Contribution of chromophores with different numbers of repeat units to overall emission of MEH-PPV: An experimental and simulation study

Article abstract of DOI:10.1016/j.polymer.2013.05.028

In the present study, poly[1-methoxy-4-(2′-ethylhexyloxy)-p- phenylenevinylene] (MEH-PPV) was synthesized, modified and characterized by FTIR, NMR, GPC, UV/vis absorption, fluorescence, and mass spectroscopy. The effective conjugation length was controlled by introducing physical and chemical defects into the chain backbone. It was observed that absorption and emission spectra of polymer chains were blue-shifted by importing structural deficiencies. Chemical defects were introduced by controlled oxidation of vinylene double bonds using m-chloroperbenzoic acid and physical defects were induced by tuning polymer-solvent interactions (varying solvent quality). It was shown for the first time that the recorded fluorescence spectrum of MEH-PPV can be reconstructed using the emission data of oligomeric constituents and the contribution of chromophores with different numbers of repeat units to overall emission can be quantified. The effect of excitation energy on the emission pattern was also investigated. It was shown that the emission contribution of shorter chromophores to overall emission is detectable using high excitation energies. Finally, atomistic molecular dynamic (MD) simulation was used to investigate the polymer chain conformation in methanol and chloroform. The results obtained from torsional angles studies confirmed the presence of a great number of induced physical defects in the backbone and, hence, a collapsed conformation of polymer chains in methanol. Evaluations of physical defects along the chain proposed the average number of coplanar phenyl rings of ～5 and 14 for polymer chains in methanol and chloroform respectively.

Full text of DOI:10.1016/j.polymer.2013.05.028

Polymer xxx (2013) 1e13
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Contribution of chromophores with different numbers of repeat units
to overall emission of MEHePPV: An experimental and simulation
study
,
b
Mahdi Samadi Khoshkhoo a, Faramarz Afshar Taromi a , Elaheh Kowsari ,
*
Elham Khodabakhshi Shalamzari a
a Polymer Engineering and Color Technology Department, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
b Chemistry Department, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
In the present study, poly[1-methoxy-4-(20-ethylhexyloxy)-p-phenylenevinylene] (MEHePPV) was syn-
thesized, modified and characterized by FTIR, NMR, GPC, UV/vis absorption, fluorescence, and mass spec-
troscopy. The effective conjugation length was controlled by introducing physical and chemical defects into
the chain backbone. It was observed that absorption and emission spectra of polymer chains were blue-
shifted by importing structural deficiencies. Chemical defects were introduced by controlled oxidation of
vinylene double bonds using m-chloroperbenzoic acid and physical defects were induced by tuning polymer
esolvent interactions (varying solvent quality). It was shown for thefirst timethat the recorded fluorescence
spectrum of MEHePPV can be reconstructed using the emission data of oligomeric constituents and the
contribution of chromophores with different numbers of repeat units to overall emission can be quantified.
The effect of excitation energy on the emission patternwas also investigated. It was shown that the emission
contribution of shorter chromophores to overall emission is detectable using high excitation energies.
Finally, atomistic molecular dynamic (MD) simulation was used to investigate the polymer chain confor-
mation in methanol and chloroform. The results obtained from torsional angles studies confirmed the
presence of a great number of induced physical defects in the backbone and, hence, a collapsed confor-
mation of polymer chains in methanol. Evaluations of physical defects along the chain proposed the average
number of coplanar phenyl rings of w5 and 14 for polymer chains in methanol and chloroform respectively.
Ó 2013 Elsevier Ltd. All rights reserved.
Article history:
Received 13 December 2012
Received in revised form
8 May 2013
Accepted 12 May 2013
Available online xxx
Keywords:
Conjugated polymers
Fluorescence spectrum decomposition
Molecular dynamic simulation
1. Introduction
semiconductors appropriate candidates for various applications
such as polymeric light-emitting diodes (PLEDs) [1e3], photovoltaic
Because of their semiconducting and luminescent properties, the
position of conjugated polymers in modern microelectronic in-
dustries has been continuously grown and the importance of this
class of materials due to their unique implementations has always
attracted a great deal of researches [1]. Solubility in common
organic solvents, ease of fabrication, high photoluminescence
quantum yields and low operating voltages are some typical reasons
for utilizing organic molecular semiconductors in technological
applications. The unique physical properties of these materials such
as their flexibility and facile processability to form useful and robust
structures jointed with acceptable photophysical behaviors always
attract a great number of research groups. Such characteristics make
diodes [4e9], laser devices [10e12], light-emitting electrochemical
cells [13e15], and chemical sensors [16,17]. One of the most
important and promising families of conjugated polymers for PLED
applications is poly(p-phenylenevinylene) (PPV) along with its de-
rivatives that have been extensively studied since the discovery of
the first polymer-based light-emitting diode [18]. Undoubtedly, the
most popular and widely utilized polymer in this class is the well-
known prototype poly[1-methoxy-4-(20-ethylhexyloxy)-p-phenyl-
enevinylene] (MEHePPV) that has been the subject of a great
number of studies during the past few decades [19e29]. Its chemical
structure contains an aromatic backbone with slightly polar
methoxy and 2-ethylhexyloxy substituents on the phenyl rings that
enable polymer to dissolve in a wide range of common organic
solvents allowing the use of facile processing techniques. The
asymmetric alkoxy side chains of the polymer also prevent the
backbone from crystallization when cast into the films. An extensive
* Corresponding author. Tel.: þ98 21 64542401; fax: þ98 21 66418107.
E-mail addresses: m.samadi.kh@aut.ac.ir, afshar@aut.ac.ir (F.A. Taromi).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.05.028
Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

2
M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
number of reports on photophysical and electronical properties of
this polymer can be found in literature [20e22,27,29e33].
