Structural study of the thermochromic transition in poly(2,5-dialkyl-/p- phenyleneethynylene)s

Article abstract of DOI:10.1021/ma051342y

The solvatochromic and thermochromic properties of poly(2,5-di-2′- ethylhexyl-p-phenyleneethynylene) have been investigated. While some important changes were observed in UV - vis absorption, calorimetric, and X-ray measurements, temperature-dependent FTI

Full text of DOI:10.1021/ma051342y

Macromolecules 2005, 38, 9631-9637
9631
Structural Study of the Thermochromic Transition in
Poly(2,5-dialkyl-p-phenyleneethynylene)s
Nolwenn Lebouch,† Se´bastien Garreau,†,‡ Guy Louarn,‡ Michel Belleteˆte,§
Gilles Durocher,§ and Mario Leclerc*,†
Canada Research Chair on Electroactive and Photoactive Polymers, De´partement de Chimie,
Universite´ Laval, Que´bec, Que´bec, Canada G1K 7P4; Laboratoire de Physique Cristalline, Institut des
Mate´riaux Jean Rouxel, B.P. 32229, 44322 Nantes Cedex 3, France; and De´partement de Chimie,
Universite´ de Montre´al, Montre´al, Que´bec, Canada H3C 3J7
Received June 23, 2005; Revised Manuscript Received September 21, 2005
ABSTRACT: The solvatochromic and thermochromic properties of poly(2,5-di-2′-ethylhexyl-p-phenyle-
neethynylene) have been investigated. While some important changes were observed in UV-vis absorption,
calorimetric, and X-ray measurements, temperature-dependent FTIR, Raman, and solid-state NMR spectra
did not reveal any significant shift of the peaks associated with the polymer backbone. These results
suggest that the chromic effects are not mainly governed by conformational changes of the polymer. From
these data and ab initio calculations performed on model compounds, it is believed that the thermochromic
transition proceeds from aggregated polymers at room temperature to “free” (weakly interacting) polymer
chains at higher temperatures.
Introduction
To better understand these interesting chromic ef-
fects, we report here a detailed spectroscopic investiga-
tion of a thermochromic poly(2,5-di-2′-ethylhexyl-p-
phenyleneethynylene). This polymer can be obtained in
either a highly or less conjugated form depending on
the temperature and will therefore be utilized as a
model compound to obtain a better description of the
molecular interactions involved in chromic and non-
chromic PPEs and related polymers.
Conjugated organic polymers1-4 have received con-
siderable attention in the past 25 years due to their
great potential for applications in microelectronics,
electrooptics, and biophotonics. For instance, the elec-
trooptical properties of semiconducting conjugated poly-
mers depend critically on their molecular structure,
doping, and morphology.5 Therefore, it is of great
interest to carefully study the structure/property rela-
tionships in those materials.
Experimental Section
Some of the conjugated polymers exhibit interesting
color changes (chromism) that, depending upon the
stimulus involved, are called thermochromism, solva-
tochromism, or affinity chromism.6,7 These polymers can
therefore detect and transduce physical or chemical
information into an optical signal. Depending upon the
polymer structure, this transition has been explained
by a conformational change of the backbone and/or
variable π-π interchain interactions.8-13 Among these
materials, poly(p-phenyleneethynylene)s (PPEs)10,14 have
received particular attention. PPEs are rigid and highly
fluorescent conjugated polymers.15-20 In the solid state,
some studies have revealed that PPEs form lamellar
structures, in which rotation around the triple bond is
restricted. A planar arrangement of the conjugated
backbone with concomitant π-π stacking is therefore
observed.21 In solution, upon the addition of a nonsol-
vent, aggregation of dialkyl-PPEs is accompanied by a
dramatic shift of the UV-vis spectrum.10,22 These
polymers show similar chromic transitions in the solid
state.10,22 Interestingly, dialkoxy-substituted PPEs do
not show any significant chromic feature.23 Finally, it
is interesting to note that poly(fluorenyleneethynylene)-
s24 and poly(2,5-thienyleneethynylene)s25 do also exhibit
some thermochromic and solvatochromic transitions,
whereas poly(3,6-carbazoleneethynylene)s26 and poly-
(2,7-carbazoleneethynylene)s27 do not.
