ma020121e

A series of new rigid amphiphilic aryl-ethynylene homopolymers and copolymers with phenyl, pyridinyl, thionyl, or carbazolyl as aryl units has been synthesized. A single side chain on the phenyl rings (alkyl-hydroxyl ester group) provides an amphiphilic c

Full text of DOI:10.1021/ma020121e

3570
Macromolecules 2003, 36, 3570-3579
Amphiphilic Phenyl-Ethynylene Polymers and Copolymers. Synthesis,
Characterization, and Optical Emission Properties
E. Ar ia s-Ma r in ,†,§ J . Le Moign e,*,† T. Ma illou ,† D. Gu illon ,† I. Moggio,†,§ a n d
B. Geffr oy‡
Institut de Physique et Chimie des Mate´riaux de Strasbourg, UMR 7504, 23 rue du Loess BP 43,
67034 Strasbourg, France, and LETI (CEA Technologies Avance´es), DEIN/ SPE, Groupe Composants
Organiques, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
Received J anuary 23, 2002; Revised Manuscript Received March 3, 2003
ABSTRACT: A series of new rigid amphiphilic aryl-ethynylene homopolymers and copolymers with
phenyl, pyridinyl, thionyl, or carbazolyl as aryl units has been synthesized. A single side chain on the
phenyl rings (alkyl-hydroxyl ester group) provides an amphiphilic character to the polymer backbone.
The polymers are characterized by NMR, elemental analysis and size exclusion chromatography (SEC);
the thermal properties are analyzed by TGA, DSC, and X-ray diffraction. The polymers with side chains
are enough amphiphilic to give stable Langmuir films on water, and the LB films can be transferred on
hydrophilic or hydrophobic substrates. The films are deposited on treated glass or silicon substrate.
Analyzed by grazing incidence X-ray analysis (GIXA) and by atomic force microscopy (AFM), they reveal
well-structured layering periods of 3.5 and 2.7-2.9 nm for the phenyl homopolymer (pPEnBz) and the
copolymers with pyridine (pPEn(Bz-co-Py)) and thiophene (pPEn(Bz-co-Ti)), respectively. These results
suggest a rearrangement in a Y-type bilayer for the pPEnBz on a hydrophilic substrate. For copolymers
pPEn(Bz-co-Py), and pPEn(Bz-co-Ti), deposited on a hydrophobic substrate, the self-assembly within one
layer consists of interdigitated alkyl chains. In solid-state films the polymers and copolymers show a
rather good photoluminescence yield and large Stokes-shifts with emission peaks at 539, 502, 569, and
563 nm for pPEnBz, pPEn(Bz-co-Py), pPEn(Bz-co-Ti), and pPEn(Bz-co-Cz), respectively. The photolumi-
nescence quantum yields are decreasing in the series by 23, 15, 12, and 7% respectively, while no emission
is observed from pPEnPy. A dichroism on LB films has been found in absorption (R ) 2.0, 2.04 and 1.3)
and in emission spectra (R ) 3.4, 4.0 and 2.6) for pPEnBz, pPEn(Bz-co-Ti) and pPEn(Bz-co-Py),
respectively. LED properties are demonstrated using the ITO/polymer/LiF-Al sandwich, yielding photon
emission at 555 and 550 nm for pPEnBz and pPEn(Bz-co-Py). These results are very important for the
future realization by LB deposition technique of more sophisticated devices for polarized emission with
further enhanced brightness and improved light efficiency.
1. In tr od u ction
this respect, the Langmuir-Blodgett (LB) is a promising
technique that provides self-organized systems11 with
good molecular order and molecular alignment, indis-
pensable features to obtain polarized light.12,13
It is now well recognized that the unique electrical
and optical properties of conjugated polymers are of
great importance for modern technology1. Indeed many
applications have been recently reported for conjugated
polymers in waveguides,2,3 fluorescent chemical sen-
sors,4 photoconductors,5 organic light-emitting diodes
(OLEDs),6-8 etc. The basic characteristics, required for
conjugated materials in OLEDs, are semiconducting
properties and high quantum yield of the photolumi-
nescence. Moreover, attracting properties of polymers
such as the tunable emission wavelengths in a wide
range of color and a good solubility, which are required
for a low cost manufacturing, are of great interest. These
properties are the privileged domains of chemists, who
can modify widely the spectral range of emission by
inserting specific electroactive substituents9 as well as
enhance the usually poor solubility of the rigid conju-
gated backbone by grafting flexible chains to the poly-
mer backbone.10 Even when these requirements are
achieved, it is necessary, however, to optimize the
quality of the emitting layer by an appropriate deposi-
tion technique, to control the film morphology, the
carrier mobility and the emission yield of the device. In
We recently reported7,14 on the preparation and
characterization of LB films of a series of oligomers
based on the phenyl-ethynylene unit (oPEn). We
observed high electroluminescent and photoluminescent
properties for the longest molecules with n g 5.7,15 The
heptamer gives rise to well oriented films by LB uniaxial
compression or mechanical rubbing and polarized light
in an emitting diode was successfully obtained.15 Fol-
lowing our first investigations on these series of phenyl-
ethynylene materials, we report here on the macro-
molecular and the electrooptical properties of polymers
and aryl copolymers of benzoate ethynylene. The alter-
nating aryl units are pyridine or carbazole as electron
acceptors; or thiophene as electron donor, which are able
to tailor the emission properties of the active layer. The
present work concerns the synthesis, the polymer
characterization, and the spectroscopic properties of
homopolymers (benzoate or pyridine) and copolymers
(benzoate-co-pyridine, benzoate-co-carbazole, and ben-
zoate-co-thiophene) and their deposition in LB films. In
addition, preliminary data concerning the intrinsic
electroluminescent properties of spin-coated films of
pPEnBz and the copolymer pPEn(Bz-co-Py) are here
presented. The good optical quality obtained with the
LB films, with emissions ranging from the blue-green
† Institut de Physique et Chimie des Mate´riaux de Strasbourg,
UMR 7504.
‡ CEA Saclay.
§ Present address CIQA, Blvd. Enrique Reyna 140, 25100
Saltillo, Me´xico.
10.1021/ma020121e CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/24/2003

Macromolecules, Vol. 36, No. 10, 2003
Phenyl-Ethynylene Polymers and Copolymers 3571
Sch em e 1. P d /Cu Cr oss Cou p lin g Rea ction for th e Syn th esis of th e Hom o- a n d Cop olym er s Stu d ied in Th is
Wor k a
a
Ar can be (11-undecanol) 2,5-dibromobenzoate, 2,5-dibromopyridine, 3,6-dibromocarbazole, or 2,5-dibromothiophene. The lateral
chains (11-undecanol) substituted on the benzoate rings improve the solubility and the amphiphilic character of the molecule.
