Controlling chain conformation in conjugated polymers using defect inclusion strategies

Article abstract of DOI:10.1021/ja2006687

The Horner method was used to synthesize random copolymers of poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) that incorporated different backbone-directing monomers. Single-molecule polarization absorption studies of these new polymer

Full text of DOI:10.1021/ja2006687

ARTICLE
pubs.acs.org/JACS
Controlling Chain Conformation in Conjugated Polymers Using Defect
Inclusion Strategies
Giannis Bounos,† Subhadip Ghosh,† Albert K. Lee,† Kyle N. Plunkett,‡ Kateri H. DuBay,‡
Joshua C. Bolinger,*,† Rui Zhang,† Richard A. Friesner,‡ Colin Nuckolls,‡ David R. Reichman,‡ and
Paul F. Barbara†
†Department of Chemistry and Biochemistry and the Center for Nano and Molecular Science and Technology, University of Texas,
Austin, Texas 78712, United States
‡Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States
S Supporting Information
b
ABSTRACT: The Horner method was used to synthesize
random copolymers of poly(2-methoxy-5-(20-ethylhexyloxy)-
p-phenylene vinylene) (MEH-PPV) that incorporated different
backbone-directing monomers. Single-molecule polarization
absorption studies of these new polymers demonstrate that
defects that preserve the linear backbone of PPV-type polymers
assume the highly anisotropic configurations found in defect-
free MEH-PPV. Rigid defects that are bent lower the anisotropy
of the single chain, and saturated defects that provide rotational
freedom for the chain backbone allow for a wide variety of
possible configurations. Molecular dynamics simulations of model defect PPV oligomers in solution demonstrate that defect-free
and linearly defected oligomers remain extended while the bent and saturated defects tend toward more folded, compact structures.
’ INTRODUCTION
previous studies, the vinyl linkages in the PPV backbone were
randomly saturated resulting in a truncation of the chromo-
phores in the chain. This leads to a blue shift of the solution
absorption spectra by an amount dictated by the degree of
backbone saturation and concomitantly limits the range of
energy funneling that occurs in typical conjugated polymer
molecules.12 In comparison to recent results,5 the discrepancies
between older synthetic efforts and today's commercially avail-
able materials deserves attention as well as more refined techni-
ques aimed at polymer conformational control. It is natural to ask
if, with proper synthetic techniques, one can controllably alter the
conformation of a single polymer chain in a controlled and systematic
manner characterizable by single molecule and theoretical methods.
In this study, we measure the polarization anisotropy of single poly-
mer chains with a variety of “morphology directing” inclusions of
different concentrations. These “defects” include groups that
preserve the high anisotropy originally observed in MEH-PPV,
rigid groupsthatresult in verylow anisotropy conformations, and a
flexible group that allows for a broad distribution of observable
anisotropies. Molecular dynamics simulations have been per-
formed on oligomers of “defected” MEH-PPV to provide insight
into the role that defects play on the local folding of the polymer
chain in solution. This collaborative effort thus demonstrates a
powerful synthetic approach for polymer morphology control.
