LB film structure of poly(2-methoxy,5-(8-methoxy-3,6-dioxa-1-undecoxy)-p-phenylene vinylene) studied by spectroscopy

Article abstract of DOI:10.1016/S1386-1425(02)00392-X

A PPV derivative, poly(2-methoxy,5-(8-methoxy-3,6-dioxa-1-undecoxy)-p-phenylene vinylene), has been synthesized by the Gilch route to study the influence of a long alkyl side chain and a di(ethylene oxide) methyl ether group on the multilayer structure obtained by Langmuir-Blodgett (LB) technique. UV-visible, PL, and FTIR spectra are applied to study the conformation and orientation of the MMDU-PPV molecules in multilayer organization. MMDU-PPV is apt to form a transferable monolayer film, in which the plane of its π system is perpendicular to the air-water interface. The adjacent conjugated main chains of MMDU-PPV in LB films are aligned in parallel fashion and packed with the plane of its π system approximately perpendicular to the layer plane and not organized to compact π-stacking structure for introducing di(ethylene oxide) methyl ether (DEOM) side chains to conjugated main chains. The long alkyl side chains are characterized by all trans-zigzag conformation and average tilt angle of 36±1.5°. The layer-by-layer multilayer of MMDU-PPV obtained by Langmuir-Blodgett technique exhibits some in-plane anisotropy and more pure photoluminescence than that of the dilute MMDU-PPV solution.

Full text of DOI:10.1016/S1386-1425(02)00392-X

Spectrochimica Acta Part A 59 (2003) 1631ꢀ1641
/
www.elsevier.com/locate/saa
LB film structure of poly(2-methoxy,5-(8-methoxy-3,6-dioxa-1-
undecoxy)-p-phenylene vinylene) studied by spectroscopy
Zhongkui Wu, Sanxie Wu, Yingqiu Liang *
Laboratory of Mesoscopic Science Materials and State Key Laboratory of Coordination Chemistry, Nanjuig University, Nanjing 210093,
People’s Republic of China
Received 5 April 2002; received in revised form 19 July 2002; accepted 23 October 2002
Abstract
A PPV derivative, poly(2-methoxy,5-(8-methoxy-3,6-dioxa-1-undecoxy)-p-phenylene vinylene), has been synthesized
by the Gilch route to study the influence of a long alkyl side chain and a di(ethylene oxide) methyl ether group on the
multilayer structure obtained by Langmuirꢀ
study the conformation and orientation of the MMDU-PPV molecules in multilayer organization. MMDU-PPV is apt
to form a transferable monolayer film, in which the plane of its p system is perpendicular to the airꢀwater interface. The
/
Blodgett (LB) technique. UVꢀ
/
visible, PL, and FTIR spectra are applied to
/
adjacent conjugated main chains of MMDU-PPV in LB films are aligned in parallel fashion and packed with the plane
of its p system approximately perpendicular to the layer plane and not organized to compact p-stacking structure for
introducing di(ethylene oxide) methyl ether (DEOM) side chains to conjugated main chains. The long alkyl side chains
are characterized by all trans-zigzag conformation and average tilt angle of 369
/
1.58. The layer-by-layer multilayer of
MMDU-PPV obtained by LangmuirꢀBlodgett technique exhibits some in-plane anisotropy and more pure
/
photoluminescence than that of the dilute MMDU-PPV solution.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: PPV and its derivative; LB film; Layer-by-layer structure; Anisotropy
1. Introduction
stand out for being able to combine a reasonably
good environmental stability to high optoelectro-
nic properties [3], and they are presently developed
as active components for organic electrolumines-
cent (EL) device. At present, active layers in light-
emitting devices (LEDs) are made mostly via
simple spinning cast technique [4], which leads in
principle to randomly oriented polymer chains and
monomerie units. It appears that controlling over
the organization of polymer film in nanoscale
might have a great influence on the optical and
electrical properties of EL device [5]. Recently,
Conjugated polymers have been the subject of
great interest in the recent literatures from both
fundamental point of view and applied science
perspective [1,2]. Among the several types of
conjugated polymers in the present investigations,
poly(p-phenylene vinylene) (PPV) derivatives
* Corresponding author. Tel.: ꢁ
25-331-7761.