Synthesis. Following procedures already described in the
literature,28,29 the starting 1,4-dichlorobenzene (30.0 g, 0.204
mol) was treated with 2-ethylhexylmagnesium bromide (73.9
g, 0.449 mol) in 600 mL of anhydrous ether under reflux for
48 h to yield the 1,4-bis(2-ethyl)hexylbenzene (31.6 g, yield
51%). The latter compound (30.0 g, 0.100 mol) was iodinated
via the addition of KIO4 (27.4 g, 0.119 mol) and I2 (30.2 g, 0.119
mol) in 300 mL of an acidic solution for 24 h at 90 °C (46.2 g,
yield 84%). To obtain the acetylic compound, the 1,4-bis(2-
ethyl)hexyl-2,5-diiodobenzene (35.4 g, 0.064 mol) was mixed
with trimethylsylilacetylene (20.0 g, 0.204 mol) in the presence
of palladium tetrakis(triphenylphosphine) (4.0 mol %) and
cuprous iodide (2.8 mol %) in 275 mL of triethylamine at 40
°C for 24 h (10.2 g, yield 32%), followed by a KOH (20 mol %)
deprotection reaction (4.53 g, yield 77%). Poly(2,5-di-2′-ethyl-
hexyl-p-phenyleneethynylene) was obtained through a Sono-
gashira coupling between the aryl acetylene moiety (0.200 g,
0.570 mmol) and the aryl halide one (0.316 g, 0.570 mmol) in
a mixture of toluene and diisopropylamine (20 mL) (Scheme
1) (0.244 g, yield 73%).29 Size exclusion chromatogry (SEC)
measurements on poly(2,5-di-2′-ethylhexyl-p-phenyleneethy-
nylene) gave a number-average molecular weight (Mn) of
18 000 g/mol and a polydispersity (Mw/Mn) of 3 relative to
monodisperse polystyrene standards. Similar procedures were
followed to synthesize poly(2,5-di-decyl-p-phenyleneethy-
nylene).
Physical Methods. Differential scanning calorimetry (DSC)
analyses were performed on a Perkin-Elmer (DSC-7) instru-
ment at a heating rate of 20 °C/min under a nitrogen flow.
Solid-state UV-vis absorption measurements were performed
on polymer films deposited from CHCl3 solutions on a quartz
cell, with a Hewlett-Packard diode array spectrophotometer
(model 8452) equipped with a temperature control unit. The
temperature was measured with a thermocouple, with ∆T ∼
2 °C.
† Universite´ Laval.
‡ Institut des Mate´riaux Jean Rouxel.
§ Universite´ de Montre´al.
* To whom correspondence should be addressed: Tel 1-418-656-
3452, Fax 1-418-656-7916, e-mail Mario.leclerc@chm.ulaval.ca.
10.1021/ma051342y CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/20/2005

9632 Lebouch et al.
Macromolecules, Vol. 38, No. 23, 2005
Scheme 1. Synthesis of Poly(2,5-di-alkyl-p-phenyleneethynylene)s
X-ray data were obtained from a X-ray diffractometer
(Siemens/Bruker) using the graphite monochromatized copper
radiation (KR ) 1.5418 Å). The instrument consists of a
Kristalloflex 760 generator, three-circle goniometer, and Hi-
Star area detector and is equipped with GADDS software. The
operation power was 40 kV, 40 mA and the collimator was
0.8 mm (diameter). The samples were inserted in thin-walled
(0.01 mm) glass capillary tubes (1.0 mm diameter). The sample
temperature is controlled with a custom-modified HCS400
Instec hot stage equipped with a STC200D controller.