With the increasing interest in conjugated polymers and their
use in low-cost, easily processed devices such as organic light-
emitting diodes (OLEDs), photovoltaics, and field effect tran-
sistors, understanding the role that morphology plays in the
electronic properties and long-term stability of these devices
is critical to creating better materials.1ꢀ4 Discerning how the
structure of a single chain evolves from a single molecule to a thin
film, a bulk heterojunction or even a higher-order structure
will lead to key insights for molecular design strategies. The
single chain structure has long been a topic of interest.5ꢀ10
Over the course of the past decade, numerous studies have
provided evidence that the prototypical conjugated polymer
poly(2-methoxy-5-(20-ethylhexyloxy)-p-phenylene vinylene)
(MEH-PPV), when isolated in a matrix of PMMA, can assume a
myriad of configurations ranging from highly amorphous structures
such as the “moltenglobule”and “defect cylinder”to highly ordered
ones such as the “toroid” and “rod”.11 Recent improvements in
synthetic methods, polymer purification techniques, and optical
characterization have shown that MEH-PPV chains uniformly fold
into highly anisotropic rod-like structures, as proposed previously.5
Recent work has demonstrated that synthetic strategies to
incorporate saturated backbone defects can disrupt the nature of
polymer backbone folding by acting as kinks in the rigid PPV
chain. Polarization anisotropy measurements and simulations
show that higher defect inclusion rates lower the anisotropy
values measured for these conjugated systems.11 In these
Received: February 10, 2011
Published: May 25, 2011
r
2011 American Chemical Society
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NMR Analysis. MEH-PPV samples were prepared via the
Horner condensation method (see Supporting Information),
which has been shown to produce trans double bonds in
preference to cis double bonds.13 NMR characterization of the
methylene resonances from the ethylhexyl group from 3.5 to
4.0 ppm provides a quantitative marker for determining the
cis/trans ratio.14,15 The methylene protons adjacent to trans
double bonds arise at 3.9 ppm, while the methylene protons
adjacent to a cis double bond appear at 3.5 ppm. Using these
characterization strategies, MEH-PPVs prepared by the Horner
method have consistently reported cis inclusion rates of
2ꢀ5%.13ꢀ15 Our synthesized MEH-PPV, without the incorpora-
tion of defect sites, showed a similar cis double bond inclusion of
5.3%. However, we have found that including engineered defect
sites in the polymer backbone can dramatically disturb the
cis/trans ratio in MEH-PPV. For example, when preparing
MEH-PPV with ortho-terphenyl defect groups at 10% or 30%
defect loading, the cis double bond inclusion rate jumped to
18.7% and 13.1%, respectively. Similarly, when saturated defects
were incorporated at 10% or 30% defect loading, the cis double
bond inclusion rate was again high at 19.0% and 15.1%, respec-
tively. The inclusion of para-terphenyl defects at 10% or 30%
loading provided polymers with cis double bond inclusion rates
of 5.8% and 7.6%, respectively, similar to that of the native
MEH-PPV. We theorize that the incorporated defects template
the polymerization and influence the chemical reaction pathway
(i.e., formation of cis or trans double bonds). The para-terphenyl
defect is a linear segment that does not undermine the linear trans
double bonds of MEH-PPV, and therefore we see little change to
the amount of cis inclusion. In contrast to the linear defect,
the ortho-terphenyl defect installs a 60ꢀ kink in the polymer
backbone. This structural modification disorients the polymer
segments away from linearity and templates a more globular
assembly. We suggest cis double bonds are incorporated at higher
rates during polymerization to allow better intramolecular chain
packing and reduce the overall energy. A similar result arises from
incorporation of the conformationally flexible, saturated defect.
During polymer growth the defects could provide a hinge-like
segment that allows the polymer segments to bend onto itself,
creating more intra-molecular associations. A globular structure
can result that would warrant the inclusion of more cis defects.
Finally, the cis double bond inclusion was slightly greater in
MEH-PPVs with 10% defect inclusion than in 30% defect
inclusion polymers.
Figure 1. (AꢀD) Chemical structures of the conjugated polymers:
MEH-PPV, para-terphenyl MEH-PPV, ortho-terphenyl MEH-PPV, and
saturated MEH-PPV, respectively. (EꢀH) Absorption and emission
spectra of the corresponding polymers are shown to the left. In (FꢀH),
10% defect inclusion is represented in black lines, and 30% defect
inclusion is represented in green lines.
’ RESULTS AND DISCUSSION
Solution Absorption and Emission Spectra. The chemical
structures of MEH-PPV and the defect copolymers are shown in
the left side of Figure 1AꢀD and each compound's correspond-
ing solution phase absorption and emission spectra on the right
side (Figure 1EꢀH), with (A) MEH-PPV, (B) para-terphenyl
defected MEH-PPV (para-terphenyl MEH-PPV), (C) ortho-
terphenyl defected MEH-PPV (ortho-terphenyl MEH-PPV),
and (D) 1,2-diphenylethane defected MEH-PPV (saturated
MEH-PPV). The para-terphenyl defect is a stiff linear molecule
that is expected to maintain the linearity of the PPV backbone;
the ortho-terphenyl defect is a highly kinked species that should
disturb the ordering of the pristine polymer, and the saturated
defect lends rotational flexibility to the polymer backbone. The
absorption maximum of defect-free MEH-PPV is 503 nm, in
good agreement with previous work, while the 10% defected
polymers each undergo substantial blue shifts (∼20 nm), and
the 30% defected polymers demonstrate shifts up to 80 nm
dependent on defect type. Previous studies have shown that the
absorption spectra of PPV type polymers are sensitive to the
concentration of saturated defects along the polymer backbone
with increased defects blue shifting the absorption to higher
energies, consistent with our results.12 Solution emission spectra
for all defected species show only a modest change upon defect
inclusion as shown in Figure 1. At 10% defect inclusion, emission
spectra are almost identical to that of MEH-PPV, and even at
30% defect inclusion, the peak of emission is blue-shifted by
<10 nm. Spectra at 30% are also broadened, masking the vibronic
band shoulder. The fact that the emission spectra of the defected
polymers are not blue-shifted suggests that even in the highly
defected species there is still a subset of lower energy, longer
chromophores present in each polymer chain, and energy
transfer to these emitting sites remains efficient in solution.