E-mail address: yqliang@ndu.edu.cn (Y. Liang).
/86-25-359-2369; fax: ꢁ86-
/
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 3 9 2 - X

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highly anisotropic optical properties have been
obtained in stretched layers [6], as in LEDs with
polarized emission with an intensity ratio up to 3:1
between light emitted parallel and perpendicular to
the stretching direction [7]. Such a film can also be
strongly birefringent with different refractive in-
dexes taken at the preferential directions [8].
Nanoscale control of the organization of poly-
article. Such a PPV derivative is soluble in many
organic solvents and apt to form a stable and
transferable monolayer at the airꢀwater interface.
/
The characteristics of Langmuir films of the
poly(2-methoxy,5-(8-methoxy-3,6-dioxa-1-unde-
coxy)-p-phenylene
formed at the airꢀ
and the results of UVꢀ
vinylene)
water interface is investigated
visible polarized spectro-
(MMDU-PPV)
/
/
mer film can be obtained when the Langmuirꢀ
/
scopic studies and preliminary linear dichroism
infrared analysis for molecular orientation of
polymeric chains deposited via LB transfer on
solid substrates are presented. The peak width at
the half height of the photoluminescence spectra of
MMDU-PPV LB films is smaller than that of its
dilute solution. It is expected that further work on
these compounds could in fact corroborate their
potential as anisotropic thin films for use in
optoelectronic devices.
Blodgett (LB) technique, or self-assembly techni-
ques, is used instead of the spinning cast techni-
que. The LB technique can precisely control the
thickness and order of a film to molecular scale,
and allow the study and modeling of polymeric
devices [9]. Using the LB deposition technique in
molecular processing has represented a powerful
method for organizing rodlike polymers [10].
Previous LB studies of rigid-rod polymers with a.
nonpolar ‘hairy-rod’ structure have established
that the flow induced in transferring monolayer
film to substrate is responsible for the formation of
anisotropic polymer film [11]. Unfortunately, not
all molecules can be processed into single-compo-
nent multilayers by this technique. The appropri-
ate materials must have the proper balance
between hydrophilic and hydrophobic properties
and between rigid and flexible moieties to facilitate
the formation of a stable monolayer phase at the
2. Experimental
2.1. Synthesis
MMDU-PPV and its intermediates were pre-
pared in accordance with the methods proposed in
the literature [15ꢀ17]. All the reaction steps are
/
airꢀwater interface. So far, most LB films of PPVs
/
described in Scheme 1 and the intermediates were
1
characterized by FT-IR and H-NMR (no shown
here)
have been prepared by using heat treatment of LB
films of a precursor polymer [12,13], which is so
called tailoring precursor route of PPV in soluble
form. Upon heat treatment, a large volume frac-
tion of the multilayer is eliminated from the film,
probably causing the disruption of film structure.
Synthesis of regioregular head to tail coupled
amphiphilic PPV derivative is another way to be
suitable to LB technique. However, the reports of
LB films of these PPV’s derivatives are few. The
LB layers of poly(2-methoxy,5-(2?-ethylhexyloxy)-
p-phenylene vinylene) (MEH-PPV), a typical EL
material produced commercially, have been built
up on quartz and ITO substrates, but it seems not
2.1.1. Synthesis of 1-bromo-2-(2-
inetlioxyethoxy)ethane (1)
To a round-bottom flask were added di(ethyle-
neglycol)methylether (12.0 g, 0.10 mol) and dry
THF (30 ml). To this solution was slowly added
phosphorus tribromide (10.0 g, 0,04 mol). The
reaction was then heated to 65 8C for 8h. After
allowing the reaction to cool to ambient tempera-
ture, 30 ml CHC13 and 50 ml water were poured
into the reaction and the layers were separated
followed by extracting of the organic layer with
water. The organic layer was dried with MgSO4
and then concentrated under vacuum to yield the
desired product in 93% (17.0 g) yield as colorless
oil.
a monolayer film at the airꢀwater interface [14].