Ta ble 1. (a ) SEC Resu lts of th e Oligom er s oP En Used To
to the yellow region, and the good molecular orientation
Bu ild a Ca libr a tion Cu r ve fr om Th eir Actu a l Molecu la r
achieved could be promising features to obtain polarized
light in LED devices.
Weigh t a n d Th eir Ca lcu la ted Elu tion a n d Volu m e (Ve) in
THF a n d (b) Com p a r a tive Molecu la r Weigh ts of Hom o
a n d Cop olym er s Obta in ed by Both th e P olystyr en e
Ca libr a tion Cu r ve (Mw (P Seq)) a n d Th a t Obta in ed w ith th e
2. Resu lts a n d Discu ssion
oP En Oligom er s (Mw (oP En eq)
)
2.1. P olym er Syn th esis a n d Ch a r a cter iza tion .
The series of homopolymers and copolymers have been
synthesized according to the chemical route given in the
literature.16 Scheme 1 shows the chemical formula of
such poly(aryl-ethynylene) homopolymers and copoly-
mers studied in this work. The benzoate, pyridine,
benzoate-pyridine, benzoate-thiophene and benzoate-
carbazole derivatives are hereafter named pPEnBz,
pPEnPy, pPEn(Bz-co-Py), pPEn(Bz-co-Ti), and pPEn(Bz-
co-Cz), respectively. Some specific synthesis details are
reported in the Experimental Part. The long side chain
(11-undecanol) grafted on the benzoate moiety improves
the solubility and introduces an amphiphilic character
to the materials through the hydroxyl groups. For all
these materials, Tables 1a and b give the determination
by SEC, the average molecular weight (Mw(PSeq)), relative
to the polystyrene (PS) or relative to the oligomer
calibration (Mh w(oPEneq)), the weight-average degree of
a. SEC Results of the Oligomers
Ve
Lhg
oligomers
Mw
(mL)
Mh w(PSeq)
Mw/Mn
(nm)
oPE1
oPE2
oPE3
oPE5
oPE7
340
607
921
1550
2179
18.22
17.82
17.23
16.58
16.15
1686
2246
3427
5460
7432
1.09
1.02
1.02
1.05
1.09
1.2
1.9
3.3
4.7
b. Comparative Molecular Weights of Homo and Copolymers
Ve
Lhg
polymers
pPEnBz
pPEnPy
pPEn(Bz-co-Py) 16.73 14 506
(mL) Mh w(PSeq) Mw/Mn Mh w(oPEneq) DPw (nm)
15.53 20 389
17.10 3762
1.71
1.21
2.96
2.17
1.42
3880
1014
1392
2902
1277
13
10
4
7
3
8.6
6.5
5.1
8.9
3.0
pPEn(Bz-co-Ti) 15.87
pPEn(Bz-co-Cz) 16.83
9083
4565
a
The Lhg represents the mean chain length from the calculated
DPw.
polymerization DPw, the polydispersity index (Mw/Mn)
and the mean chain length (Lhg), respectively. Figure 1
shows the PS calibration and the specific calibration
obtained with the homologous oligomers oPEnBz having
estimation is more evident at low molecular weights,
while according to the (Mw(oPEneq)) calibration curve, the
tendency for higher Mw seems to have a better correla-
tion with those obtained with the PS calibration curve.
The homopolymer pPEnBz, bearing the (11-undecanol)
benzoate group per phenyl unit shows the highest
Mh w(oPEneq) ≈ 3880 Da, while the unsubstituted pyridine
derivative, pPEnPy, is only 1014 Da. Similar results
were already reported for oligo(aryl-ethynylene)s in the
pioneer work of Sanechika et al.20 A drastic increase of
the molecular weight Mw can be observed in the coupling
reaction by using a cosolvent to triethylamine, i.e., THF
(5% v/v). In these conditions Mh w(oPEneq) rises to 7887;
1, 2, 3, 5, and 7 repeat units.7,17 DPw and Lhg are
calculated from Mh w(oPEneq). Indeed several authors such
as Giesa18 and Tour19 have already highlighted that the
molecular weights calculated from PS standards are not
reliable for rigid rod polymers. Size exclusion chroma-
tography (SEC) affords the measurements of the hy-
drodynamic volumes of macromolecules, which are
greatly different for a rigid rod chain and a random coil
of a PS chain. From the present experimental data, it
can be seen how the molecular weight of the rigid rod
aryl-ethynylene polymers can be strongly overesti-
mated and directly affected by the number of alkyl
chains substituted on the aromatic rings. This over-
DPw, 25 while Mh w(PSeq) ) 33 600. However, the polym-
erization yield is much lower (75%); this decrease is due

3572 Arias-Marin et al.
Macromolecules, Vol. 36, No. 10, 2003
F igu r e 1. SEC calibration curves, line with black dots (b);
PS calibration curve (Mw(PSeq)), line with open squares (0).
Specific calibration curve from rigid benzoate ethynylene
oligomers having 1, 2, 3, 5, and 7 repeat units (Mw(oPEneq)). THF
as elution solvent at the rate of 1 mL/min.
F igu r e 3. Temperature dependence of X-ray diffraction
patterns for (a) pPEnBz and (b) pPEn(Bz-co-Py) powders in
the second heating. The relative X-ray intensities were nor-
malized at 2θ ∼ 20°.
F igu r e 2. DSC of pPEnBz and pPEn(Bz-co-Py). The second
heating and cooling cycles were performed at scanning rates
of 5 °C min-1 under nitrogen.
to the fusion of the alkyl or aromatic parts for both
polymers, since the π-π interaction between the aryls
is very strong, in particular for the copolymer that forms
hydrogen bonds between the nitrogen and the alcohol
group.
to an insoluble polymer fraction, which is formed during
or after purification mainly by an aggregation phenom-
enon.21
The X-ray diffraction of pPEnBz (Figure 3a) at low
temperature exhibits two sharp reflections in the low-
angle region at 2θ ) 2.52°, corresponding to reciprocal
spacings of 3.5 ( 0.2 nm in the ratio 1:2, which are
indicative of a lamellar system. In the wide-angle region,
two broad bands (at 2θ ) 20.3 and 24.3°), corresponding
to spacings of 0.45 and 0.35 nm, are respectively
attributed to the average spacing of the alkyl chains and
to van der Waals interactions between phenyl rings.
