Influence of branching on the Chiral self-assembly of poly(phenylene ethynylene)

Article abstract of DOI:10.1002/pola.27443

In this contribution, we report the synthesis of chiral all-conjugated branched poly(phenylene ethynylenes) with a controlled amount of branching. Subsequently, the selfassembly of these PPEs is studied by means of UV-vis, fluorescence spectroscopy, and D

Full text of DOI:10.1002/pola.27443

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Influence of Branching on the Chiral Self-Assembly of Poly(phenylene
ethynylene)
Joost Steverlynck, Pieter Leysen, Guy Koeckelberghs
Laboratory for Polymer Synthesis, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium
Correspondence to: G. Koeckelberghs (E-mail: guy.koeckelberghs@chem.kuleuven.be)
Received 7 August 2014; accepted 9 October 2014; published online 00 Month 2014
DOI: 10.1002/pola.27443
ABSTRACT: In this contribution, we report the synthesis of chi-
influence of branching and self-assembly on the chiral expres-
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ral all-conjugated branched poly(phenylene ethynylenes) with
sion of these polymers. 2014 Wiley Periodicals, Inc. J. Polym.
a
controlled amount of branching. Subsequently, the self-
Sci., Part A: Polym. Chem. 2014, 00, 000–000
assembly of these PPEs is studied by means of UV–vis, fluores-
cence spectroscopy, and DSC and the influence of branching is
investigated. Finally, CD-spectroscopy is used to study the
KEYWORDS: branched; chiral; conjugated polymers; self-assem-
bly; supramolecular structures
supramolecular organization is lacking. Very recently the
group of Luscombe reported the synthesis of branched P3HT
using direct arylation in which the branching can be varied
by adjusting the base and ligand.27 Finally, hyperbranched
PPE-PPV copolymers were polymerized by Kub et al.28 In
this article, we describe the synthesis of a series of branched
PPE’s (Fig. 1) and investigate the influence of branching on
the self-assembly and chiral expression.
INTRODUCTION The properties of conjugated polymers do
not only depend on their chemical structure, but to a large
extent on their supramolecular organization. Until now
mainly linear conjugated polymers have been studied. The
reason is twofold. First, the desired opto-electronical proper-
ties for the current applications are mostly satisfied by these
1-D systems. Second, the synthesis of these branched sys-
tems is synthetically challenging. Nevertheless, branching
affects the supramolecular organization of polymers and, as
such, can lead to new properties which in turn can make
new applications possible. For nonconjugated polymers
branching has shown to result in a lower viscosity and glob-
ular structure, leading to applications as coating and drug
delivery by encapsulation.1 For conjugated polymers we
could think of solar cell applications.2 They are also very
promising as active materials for light-emitting devices as
they are more stable, have better solubility, and do not suffer
as much from aggregate-based fluorescence quenching as
their linear counterparts.3–11 Some even show aggregation-
induced emission.12 Crosslinkable hyperbranched polymers
are a solution for the solubility problem when preparing
organic-processable multilayer devices.13,14 As they are fluo-
rescent and can form a porous network they can be used as
highly sensitive chemosensors.15 Another interesting prop-
erty is the possible modification of end-groups allowing tun-
ability of the optical properties.16 Although quite some
RESULTS AND DISCUSSION
Monomer Synthesis
The synthesis of the “linear” monomers 1 and 3 (Scheme 1)
started from hydroquinone, which was alkylated twice by
the chiral (S)-dimethyloctylbromide.29 Subsequent iodination
renders monomer 1. Monomer 3 is formed by reaction of
monomer 1 with trimethylsilylacetylene followed by depro-
tection of the silyl group with TBAF.3H2O.30
The synthesis of the “branched” monomer 4 (Scheme 1) started
from commercially available 2-iodo-dimethylterephtalate. This
product was reduced to the dialcohol with DIBAL and without
further purification oxidized with PCC to form 2-iodoterephtal-