/
A synthesis of a PPV derivative (see Scheme 1)
carrying long hydrophobic n-hexadecyloxy side
chains and hydrophilic di(ethylene oxide) methyl
ether (DEOM) side chains is reported in this

Z. Wu et al. / Spectrochimica Acta Part A 59 (2003) 1631ꢀ
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Scheme 1. Synthesis of MMDU-PPV.
2.1.2. Synthesis of 4-(n-hexadecyloxy) phenol (2)
To a round-bottom flask were added hydro-
quinone (14.5 g, 0.13 mol), NaOH (4.0 g, 0.10 mol)
and ethanol (100 ml). To this solution was added
1-bromohexadecane (30.5 g, 0.10 mol), and a N2
atmosphere was established. The reaction was then
heated to reflux for 6 h. After allowing the reaction
to cool to ambient temperature, the precipitate
was suction-filtered and washed with hot water
five times., and dried in a vacuum oven at 50 8C
for 24 h. The product was purified by recrystalli-
zation twice from ethanol to give 27.0 g (81%) of 2.
reaction was then heated to reflux for 12 h. After
allowing the reaction to cool to ambient tempera-
ture, the precipitate was suction filtered and
washed quickly with ethyl ether several times,
and dried in a vacuum oven at 50 8C for 24 h, A
white plate crystals were recovered in ethanol,
dissolved in CHCl3, again, recovered in ethanol
and dried in a vacuum oven at 50 8C for 24 h. The
yield of 3 was 76% (6.7 g)
2.1.4. Synthesis of 1,4-bis(bromomethyl)-2-(n-
hexadecyloxy), 5-(8-methoxy-3,6-dioxa-1-
undecoxy) benzene (4)
2.1.3. Synthesis of 1-(n-hexadecyloxy), 4-(8-
methoxy-3,6-dioxa-1-undecoxy) benzene (3)
To a round-bottom flask were added 2 (6.7 g,
0,02 mol), KOH (1,3 g, 0.022 mol), and ethanol
(50 ml). To this solution was added 1 (3.86 g, 0.02
mol), and a N2 atmosphere was established. The
To a round-bottom flask were added 3 (5.2 g,
0,012 mol), paraformaldehyde (2.5 g, 0.08 mol),
acetic acid (20 ml), and 30% HBr in acetic acid (20
ml). A N2 atmosphere was established followed by
heating the reaction firstly to 70 8C for 1.0 h, a
while precipitate appeared in the solution, the

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precipitate was dissolved again with heating to
85 8C, and then keeping reaction at the tempera-
ture of 85 8C for 2.0 h. After allowing the reaction
to cool to ambient temperature, the reaction was
neutralized by Na2CO3 (aq). The precipitate was
suction filtered and washed with water until the
water tested natural, and dried in a vacuum oven
at 50 8C for 24 h. The product was purified by
recrystallization twice from hexane to give 4.73 g
(63%) of 4.
MMDU-PPV with a concentration of 160 mg mlꢂ1
onto pure water without the adding of acid (pH
Â
/
6.0). After this, 15 min were allowed for solvent
evaporation and then the monolayer was com-
pressed at a typical rate of 20 mm minꢂ1
Isotherms of surface pressure versus mean mole-
cular area/repeating unit are measured at 129
.
/
0.2 8C. The molecular weight/repeating unit of
460.0 g molꢂ1 was adopted for all calculations.