Polarization Modulation Depth. The absorption polariza-
tion modulation was measured for ensembles of single polymer
chains of MEH-PPV with the conformation directing groups
discussed above as well as “pure” MEH-PPV for comparison.
This method has been discussed in detail in previous studies.5,11
In brief, using the microscope apparatus described in the sup-
porting information, linearly polarized excitation is modulated
such that the angle of polarization is rotated 180ꢀ. As the angle of
the polarized light matches the orientation of a transition dipole
in a fluorescent species, the increase in excitation rate becomes
apparent from the increased fluorescence intensity. This inter-
play between fluorescence intensity and polarization angle, θ, is
described by the following equation
IðθÞ ꢁ 1 þ Mcos 2ðθ ꢀ ϕÞ
where ϕ is the orientation when the emission intensity is
maximized. M is the polarization modulation depth. For a single
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Figure 3. Experimental histograms of modulation depth, M, obtained
with 457 nm excitation of (A) 30% para-terphenyl MEH-PPV (362
molecules), (B) 30% ortho-terphenyl MEH-PPV (862 molecules), and
(C) 30% saturated MEH-PPV (1033 molecules).
and the 10% defected compounds in this study are presented
in Figure 2, and the modulation depth histograms for the three
30% defected MEH-PPV compounds studied are presented in
Figure 3. Although Figure 2 only presents results obtained with
488 nm excitation, excitation at 458 nm was also done, resulting
in the same general trends for the modulation depth distribu-
tions with an ensemble average modulation depth slightly lower
than that obtained at 488 nm (data shown in the Supporting
Information, Figure S2).
Molecular Dynamics. Replica exchange molecular dynamics
(REMD)17 was used to probe the canonical ensemble of struc-
tures at 300 K for a set of 10 oligomer variants for each type of
defect (30% defected) as well as five oligomer variants for the
defect-free MEH-PPV (see Supporting Information for details),
all starting from extended configurations. Representative struc-
tures for the various chains, chosen to display a range of aniso-
tropies, are shown in Figure 4, as are the average radii of gyration,
anisotropy order parameters, and modulation depths. Two parti-
cularly noteworthy structures are also shown in Figure 5.
Radii of gyration are calculated from the coordinates of just
the initial carbon atom from each monomer, as are the vectors
representing the orientation of the chain between each monomer.
A set of 15 atom coordinates is therefore used to calculate
the center of mass and then the distances contributing to the
radius of gyration for each structure, and a corresponding set of
14 vectors is used to determine the anisotropy for each structure.
Statistics are calculated on 400 structures from each REMD run,
from structures obtained at regular intervals for the last 4 ns of
each 5 ns run. The anisotropy order parameter, A, is determined
from A = (3Sc)/(2 þ Sc), and Sc, the order parameter of the
polymer chain, is calculated as Sc = 0.5Æ3(cos2 β) ꢀ 1æ, where the
angled brackets indicate an average over the full set of chain
vectors for each structure and β is the angle between each chain
vector and the principal internal axis of those vectors.5 A value
of A = 1.0 indicates complete alignment of the chain vectors while
a value of A = 0.0 indicates no alignment. The distribution of
anisotropies was then converted to a distribution of modulation
depths to better compare to the experimentally measured values,
as described in ref 5. Average values of the radius of gyration, the
Figure 2. Experimental histograms of modulation depth, M, obtained
with 488 nm excitation of (A) MEH-PPV (981 molecules), (B) 10%
para-terphenyl MEH-PPV (424 molecules), (C) 10% ortho-terphenyl
MEH-PPV (454 molecules), and (D) 10% saturated MEH-PPV (524
molecules).
dipole or a collection of dipoles with similar orientation, M = 1.