It is well known that optical, electrical and optoelectronic
properties of conjugated polymers directly depend on the extent of
effect of polymer chains in solid state from their conformation in
solution has been confirmed by many research groups [42e44]. It is
well accepted that solvents strongly affect the morphology of
prepared films and, hence, the light-emitting behavior of devices.
An understanding of chain conformation in solid state determines
the electroluminescence properties and helps to improve the effi-
ciency of electroluminescent devices. Technological importance of
chain conformation in solution phase has motivated simulation
studies by different research groups [19,24,25,28,45,46]. Over the
course of past decade, many conformations ranging from “molten
globule”, “toroid”, “rod”, “defect coil”, and “defect-cylinder” have
been assumed for isolated MEHePPV chains in poly(methyl
methacrylate) (PMMA) matrix [19]; however, a recent work has
proposed that an approximately defect-free MEHePPV chain
adopts a rod-like structure in matrix [41]. It is of great importance
to understand the solution properties of MEHePPV chains to better
control the properties of subsequently formed thin films through
the so-called memory effect [21]. Since a few number of reports on
simulation of conformation of MEHePPV chains in solution phase
can be found in literature [24,28,45,47], it seems that more studies
are needed in this field.
In this study, at first the synthesis and modification of MEHe
PPV by controlled oxidation of vinylene double bonds have been
reported. Electron delocalization was confined in shorter sequences
(with controlled number of repeat units) by introducing chemical
defects. The recorded fluorescence spectra were well reconstructed
using emission data of oligomeric constituents. The contribution
from each chromophore with different numbers of monomer units
to overall emission was determined based on reconstruction of
emission profiles. To our best of knowledge, this is the first report
on resolving the fluorescence spectrum for MEHePPV in terms of
its oligomeric units. Next, the physical controlling of conjugation
length, inducing by polymeresolvent interactions, was investi-
gated. Effect of excitation energy on emission pattern was
completely studied and the same procedure for reconstruction of
the emission profiles was applied. Finally, MD simulation was used
to investigate the conformation of polymer chains in two different
solvents. Moreover, detailed studies on torsional angles around the
single CeC bond and the angles between planes, defined by two
successive phenyl rings, have been presented.
delocalization of
p-electrons over conjugated sequences i.e. effec-
tive conjugation length [29,34]. Several studies have found the
effective length of electron delocalization of w10e15 monomer
repeat units for MEHePPV [19,35], in which
p-orbitals are in a
coplanar arrangement. A control over photophysical behavior of a
conjugated polymer can be achieved by controlling the average
conjugation length [20,29]. Several approaches to control the
electron confinement such as partial elimination of precursors [36],
direct post-functionalization of the conjugated backbone
[26,33,37], selective elimination of organic soluble precursor [22],
and introducing tetrahedral defects into the backbone [38,39] have
been reported by different research groups. For instance, Zhang
et al. utilized a commonly used radical initiator, azobisisobutyr-
onitrile (AIBN), to regulate the conjugated sequences by radical
addition onto the double bonds of the conjugated backbone [33]. In
this case, free radicals, generated through decomposing of hydro-
lyzed AIBN, randomly attacked on vinylene linkages along the
chain. With increasing the reaction time, the concentration of de-
fects was increased and resulted in confining delocalized electrons
in shorter conjugated blocks. In another work reported by Park
group, controlled oxidation of double bonds situated between
phenyl rings was used for restricting the polymer’s exciton delo-
calization length [23,26]. In this case, the epoxide rings, which were
formed through the oxidation reaction, restricted delocalized
electrons in shorter conjugated sequences that led to a controlled
red shift in absorption and fluorescence spectra.
A control over conjugation length can also be achieved by
adjusting the solvent quality [20,27,29]. Physical defects such as
twisted points along the chain can break the electronic conjuga-
tion; the number of which is dictated by chain conformation. These
structural deficiencies lead to energy absorption by shorter conju-
gated segments (the average length of shorter blocks relies on the
extent of chain collapse). As the conformation of polymer chains in
solvents depends on local polymeresolvent interactions, the con-
trol over solvent quality can control the amount of physical defects
as well as the extent of
p-electron delocalization [20,21,27,29,32].
The quality of solvent and its interaction with polymer backbone
can also affect intramolecular dynamic of phenyl rings. Traiphol
et al. have shown that how an aromatic solvent can couple strongly
with phenyl rings causing the restriction of the ring rotation [32]. It
has been shown that intramolecular dynamic of phenyl rings cau-
2. Experimental section
MEHePPV was synthesized in our laboratory according to the
procedure described in the Supplementary material (Scheme 1A).
Controlled oxidation of polymer chains was performed by the
method recently reported in the literature (Scheme 1B, see
Supplementary material for more details) [23,26]. IR spectra were
acquired on Equinox 55 FTIR/FTNIR spectrometer from Bruker
Corporation in diffuse reflectance mode using 1:200 KBr pellets. 1H
and 13C NMR spectra were obtained on Bruker Avance 500 and
400 MHz spectrometers using a 5 mm NMR tube at 298 K (sample
concentration was about 5 mg in 1 ml chloroform-d). Mass spectra
were recorded on a FINNIGAN-MAT 8430 spectrometer operating
at an ionization potential of 70 eV, in m/z. Molecular weight of the
polymer was measured in THF against polystyrene (PS) standards
with a solvent flow rate of 1.0 ml/min on a PLgel MIXED-C
ses the out-of-plane twisting of
p-orbitals which in turn can
interrupt delocalization of -electrons along the backbone [40].
p
The other parameter that can affect photophysical behavior of
conjugated polymers in solutions is the formation of chain associ-
ations. The chain organization and photophysical properties of
MEHePPV in poor solvents or in concentrated solutions are
different from isolated chains. Interchain association of conjugated
backbones, including aggregates and agglomerates, have been well
studied and their effects on optical properties are well reported in
literature [30,31].