They are related respectively to a liquidlike order of the
side chains within the lamellae and to a weak phenyl
π-π interaction between rigid cores. On heating, the
peaks at small angles vanish, corresponding to the
disappearance of the lamellar structure. However, sharp
X-ray diffraction peaks superpose on these signals and
indicate the presence of microcrystals. Therefore,
pPEnBz appears to exhibit the coexistence of two phases
consisted of a disordered LC phase and of microcrystals.
In the LC phase, the layer spacing being twice the
length of a repeat unit in an extended conformation, the
molecules are arranged in a double layer with a comb
like conformation (Figure 4a). Such a two-phase system
may explain the very broad endothermic transition
observed in DSC (see above).
2.2. Molecu la r Str u ctu r e a n d Th er m a l P r op er -
ties. By thermogravimetric analysis, the homo- and
copolymers show a good stability in nitrogen or in air
up to 180 °C, where the mass loss is less than 2%. The
thermal properties are observed first by optical micros-
copy with polarized light. At room temperature, pPEnBz
or pPEn(Bz-co-Py) appears as a highly viscous and
strongly birefringent paste. On heating the polymers
become fluid at around 100 °C, and they remain bire-
fringent until the beginning of decomposition around
180-200 °C. The birefringent textures of the two
homopolymers are not typical of mesomorphic textures
reported in the literature, making difficult the deter-
mination of the nature of the mesophases. As for
copolymers, neither pPEn(Bz-co-Ti) nor pPEn(Bz-co-Cz)
are birefringent at room temperature; they become fluid
at 110 and 145 °C, respectively.
The thermal and structural behaviors of the polymers
have been also studied by DSC and X-ray diffraction
on powders. Figure 2 shows the thermograms for the
second heating and cooling of pPEnBz and pPEn(Bz-
co-Py). For pPEnBz, a broad endothermic transition is
observed between 37 °C and 128 °C (∆H ) 14.8 J /g).
On cooling, the thermogram is characterized also by a
broad exothermic transition between 129 and 14 °C (∆H
) -15.9 J /g) with a peak at 98 °C. On heating, pPEn(Bz-
co-Py) shows a large exothermic transition ranging from
30 to 140 °C (∆H ) 27.2 J /g) with a main peak at 109
°C and a shoulder at 124 °C. On cooling, two exothermic
peaks are observed at 104 and 80 °C (total ∆H ) -28.4
J /g). Indeed it is difficult to assign the enthalpic peaks
The X-ray diffraction pattern of pPEn(Bz-co-Py) in
Figure 3b, shows at small angles, a sharp reflection at
2θ ) 3.25 corresponding to a lamellar system with
spacing of 2.7 ( 0.2 nm. In the wide-angle region, two
broad bands (at 20.16 and 25.30°) are observed, which
were assigned to the alkyl spacers (here 0.45 nm) and
to the aromatic interactions (0.36 nm), indicating a
liquidlike order within the layers. All the patterns

Macromolecules, Vol. 36, No. 10, 2003
Phenyl-Ethynylene Polymers and Copolymers 3573
F igu r e 5. Pressure-area isotherms of the homo- and copoly-
mers in Langmuir films at 30 °C and the related BAM
pictures: (a, b) for pPEnBz; (c, d) for pPEn(Bz-co-Ti); (e, f) for
pPEn(Bz-co-Py). The pictures correspond to the surface pres-
sure-molecular area/repeated unit: (a) ∼ 0 mN/m at 0.35 nm2/
repeated unit; (b) 20 mN/m, at 0.20 nm2/repeated unit; (c) ∼ 0
mN/m at 0.85 nm2/repeated unit; (d) 17 mN/m, at 0.21 nm2/
repeated unit; (e) 17 mN/m, at 0.21 nm2/repeated unit; (f) 35
mN/m at the collapse pressure, respectively.
can be understood on the basis of the possible conforma-
tions of the main chain. According to the CPK models,
the bond angles on the thiophene or carbazole unit with
the ethynylene groups are 125 and 122°, respectively,
giving rise to a statistic disorder.
F igu r e 4. Molecular models deduced from X-ray diffraction
powder: (a) for pPEnBz, a three-dimensional view, a side view,
and a top view respectively of the smectic-A mesophase
arrangement; (b) for pPEn(Bz-co-Py), a view showing the
hydrogen bond interactions between the lateral alcohol group
and the pyridine moiety.
2.3. La n gm u ir F ilm s Ch a r a cter iza tion . All the
polymers are amphiphilic enough to form stable Lang-
muir and Langmuir-Blodgett (LB) films. Figure 5
shows the pressure-area isotherm corresponding for
pPEnBz (continuous line), pPEn(Bz-co-Ti) (dotted line),
and pPEn(Bz-co-Py) (dashed line). The pPEnBz shows
a liquid expanded behavior already observed on the
oPE5 and oPE7 oligomers7,11 with the same typical high
compressibility at low pressures. At a molecular area
close to 0.27 nm2/repeated unit, the pressure levels up
and the compressibility decreases. The isotherm is
perfectly reversible after several compression-decom-
pression cycles and no hysteresis is observed for surface
pressures lower than 20 mN/m and temperatures higher
than 30 °C. Figure 5, parts a and b, shows the Brewster
angle microscopic (BAM) pictures at successive com-
pression steps of the isotherm. The deposition of pPEn-
registered as a function of temperature are character-
istic of a single LC phase. The 2.7 nm spacing suggests
a double layer structure with interdigitation of the side
chains. This period is consistent with the model depicted
in Figure 4b. Moreover, the interdigitation allows the
possible formation of hydrogen bonds between the OH
group of the side chain and the nitrogen of the pyridine.
For pPEn(Bz-co-Ti) and pPEn(Bz-co-Cz), the diffracto-
grams are significantly different, as they reveal no
periodic structure at low angle, while in the wide-angle
region a diffuse broad band is observed. The diffracto-
grams confirm the quasi-amorphous nature of such
copolymers as observed by thermal and optical analysis.
The lack of order of thiophene and carbazole copolymers

3574 Arias-Marin et al.
Macromolecules, Vol. 36, No. 10, 2003
F igu r e 6. AFM images of a double LB layers on Si wafers: (a) pPEn(Bz-co-Py), the section analysis showing a single step defect
of 2.8 ( 0.1 nm; (b) pPEnBz, the image taken near one of the Si borders showing a step defect of 3.5 ( 0.02 nm. The scan was in
tapping mode with a scanning rate of 0.39 Hz.