Fourier transform infrared (FTIR) experiments were per-
formed with a Nicolet Magna 560 spectrometer on thin films
coated on NaCl disks for ex situ experiments or on BaF2 disks
for in situ experiments. For in situ measurements, a calibra-
tion curve was obtained by measuring the “real” temperature
in the center of a BaF2 disk with a thermocouple vs the
displayed temperature on the temperature control unit. The
subsequent linear regression formula calculated was then used
to obtain the real temperature around the sample.
Raman studies were performed with a multichannel Jobin-
Yvon T64000 spectrophotometer connected to a CCD detector
in the visible range. The excitation line was 676.4 nm. FT
Raman spectra were recorded with a RFS 100 Bruker spec-
trometer (excitation wavelength ) 1064 nm). In situ Raman
experiments were carried out by increasing the temperature
by 10 deg steps between 30 and 200 °C.
Figure 1. Ground-state energy surface of (2-ethylphenylene-
ethynyl-2-ethylphenylene) model compound.
13C CPMAS NMR experiments were carried out on a
BRUKER AVANCE300 (Bruker Biospin Inc, Milton, ON,
Canada) spectrometer, equipped with a 4 mm CPMAS NMR
probe. The 13C carrier frequency was 75 MHz, and the MAS
frequency was set to 10 000 Hz. The contact time for cross-
polarization was 1 ms, followed by a proton decoupling (type
TPPM) of 40 ms. Typically between 7K and 12K scans were
recorded at room temperature and 135 °C.
Theoretical Methods. Ab initio calculations were per-
formed on a Pentium 4 (2.57 GHz) personal computer with 1
Gb RAM using the Gaussian 03W program, revision B.05.30
The conformational analysis of the model compound was done
by changing the torsional angles θ between phenylene rings
and the ethynyl bond by 15° steps. The geometries were
optimized at the HF level with the 6-31G* basis set. The Berny
analytical gradient method was used for the optimizations. The
requested HF convergence on the density matrix was 10-8, and
the threshold values for the maximum force and the maximum
displacement were 0.00045 and 0.0018 au, respectively.
Figure 2. Solvent-dependent UV-vis absorption spectra of
poly(2,5-di-2′-ethylhexyl-p-phenyleneethynylene).
tion. Recently, a very flexible backbone was also pre-
dicted for poly(N-octyl-2,7-carbazoleneethynylene).27
UV-vis Absorption Measurements. As shown in
Figure 2, the absorption spectrum of poly(2,5-di-2′-
ethylhexyl-p-phenyleneethynylene) in chloroform (good
solvent) exhibits one broad feature centered around 386
nm, typical for dialkyl-substituted PPEs.22 The absorp-
tion spectrum changes dramatically if a nonsolvent
(methanol) is added to the chloroform solution. Indeed,
the main band is red-shifted by about 10 nm, whereas
a new sharp peak appears at 428 nm, which becomes
more intense with the addition of the nonsolvent. These
results are in excellent agreement with those previously
reported by Bunz et al. for similar PPEs.10,22 The
absorption spectrum of poly(2,5-di-2′-ethylhexyl-p-phe-
nyleneethynylene) in the solid state, at room tempera-
ture, shows two distinct bands at 396 and 428 nm (see
Figure 3). Upon heating, the band located at 396 nm is
blue-shifted by about 10 nm, whereas the peak at 428
Results
Conformational Analyses. The ground-state energy
surface obtained from theoretical HF/6-31G* ab initio
calculations performed on alkyl-substituted (2-ethylphe-
nylene-ethynyl-2-ethylphenylene) isolated model com-
pound is shown in Figure 1. A coplanar anti structure
is favored, but the rotation along the triple bond is
essentially unrestricted in the gas phase. The alkoxy-
substituted (2-methoxyphenylene-ethynyl-2-methox-
yphenylene) model compound exhibits a very similar
ground-state energy surface (results not shown). As
revealed by these conformational analyses, the corre-
sponding dialkyl-substituted (and dialkoxy-substituted)
PPEs should possess more or less the same conforma-

Macromolecules, Vol. 38, No. 23, 2005
Poly(2,5-dialkyl-p-phenyleneethynylene)s 9633
Figure 3. Temperature-dependent UV-vis absorption spectra of a poly(2,5-di-2′-ethylhexyl-p-phenyleneethynylene) thin film.