2.3. Fabrication of LangmuirꢀBlodgett films
/
2.1.5. Polymerization of 1,4-bis(bromomethyl)-2-
(n-hexadecyloxy),5-(8-methoxy-3,6-dioxa-1-
undecoxy) benzene (MMDU-PPV)
LB films were fabricated onto both CaF2
crystals and quartz plates by vertical dipping
method at a surface pressure of 20 mN mꢂ1 and
a dipping speed of 2.0 mm minꢂ1. The substrate
surface was set to be parallel or perpendicular to
the moving direction of the barrier. Prior to
deposition, the CaF2 crystals were washed with
hot CHC13, for 24 h in a Soxhlet apparatus and
the quartz plates were treated with saturated
K2Cr2O7/concentrated H2SO4 solution for 6 h
and then carefully washed with ultrapure water.
In course of deposition, after each cycle of the
upward and downward stroke, the substrate was
kept dry completely in air for at least 15 min, so
that the following monolayer could be favorably
transferred. The transfer ratio for Z-type of films
To a round-bottom flask, well flushed with N2
and equipped with a rheostat as the mechanical
stirrer, was added a solution of potassium tert-
butoxide in THF (20 ml, 1.0 M), 4-methoxyꢀ
/
phenol (1.0 mg), and dry THF (20 ml). The
solution of 4 (2.5 g, 4.0 mmol) in dry THF (60
ml) was then added at a rate of 15 ml hꢂ1 with
syringe. After complete addition of the monomer,
the reaction was stirred for an additional 18 h and
poured into rapidly stirred methanol, and the
resulting polymer was collected by suction filtra-
tion. The polymer was purified by two additional
precipitations, from THF into methanol. Then the
polymer was collected and dried under a reduced
pressure to give 1.08 g (56%) of MMDU-PPV. IR
(KBr), cmꢂ1; 3058, 2920, 2850, 1598, 1504, 1468,
was of the order of 0.890.1 for both CaF2 and
/
quartz plates, respectively. The forward (back-
ward) speed of the barriers was set to be 10.0 mm
minꢂ1
.
1422, 1388, 1355, 1256, 1206, 1050, 966, 851, 822,
1
721, 697. H-NMR (500MHz, CDC13): 7.10ꢀ
/
7.43
7.01 (m, 2H, vinyl protons),
4.20 (m, 7H, OCH2CH2O, OCH2 and
OCH3), 1.81 (m, 2H, OCH2CH2), 1.48ꢀ0.80 (m,
(m, 2H, PhH), 6.80ꢀ
/
3.87ꢀ
/
2.4. Spectra measurement
/
29H, CH2 and CH3).
UVꢀ
/
visible absorption spectra were obtained on
Shimadzu UV-3100-VIS-NIR spectrophot-
a
2.2. Monolayer characterization
ometer, and PL spectra were obtained on a
Aminco Bowman series 2 Luminescence Spectro-
meter (USA). Fourier transform infrared (FTIR)
transmission spectra were collected through a
IFS66V FT-IR spectrophotometer (Bruker, Ger-
man) equipped with a DTGS detector, 20 scans for
those KBr pellets and 300 scans for those LB films
on CaF2 substrates. An ednalitc polarizer (USA)
The experiments for monolayer spreading were
performed on a KSV-5000 LB system (KSV
Instruments, Helsinki, Finland) equipped with
computer controls. A Wilhelmy plate was used as
the surface pressure sensor and situated in the
middle of the trough. Two barriers compress or
expand symmetrically at the same rate from two
sides of the trough. Monolayers were obtained by
spreading 300 ml chloroform solution composed of
was employed for UVꢀvisible polarization and a
/
KRS-5 polarizer for IR polarization measure-
ments of LB films. For linear dichroism studies,

Z. Wu et al. / Spectrochimica Acta Part A 59 (2003) 1631ꢀ
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1641
1635
behind a rotating polarizer the sample frame
composed of aluminum strips could be rotated to
change the incidence angle in CaF2 plate. All
FTIR spectra were collected for 500 interfero-
According to CPK. model [13], the repeating unit:
2
area of 54 A might be accounted for the orienta-
˚
tion of the aromatic rings and the alkyl chains
water interface. While the per
tilting to the airꢀ
/
2
repeating unit surface area of 30 A , close to the
grams with the resolution 4 cmꢂ1
.