For an isotropic distribution of dipoles where the spacing
between dipoles is much smaller than the diffraction limit of
light, M = 0; that is, no modulation should be observed.
In the case of a conjugated polymer such as MEH-PPV, a
single chain consists of multiple chromophore segments, each
composed of a set of sequential monomers, that are capable of
absorbing light.16 Recent polarization modulation measurements
of defect-free MEH-PPV reveal a single distribution of modula-
tion values centered at ∼0.7, suggesting that each individual
polymer is folding into a well-ordered structure in which the
majority of the chromophore dipoles are aligned. The resulting
distribution of observed modulation depth values is due to
variation in single-chain folding from molecule to molecule
and a lack of preferential orientation in the film where a molecule
tilted out of the sample plane will have a smaller observed
modulation depth than an identical molecule lying flat in the
sample plane.5 These results are qualitatively consistent with
the picture that has emerged from coarse-grained Monte Carlo
simulations of interacting beads on a chain.5
In this work, a 488 nm excitation was used to democratically
interrogate the chromophores in the defect-free MEH-PPV as
well as in the 10% defected MEH-PPV species, as this wavelength
is near the peak of the solution absorption for these species
(Figure 1). For the 30% defected MEH-PPV species, 488 nm
excitation is on the red edge of the absorption spectrum and is
therefore likely to excite only a lower-energy subset of chromo-
phores, resulting in nondemocratic excitation of the polymer
chain which artificially creates high anisotropy values (data shown
in the Supporting Information, Figure S1). Therefore, 458 nm
excitation was used to probe the 30% defected chains. Polariza-
tion modulation depth histograms for the defect-free MEH-PPV
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Figure 5. Additional structures of (A) ∼33% ortho-terphenyl MEH-
PPV and (B) ∼33% saturated MEH-PPV. Saturated defects discussed in
the text are circled in B.
Figure 4. Representative molecular structures from REMD simulations
of short 15 unit oligomers of (A) MEH-PPV, (B) ∼33% para-terphenyl
MEH-PPV, (C) ∼33% ortho-terphenyl MEH-PPV, and (D) ∼33%
saturated MEH-PPV. The corresponding radius of gyration, anisotropy
order parameter, and modulation depths are also shown. See the
Molecular Dynamics section for details.
large, at 28.0 Å, and the anisotropy order parameter is also quite
high, with a value of 0.92 and a corresponding average modula-
tion depth of 0.88. Such stiff chains will nonetheless fold at longer
chain-lengths,11 but the stiffness demonstrated in these oligo-
mers suggest that it will be favorable for these molecules, even
once folded, to adopt a set of stiff, highly ordered, and anisotropic
configurations qualitatively consistent with the single molecule
data shown above.
anisotropy order parameter, and the corresponding modulation
depths were then calculated.
The simulations were done both in implicit toluene and in the
gas phase (data not shown), and the resulting structures were
quite similar. This suggests that mean field solvation effects do
not greatly influence the stability and internal alignment of these
collapsed oligomers at 300 K. However, it is important to note
that both toluene and gas-phase simulations neglect the effect
of PMMA, the carrier polymer, as well as the entire spin-casting
process, on the final morphology of the spin-cast conjugated
polymers. Such effects are not negligible, especially in light of
recent work demonstrating significant changes in polymer mor-
phology when toluene solvent vapor annealing is performed on
single molecule MEH-PPV/PMMA systems.10 Results shown
are from all-trans oligomers in implicit toluene, and their statistics
are calculated over the set of five unique polymer sequences
where defects were placed probabilistically (see Supporting
Information) for both the nondefected MEH-PPV and the
defected MEH-PPV chains.