Figure 4. DSC thermogram of poly(2,5-di-2′-ethylhexyl-p-
phenyleneethynylene).
nm decreases in intensity and completely disappears
when the temperature reaches 130 °C. Then, the result-
ing high-temperature solid-state spectrum is very simi-
lar to that measured in chloroform (good solvent). Upon
cooling, the poly(2,5-di-2′-ethylhexyl-p-phenyleneethy-
nylene) returns partially to its previous state. Indeed,
both absorption bands reach their initial wavelengths,
but without recovering their full intensities.
Calorimetric Measurements. In parallel to these
optical measurements, DSC thermal analyses were
performed on the poly(2,5-di-2′-ethylhexyl-p-phenyle-
Figure 5. X-ray diffraction data measured at various tem-
peratures for a poly(2,5-di-2′-ethylhexyl-p-phenyleneethy-
nylene) powder.
neethynylene). The thermogram reported in Figure 4
shows two distinct transitions: a broad endotherm with
a peak near 125 °C upon heating and an exothermic
peak at 75 °C upon cooling. The endothermic peak
corresponds to an energy of 4 J/g. When the temperature
is decreased, the exothermic peak appears at a lower
temperature but with a comparable energy (4 J/g).
However, when the temperature exceeds 140 °C in the
heating cycle, the peak upon cooling is not observed
anymore, and in a second cycle, no transition is ob-
served.
X-ray Diffraction Measurements. To obtain more
information about this solid-state transition, X-ray
diffraction analyses were performed at various temper-
atures (see Figure 5). At ambient temperature, the
X-ray diagram exhibits a peak at 2θ ) 7°, consistent
with an interlamellar distance of 12.6 Å, and two broad
peaks at 2θ > 15°, corresponding to d values of 4.9 and
4.2 Å. This diffraction pattern at room temperature is
very close to the one previously reported for the same
polymer by Lieser et al.,21 who attributed the two latter
diffraction peaks to arene-arene packing distance.
Upon heating, the overall peak intensities decrease and
the peaks broaden. Above 130 °C, the material appears
to be mainly amorphous. Upon cooling, a partial recrys-
tallization is observed, the three peaks reappearing from
90 °C but without recovering their full intensities.
FTIR Spectroscopy. In situ FTIR spectra recorded
on a KBr pellet do not show any significant modification
upon heating (Figure 6). For instance, the intense bands
at 1379, 1442, 1460, and 1500 cm-1, which can be
assigned to the CH2 and CH3 bending vibrations, remain
at about the same position. Moreover, many of the
weaker bands of the spectra, which are attributed to
phenyl rings (like the C-H stretching band at 3021
cm-1 or the CdC stretching band at 1596 cm-1), do not
shift during the heating cycle. However, if one compares
the in situ spectral position of the antisymmetric CH
stretching band of the CH2 groups, located at 2925 cm-1
,
a slightly hypsochromic shift is observed when the
temperature is increased (see Figure 7). This well-
known behavior corresponds to the transformation from
a “trans” to a “gauche” conformation of these groups.9

9634 Lebouch et al.
Macromolecules, Vol. 38, No. 23, 2005
Figure 6. FTIR spectra of a poly(2,5-di-2′-ethylhexyl-p-
phenyleneethynylene) thin film at various temperatures.
Figure 9. Solid-state 13C CP/MAS NMR spectra of poly(2,5-
di-2′-ethylhexyl-p-phenyleneethynylene) at 25 and 135 °C.
Figure 7. Evolution of the position of the CH2 antisymmetric
stretching upon a heating-cooling cycle of a poly(2,5-di-2′-
ethylhexyl-p-phenyleneethynylene) thin film.