The fabrication process of PLED devices requires the formation
of a thin film from polymeric solution. Because of favorable local
pe
p
interaction and rigid-rod nature of MEHePPV chains [41], they
(300 ꢀ 7.5 mm, 5
mm PS gel) column from Agilent Technologies
tend to form aggregates in solid state. Formation of aggregates is an
important issue as the polymer chains in aggregates represent
different optoelectronic properties from that of the isolated chains
due to formation of novel electronic species [30,31]. It should be
noted that optical and electronic properties of the final films
depend sensitively on the conformation of polymer chains in the
solution which is used to spin-coat the films [21]. The memory
using ETA-2020 e RI e and viscosity detector from Fa.Bures and
miniDAWN-LS detector from Wyatt Technology. Absorption spectra
were recorded on a double beam PerkineElmer Lambda 45 UV/vis
spectrometer using a 1 mm thick cuvette. Measurements of pho-
toluminescence (PL) emission and excitation spectra were carried
out on a PerkineElmer LS55. For PL measurements, the solutions
were made by adding the freshly synthesized polymer to solvent,
Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
3
Scheme 1. (A) The synthetic route to the monomer and polymer. (B) Oxidation of MEHePPV with m-chloroperbenzoic acid.
stirring, and sonicating for an appropriate amount of time (polymer
concentration were ranged from 0.001 to 0.05 mg/ml). Freshly
prepared solutions were filtered through 0.45 mm filter and kept in
bending, torsion, and inversion components, respectively. The
chemical structures and partial charges of polymer chain, chloro-
form and methanol are shown in Fig. 1, and their force field pa-
rameters used to calculate the intra- and intermolecular
interactions are summarized in Table S1. Detailed information on
construction of simulation systems can be found in the
Supplementary material.
a refrigerator (w4 ꢁC) for about 24 h or more to precipitate the
aggregates which were in a form of colloidal particles. The top
portion of samples which were clear solutions, probably saturated
by majority of the isolated chains, were used for spectroscopic
measurements.
4. Results and discussion
3. Simulation protocols
4.1. Controlled oxidation
In the simulation part, methanol molecule was considered as
three interaction sites (oxygen atom, methyl group and hydrogen
atom) and the total potential energy was represented as the sum of
intra- and intermolecular potentials. As the target is to simulate
polymer chain conformation in solvent at 293 K, molecular flexi-
bility of solvent molecules may not be an important factor and the
rigid models for intramolecular interactions can be used. Accord-
ingly, bond stretch and angle bend interactions for methanol
molecules were described using the harmonic equations of the
DREIDING force field [48]. The more deterministic intermolecular
interactions were described by optimized potentials for liquid
simulations (OPLS) with adjusted parameters proposed by Honma
et al. [49]. Chloroform molecules were simulated using the
DREIDING potentials as it was previously tested and successfully
used by Hubel et al. [50]. The atomic charges of the mentioned
solvents were adopted from the reported values in corresponded
references. Polymer chains were also described by the explicit all-
atom model using DREIDING force field [48]. The total potential
energy is:
m-Chloroperbenzoic acid is an effective and widely used
oxidative reagent for epoxidation of ethylene backbones [51,52].
Using harsher oxidative reagents can lead to pernicious effects on
polymer properties. Upon adding m-chloroperbenzoic acid, it sta-
tistically reacts with the ethylene moieties along the polymer chain
and forms epoxide rings, which can break the conjugated bands,
thereby decreasing the average exciton delocalization length.
When it was added to the solution of MEHePPV in chloroform, the
solution color gradually changed from red-orange to yellow to
clear. With an increased reaction time, the emission wavelength of
the solution changed from 554 nm (pristine polymer) to 476 nm
(heavily oxidized polymer) and the absorption spectrum of the
sample was blue-shifted from 493 nm to 369 nm as shown in Fig. 2.
Fig. 2A shows a digital photograph taken under UV lighting
(exciting wavelength at 365 nm) for pristine and oxidized MEHe
PPV samples. As can be seen, the samples exhibited red-orange,
yellow, green-yellow, green, blue-green, and blue fluorescence
emission signatures, indicating an effective decrease in the exciton
delocalization length. The observed blue-shift is attributed to an
increase in the number of conjugation breaks per chain due to the
formation of epoxide groups as the reaction proceeds (Fig. 3A,
inset). The adsorption spectrum of pristine MEHePPV in
Etotal ¼ EvdW þ EQ þ Ebond þ Eangle þ Etorsion þ Einversion
(1)
where Etotal, EvdW, EQ, Ebond, Eangle, Etorsion, and Einversion are the total
energy and the van der Waals, electronic, bond-stretching, angle-
Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

4
M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
Fig. 1. (A) Five randomly generated initial states of polymer chains. (B) Chemical structure and partial charges of MEHePPV, chloroform, and methanol.
chloroform is very broad and featureless, indicating the distribu-
tion of chromophores and concentrations of tetrahedral defects.
The synthesis procedure for MEHePPV chains can randomly
introduce these defects, where conjugated links are replaced with
tetrahedral links, thus interrupting conjugation. Based on a previ-
ously published correlation, spectroscopic analysis of our sample
was performed which led to a mole percent of defect estimate of
w4% [22].
As shown in Fig. 3A, the formation of epoxide rings was
analyzed and confirmed by FT-IR spectra of pristine and oxidized
polymers. The trans-vinylene CH stretch in the vibrational spec-
trum of pristine MEHePPV that appeared at 3056 cmꢂ1 completely
disappeared in the oxidized counterpart which indicates a signifi-
cant decrease in the number of C]C bonds. Simultaneously, the
appearance of a new peak at 800 cmꢂ1, which is known as a typical
value for an asymmetric stretch of an epoxide ring, confirmed the
reaction of m-chloroperbenzoic acid with conjugated C]C groups
along the MEHePPV chains. However, there were no significant
peaks in the range of 1600e1800 cmꢂ1 (carbonyl stretching region)
in oxidized MEHePPV samples, indicating no chain scission in the
polymer backbone. The lack of evidence for significant C]O for-
mation supports the previously reported high quantum yields for
oxidized MEHePPV chains [26].