Defect-Free MEH-PPV. Figure 2A presents modulation depth
data for 16 kDa (∼60 monomer length chain) MEH-PPV without
defects present. Consistent with previous results from studies done
on MEH-PPV with a higher molecular weight, modulation depths
for these shorter, defect-free, MEH-PPV chains are distributed
around a single value with an average modulation depth of
0.67. One important difference to note in this work as compared
to the previous studies on longer MEH-PPV chains is the
increased signal-to-noise ratio for these very low molecular
weight chains. As more error is introduced into the measurement,
the modulation depth histograms become broader, as can be seen
in the increased number of molecules with modulation depths
above the physically realistic value of one as compared to pre-
vious studies.5,10ꢀ12 As can be seen in Figure 4A, the defect-free
MEH-PPV oligomer demonstrated no propensity to fold during
the course of the REMD simulations, adopting instead an
elongated conformation with only slight fluctuations away from
linear configuration. The resulting radius of gyration is rather
Para-Terphenyl MEH-PPV. This linear defect preserves the
extended, stiff backbone configuration common to PPV polymers
and is not expected to introduce dramatic changes in the
polymer's overall morphology. Indeed, the modulation depth
histograms of both 10% and 30% para-terphenyl MEH-PPV
(Figures 2B and 3A) present nearly identical distributions in both
their shape and average modulation depth (0.67 and 0.64,
respectively) to that of the defect-free MEH-PPV. Likewise,
the structures that are observed during our simulations of 33%
para-terphenyl MEH-PPV (Figure 4B) are also quite similar to
those for the defect-free MEH-PPV, although elongated since a
single para-terphenyl group is longer than the corresponding
MEH-PPV monomer. The radius of gyration (36.3 Å) is there-
fore greater, but the anisotropy order parameter (0.91) and
the modulation depth (0.87) are nearly the same. These results
together confirm our hypothesis that a stiff, linear defect would
preserve the highly anisotropic nature of the single-molecule
MEH-PPV morphology. The only observed effect of the para-
terphenyl defect inclusion in our study, namely, the shift in peak
absorption, can be attributed to the corresponding disruption
to the average conjugation length, since the π conjugation is
reduced by the nonplanar geometry of the phenyl rings in the
terphenyl group.18
Ortho-terphenyl MEH-PPV. Ortho-terphenyl MEH-PPV
demonstrates the largest deviation in modulation depth as
compared to defect-free MEH-PPV (see Figures 2C and 3B).
The average value of the modulation for the 10% ortho-
terphenyl MEH-PPV is still relatively high at 0.56, but the
distribution of values is now much broader, with an almost
equal probability of a polymer chain having a modulation
depth anywhere from 0.1 to 1. This wide range of modulation
depths suggest that, at 10% defect incorporation, the polymer
chain can assume a wide variety of conformations ranging
from nearly amorphous to highly folded.
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Given the random nature of the chain-growth synthesis
coupled with the low molecular weights of the compounds
studied, the population of single molecules with high modulation
depths may consist mainly of molecules with few, if any, defects.
It is also possible that the 120ꢀ bend from the linear PPV
backbone provided by the ortho-defect might enhance the ability
of the chain to fold into a more compact, but still highly aligned
structure. An example of such a structure can be seen in Figure 4C,
where the regular placement of the ortho-terphenyl defects
promotes alignment in the folded structure. However, in longer
chains or at higher defect concentrations, it would be less likely
that the random placement of these “kinks” would allow for such
alignment. Indeed, the 30% ortho-terphenyl MEH-PPV com-
pound demonstrated the lowest average modulation depth
measured in this study, with a value of 0.40 (see Figure 3B).
The distribution of modulation depth values is almost the exact
opposite of the distribution of the 30% para-terphenyl MEH-
PPV, with the majority of modulation depth values below 0.5.
At 30% defect concentration, the majority of polymer chains
now appear incapable of folding into the highly anisotropic
structures that have been reported here and elsewhere. While
most structures are likely to be adopting a more amorphous
collapsed structure similar to a “molten globule” type conforma-
tion (see the first structure in Figure 4C), the modeled structure
shown in Figure 5A demonstrates another possibility, wherein
significant alignment in differing directions could offset one
another. Another thing to consider is the tilt angle of the polymer
molecules in the PMMA host. For a highly aligned, anisotropic
polymer molecule, out-of-plane tilting will only decrease the
observed modulation depth. However, in a polymer molecule
with aligned regions with different orientations, the tilt angle of
the molecule can either increase or decrease the modulation
depth observed, perhaps explaining the tail to higher values for
the highly defected ortho-terphenyl MEH-PPV.