Figure 8. Raman spectra of poly(2,5-di-2′-ethylhexyl-p-phe-
nyleneethynylene) at various temperatures.
Figure 10. Expanded aliphatic regions of the 13C MAS spectra
at 25 and 135 °C.
However, the actual shift is smaller than that observed
previously for chromic polythiophenes,9 and it seems to
precede the thermochromic transition.
NMR Measurements. Solid-state 13C CP/MAS NMR
spectra of the poly(2,5-di-2′-ethylhexyl-p-phenyleneethy-
nylene) recorded at room temperature and at 135 °C are
shown in Figure 9. The assignment of each resonance
line was mainly made on the basis of the assignments
established in solution. The peaks in the region between
120 and 150 ppm can be assigned to the aromatic main-
chain carbon atoms, and the peaks in the region
between 0 and 50 ppm can be assigned to the aliphatic
Raman Spectroscopy. Figure 8 shows the in situ
Raman spectra obtained at 676 nm on poly(2,5-di-2′-
ethylhexyl-p-phenyleneethynylene) thin film. On the
contrary to a previous study performed on chromic
polythiophenes,9 we do not observe any significant
change in the positions of the main bands of the spectra
upon heating. Indeed, the stretching band of the ethynyl
bond, located at 2196 cm-1, is shifted by only 3 cm-1
when the temperature is increased. Similarly, the bands
located at 1595 and 1525 cm-1 related to C-C(-H) and
C-C(-alk) stretching vibrations, respectively, do not
move, like the CH bending band (1217 cm-1) that
decreases in intensity without any significant spectral
shift.
side-chain carbon atoms. Figure 10 shows expanded 13
C
CP/MAS NMR spectra of the poly(2,5-di-2′-ethylhexyl-
p-phenyleneethynylene) in the range 0-60 ppm from
room temperature to 135 °C. At room temperature, the
peaks at 11.6 and 14.49 ppm correspond to the methyl
groups of the side chain. The peaks from 23 to 40 ppm
correspond to the methylene groups. The peak at 23.7

Macromolecules, Vol. 38, No. 23, 2005
Poly(2,5-dialkyl-p-phenyleneethynylene)s 9635
Scheme 2. Assignments for
spectroscopy is a very useful technique because it is
complementary to infrared spectroscopy. Indeed, ac-
cording to the group theory, when molecules possess a
center of symmetry, the in-plane “gerade” vibration
modes are only Raman-active, whereas the “ungerade”
modes are only infrared-active. Thus, the combination
of FTIR and Raman measurements should provide a
complete assignment of the vibrational bands of the
molecules investigated. As mentioned above, tempera-
ture-dependent Raman spectra did not show any sig-
nificant spectral shift, in sharp contrast to chromic
polythiophenes.9 As the temperature increases, a slight
broadening and intensity decrease of all Raman peaks
are observed. When comparing these results with other
work on the characterization of polymers with Raman
spectroscopy, one can found additional insights into two
previous studies. For instance, Davidson’s group31 used
Raman spectroscopy to monitor phase transition in
aliphatic ketones, and they observe broadening of the
bands with a decrease of the crystallinity. They also
observe a complete disappearance of some bands when
the sample reaches a transition temperature pointing
to a complete phase transition. A similar feature can
be observed here with PPE: no shift of the Raman peaks
but rather a broadening of the bands with increasing
temperature. The decrease in the band intensities upon
heating can also be related to a decrease of the inter-
chain interactions as the molecular mobility within the
material is activated by temperature. This loss of order
correlates extremely well with the observed decrease in
intensity observed by X-ray analyses which were ex-
plained by a transition from an ordered state to a
disordered one upon heating. Finally, a Raman study
was also carried out on another chromic conjugated
polymer: polydiacetylene. This polymer is believed to
show chromism through transitions from a planar to a
nonplanar form of the conjugated backbone. Kiefer’s
group32 used Raman spectroscopy to prove that model.
They did not observe any decrease in intensity in their
Raman spectra but only shifts in wavenumbers, which
would mean that we do not have with our PPE the same
kind of behavior.