Notably, the photoluminescence (PL) spectra for more oxidized
samples were highly structured with multiple spectral shoulders.
According to an earlier report by Meier et al. [34], PL maxima at 413,
460, 496, 521, and 534 nm are characteristics of dialkoxy-
substituted oligo(1,4-phenyleneethenylene) with n ¼ 2, 3, 4, 5,
and 6 monomer units, respectively. Interestingly, the main char-
acteristics of obtained fluorescence spectra manifest themselves at
these points. The PL spectra of oxidized polymer chains are su-
perpositions of those from isolated segments with different de-
grees of conjugation lengths. Thus, the excitons in these blocks are
restricted by chemical defects and epoxide groups in the conju-
gated backbone. For a detailed investigation of the way in which
the emission intensity and the positions of the peaks varied during
the oxidation process. As it can be seen, in parallel with the
sequential blue-shift of emission spectra, the intensities were
increased and then decreased after passing
a maximum at
approximately 500 nm (see the Supplementary material for addi-
tional discussion). It is nicely demonstrated in Fig. 3B that further
oxidation doses did not lead to further significant wavelength shifts
and the emission lmax remained constant at the end of the reaction.
Thus, it is important to monitor the reaction to prevent over-
oxidation. It is reasonable to consider the chains of MEHePPV as an
ensemble of conjugated segments of chromophores linked by
physical and chemical defects. Assuming this scenario, one should
be able to decompose each emission spectrum into a sum of
emission data of coplanar phenyl rings with different conjugation
lengths. To investigate this possibility, we decide to rebuild the
emission spectra from the emission profiles of shorter constituent
chromophores. One difficulty here is to have the emission spectra
of correspond oligomers with different repeat units. Another point
is the trend in which the emission of oligomers varied with
increasing the number of repeating units. Recently, Tilley et al. [53]
reported the preparation of MEHePPV oligomers and described
their fluorescence and other photophysical properties. Unfortu-
nately, their work only focused on MEHePPV oligomers with 3, 4,
and 5 repeat units. However, to reconstruct the emission spectrum
of polymer chains from corresponding constitutive chromophores,
emission data from conjugated sequences with larger numbers of
repeating units are also needed, as implied by a comparison of the
works of Meier and Tilley. According to the results reported by
Tilley et al. [53], the emission maxima of trimer, tetramer, and
pentamer oligomers, based on the polymer backbone structure of
MEHePPV, occurred at 450, 494, and 520 nm, respectively, which
are very close to the emission maxima of dialkoxy-substituted
oligo(1,4-phenyleneethenylene) with the same repeat units re-
ported by Meier (460, 496, and 521 nm). Moreover, the emission
maxima of MEH-stilbene (a dimer) was detected at 414 nm by
Traiphol et al. [31], which is in good agreement with the reported
emission maxima of dimers in Meier’s work (413 nm) [34]. In
addition, the convergence of the emission wavelength with
delocalization of
p-electrons is confined, fluorescence measure-
ments were performed during the oxidation reaction (Fig. 3B). The
first point that can be obtained from Fig. 3B is the trend in which
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emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
5
Fig. 3. (A) Infrared spectra of MEHePPV (above) and oxidized MEHePPV (below). The
sample used for oxidized MEHePPV was the polymer with the highest oxidation de-
gree with an emission at lmax ¼ 476 nm as shown in Fig. 2C. (B) Photoluminescence
spectra of MEHePPV solutions in chloroform at a concentration of approximately
0.025 mg/ml during the oxidation reaction (exciting wavelength at 350 nm).
Fig. 2. (A) Digital photograph of luminescence from 0.1 mg/ml pristine and oxidized
MEHePPV solutions in chloroform under UV lighting (exciting wavelength at 365 nm).
(B) UVevisible absorption spectra of pristine and oxidized MEHePPV solutions in
chloroform at a concentration of 0.025 mg/ml. (C) Normalized photoluminescence
spectra of pristine and oxidized MEHePPV solutions in chloroform at a concentration
of 0.025 mg/ml.
increasing the number of repeating units has been well demon-
strated for different conjugated oligomers [34]. It is well accepted
that the exciton delocalization length of MEHePPV is approxi-
mately 10e15 repeat units [19,35]; therefore, molecular weight is
an insignificant factor on average effective conjugation length and
optical properties of high molecular weight polymers often corre-
spond to those of oligomers with a sufficient chain length [34]. As
one would expect, the convergence behavior of MEHePPV was
similar to other conjugated polymers. Therefore, by comparing the
limiting value of emission wavelength at w554 nm for MEHePPV
with that reported by Meier et al. [34], the emission wavelengths
for oligomers of dialkoxy-substituted phenyleneethenylenes can be
used as a good approximation instead of emissions from MEHePPV
oligomers. According to the above discussion, the main reported
characteristics were chosen to reconstruct the emission profiles. As
a consequence, it is assumed that each PL spectrum is the result of
five components. Four components are the emissions resulted from
intrachain singlet excitons in chromophores with 2, 3, 4, and 5
repeat units centered at approximately 415, 460, 492, and 521 nm,
respectively. Emissions from chromophores with more than 5
repeat units occurred in the range of 534e552 nm; therefore, due
to the small change of emission wavelength in this range, they were
considered as one component. Emission from low-energy aggre-
gates (small shoulder approximately at 600 nm) was also included
in this component. Fig. 4 presents a typical spectral decomposition
for emissions taken from the sample 6, 16, and 36 min after starting
the oxidation reaction. The reconstructed spectra matched very
well with the recorded one. To the best of our knowledge, this is the
first report describing contributions to PL from excitons confined to
short constituent blocks that were clearly resolved. It should be
considered that the spectra reported for MEHePPV trimers, tetra-
mers, and pentamers were bimodal and had spectral shoulders
[53]. Therefore, due to similarity of the patterns, a combination of
two GaussianeLorentzian peaks was used as a characteristic
emission of components with 2, 3, 4, and 5 repeat units which are
in reasonable proportions with the profiles reported by Tilley et al.
Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

6
M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
interactions, hence allowing polymer chains to adopt extended
conformations, resulting in an increase of effective conjugation
lengths [20,27,29]. In addition, at this low-energy excitation
wavelength, the other electronic species do not absorb any photons.
Moreover, as the fluorescence spectra often reflect the properties of
the conjugated segments with the lowest transition energy gaps, it
is unlikely to have significant contributions from shorter conjugate
sequences in emission profiles. We will return to this point in the
next section where it will be indicated that, in such a good solvent,
there is no considerable contribution from chromophores with 2, 3,
4, and 5 repeat units to overall emission. Thus, the polymer emis-
sion in the aforementioned condition can be considered as a
representative emission from chromophores with more than 5
repeat units and low-energy aggregates. A discussion on this issue
is postponed to the following section.
With respect to the decomposition of each spectrum, the con-
tributions of different components to overall emission were
determined by evaluating the fractional area under the emission
curves. From the results presented in Fig. 5, one can recognize three
different behaviors contributed by conjugated blocks to overall
emission by continuing the oxidation reaction: (i) the emission
contribution from chromophores with 2e3 repeat units continu-
ously increased by lengthening the reaction time (Fig. 5A); (ii) the
emission contributions from chromophores with 4e5 repeat units
increased initially and then decreased (Fig. 5B); and (iii) the
emission contribution of chromophores with more than 5 repeat
units and low-energy aggregates continuously decreased
throughout the oxidation reaction (Fig. 5C). Detailed discussion on
these different behaviors has been provided in the Supplementary
material.
The obtained results from this section encouraged us to examine
the same procedure in cases where the electronic conjugation was
physically broken by adjusting the polymeresolvent interactions
rather than chemical modification of the structures. In the next
section, the effect of solvent quality on effective conjugation length
and the effect of excitation wavelength on emission profile have
been completely investigated for polymer chains in different sol-
vents. It will be shown that increasing excitation energy led to the
manifestation of shorter chromophores in overall emissions.
4.2. Physical confinement of delocalized p-electrons
Polymer chains can twist to form various conformations in
different solutions. Their conformation, which is a dominant factor
in controlling the photophysical behaviors, can be finely tuned by
controlling polymeresolvent interactions [20,21,29e32]. The
effective conjugation length of polymer chains can be considered as
the extent of
p-electrons delocalization in each conjugated block.
Based on the magnitude of polymeresolvent interactions, the
extent of chain collapse is changed which can control delocalization
of p-electrons over the entire chain [29e32]. Rather than chemical
Fig. 4. Typical spectral decomposition into the sum of emission data of conjugated
chromophores for emissions taken from the samples (A) 6 min, (B) 16 min, and (C)
36 min after starting the oxidation reaction.
modifying the polymer backbone presented in the previous section,
a control over conjugation length can also be achieved using sol-
vent with different qualities. As discussed in the previous section,
single molecule of MEHePPV can be viewed as a chain consisting of
quasi-localized chromophores linked by chemical and physical
defects. Therefore, adjusting the quality of solvent should give the
possibility of adjusting the conformation of the chains in solution,
and hence, the number of physical defects along the polymer
backbone. In the case of MEHePPV, multichromophoric picture is
accounted for the photophysical behavior such as line shape and
line width of the optical absorption and emission spectra [22]. Here
we elaborate in more detail on physically controlling the conjuga-
tion length of MEHePPV chains using different solvents and its
relationship with photophysical characteristics.
[53]. The first peak in each component represents the main char-
acteristic of oligomer emission and the second smaller peak is
related to vibronic structures. In peak synthesis for these compo-
nents, the line width (full width at half-maximum, FWHM) of the
GaussianeLorentzian peaks was kept essentially constant in a
particular spectrum. For contribution of chromophores with more
than 5 repeat units and low-energy emissions from aggregates, the
emission spectrum of polymer solution in chloroform at excitation
wavelength of 500 nm was used. This assumption was adopted
because there are convincing evidences that MEHePPV tends to
unfold in chloroform to maximize favorable polymeresolvent
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emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
7
intensity changes, the overall pattern of emission is independent of
different excitation energies (see Fig. 6A and B). In parallel, exci-
tation profiles detected at different emission wavelengths are also
imitating from a same pattern without any special features that
indicate the presence of only one type emitters in the conjugated
chains [29]. From the obtained results, a significant intrachain en-
ergy transfer from short conjugated sequences to those with lowest
transition HOMOeLUMO energy gap is inferred [22]. It is well
known that the mentioned solvents can dissolve MEHePPV to a
considerable amount. In such good solvents, due to favorable
polymeresolvent interactions, polymer chains can extent their
conformation which results in more statistically coplanar
arrangement of phenyl rings. This important factor promotes
delocalization of p-electrons along the backbone and allows poly-
mer chains to adopt a conformation with highest effective conju-
gation length. Therefore, resemblance of excitation and emission
spectra in these solvents can be rationalized by considering the
nature of MEHePPV chromophores.
As it was mentioned before, the state of chain conformation can
be adjusted using different solvents. Because the formation of
chromophores in MEHePPV is driven by the relatively coplanar
conformation of phenyl rings, twists along the backbone disrupt
the p-conjugation leading to light absorption by shorter conjugated
blocks. This is what occurred for polymer chains in poor solvents.