˚
value of the related macromolecules at the airꢀ
/
water interface [13,17], suggests formation of a
MMDU-PPV monolayer with the plane of its p-
3. Results and discussion
system nearly perpendicular to the airꢀwater
/
interface (see Scheme 2). The declining slope of
3.1. Behavior of MMDU-PPV monolayer
the p ꢀA isotherm in the second state, character-
/
istic of flexibility of the monolayers, indicates that
MMDU-PPV is not arranged in a rigid crystalline
MMDU-PPV is not a classical surfactant with
charged headgroups, but it has structural compo-
nents that can be ranked as more or less polar. The
DEOM groups of MMDU-PPV are the polar
portion of the macromolecule, and the hexadecy-
loxy groups constitute the nonpolar tails, Spread-
ing the chloroform solution of MMDU-PPV onto
2
˚
p-stacked structure. The repeating unit of 14 A ,
which is close to the half of area in the second
state, gives that the monolayers of MMDU-PPV
have collapsed to the multilayers.
3.2. UVꢀvisible and PL spectra
/
the airꢀ/water interface carefully obtains a uniform
dispersion. The corresponding pressure area (p ꢀ
/
The UVꢀ/visible absorption of conjugated poly-
A) isotherm is shown in Fig. 1. In Fig. 1, MMDU-
PPV molecules exist three kinds of aggregation
mers from the pꢀ
/
p transition is very sensitive to
the direction of polarization of an incident light,
which enables the use of polarized light to probe
the structural anisotropy within the films [18]. LB
films were obtained by setting the substrate surface
parallel to the moving direction of the barrier at 14
and 30 mN mꢂ1 surface pressure, respectively. P-
Polarization is defined so that the electrical field is
parallel to the substrate edge (dipping direction).
S-Polarization is orthogonal to P-polarization.
Both P and S polarization directions are perpen-
dicular to the light transmission direction. Fig. 2
states at the airꢀ
/
water interface in the process of
2
compression: the first at about 54 A per repeating
˚
2
unit, the second at about 30 A per repeating unit,
2
and the third at about 14 A per repeating unit.
˚
˚
shows UVꢀ
/
vis spectra of LB films, which is
prepared at the surface pressure of 14 mN mꢂ1
,
corresponding to different polarization directions.
The absorption under P-polarization mode is the
strongest, while that under the S-polarization
mode is the weakest. The absorption at any other
polarization angle falls between two extremes.
Since the absorption of UV light is the strongest
when the electric vector of the incident light is
parallel to the backbone of a conjugated polymer
[19,20], the results indicate that MMDU-PPV
backbones preferentially align parallel to the
dipping direction. There is a natural tendency of
rigid rods to align perpendicular to the direction of
compression and parallel to lines of flow between
Fig. 1. Isotherm corresponding to the first compression of a
floating film of MMDU-PPV (on water subphase at tempera-
ture of 14 8C) spread from a dilute solution. Inset is the
orientation of repeating unit of MMDU-PPV at the airꢀ
/water
interface.

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/
Scheme 2. Molecular structure and aggregating state of MMDU-PPV monolayer at the airꢀwater interface.
/
pression. These observations again confirm the
preferential order and edge-on arrangement of the
polymer backbone in the film [21]. The polarized
UVꢀvisible absorption of LB films obtained at the
/
surface pressure of 30 mN mꢂ1 (not shown here)
reveals no any anisotropy, which further confirms
the results shown by the p ꢀA isotherm that the
/
MMDU-PPV monolayers under the pressure of 30
mN mꢂ1 have collapsed to multilayers.