Solid-state nuclear magnetic resonance (NMR) has
recently made considerable progress for the study of
molecular systems that are insoluble or do not exhibit
long-range structural order. In addition to the study of
molecular structure, solid-state NMR also provides
unique means to probe molecular dynamics. Using this
technique, we aimed, for the first time, to characterize
the conformational behavior of the main chain and/or
the alkyl side chains of PPEs over a wide range of
temperatures by 13C CP/MAS (cross-polarization/magic
angle spinning) NMR spectroscopy and to elucidate the
relation between the structure and the observed phase
transition. To obtain more information concerning the
conformation of the polymer chains, it is first necessary
to accurately assign the peaks in the 13C NMR spec-
trum. Figure 10 shows expanded 13C CP/MAS NMR
spectra of the poly(2,5-di-2′-ethylhexyl-p-phenyleneethy-
nylene) in the range 0-60 ppm from room temperature
to 135 °C in order to clarify the temperature changes of
the side-chain conformation. The methyl signals be-
tween 11 and 15 ppm consist of two peaks, correspond-
ing to the two different kinds of CH3 arising one from
the ethyl group of the side chain and the other from the
hexyl part of the side chain. The intensity of the two
peaks decreases with an increase in temperature with
Poly(2,5-di-2′-ethylhexyl-p-phenyleneethynylene)
ppm corresponds to the R-CH2, the one at 28.9 ppm
corresponds to the â-CH2, the one at 33.2 ppm corre-
sponds to the internal CH2, and the one at 40.8 ppm
corresponds to the γ-CH2 (Scheme 2). Upon heating,
neither the peaks of carbon atoms associated with the
main chain nor the side chains shift, but we rather
observed a change in shape and line widths for the
peaks of the aliphatic region and the aromatic region.
Discussion
From the optical properties and X-ray diffraction data
obtained for poly(2,5-di-2′-ethylhexyl-p-phenyleneethy-
nylene) in the solid state (thin film) at various temper-
atures, a relatively clear transition was observed upon
heating/cooling. Indeed, from the UV-vis spectra, it is
observed that the band detected at 428 nm is completely
vanished upon heating at 130 °C. In agreement with
these results, X-ray data show that the interlamellar
organization peak at 7° almost disappears around 130
°C, indicating a disordering in the material. When
cooling back the samples, they partially return to their
initial state, but for both types of measurements, a total
recovery of the peak intensities is not obtained. By
plotting the intensity vs temperature for both UV-vis
and X-ray diffraction data (figure not shown), a clear
transition can be observed between 90 and 130 °C upon
heating. However, in the cooling cycle, the physical
transition occurs at lower temperatures, this hysteresis
being probably related to the high viscosity of the
system.
This physical transition between two different states
of the polymer can be associated with the thermal
transitions observed in the DSC measurements. This
physical transition seems to proceed from an ordered
state (with the presence of three main diffraction peaks)
to a disordered form at temperatures higher than 130
°C. To obtain a better understanding of the nature of
the molecular interactions involved in these tempera-
ture-dependent measurements, spectroscopic studies
using FTIR and Raman techniques were carried out. In
principle, these techniques should detect movements
within the side chains and/or the backbone of the
polymer during the thermal transition.9 As mentioned
above, FTIR spectra did not show any significant change
during the thermal transition. Indeed, only small spec-
tral changes involving the alkyl chains occurred during
the heating cycle and were attributed to weak modifica-
tions from “trans” to “gauche” conformations. However,
the spectral shifts observed are smaller than those
reported for chromic polythiophenes.9 Usually, Raman

9636 Lebouch et al.
Table 1. Normalized Area and Full Widths at
Macromolecules, Vol. 38, No. 23, 2005
Half-Maximum of the Different Peaks at 25 and 135 °C
25 °C 135 °C
peak (ppm)
%
fwhm
%
fwhm
12
14.5
24
7.6
4.6
19.5
16
2.87
1.83
3.66
5.95
4.3
4.86
1.86
3.13
4.93
1.92
4
1.9
1.8
6.4
3.2
6.6
45.15
2
1.15
1.57
1.54
1.84
2.85
1.37
2.73
3.95
1.7
29
33.5
41
95
124
134
142
7.5
22
2.3
4.5
12.4
4
12.7
5.3
3.75
Figure 11. Solvent-dependent UV-vis absorption spectra of
poly(2,5-didecyl-p-phenyleneethynylene).