The emission and excitation profiles of MEHePPV in methanol and
n-hexane are shown in Fig. 6C and D, respectively. The first point
that should be highlighted in these figures is the obvious blue-shift
of emission spectra which is caused by the presence of high fraction
of physical defects along the backbone due to extreme collapse of
polymeric chains. The second point is that for polymer chains in
such poor solvents, the fluorescence spectra have become more
structured in comparison with featureless emission profiles of
polymer chains in chloroform, toluene and THF. This is due to the
fact that in methanol there must simply be fewer long conjugated
sequences for the energy to migrate to following excitations.
Emerging of these shoulders is an evidence for suppression of en-
ergy transfer from short excited electronic species to their neigh-
boring longer chromophores. Therefore, emission from shorter
chromophores can be also detected in overall emission spectrum.
Moreover, excitation profiles recorded at different emission wave-
lengths indicate multiple absorption processes which proves the
existence of at least two distributions of electronic species with
distinct ground- and excited-state properties [29,30]. The existence
of these distinct species, well packed and isolated chains, have been
investigated and confirmed by several research groups [20,29,54e
56]. Although, as explained in experimental section, the portion of
solutions which are probably saturated by majority of the isolated
chains were used for spectroscopic measurements, however; it is
expected that polymer chains in an unfavorable environment such
as methanol take different pathways of molecular rearrangements
includes [30,57]: (i) aggregates which refer to those interchain as-
sociations that alter the photophysical characteristics of chromo-
phore, (ii) agglomerations which beyond them, chromophores do
not interact electronically, and (iii) collapsed isolated chains. For
MEHePPV in methanol, excitation at 455 nm results in a broad and
approximately featureless emission spectrum centered at 521 nm
with a small shoulder at 491 nm. Using excitation energies at the
blue-edge of the absorption band at lex ¼ 335 and 380 nm yields a
structured emission spectrum centered at 491 nm with obvious
shoulders at 413, 459 and 521 nm. Again the main characteristics of
obtained fluorescence spectra have been emerged in the maximum
emission wavelengths of oligomeric counterparts reported in
literature [31,34,53]. Therefore, the same procedure described in
the previous section was applied to develop the contribution of
each chromophore to overall emission profile at different excitation
Fig. 5. Three different behaviors were detected for contributions of conjugated se-
quences with different numbers of repeat units to overall emission spectra. The results
were obtained by evaluating the fractional area under the curves of each component
for (A) blocks with 2e3 repeat units, (B) blocks with 4e5 repeat units, and (C) blocks
with >5 repeat units and emission from low-energy aggregates.
The emission and excitation profiles of MEHePPV in different
solvents are illustrated in Fig. 6. In the case of MEHePPV solutions
in chloroform, toluene, and THF, the emission spectra are inde-
pendent of excitation energies and mainly consist of a peak at
w554 nm with a shoulder near 595 nm (data for THF are not pre-
sented due to similarity). The main peak is attributed to intrachain
singlet exciton undergoing fast emission on a picosecond time scale
and the shoulder which manifests itself at the red edge of emission
spectra is related to aggregates interchain exciton decay with life
times on the nanosecond scale [27,30]. Apart from absolute
Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

8
M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
Fig. 6. Each part depicts the emission spectra at different lexc (red profiles) and excitation profiles at different lemi (blue ones) of MEHePPV solutions in (A) chloroform, (B) toluene,
(C) methanol, and (D) n-hexane. The general features of spectra for polymer chains in THF are the same as those represented for chloroform and toluene. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
wavelengths. A typical decomposition of fluorescence spectra for
MEHePPV in methanol is represented in Fig. 7. Again an excellent
match between reconstructed and recorded spectra was obtained.
It is obvious that the relative intensity changes at different wave-
lengths are proportional to contribution of corresponded
components to overall emission. As a result, the contribution per-
centage of different chromophores can be obtained by simply
dividing the area under each component to the area of the overall
emission spectrum. It is nicely demonstrated in Fig. 7 that polymer
emission spectra for different excitation wavelengths can be well
Fig. 7. Typical decomposition of emission spectra for polymer chains in methanol with different excitation energies.
Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
9
reconstructed from emission profiles of defined oligomeric com-
ponents. Therefore, the same procedure was applied for the emis-
sion spectra of polymer chains in other solvents and the results are
listed in Table S2 (details on contribution of chromophores to
overall emission can be found in the Supplementary material).
The net result that can be obtained from the above discussion is
that the quality of solvent can significantly alter the photophysical
behavior of the system. It is shown that how electronic conjugation
can physically be broken by adjusting polymeresolvent in-
teractions. Overall emission spectra for different systems can be
well resolved in terms of defined shorter constituent components
and their emission contributions can be quantified. It is also shown
that high-energy excitation wavelengths lead shorter chromo-
phores to manifest themselves in overall emission spectrum.
Finally, it is pertinent to add here that in conjugated polymers,
before radiative relaxation of each chromophore, the excitation
energy tends to migrate to longer conjugated blocks [58,59]. Since
the fluorescence spectra often reflects the chromophores with the
lowest transition energy gap [22,39,60], PL may be more compli-
cated in terms of the fraction of each type of chromophores pre-
sent; however, it is worth pointing out that in so-far presented
discussion, the aim has been their relative contribution in overall
emission and not their real populations. As it was shown before,
even in very dilute solutions, conjugated polymers experience
intrachain energy transfer to longer conjugation length segments;
the extent of which is proportional to probability of finding long
conjugated sequences [22]. In the next section, to verify the ob-
tained experimental results, we will focus on simulation of the
conformation of a typical MEHePPV chain in both chloroform and
methanol (as solvents with good and poor qualities, respectively).
4.3. Simulation of polymer chain conformation
The obtained experimental results, presented in previous sec-
tions, encouraged us the run of following simulation study in order
to more detailed conformational analysis. The conformational
study of polymer chains in chloroform and methanol has been
carried out by means of MD simulation as described in Section 3. A
range of random possible conformations that can be adopted by a
typical MEHePPV chain with 100 repeat units are represented in
the first column of Fig. 8. These conformers were randomly
generated, minimized, and utilized as five initial structures in ten
MD simulations in chloroform and methanol. Starting from
different initial structures led to conformations with the same final
energies for each solution. Typical conformations of polymer chain
at equilibrium phase in two solvents (with the corresponded initial
structures used for MD simulations) are also presented in Fig. 8.