Interactions between polymers or polymer seg-
ments strongly influence the electronic properties
of conjugated polymer films. For example, the
large absorption spectral shift measured in aggre-
gates of oligo-phenylene vinylene [22] and the
rapid spectral diffusion observed in time-resolved
photoluminescence spectra of amorphous PPV
films [23] are quite related to these interactions.
Fig. 2. Polarized UVꢀvisible spectra of MMDU-PPV LB films
(15 layers on both sides of the plate, prepared at the surface
pressure of 14 mN mꢂ1).
/
Fig. 3 shows the UVꢀvis absorption spectra of
/
both LB films and the solution of MMDU-PPV.
The maximum peaks of the absorption are found
at 489 and 485 nm, respectively. The broad
ground-state absorption spectrum for LB films
shows a small red shift compared with that for the
solution spectrum. This characteristic is in contrast
the water and the substrate induced by dipping
[11]. Thus, to obtain the films with more obvious
anisotropy, optimal deposition geometry can be
chosen (in a standard LB trough) to place the
substrate plane parallel to the direction of com-

Z. Wu et al. / Spectrochimica Acta Part A 59 (2003) 1631ꢀ
/
1641
1637
MMDU-PPV.Therefore, this negligible red shift
in MMDU-PPV films compared with the solution
spectrum may be the result of solid-state aggrega-
tion [28]. It seems that the introduction of DEOM
to the conjugated main chains may favor forma-
tion of J-stacking [29] between benzene rings in LB
films and avoid the compact H-stacking structure,
in which the stronger interaction of the conjugated
repeating units can lead to large absorption
spectral shift. The absorption edge of MMDU-
PPV is about 690 nm, which corresponds to 1.80
eV of optical band gap, smaller than that of the
parent PPV (Eoptꢂ2.39 eV). The steric interaction
/
of side groups between the conjugated repeating
units and the ordered arrangement of conjugated
main chains in LB films of MMDU-PPV might
account for the narrowed bandgap.
Fig. 3. Absorption spectra of MMDU-PPV; (a) multilayer on a
quartz plate (15 layers on both sides of the plate); (b) solution in
The PL spectra of MMDU-PPV pumped by UV
chloroform (1ꢃ
10ꢂ5 mol lꢂ1).
/
light (lex
ꢂ350 nm, Xe lamp) of 5-layer LB films
/
onto the quartz substrate and the solution in
CHCl3 display some similar feature(as shown in
Fig. 4). The sharp main peaks for LB films and for
the solution sample are at 584 and 586 nm,
respectively, indicating that the emission area lies
in the region of orange. No obvious shift in the PL
spectrum of LB films relative to that of the dilute
to that of many other soluble conjugated polymers
where large solvatochromism) (the spectra in
dilute solution show large blue shifts relative to
that obtained in the solid state) has been observed
[24ꢀ26]. Conformational changes of p-conjugated
/
polymers are inherently coupled to the electronic
structures of the p-electron system. The broad,
longest wavelength absorbance of LB films can be
attributed to the pꢀp electronic transition asso-
/
ciated with the p-conjugated polymer backbones.
In the previous literature, polydiacetylenes,
poly(thiophene)s and PPVs exhibit color change
(chromism), which is solvent (solvatochroinism)
and/or temperature (thermochromism) dependent.
But the interpretation of these transitions for
conjugated polymers has been controversial be-
cause of the difficulties in separating the contribu-
tions of the conformational changes in the
polymer backbone and the aggregation phenom-
enon of the macromolecules to the observed
chromism [27]. However, in MMDU-PPV, the
side groups mat render the polymer soluble are
well separated from those on the adjacent alter-
nating chromophore units. The steric interaction
between side groups, which may twist the con-
jugated repeating units from planar configuration
and cause solvatochromism, is negligible in
Fig. 4. PL spectra of MMDU-PPV: (a) multilayer on a quartz
plate (five layers on both sides of the plate); (b) solution in
chloroform (1ꢃ
10ꢂ5 mol lꢂ1).