Bz solution has led to the coexistence of two bidimen-
sional phases; isolated solid blocks randomly distributed
in a gaseous phase. The coalescence of these blocks
during compression (π between 15 and 20 mN/m)
(Figure 5a) yields the disappearance of defects and the
formation of a homogeneous film (Figure 5b). At this
point, the specific molecular area, extrapolated at zero
pressure, is A0 ) 0.20 ( 0.02 nm2/repeated unit, the
same value than that was found for oPE7 (A0 = 0.21 (
0.02 nm2/repeated unit). An irreversible domain begins
for a pressure higher than 20 mN/m, where the BAM
shows an increase of defects, the molecular layer becom-
ing more and more heterogeneous up to the collapse at
pressure near 30 mN/m. During the decompression step,
the film cracks forming solid blocks that will reassemble
in the next compression cycle into a new homogeneous
film.
For pPEn(Bz-co-Ti), the isotherm at 30 °C shows the
same behavior than for pPEnBz. At high molecular area,
in the range 0.85-0.30 nm2/repeated unit, isolated solid
blocks are distributed in a homogeneous gas phase
(Figure 5c). For pPEn(Bz-co-Py), the isotherm shows a
gas-liquid-phase transition, which occurs at 0.51 nm2/
repeated unit and then, in the range 0.51-0.23 nm2/
repeated unit, a heterogeneous liquid expanded phase.
The films of pPEn(Bz-co-Ti) and pPEn(Bz-co-Py) become
homogeneous around 17 mN/m without any defect as
observed by BAM (Figure 5, parts d and e). The
molecular areas, extrapolated from the isotherms at zero
pressure, are A0 ) 0.23 ( 0.07 and 0.27 ( 0.07 nm2/
repeated unit for the Ti and Py copolymers, respectively.
At lower areas, for the two copolymers the compress-
ibility decreases and the pressure rises until the collapse
is reached at 40 and 43 mN/m for the Ti and Py
copolymers, respectively. Figure 5f shows a typical BAM
image for the collapsed (Bz-co-Py) film. On the contrary,
for pPEn(Bz-co-Cz), the Langmuir film is always char-
acterized by the presence of solid blocks randomly
distributed that never coalesce to form a homogeneous
film.
2.4. Ch a r a ct er iza t ion of La n gm u ir -Blod get t
F ilm s. For pPEnBz and copolymers, a good transfer of
the Langmuir films onto substrates is possible at
temperatures close to 60 °C. At lower temperatures, only
fragments of films could be transferred, probably be-
cause of the too rigid properties of the monolayers. The
transfer of the pPEnBz LB film is studied on hydrophilic
and hydrophobic substrates. Poor transfer ratios (TR <
0.3) are obtained on hydrophobic substrates, while on
hydrophilic substrates, the transfer ratio reaches 0.98
( 0.04. The transfers are only possible by lifting, which
means that the multilayer presents an X-type molecular
arrangement. Moreover drying at air for 2 or 3 h is
required after every upstroke in order to get a well-
structured multilayer film.
The transfer and deposition of pPEn(Bz-co-Ti) and
pPEn(Bz-co-Py) LB films onto hydrophobic substrates
were performed only by immersion-lifting cycle, favoring
a Z-type molecular assembly. A good transfer ratio of
1.2 ( 0.3 is obtained for the first deposition on hydro-
phobic surfaces but it decreases slightly for the succes-
sive immersions. On the contrary, the transfers on
hydrophilic substrates are only possible for the first
layer with a TR of 1.3 ( 0.3. For the second and
following transfers the TR decreases down to 0.5 ( 0.1,
thus no stable multilayers of copolymers could be
obtained on hydrophilic substrates. Figure 6a shows the
AFM image of a transferred layer of pPEn(Bz-co-Py) on
a wafer of hydrophilic silicon and its Z profile along the
cross-section. The average roughness (Rq) is 0.35 nm for
a surface of 6.25 µm2 and an average height of the step
of 2.8 ( 0.1 nm is measured. This value matches well
with the thickness of a double layer according to the
model proposed in Figure 4b. Similar thickness was
found for a transferred layer of pPEn(Bz-co-Ti) on
silicon, where the average height of the step is 2.9 (

Macromolecules, Vol. 36, No. 10, 2003
Phenyl-Ethynylene Polymers and Copolymers 3575
F igu r e 8. Optical absorption spectra in CHCl3 solution
corresponding to (a) pPEn(Bz-co-Ti), (b) pPEnBz, (c) pPEn(Bz-
co-Py), and (d) pPEnPy (in L g-1 cm-1). The insert shows the
normalized photoluminescence emission spectra in LB films
of (a) pPEn(Bz-co-Ti), (b) pPEnBz, and (c) pPEn(Bz-co-Py).
Figure 7b reports the experimental and the simulated
diffraction pattern of pPEn(Bz-co-Py) for five transferred
layers with a total thickness of 12.9 nm. For this
copolymer, Kiessig fringes and Bragg peak are observed;
they indicate a good periodic structure of the multilayer.
The Bragg peak centered at 2θ ) 3.23° corresponds to
a periodicity of 2.7 nm for the multilayer, while the
thickness of a double layer is 2.8 nm. Furthermore, the
2.8 nm height found by AFM confirms that the molec-
ular arrangement in the film is the same as in the
mesomorphic phase characterized by X-ray diffraction,
corresponding to the interdigitated alkyl chain self-
assembly already described in Figure 4b. A similar
result was observed for pPEn(Bz-co-Ti), where a thick-
ness of 2.9 ( 0.3 nm corresponds also to a double and
interdigitated layer. All the structural parameters of the
different films are collected in Table 2. We attribute the
good stability of the polymer layers on hydrophobic
substrates to the presence of hydrogen bonds and also
the π-π interactions between the phenyl rings.