Apparently, solvent quality has significant effect on chain
Fig. 8. Initial states used for MD simulations and a typical conformation of 100-unit MEHePPV chains at equilibrium phase in methanol and chloroform.
Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

10
M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
conformation. As it can be seen, polymer chains in chloroform tend
to unfold to maximize favorable polymeresolvent interactions and
adopt an extended conformation, resulting in the increase of
effective conjugation length. On the other hand, it is nicely
demonstrated in Fig. 8 that polymer chains tend to adopt a collapse
conformation in methanol to minimize the number of repeating
units interacting with solvent molecules. Here, the obtained chain
conformations are the consequence of competitions between
inherent stiffness of polymer backbone and intermolecular inter-
action of MEHePPV chain with poor solvent. As a consequence of
unfavorable interactions, a greater number of torsional defects are
present along the backbones, which in turn, lead to shorter
conjugation lengths. Irrespective of which initial state is used for
simulations, the radius of gyration (Rg) and end-to-end distance
(DEE) of final conformations in chloroform are higher than their
counterparts in methanol for all the cases. Note that the mean coil
size often reflects the solvent quality for the polymer in a specific
solvent. Thus, the results given in Fig. 8 clearly support the effective
role of solvent in controlling the chain conformation. The obtained
structures are in reasonable agreement with experimental results
based on fluorescence spectroscopy presented in previous sections.
To analyze the conformation of the chains in more detail, the
torsion angles around the vinylene single CeC bonds, 4 and 4 , for all
the cases are collected along the backbone during equilibration phase
at each step of MD runs (for 1000 step of 1 fs) and the net results are
presented in Fig. 9. The distributions reported in Fig. 9A clearly
indicate that the resulting torsion angles (4) oscillate around 0 and
ꢃ180ꢁ. The symmetryof histograms isrelated totwo possiblerotation
signs around the single CeC bond. In the case of polymer chains in
chloroform, the distribution is narrower and the population of
dihedral angles around ꢃ180ꢁ is less in comparison to their coun-
terparts in methanol. The presence of two peaks (at 0 and ꢃ180ꢁ) is
due to the different orientations of the vinyl double bond and the
alkoxy side chains on the neighbor phenyl rings (Fig. 9C). The smaller
population of torsion angles around ꢃ180ꢁ can be attributed to steric
effects between substituents on phenyl rings and the nearby
hydrogen atoms of the neighboring vinylene linkage as shown in
Fig. 9C. If the population of torsion angles around 0 and ꢃ180ꢁ is
defined as S1, S2, and S3 respectively, the ratio of S1 to S2 þ S3 indicates
the way in which vinyl double bonds tend to orient themselves
relative to the adjacent alkoxy side chains (see Fig. 9). In chloroform,
this ratio is equal to 8.2 while this number for polymer chains in
methanol is equal to 2.7. Extended conformation of MEHePPV in
chloroform allows the polymer chains to rotate quite freely. The
higher amount of this ratio in chloroform solutions is presumably
attributed to easier dynamic of polymer chains in this media. Because
of favorable environmental interactions with solvent molecules,
polymerchains can freely rotate andadopt theconformation inwhich
alkoxy side chains have enough distance from neighboring vinylene
linkages to avoid steric effects. However, in methanol polymer chains
tend to fold to minimize unfavorable interactions with solvent mol-
ecules, therefore; relative to chloroform solutions, the population of
torsion angles around ꢃ180ꢁ has slightly grown (although it is less
than the population of torsion angles centered at 0ꢁ yet).
The other characteristic that can be helpful in conformational
analysis, especially for detecting the presence of conjugation breaks,
is the angle between planes defined by two successive phenyl rings,
F
¼ 4 þ 4 . The distributions of torsion angles,
F, between two
1
2
successive rings for polymer chains in chloroform and methanol are
shown in Fig. 9B. The results indicate a broad histogram for polymer
chains in poor solvent, while, the distribution is narrower in chlo-
roform. As a result, less coplanar phenyl rings can be found along the
1
2
polymer backbone in methanol. Here, the torsion angles
70ꢁ mean that ethylhexyloxy substituents lie on the same side of the
backbone while the torsion angles
of w110e180ꢁ mean that they
F of w0e
F
are placed around the backbone. If we define the population of the
first group as P1 and the population of second one as P2, then the
ratio of Pi to P1 þ P2 represents the fractional population of each
group. Calculations reveal that in chloroform, most of the ethyl-
hexyloxy substituents lay on the same side of the backbone while in
methanol, they are randomly positioned around the backbone due
to torsional defects. The obtained results clearly support the
conclusion that unfavorable interactions between methanol mole-
cules and polymer chains force the backbone to adopt a tightly
coiled conformation, induce a greater number of torsional defects,
and, hence, lead to shorter conjugation lengths.
Fig. 9. (A) The net results for distribution of the torsion angles, 4, for five MD simulations of 100-unit MEHePPV in chloroform and methanol. (B) The net results for distribution of
torsion angles, V, between two successive phenyl ring for five MD simulations of 100-unit MEHePPV in chloroform and methanol. (C) Different orientations of the alkoxy side
chains on the phenyl rings and the neighbor vinyl double bonds depend on the rotation around the vinylene single CeC bonds.