no appearance of any other peak, which means that
these side-chain methyl carbon atoms are rather in an
immobile region. In the region corresponding to the
methylene groups, four peaks appear at 25 °C, whereas
at 135 °C at least five peaks appear in the same region.
The assignments of the peaks are not shifted; only the
line widths of the peaks become thinner at higher
temperatures, which means these carbon atoms turn to
be more mobile at higher temperatures. The chemical
shifts of the respective CH2 resonance lines at 25 and
135 °C have been found to be in good agreement with
the values measured in solution, indicating that all C-C
bonds of the CH2 sequences undergo rapid trans-
gauche exchanges in these two cases, as observed in the
solution state. This is in agreement with what has been
found from our analyses of infrared and Raman spectra.
The increasing mobility of the molecules is also reflected
by the full width at half-maximum (fwhm) of the C-C
resonance, which narrows with increasing temperature
as shown in Table 1. This happens for all carbon atoms
of the molecule, but it is more pronounced for the carbon
atoms of the aliphatic side chains.
Moreover, as mentioned in the Introduction section,
the phenyleneethynylene units can exhibit π-π stack-
ing. This phenomenon can be specifically probed in the
solid state in contrast to solution state, where these
effects are removed due to molecular tumbling and
dilution. The resonance lines assigned to the phenylene
carbon atoms at 25 and 135 °C are different in the solid-
state CP/MAS spectra (141.7, 134.5, and 123.7 ppm)
when compared to those obtained in solution (141.2,
133.5, and 123.3 ppm). In the solid state, the phenylene
rings are located in a position not significantly deviating
from the coplanar position. Such a coplanar structure
may be destroyed by a rapid rotation of each phenylene
ring at least at high temperatures like what happens
in solution, resulting in significant changes in the
chemical shifts. However, we only observed a change
in the chemical shifts from the solution spectrum to the
solid-state spectra but no shift between the two solid-
state spectra recorded at 25 and 135 °C, indicating that
the phenylene rings are not undergoing rapid rotation
in the solid state, and even at high temperatures (135
°C), they seem to remain coplanar. It is also apparent
from Table 1 that the half-widths of the aromatic carbon
atoms are larger than those of other aliphatic carbon
atoms. This difference is believed to originate from
inhomogeneities in the molecular motion. These carbon
atoms have therefore less mobility than the other carbon
atoms. In addition to the 13C signals of the aromatic
carbon atoms in the backbone, the first carbon atom of
the side chain linked to the phenylene unit is also very
intense at 135 °C compared to the aliphatic resonance
peaks of the rest of the side chain. This reflects that
the rigid components include both the backbone and the
beginning of the side chain.