/

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/
solution would be an implication that the polymer
main chains are separated from each other and
excimer emissions are suppressed in both LB films
and the dilute solution. In addition, the peak width
at the half height of LB films is smaller than that
of the dilute solution, suggesting the introduction
of DEOM to PPV might improve the monochro-
malicity of emission light in solid state.
The UVꢀvis absorption spectra of MMDU-
/
PPV LB films prepared at surface pressure of 14
mN mꢂ1 with various numbers of layers are in
Fig. 5. The linear increase in absorbance with the
number of layers confirms that the transfer process
of Lungmuir films is uniform and the layer-by-
layer deposition of MMDU-PPV films at molecu-
lar level is successfully controlled.
Fig. 6. FTIR absorbance spectrum of MMDU-PPV LB multi-
layers on CaF2 plate. See the text for the discussion of many
characteristic peaks.
3.3. FTIR spectra of MMDU-PPV LB films
pressure of 14 mN mꢂ1. A weak feature near 2955
cmꢂ1 is assigned to the CH3 antisymmetric
stretching mode of hydrocarbon tail; two strong
bands at 2920 and 2850 cmꢂ1 are attributed to the
antisymmetric and symmetric CH2 stretching vi-
bration of long polymethylene hydrocarbon
chains, respectively. The band at 1734 cmꢂ1 is
attributed to the carbonyl absorption of a ester or
carboxylic acid; a weak band at 1678 cmꢂ1 is
Fig. 6 shows the FTIR spectrum of 15-layer
MMDU-PPV LB films obtained at the surface
attributed to CÄ/O stretching vibration of the
aromatic aldehydes; a weak peak at 1600 cmꢂ1
can be assigned to the antisymmetric stretching
vibration in the aromatic rings due to the forma-
tion of carbonyls [30], these peaks suggest that
MMDU-PPV has been oxidized in some extent. A
band at 1508 cmꢂ1 is assigned to the stretching
vibration of semicircular phenyl rings; a strong
peak at 1467 cmꢂ1 is attributed to the CH2
scissoring mode of long polymethylene hydrocar-
bon chains; a band at 1414 cmꢂ1 is attributed to
the scissoring mode of the CH2 connected to oxide
atom; a band at 1208 cmꢂ1 is attributed to the
stretching mode of CÃ
/
O in the benzene rings. It is
well-known that the frequency of CH2 antisym-
metric and symmetric stretching vibrations bands
are sensitive to the degree of conformational order
Fig. 5. Absorption spectra of MMDU-PPV LB multilayers
deposited on quartz. The number of layers deposited is
indicated. The absorbance at 489 nm as a function of the
number of layers is given in the inset.
of alkyl chains [31ꢀ
/
33]. When the alkyl chains are
highly ordered (trans-zigzag conformation), the

Z. Wu et al. / Spectrochimica Acta Part A 59 (2003) 1631ꢀ
/
1641
1639
bands appear near 2920 and 2850 cmꢂ1, respec-
tively, and if conformational disorder is included
in chain, they shift upward to higher wavenumber
depending upon the content of gauche conformers.
Therefore, the observations in Fig. 6 suggest that
the polymethylene hydrocarbon chains of
MMDU-PPV in LB multilayers are ordered. The
band pattern and splitting of the CH2 scissoring
mode have been used extensively to characterize
subcell packing in LB films since this peak can be
spilt by the dimer local crystal field of adjacent
methylene groups. Usually the splitting of d(CH),
1470 cmꢂ1 peak, provides an indication of
orthorhombic-type packing with two molecules
in one unit cell. On other hand, if only a single
peak is observed at 1468 cmꢂ1, the crystal packing
will be triclinic with one molecule per unit cell or,
rarely, hexagonal [34]. A single peak near 1467
cmꢂ1 in Fig. 6 assigned to the scissoring mode of
alkyl side chain is indicative of triclinic subcell
packing of polymethylene hydrocarbon chains in
Fig. 7. Rotating polarized transmission FTIR spectra of
MMDU-PPV LB films (21 layers). Ap (iꢄ0) (solid line), As
(iꢄ0) (dotted line), and Ap (iꢄ60) (dash-dotted line). Inset is
/
/
/
the transition dipolar moment u of a molecule positioned by the
angles v and u in the trihedral OXYZ in nondegenerate case.
multilayer LB films of MMDU-PPV [35ꢀ39].