2.5. P h otolu m in escen ce a n d Electr olu m in escen t
P r op er ties. Figure 8 shows the spectroscopic absorp-
tion of the polymer solutions of pPEnBz, pPEnPy,
pPEn(Bz-co-Py), pPEn(Bz-co-Ti), and pPEn(Bz-co-Cz)
respectively, together with the PL emission spectra of
their corresponding LB films (in insert). The optical
properties are reported in Table 3. As expected from the
electron charge transfer between the phenyl and
thiophene as electron donor unit in pPEn(Bz-co-Ti), the
excitonic peak is red-shifted from 404 to 412 nm with
respect to the homopolymer pPEnBz. For pyridine as
the electron-acceptor unit in the copolymer pPEn(Bz-
co-Py), the excitonic peak is blue shifted from 404 to
390 nm. This effect is also observed in the energy shift
of the emission spectra. While the photoluminescence
of pPEnBz reaches a maximum of emission at 539 nm,
the pPEn(Bz-co-Ti) copolymer is red shifted to 569 nm,
and for the pPEn(Bz-co-Py) copolymer, the emission is
blue shifted to 502 nm. The pPEnPy does not show any
emission. The same behavior is also observed in the
Raman spectra, where the -CtC- stretching vibration
energy centered at 2186, 2198, and 2206 cm-1 for
pPEn(Bz-co-Ti), pPEnBz, and pPEn(Bz-co-Py), respec-
tively, moves to lower frequencies as a result of the
increase in the π-electron delocalization of the conju-
gated backbone. From these results, it is clear that the
electronic nature of the aryl group affects the conjuga-
tion and induces the photon modulation of these materi-
als from the blue-green to the yellow region.
F igu r e 7. Grazing incidence X-ray reflectivity plots for (a)
one (1) and six (2) double LB layers of pPEnBz deposited on
silicon wafer and ITO, respectively, and (b) five double and
interdigitated LB layers of pPEn(Bz-co-Py) on ITO. The
scattered points show the experimental data, and the continu-
ous line shows the simulated reflectivity patterns obtained by
GIXA.
0.02 nm, suggesting also a possible interdigitation of the
alkyl chains giving place to double layer formations. The
transferred layer surface of pPEnBz on hydrophilic
silicon is also quite flat, as observed in Figure 6b. The
calculated roughness Rq is 0.8 and 0.32 nm for surfaces
of 25 and 9.7 µm2 respectively. A line scan on a defect
near the border shows an average height of a step near
3.50 ( 0.02 nm, corresponding to a value that is
consistent with a double layer thickness.
The grazing incidence X-ray scattering spectra cor-
responding to a double layer and a multilayer of pPEnBz
are reported in Figure 7a. The plot shows the X-ray
intensity vs the scattering angle for one and six trans-
ferred layers respectively and the related GIXA simu-
lated reflection patterns, which are represented as
continuous lines. The Kiessig fringes as well as the
Bragg peak are clearly visible in the experimental
curves, which indicates that the transferred films are
well structured. The Bragg peak at 2θ ) 2.3° on the
multilayer (2) indicates a well-defined internal periodic
electron density corresponding to a layered organization
with a spacing of 3.3 nm, in agreement with the lamellar
spacing determined already by X-ray diffraction on
pPEnBz powder (3.5 ( 0.02 nm). These results are likely
consistent with the double layered organization on
silicon (3.60 ( 0.05 nm) and the thickness determined
on curve 1 by grazing incidence X-ray scattering (3.6 (
0.5 nm) and by AFM (3.50 ( 0.02 nm). Nevertheless,
these values do not match well with the periodicity
calculated either from the Bragg peak in a multilayer
film on ITO (3.3 ( 0.1 nm) or with a single layer model
of molecules, where the side chains are in an extended
conformation (2.0 nm). Hence, this result suggests a
structural self-rearrangement in double layer, in a
Y-type assembly with a slight tilt of the side chains
within the LB film as found for the oPE7 oligomer.
To prepare electroluminescent diodes, the photo-
luminescence quantum yield (ΦPL) is measured as a

3576 Arias-Marin et al.
Macromolecules, Vol. 36, No. 10, 2003
Ta ble 2. Str u ctu r a l P a r a m eter s of th e p P En Bz Hom op olym er a n d th e p P En (Bz-co-P y) a n d p P En (Bz-co-Ti) Rela ted
Cop olym er s in LB Mu ltila yer s Deter m in ed by X-r a y Reflectom etr y (GIXA)
LB
transfer
Bragg
peak (nm)
calcd thickness
from GIXA (nm)
av roughness
(nm)
sample
pPEnBz/Si (Figure 7a)
pPEnBz/ITO (Figure 7a)
pPEn(Bz-co-Py)/Si
pPEn(Bz-co-Py)/Si (Figure 7b)
pPEn(Bz-co-Ti)/Si
1
6
1
5
1
3.6 ( 0.50
16.3 ( 0.1
2.8 ( 0.2
12.9
0.33 ( 0.04
0.65 ( 0.3
0.34 ( 0.02
0.63 ( 0.01
0.35 ( 0.04
3.3 ( 0.1
2.7
2.9 ( 0.3
Ta ble 3. Sp ectr oscop ic Da ta of th e Hom op olym er s a n d Cop olym er s in CHCl3 a n d in Th in F ilm s, Wh er e th e Excita tion
Wa velen gth Is a t th e Ma xim u m of th e Absor p tion Ba n d s
λ
max(abs, CHCl3)
(nm)
ꢀ (L g-1
λ
max(abs, film)
(nm)
λ
max(PL, film)
(nm)
ΦPL
λ
max(EL)
(nm)
EL material
cm-1
)
(%)a
pPEnBza
pPPEnPy
380
327
371
405
378
374
364
46.9
30.4
56.5
66.53
45.2
56.5
50.3
404
362
390
412
404
382
374
539
no
23
555
550
pPEn(Bz-co-Py)
pPEn(Bz-co-Ti)
pPEn(Bz-co-Cz)
oPE7a
502
569
563
516
504
15
12
7
19
24
527
oPE5a
a
Films deposited by spin coating; the total thickness is in the range of 250-300 nm.
Ta ble 4. F lu or escen ce Qu a n tu m Yield s (ΦP L, %) a n d Absor p tion Coefficien t (A) of P h en yl-Eth yn ylen e Oligom er s a n d
P olym er s Mea su r ed by th e In tegr a tion Sp h er e Meth od , on th e LB a n d Sp in -Coa ted F ilm s vs th e Th ick n essa
a. LB Films
oPE7
1 LB layer
4 LB layers
8 LB layers
spin-coated film
thickness (nm)
ΦPL (%)
A
3.7
9.0
0.08
14.8
16.0
0.23
29.6
20.0
0.37
300.0
19.0
0.85
b. Spin-Coated Films
pPEnBz
Bz-co-Ti
Bz-co-Ti
Bz-co-Py
Bz-co-Py
Bz-co-Cz
polymer
coated film
2 LB layers
coated film
2 LB layers
coated film
coated film
ΦPL (%)b
A
23.0
0.77
3.0
0.33
12.0
0.65
11.0
0.16
15.0
0.74
7.0
0.09
a
b
The excitation wavelength for the fluorescence quantum yields is at 411 nm. The thickness of all spin-coated films is in the range
250-300 nm
function of the film thickness for the polymers including
different heteroatoms. Table 4, parts a and b, reports
the results of ΦPL and the absorption coefficient (A) for
the different oligomers and polymers. As a comparative
study, different numbers of oPE7 LB layers showed that
the PL yield is directly affected by the film thickness.