In the simulations of 33% ortho-terphenyl MEH-PPV we
observe an average radius of gyration of 12.1 Å, which is
significantly lower than that of either the defect-free MEH-
PPV or the para-terphenyl MEH-PPV. The anisotropy order
parameter is similarly reduced, to a value of 0.75 and a modula-
tion depth of 0.67. The observed trend of higher ortho-terphenyl
defect concentrations to have lower modulation depths, more
compact structures, and lower anisotropy values generally con-
firms our expectation that, at a high enough number of defects
per chain, the folding of MEH-PPV into a highly aligned, aniso-
tropic structure is disrupted.
Saturated Defect MEH-PPV. Results from the 10% saturated
defect polymer (see Figure 2D) give an average modulation
depth nearly identical to that measured for the defect-free MEH-
PPV (0.69); however, the measured distribution is much sharper
suggesting a more uniform distribution of conformations. Con-
trary to previous studies, our results suggest that the inclusion of
a saturated backbone site at modest inclusion rates does not
hinder highly anisotropic polymer folding12 and may in fact
enhance it in small quantities by adding a flexible hinge that
enables the chain to fold into highly anisotropic rods.
anisotropically fold; however, it is equally likely for chains to
adopt a more disordered, isotropic conformation. This can be
seen in the range of structures from the simulation shown in
Figure 4D, where both a disordered collapsed structure and a
highly anisotropic folded structure are shown. For these ∼33%
saturated defect MEH-PPV structures, the average radius of
gyration is 20.5 Å, falling in between that of the ortho-terphenyl
defect and the defect-free chain. The average anisotropy value
is 0.87 with a corresponding modulation depth of 0.83, which
are slightly higher than expected; however, the short chains
explored in this simulation most likely bias the results toward
more aligned structures since stiff segments of different lengths
together can be better fit together when there are fewer segments.
The two-fold ability of this defect either to bend or to continue
the linear chain is highlighted in Figure 5B, where four different
saturated defects are circled. These defects show the range
possible effects on the overall morphology. On the far left, a
saturated defect induces a kink in the chain, enabling it to fold
back on itself while still preserving the aligned nature of the
straight chain. In the middle of the structure, there is a saturated
defect in the top chain that is able to maintain its interaction with
the defect-free segment of the lower chain, enhancing the
stability of the anisotropic configuration. On the right, two
saturated defects, one on each chain, have associated themselves
such that the chains do not need to bend for them to continue
their chain-to-chain association on the other side of the defects.
Given this range of possible configurations for the saturated
defect, short segments with even high proportions of saturated
defects may remain highly anisotropic. However, when the
alignment of the defects is less favorable, disordered structures,
such as that shown on the left in Figure 4D, may result.
Cis Defects. The substantial increase in cis defects in the ortho-
terphenyl MEH-PPV and saturated MEH-PPV compounds as
compared to the defect-free MEH-PPV and para-terphenyl MEH-
PPV suggests that some kind of templating of the cis defect
occurs during the chain growth synthesis of polymers with these
two defects, due perhaps to the steric constraints imposed by
these designed defects. The anticipated result is a decrease in the
linearity of the backbone of the polymer, which is likely to then
enhance the expected isotropic nature of these defected poly-
mers. The 30% saturated MEH-PPV and 30% ortho MEH-PPV
were refluxed with I2 in toluene to convert the cis bonds to trans,
resulting in compounds that contained only ∼5% cis defect
(see the Supporting Information).
Interestingly, with the removal of a majority of the cis defects,
the 30% saturated MEH-PPV and 30% ortho MEH-PPV demon-
strate identical solution absorption spectra and similar anisotropy
histograms (see SI, Figures S3 and S4) with only the saturated
defect compound showing only a minor shift in the anisotropy.
The lack of significant structure change with the removal of
cis defects is surprising and suggests a limitation in the role that
intuition plays in predicting the how backbone defects can effect
polymer morphology, especially in these already highly defected
species.