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emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
11
It is well known that planarization of phenyl rings is responsible
for delocalization of -electrons within conjugated chains. In a
also imitating the same pattern, without any special features, that
indicates the presence of only one type emitters in the conjugated
chains. In poor solvents, the emission spectra were observed to be
apparently blue-shifted and became more structured with multiple
peaks and spectral shoulders. This was attributed to the presence of
high fraction of physical defects along the backbone due to extreme
collapse of polymeric chains in such poor environment. In addition,
excitation profiles recorded at different emission wavelengths
indicated multiple absorption processes which proved the exis-
tence of at least two distributions of electronic species (i.e. well
packed and isolated chains). We have shown for the first time that
the fluorescence spectrum of MEHePPV can be well resolved using
emission data of oligomeric constituents and the contributions
from various chromophores to overall emission are able to be
evaluated. Through this way, emission contributions from different
chromophores during oxidation reaction were quantified and three
behaviors were detected. These behaviors belonged to very short
chromophores with 2e3 repeat units (their contribution continu-
ously grows during the reaction), intermediate chromophores with
4e5 repeat units (their contribution increase at first follow by a
decrease), and long chromophores with more than 5 repeat units
(their contribution continuously decrease). It was also shown that
for MEHePPV chains in chloroform, toluene, and THF, the majority
of emission comes from long chromophores with more than 5
repeat units due to extended conformation of polymer chains in
these solvents, less population of shorter chromophores and
effective energy transfer through the chain. On the other hand,
chromophores with 4 and 5 repeat units have the main role in
emission from conjugated chains in methanol and n-hexane. This is
because of collapsed conformation of polymer chains, high fraction
of physical defects, and, hence, more conjugation breaks along the
backbone. Atomistic MD simulation was used to investigate the
conformation of the chains in methanol and chloroform. Simula-
tion results demonstrated the significant effect of solvent on chain
conformation. It was obviously observed that in all the cases, Rg and
DEE of polymer chains at equilibrium phase in chloroform are
higher than their counterparts in methanol. Detailed studies on
torsional angles during dynamic of the chain well confirmed the
higher number of physical defects and, as a result, lower effective
conjugation length of polymer chains in methanol respect to
chloroform. Evaluations of physical defects along the chain pro-
posed the average number of coplanar phenyl rings of w5 and 14
for polymer chains in methanol and chloroform respectively.
p
recent study, Leener and coworkers explored the relationship be-
tween conformational disorder and the electronic structure in the
excited states by combining MD simulation to semiempirical
quantum-chemical calculations [25]. They calculated the charge
transfer character in order to investigate the effect of deviations
from planarity on conjugation along the chain. According to their
report, the cutoffs of 50ꢁ < 4 < 130ꢁ (for when 4 ¼ ꢂ4 ) and
1
2
70ꢁ < V < 110ꢁ (for when 4 ¼ V) are considered as conjugation
1
breaks where the charge transfer character is decreased to 10% of its
initial value in the fully planar conformation. The reported cutoffs
are, however, evaluated for two special cases; therefore, we cannot
consider them as physical breaks in our case where we have un-
systematic distribution of dihedral angles. In another work, Brédas
et al. investigated the evolution of electronic properties of con-
ducting polymeric materials based on aromatic rings (poly(p-phe-
nylene), polypyrrole, and polythiophene) as a function of the
torsion angle along the chain [61]. They have shown that deviation
from coplanarity has to remain lower than 40ꢁ to achieve a suffi-
cient overlapping of
p-orbitals to create a conduction bond. Their
results have indicated that up to torsion angle of w40ꢁ, the elec-
tronic properties are not significantly tailored with respect to the
coplanar situation. Therefore, this angle (for V) was chosen to
recognize physical defects in order to extract the number of
coplanar conjugated units along the backbone. Through this way,
torsional analysis was carried out on 5000 generated conforma-
tions of 100-unit MEHePPV chains at equilibration phase and the
number of defects was extracted according to the mentioned
criteria. Calculations revealed the number of coplanar phenyl rings
of w14 for polymer chains in chloroform which is in well accor-
dance with extended conformation of MEHePPV in this solvent
[19,35]. For polymer chains in methanol, evaluating the number of
defects led to average number of coplanar phenyl rings of w5
which is in excellent agreement with obtained experimental results
presented in this work. It was shown in previous section that the
majority of emission of conjugated chains in methanol comes from
chromophores with 4 and 5 repeat units. Two significant spectral
features that have been left out of present simulation are cis and
tetrahedral defects which lead to various chain conformations in
different solvents. As the defect mole percent of our sample was
estimated to be w4%, we ignored to include the defects in present
simulations and detailed investigation of chains containing such
deficiencies is left to the future works. It should be remembered
that the incorporation of such these defects has serious effects on
the conformations [24,28,45,46].
Acknowledgments
The authors gratefully acknowledge High Performance
Computing Research Center (HPCRC) of Amirkabir University of
Technology (Tehran Polytechnic) due to computational supports.
Department of Chemistry, Amirkabir University of Technology
(Tehran Polytechnic) is also appreciated for supporting character-
ization facilities.
5. Conclusions
In the present study, the effective conjugation length was
controlled by introducing either chemical or physical defects into
the backbone. In both cases, absorption and emission spectra were
blue-shifted by importing structural deficiencies. Introducing
chemical defects into the main chain was carried out using m-
chloroperbenzoic acid which statistically reacted with the ethylene
moieties along the polymer chain and formed epoxide rings.
Physical defects were induced by tuning polymeresolvent in-
teractions. The effects of excitation energy on emission pattern and
photophysical properties of polymer were completely investigated
in five different solvents. It was shown that the emission contri-
bution of shorter chromophores to overall emission is detectable
using high excitation energies. It was observed that in good sol-
vents, apart from absolute intensity changes, the overall pattern of
emission is independent of different excitation energies. Moreover,
excitation profiles detected at different emission wavelengths were
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.polymer.2013.05.028.
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Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028

M.S. Khoshkhoo et al. / Polymer xxx (2013) 1e13
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Please cite this article in press as: Khoshkhoo MS, et al., Contribution of chromophores with different numbers of repeat units to overall
emission of MEHePPV: An experimental and simulation study, Polymer (2013), http://dx.doi.org/10.1016/j.polymer.2013.05.028