/
To evaluate the orientation of the polymethy-
lene hydrocarbon side chains, Chollet’s treatment
of the linear dichroism of FTIR spectra is ex-
2pd
nꢄ
tended [34,40ꢀ42]. A detailed description of the
/
method will not be repeated here since similar
theory has been shown in the literature. The
experimentally measured dichroic ratio b(i) is
related to the angle f, between transition moment
u and the surface normal of the substrate (see Fig.
l
AP(i)
n1n3(n1 ꢁ n3)
b(i)ꢄ
ꢄ
Ap(0) (n1 cos r ꢁ n3 cos i)
ꢀ
n12
ꢁ
Žctg2 f cos i cos r
7
Ap(i) and A (0) are measured band intensities in
inset). b(i)ꢄ
Š
P-polarizcd absorption spectra with the incidence
/
A (i)jA (0)ꢄAp(i)jA (0),where
/
Š
Š
Š
ꢀ
sin2 i
ꢁ
n24
Žcos2 v
n1n3
angle of i and normal incidence angle (iꢄ
/
0) with
where, iꢄ
/
608, n1ꢄ
/
1 (air), n2ꢄ1.5 (LB films), n3
/
respect to the film plane, respectively. Base on the
assumptions proposed by Hsu et al for extended
application of Chollet’s treatment [34], equations
can be derived directly from the Frcsnel equations
and Descartes laws:
is the index of calcium fluoride which is 1.42.
The polarization of fine and very intense lines at
2920 cmꢂ1 is well known. This vibration, polar-
ized in the crystal plane ac and perpendicular to
the chain axis, is used to study the orientation of
chains in acid layers. Fig. 7 shows the polarized
infrared transmission spectra for a 21-monolayers
MMDU-PPV LB films in the CH2 stretching
As(0)
aꢄ
Ap(0)
vibration region. It is found that a(2919)ꢄ
/As(0)/
Ap(0)ꢄ1.41, then Žcos 2vꢄ 0.066, the angle
/
/
ꢂ
/
(1ꢂa)=(1ꢁa)ꢄŽcos 2v
n1 sin iꢄn3 sin T
v being measured in the XY plane from the OX
axis (Figure 7 inset) which, in all the infrared
experiments, coincides with the dipping direction.

1640
Z. Wu et al. / Spectrochimica Acta Part A 59 (2003) 1631ꢀ1641
/
The fact that Žcos 2v is not zero implies a
molecular distribution anisotropy in the XY plane.
A positive value of this orientation parameters is
interpreted by assumption that the polymer layer
consists of crystallites with the crystal plane ac
oriented preferentially, parallel to the dipping
direction.
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The tilt angle of chains with the OZ axis is then
obtained from the b(608)ꢂ
which is found equal to 0.982 for the line at 2920
/
Ap(608)/Ap (iꢄ0)
/
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fꢄ548, which corresponds to a tilt angle of 368
/
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392 (1998) 161.
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/
those obtained with stearic acid monolayers in
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A conjugated polymer, MMDU-PPV, is suc-
cessfully dispersed at the airꢀwater interface to
/
form a long-term stable monolayer, which is
organized with the plane of its p system nearly
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/
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monolayer is apt to be deposited on the substrates
in Z type structure by LB technique. The con-
jugated main chains of MMDU-PPV in multilayer
LB films are aligned in a side-by-side parallel
fashion and packed with the plane of its p system
approximately perpendicular to the layer plane.
The long alkyl side chains of MMDU-PPV in
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This work is financially supported by NSFC,
No. 20273029.

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