The computational modeling results presented above neglect
cis defects, instead focusing on chains with all trans linkages. The
placement of cis linkages in the polymer is currently unknown,
and given their higher concentrations in the more disordered
polymers, it is likely that they are formed when two smaller
chain segments are already associated with one another and can
only be joined through a cis linkage. Thus, their placement may
be poorly represented by random insertion in the simulated
In contrast, the modulation depth values for 30% saturated
MEH-PPV (see Figure 3C) display a very broad and nearly static
distribution between 0.2 and 0.9, with an average modulation
depth value of 0.54. This distribution is quite similar to that of the
10% ortho-terphenyl MEH-PPV and characteristic of chains that
adopt a wide array of morphologies. The data suggests that even
at this high level of defect inclusion, it is still possible for chains to
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Table 1. Incorporation of cis Double Bonds in MEH-PPV
Backbone
morphology plays in conjugated polymer devices and how the
properties of the individual components influence the character-
istics of the bulk material.
polymer
MEH-PPV
% cis
5.3
18.7
13.1
19.0
15.1
5.8
’ ASSOCIATED CONTENT
10% ortho-terphenyl
30% ortho-terphenyl
10% saturated
S
Supporting Information. Complete procedures and de-
b
tails of the microscope apparatus, molecular dynamics and
synthesis, NMR spectra, and additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
30% saturated
10% para-terphenyl
30% para-terphenyl
7.6
’ AUTHOR INFORMATION
structures, but how to bias their placement appropriately in our
simulations is not clear. Even so, we performed additional
simulations including cis defects at the frequencies shown in
Table 1 on the same set of five probabilistically defected chains as
examined for the trans case. As expected, the inclusion of cis
linkages broadens the distributions of anisotropy values, such
that they encompass more of the lower modulation depths
observed experimentally, explaining in part why our calculated
average modulation depth values are significantly higher than
those measured experimentally. However, our set of cis structures
is too small to represent well the combinatorial complexity that
this additional structural defect introduces (see Supporting
Information), and we thus focused here on the results of our
all-trans simulations.
Corresponding Author
jcb2873@mail.utexas.edu
’ ACKNOWLEDGMENT
This work was supported as part of the program “Molecular
Tools for Conjugated Polymer Analysis and Optimization”, a
Center for Chemical Innovation (CCI, phase 1) under NSF Award
No. CHE-0943957. We would like to thank Emilio Gallicchio for
helpful conversations regarding the adjustment of the AGBNP
potential for toluene, and Takuji Adachi for the use of his program in
converting anisotropy values to modulation depth distributions.
’ REFERENCES
’ CONCLUSIONS
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In summary, the linear nature of the para-terphenyl defects
does not hinder efficient folding of the PPV chain and may in
fact improve the alignment of the chromophores in the polymer
at higher inclusion rates. The ortho-terphenyl defects provided
the largest changes to the modulation depth data suggesting that,
even at mild inclusion rates, these defects hinder chain alignment
and at higher inclusion rates dramatically shift the single chain
distribution toward less-ordered structures with lower aniso-
tropies. The effect of the saturated defect, biphenylethane, on the
single molecule anisotropy was surprisingly concentration-
dependent; a low inclusion ratio results in a conformations with
higher and more uniform anisotropies, while a high inclusion
ratio results in a nearly equal probability of both low and high
anisotropy conformations.
By combining synthetic, spectroscopic, and theoretical approaches
we have laid the groundwork for being able to design polymer
morphologies whereby the addition of different directing groups
to the chain would influence the single molecule conformation of
the conjugated polymer MEH-PPV in the manner desired. Single
molecule measurements demonstrate that defect-free MEH-PPV
and a linear defect that preserves the backbone geometry of
PPV-type polymers results in highly anisotropic single molecule
conformations, a highly bent defect results in a more isotropic
conformation, and a defect that allows for rotational freedom
of the backbone yields a concentration-dependent effect. At
low concentrations, this defect still allows for anisotropic con-
formations. However, at higher concentrations it leads to chains
that can adopt a wide range of conformations. Molecular
dynamics simulations on model oligomeric systems show quali-
tative agreement with the experimental results, providing insight
into the local folding induced by the different types of defects
used in this study. Understanding polymer conformation at the
single molecule level will lead to better insight into the role that
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