In previous studies, a correlation between the chromic
properties of several conjugated polymers and the
rotational energy barrier between adjacent moieties
(based on ab initio calculations performed on dimer
model compounds) has been established.33 For instance,
polymers having nonplanar structures with an energy
barrier against planarity smaller than 1.3 kcal mol-1
(e.g., poly[1,4-(2,5-dioctyloxyphenylene)-2,5-thiophene]
and poly[1,4-(2,5-dioctylphenylene)-2,5-furan]) have re-
vealed interesting chromic effects. On the other hand,
coplanar polymers with rotational barrier close to zero
(e.g., poly[1,4-(2,5-dioctyloxyphenylene)-2,5-furan) can
maintain a highly conjugated conformation (coplanar or
nearly coplanar), even at high temperatures, resulting
in the absence of significant chromic effects. According
to these results, PPEs, which are predicted to be
coplanar with a very weak barrier to rotation, should
not show any significant thermochromic and solvato-
chromic effect (or only small bandwidth changes without
any significant spectral shift). Moreover, on the basis
of their respective conformational analyses, dialkoxy-
substituted and dialkyl-substituted PPEs should show
similar chromic behaviors. However, UV-vis spectra
reported above clearly show that the optical properties
of dialkyl-PPEs are much affected by the presence of a
bad solvent or by changing the temperature of the thin
film whereas dialkoxy-substituted PPEs are not.23 An
important feature for the determination of the struc-
tural changes happening during the chromic transition
is the presence of a second absorption band at 428 nm
in the UV-vis absorption spectra, which appears si-
multaneously by addition of a nonsolvent in solution or
in the solid state at room temperature. This band is
included in the envelope of conformations of the main
band centered at 386 nm. To determine whether this
band represents the most planar conformation of the
poly(2,5-di-2′-ethylhexyl-p-phenyleneethynylene), we de-
cided to synthesize another dialkylated PPE with a
different kind of substituent, the poly(2,5-di-decyl-p-
phenyleneethynylene). The substitution by n-decyl side
chains, which are linear compared to the previous PPE
with ethylhexyl side chains, should provide a better
stacking of the polymer chains between themselves. The
absorption spectra of this new dialkylpolyphenylene-
ethynylene shows the same kind of pattern as the
former poly(2,5-di-2′-ethylhexyl-p-phenyleneethynyl-
ene): a large band centered near 395 nm in solution
(100% chloroform) and a new band appearing at 439 nm
upon the addition of a nonsolvent (Figure 11). This band
at 439 nm does not belong anymore to the large envelope

Macromolecules, Vol. 38, No. 23, 2005
Poly(2,5-dialkyl-p-phenyleneethynylene)s 9637
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of all the conformations observed in solution and seems
to be related to another structure of the polymer which
has its proper UV-vis signature. These results support
our previous conclusions about the nature of the con-
formational change happening during the chromic tran-
sition. Therefore, one explanation for these chromic
effects could involve interchain interactions giving rise
to an excitonic effect.34 Thus, the band located around
428 nm for poly(2,5-di-2′-ethylhexyl-p-phenyleneethy-
nylene) would involve head-to-tail molecular arrange-
ments (J-aggregates). Similar arguments were previ-
ously raised by Lieser et al. on the basis of their electron
microscopy experiments.21 These explanations are also
supported by recent results on PPEs showing a strong
dependence of the UV-vis absorption features at room
temperature with the bulkiness of the side chains and
interchain distances.35 Finally, it is worth noting again
that poly(fluorenyleneethynylene)s24 and poly(2,5-thie-
nyleneethynylene)s25 do show some thermochromic and
solvatochromic transitions whereas poly(3,6-carbazole-
neethynylene)s26 and poly(2,7-carbazoleneethynylene)-
s27 do not. On the basis of their conformational analyses,
it is difficult to explain these different behaviors, and
one possible explanation could be related to different
interchain interactions in these polymeric systems.
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Conclusion
The thermochromic transition of poly(2,5-di-2′-ethyl-
hexyl-p-phenyleneethynylene) has been studied by using
a combination of complementary techniques. From UV-
vis, X-ray, and calorimetric measurements, a structural
change from an ordered to a disordered state of the
polymer has been observed. In sharp contrast to ther-
mochromic polythiophenes, temperature-dependent vi-
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significant spectral shift. Moreover, solid-state NMR
spectra did not reveal any important modification of the
peaks related to the conjugated backbone upon heating.
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observed cannot be associated with important confor-
mational changes of the polymer backbone. It is there-
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polymer chains. This assumption is in agreement with
calculations performed on model compounds.
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Acknowledgment. This work was supported by
NSERC grants. The authors are grateful to Professor
M. Auger for helpul discussions about the NMR solid-
state measurements and Rodica Plesu for her help and
valuable advice for X-ray and DSC measurements.
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