A maximum ΦPL is given for films having eight double
layers with a total thickness of 29.6 nm. For higher
thickness a clear decrease of yielding is observed; i.e.,
for a spin-coated film of 300 nm, the ΦPL is 19%. This
is consistent with the increase of the absorption coef-
ficient (A) with thickness. Similar results are found by
Paloheimo et al.,22 who found the highest ΦPL for films
having nine LB layers of thiophene oligomers. Thus,
work is now in progress on LED’s by taking into account
the LB layers number. Another factor that affects the
ΦPL is the heteroatoms such as O, S, and N. While for
pPEnBz films ΦPL is about 23%, the copolymers in the
series Py, Ti, and Cz show markedly lower photolumi-
nescence yields downward to 15, 12 and 7% respectively.
To check if a well-ordered arrangement of the conju-
gated chains has been induced through the LB process,
the anisotropic optical properties have been investi-
gated. A rather good dichroism has been found in
absorption (R ) 2.0, 2.04 and 1.3) and in emission (R )
3.4, 4.0 and 2.6) spectra for pPEnBz, pPEn(Bz-co-Ti),
and pPEn(Bz-co-Py), respectively. These properties
highlight the emission dichroic ratio, which always
presents higher values than in absorption. This phe-
nomenon, which has been already observed by Neher23
and Chen,24 was ascribed to the possible energy transfer
that can take place after excitation in the domains of
well-aligned molecules. However, it has not been well
elucidated yet.
Recently we reported that oPE7 oligomer containing
the (11-undecanol) benzoate moiety is an electrolumi-
nescent material operating at low voltages (4.5-6.0 V)
in LED devices. Moreover, a rubbing process of the films
allowed to obtain polarized electroluminescence.12 In the
present work, we investigate the inherent electrolumi-
nescent properties of the corresponding series of homo-
and copolymers. Hence, two devices were prepared by
spin coating pPEnBz and pPEn(Bz-co-Py) to obtain 550
and 300 nm thick films, respectively. We did not try to
optimize the diode performances with regard to the film
thickness, the film morphology, or the charge injection
process. Under voltage bias in the forward direction, the
two diodes of pPEnBz and pPEn(Bz-co-Py) show a
yellow and blue-green emission at low threshold voltages
of 3.8 and 4.2 V, respectively. The luminescence-voltage
and the current density-voltage characteristics are
displayed in Figure 9. The two plots show a typical diode
behavior where the current follows well the lumines-
cence due to the orientation of internal dipoles as
suggested by Zou et al.25 With increasing forward
voltage, both the current density and the emitting light
intensity increase rapidly after 3.8 and 4.2 V. However,
a marked difference on EL intensity saturation is seen
between the two diodes. For pPEnBz the LED reaches
a maximum irradiance of about 3.2 µW cm-2 at typical

Macromolecules, Vol. 36, No. 10, 2003
Phenyl-Ethynylene Polymers and Copolymers 3577
sophisticated devices with further enhanced brightness,
polarized emission, and improved light efficiency.
4. Exp er im en ta l Section
4.1. Ma ter ia ls. The following chemical reactives, palladium
chloride (PdCl2), triphenylphosphine (TPP), copper(I) iodine
(CuI), trimethylsilylacetylene (TMSA), 2,5-dibromobenzoic
acid, 11-bromo-1-undecanol, 1,5-diazabicyclo[4.3.0]non-5-ene
(DBN), tetrabutylammonium fluoride in THF (TBAF) 2,5-
dibromopyridine, 3,6-dibromocarbazole, and 2,5-dibromo-
thiophene, were obtained from Aldrich and Lancaster. CH2-
Cl2, CHCl3, C6H6, methanol and THF were purchased from
Prolabo. Dry triethylamine was obtained by vacuum transfer
from KOH. All the chemical materials were used without
further purification.
F igu r e 9. Luminance-voltage plot of the (ITO/Film/LiF-Al)
LEDs of pPEn(Bz-co-Py) (circles) and pPEnBz (triangles). The
insert figure shows the current density-voltage behavior for
the two related diodes.
4.2. Syn th esis of Hom o- a n d Cop olym er s. The monomer
diethynyl benzoate was synthesized by using the following
pathway:
F igu r e 10. Normalized absorption, photoluminescence (PL),
and electroluminescence (EL) spectra of a spin-coated film of
pPEnBz. The emission intensity of the LED is measured in a
(ITO/film/LiF-Al) device.
Syn th esis of (11-Un d eca n ol) 2,5-Dibr om oben zoa te (2).
A round-bottom flask was charged with 2,5-dibromobenzoic
acid (12 mmol, 3.360 g), 11-bromo-1-undecanol (12 mmol, 3.012
g) and benzene (40 mL). The mixture was heated at 85 °C and
stirred until total product dissolution. Later, DBN (12 mmol,
1.48 mL) was added. The mixture was refluxed overnight, and
after cooling, water (100 mL) was added. The mixture was
transferred to a separated funnel and the organic layer
extracted with benzene and dried with MgSO4. After filtering
off the drying agent, the solvent was removed under vacuum
and the crude product was chromatographied through a silica
gel column using CH2Cl2/CH3OH, (1 L/4 mL) as eluent to afford
currents of 20 mA cm-2. For a pPEn(Bz-co-Py) device,
the corresponding values are 1.2 µW cm-2 and 2.8 mA
cm-2, respectively. Even if these last emission properties
are too low to justify a direct application of this material,
it points out that pPEn(Bz-co-Py) can be used as an
efficient holes transport layer by the fact that pyridine
is a good electron acceptor. The EL spectrum of pPEnBz
is represented in Figure 10. The maximum emission is
centered at 555 nm, 14 nm red-shifted with respect to
the maximum emission of the PL spectra at 539 nm.
This shift of the emission peak can be attributed to a
localized recombination of the charges close to one of
the electrodes. The calculated corresponding external
quantum efficiency was close to 10-5 % photon per
injected electron.
1
a pale yellow powder (2). Mp: 59-62 °C. H NMR (200 MHz,
CDCl3): δ (ppm) 7.9(d, 1H, -PhH), 7.53 (d, 1H, -PhH), 7.46
(dd, 1H, -PhH), 4.34 (t, 2H, -COOCH2-), 3.65 (t, 2H, -CH2-
OH), 1.78(q, 2H, -CH2-â-COO), 1.59(q, 2H, -CH2-â-OH),
1.30 (m, 14H, -CH2-). Anal. Calcd for C18H26Br2O3: C, 48.01;
H, 5.82; O, 10.66. Found: C, 48.40; H, 5.89; O, 10.31.
Syn th esis of (11-Un d eca n ol) [2,5-Bis((tr im eth ylsilyl)-
eth yn ylen e)]ben zoa te (3). A two-neck round-bottomed flask
containing previously degassed triethylamine (100 mL) was
charged with PdCl2 (0.88 mmol, 0.156 g), CuI (0.44 mmol, 0.083
g), and TPP (3.96 mmol, 1.03 g) under argon. The mixture was
heated at 80-85 °C for 20 min. Then, (2) (8.8 mmol, 4 g) was
added and stirred vigorously for 10 min. Finally, TMSA (22
mmol, 3.1 mL) was added via cannula in one step and stirred
for 16 h. After cooling, the mixture was washed with a
saturated solution of ammonium chloride. The organic phase
was extracted with diethyl ether (2 × 100 mL), washed twice
with distilled water, and dried with MgSO4. After solvent
evaporation, the viscous syrup was passed through a silica gel
column using CH2Cl2/THF, (75/25 v/v) as eluent to obtain a
brown oil (3). 1H NMR (200 MHz, CDCl3): δ (ppm) 7.98 (d,
1H, -PhH), 7.51 (s, 2H, -PhH), 4.32 (t, 2H, -COOCH2-), 3.64
(t, 2H, -CH2OH), 1.78(q, 2H, -CH2-â-COO), 1.59(q, 2H,
-CH2-â-OH), 1.30 (m, 14H, -CH2-), 0.26 (s, 18H, SiCH3).
Anal. Calcd for C28H44O3Si2: C, 69.36; H, 9.19; O, 9.90.
Found: C, 69.12; H, 9.17; O, 10.5.
3. Con clu sion s
In conclusion, we report in the present work, the
synthesis, the structural and optical properties of new
poly(phenyl-ethynylenes), pPEnBz, pPEnPy, and the
related copolymers pPEn(Bz-co-Ti), pPEn(Bz-co-Py), and
pPEn(Bz-co-Cz). All these polymers give films of good
optical quality by spin coating or by LB film deposition
techniques. The different conformations of the polymer
chains induced by the alkyl-substituted phenyls lead to
specific supramolecular assembly of the molecules in the
films. The homopolymer benzoate and the copolymers
have been processed into materials showing a photo-
emission in the range of blue-green to yellow. Such films
can be applied as active layers in electroluminescent
diodes. Preliminary results of LEDs based on pPEn(Bz-
co-Py) suggest also the possibility of using the copolymer
as holes transport material in order to increase the
electroluminescent yields. The polarized photolumines-
cence observed on polymers from oriented LB films is
also very important for the future realization of more
Syn th esis of (11-Un d eca n ol) 2,5-Dieth yn ylben zoa te6
(4). A round-bottom flask charged with (3) (7.2 mmol, 3.51 g)
and wet THF (100 mL) was cooled to -20 °C. Then a 1 M

3578 Arias-Marin et al.
Macromolecules, Vol. 36, No. 10, 2003
solution of TBAF (2 mmol, 2 mL) was added, and the reaction
was stirred for 5 s and then stopped by passing the reaction
mixture through a plug of silica gel. The crude product was
chromatographied using CH2Cl2/ THF (90/10 v/v) as eluent to
obtain a red viscous liquid. 1H NMR (200 MHz, CDCl3): δ
(ppm) 8.05 (s, 1H, -PhH), 7.57 (s, 2H, -PhH), 4.34 (t, 2H,
-COOCH2-), 3.64 (t, 2H, -CH2OH), 3.48 (s, 1H, -CtCH),
3.22 (s, 1H, -HCtC-), 1.76(q, 2H, -CH2-â-COO), 1.58(q, 2H,
-CH2-â-OH), 1.30 (m, 14H, -CH2-). Anal. Calcd for
C22H28O3: C, 77.64; H, 8.23; O, 14.13. Found: C, 77.41; H, 8.34;
O, 14.54.
ments. The monolayers were compressed with a typical speed
of 10.5 and 1.5 mm/min for the pPEnBz, pPEn(Bz-co-Ti) and
pPEn(Bz-co-Py), respectively. Isotherms were reproducible
from run to run and showed no noticeable hysteresis. LB films
of pPEnBz were obtained by transfer only on hydrophilic glass
slides, ITO, and silicon wafers (100) at surface pressures
ranging from 18 to 20 mNm-1. Transfers on substrates started
always from below the surface with a lifting speed of 3 mm/
min. On the other hand, Ti and Py copolymers were only
transferred on hydrophobic substrates and the substrates
themselves started from above the surface with a down speed
of 2 and 1.5 mm/min, respectively.
Hyd r op h ilic Su r fa ces. Prior to transfer, glass substrates
were cleaned using the following procedure: the plates were
immersed in a hot detergent solution (Decon 90, 4% v/v) over
5 h and rinsed 10 times with hot water, or were treated by a
hot sulfochromic solution and rinsed with ultrapure water in
an ultrasonic bath over 20 min, and then dried in nitrogen
flux and heated at 50 °C. Si substrates were treated according
to a modified RCA procedure.31
P olym er iza tion of h om o- a n d cop olym er s was achieved
by a Pd/Cu cross-coupling reaction.26-28
A typical procedure is described as follows: A two-neck
round-bottomed flask containing previously degassed and dried
triethylamine (100 mL) was charged with PdCl2 (0.145 mmol,
26 mg), CuI (0.072 mmol, 14 mg), and TPP (0.432 mmol, 114
mg) under argon. The mixture was heated at 80-85 °C for 20
min. Then, the dibromoaryl was added: (11-undecanol) 2,5-
dibromobenzoate (1.45 mmol, 648 mg) or 2,5-dibromopyridine
or 3,6-dibromocarbazole or 2,5-dibromothiophene and the
mixture stirred vigorously for 10 min. Finally, (11-undecanol)
2,5-diethynylbenzoate (1.45 mmol, 490 mg) was added via a
cannula in one step and stirred overnight. After cooling, the
mixture was filtered off to eliminate the ammonium salt
formed and the organic phase was partially evaporated and
washed with a saturated solution of sodium diethyldithiocar-
bamate.29 The organic phase was extracted with chloroform
and washed twice with distilled water and the solvent re-
moved. The pasty residue was dissolved in 6 mL of chloroform
and poured into 50 mL of cold methanol to precipitate and
then centrifuged. The product was recuperated in 5 mL of
chloroform yielding between 80 and 90%. All the optical
characteristics are collected in Table 3. The homopolymers and
copolymers were purified from residual monomers and shortest
oligomers by preparative size exclusion chromatography. They
were characterized by 1H and 13C NMR as well as by elemental
analysis.30
H yd r op h ob ic Su r fa ces (OTS P r ep a r a t ion , Sa giv32
Meth od ). The glass substrates and the Si wafers were first
treated following the same procedure used to have hydrophilic
surfaces (described above). Then they were rinsed with water
or ethylic alcohol and treated with an octadecyltrichlorosilane
(OTS) solution (2% in heptane) in an ultrasonic bath for 10
min at 60-80 °C. Finally, the substrates were rinsed succes-
sively with chloroform and heated in an oven for 3 h at 120
°C.
F ilm s Ch a r a cter iza tion . The grazing incidence X-ray
studies of LB films were performed on
a X′PERT-MPD
apparatus from Philips, (with nickel â filter, programmable
divergence slit (1/32°), parallel plate collimator, flat Ge mono-
chromator, and a Xe detector). A Cu KR beam at wavelength
of 0.1542 nm was used. All measurements have been recorded
immediately after the LB transfers. Data were analyzed by
using GIXA (V2.1) software from Philips Electronics Instru-
ment.
4.3. In str u m en ts a n d Meth od s. Ma cr om olecu la r Ch a r -
a cter iza tion . SEC has been performed on a Waters chro-
matograph using a refractive index detector and THF as
effluent at 1 mL/min. The thermogravimetric analysis has
been carried out on a Mettler TA 300. Differential scanning
calorimetry was performed on a Perkin-Elmer DSC 7 at a rate
of 5 °C/min for both the heating and cooling cycles. The X-ray
diffraction study has been realized on a Debye-Scherrer-type
chamber with 900 nm of circumference and equipped with a
Mettler FP52 heating stage. The rotating anode and generator
were from Marconi-Avionics.
The AFM measurements were carried out with a Dimension
3100 from Digital Instruments operating at ambient atmo-
sphere. The images were recorded at room temperature using
the tapping mode at resonance frequency of 290-420 kHz.
From the AFM recorded images, the roughness of the surface
topography was characterized applying the standard deviation
of the height values, denoted Rq, and obtained as follows: Rq
) [(1/N)∑i(Zi - Zm)2]1/2, for i from 1 to N, where Zm is the
average of the Z values within the given image, Zi is the
current Z value and N is the number of points of the image.
P h otolu m in escen ce Qu a n tu m Yield s. ΦPL was measured
in solid films using an integrating sphere (TRC-060-SL from
Labsphere) and a laser excitation at 411 nm (PPm04 from
Power Technology). The integrating sphere is a hollow sphere,
coated inside with a diffusely reflecting material. The flux
received at an aperture in the sphere (the exit port) is
proportional to the total amount of light within the sphere,
irrespective of its angular distribution. The samples were
measured under pure nitrogen atmosphere, and ΦPL was
calculated by taking into account the incident light absorbed
directly by the polymer film and after internal reflection.33
Electr olu m in escen t Devices. Electroluminescent diodes
were constructed with spin-coated films of pPEn(Bz-co-Py)
from chloroform solutions and sandwiched between two elec-
trodes (ITO/Film/LiF-Al).34 The film deposited on top of ITO
(15 Ω/cm2) was previously dried for 12 h in a 10-7 mb vacuum
chamber. The layer thickness of spin-coated and LB films were
always determined by X-ray reflectivity and checked by a
profilemeter DEKTAK 3. A LiF/Al cathode then covered the
film as an electron injection.35 The thickness of these evapo-
rated layers, controlled by the quartz balance monitor was of
1.5 and 100 nm, respectively. The analyzed area of the EL
devices was 0.33 cm2. Diodes were studied in air, electrolu-
minescence and photoluminescence spectra being recorded
with a broadband J obin-Yvon spectrometer coupled to an
amplified Hamamatsu-CCD multichanel detector and with a
F 4500-Hitachi fluorescence spectrophotometer, respectively.
1H and 13C NMR data were obtained at room temperature
with a Bru¨ker AC-200F (200 MHz) spectrometer using CDCl3
as chemical shift standard.
Sp ectr oscop ic Ch a r a cter iza tion . UV-visible absorption
spectra were measured in CHCl3 and in casting and LB films
using a Hitachi U-3000 spectrophotometer equipped with
polarizers. The chain orientation was evaluated by the dichroic
ratio, R ) A|/A where A| and A are the absorbance in parallel
and normal polarization of light relative to the dipping
direction. The emission spectra have been obtained with a PTI
C60-1717 spectrofluorimeter from Photon Technology Inter-
national. Raman scattering measurements were made with a
1064 nm Nd:YAG laser line and the diffused intensities were
detected on a Bru¨cker FTIR spectrometer IFS 66.
La n gm u ir a n d La n gm u ir )Blod gett F ilm s. Solutions
were prepared using CHCl3 (Analysis Grade, Carlo Erba) at
2-2.5 mg/mL concentrations. Volumes of 10 to 60µL were
spread using a microsyringe. Spreading solutions were left 15
to 20 min to equilibrate before the compression started. Data
were collected with a KSV LB5000 system (KSV Instruments)
using a symmetrical compression Teflon barrier in a clean dust
free environment. The trough temperature was controlled to
(0.1 °C. The ultrapure water (F ) 18.2 MΩ.cm) used for the
subphase was obtained from a Milli-RO3-plus and Milli-Q185
ultrapurification system from Millipore. The Wilhelmy plate
method (platinum) was used for surface pressure measure-

Macromolecules, Vol. 36, No. 10, 2003
Phenyl-Ethynylene Polymers and Copolymers 3579
Emitting light was measured with a calibrated 1 cm2 area
silicon photodiode (Hamamatsu) placed in front of the device
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Ack n ow led gm en t. We wish to acknowledge the
Mexican National Council for Science and Technology
through the program CONACyT/SFERE for Scholarship
112097 to E.A.-M. and the CNRS through the GDR
“Mate´riaux pour l’Optique nonline´aire” (GDR 1181) for
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