dehyde.31 The aldehyde functions were subsequently converted
to alkyne functions using the Ohira-Bestmannreaction.
Polymer Synthesis
The polymers were prepared by means of the Sonogashira
reaction. The “linear” 1 and 3 monomers were combined
with the “branched” monomer in well determined quantities
(Table 1). Iodobenzene (5) was added as a chain stopper.
hyperbranched
conjugated
polymers
have
been
reported,17–26 no series of all-conjugated branched polymers
with a controlled amount of branching has been prepared
and a systematic study on the influence of branching on the
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 Structure of the polymers.
The polymerization was carried out in THF with Pd(PPh3)4
as catalyst, Et3N as base and CuI as cocatalyst (Scheme 2).
A chain stopper was used to limit the molar mass and
crosslinking to obtain soluble material and a fixed degree
of polymerization (DP) of 15. A similar DP is necessary to
compare the different polymers. Iodobenzene is used and
not phenylacetylene in order to avoid terminal acetylene
groups and possible concomitant Glaser couplings after
polymerization. In total,
5 polymers were synthesized,
which varied in the amount of monomer 4 present in the
feed (0, 2.5, 5, 10, and 20%), rendering polymers P1, P2,
P3, P4, and P5, respectively. After synthesis these polymers
are purified by Soxhlet extraction with methanol followed
by chloroform. The chloroform fraction was concentrated
and the polymer was precipitated in methanol. The yields
of the chloroform-soluble fractions are depicted in Table 1.
Roughly, the yield drops as the amount of 4 in the feed
increases, which can be attributed to crosslinking resulting
in an insoluble fraction that remained in the Soxhlet
thimble.32
SCHEME 1 Synthesis of the monomers.
Apart from the peak from the aromatic proton of the “linear”
unit at 7 ppm,33,34 also additional peaks are observed, which
can be attributed to the branching unit (if present) and ter-
minal units (Fig. 2). A complete assignment of the aromatic
1
peaks is shown in Figure 3 for P5 using the H NMR spectra
of already reported model compounds (Supporting Informa-
tion Fig. S6). Given the fact that iodobenzene is added as
chain stopper, two possible end groups are present: either
from 5 (signals g, h, and i) or from monomer 1 (signals e,
and f). Note that no end-groups from 4 are found, which can
be explained by its small presence. The fact that signals e
and f diminish with increasing amount of branching is in
line with this assignment. The branching unit has resonances
at b, c, and d. The increase of b is clearly in line with
GPC and NMR
The dispersities and molar masses (Table 1) were deter-
mined via GPC against polystyrene standards and lay in the
same range, making a comparison possible. In general, a
decrease of molar mass with the degree of branching is
observed, which on one hand is in line with a smaller hydro-
dynamic volume originating from branching, but can on the
other hand also be attributed to the fact that only the
chloroform-soluble fraction is measured.
increasing amount of
4
for P1-> P5. Since each
TABLE 1 Ratio of Monomers Used, Yields, Number-Averaged Molar Mass, Dispersity, Degree of Branching and DP
Degree of
Branchingc (%)
-
D
Polymer
1 (mmol)
3 (mmol)
4 (mmol)
5 (mmol)
Yield (%)
Mn (kg/mol)a
DPb,c
P1
P2
P3
P4
P5
0.250
0.244
0.238
0.225
0.200
0.250
0.244
0.238
0.225
0.200
0.000
0.013
0.025
0.050
0.100
0.0357
0.0482
0.0607
0.0857
0.135
54
58
72
37
14
6.4
7.8
5.5
5.9
5.3
1.8
1.7
1.8
1.8
1.7
11
11
11
12
12
0
2.2
4.5
8.9
12
a
c
Determined by GPC in THF toward poly(styrene) standard.
Degree of polymerization.
Determined by 1H NMR.
b
2
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SCHEME 2 Synthesis of the polymers.
FIGURE 2 1H-NMR spectra of P1-P5 for (A) aromatic and (B) aliphatic region in CDCl3.
incorporation of 4 results in an additional end-group, an
increase of the signals g, h, and i is expected for P1-> P5.
The amount of branching is then calculated from
in chloroform adding more and more of the poor solvent
methanol. The solvatochromism spectra are found in Sup-
porting Information (Figs. S7–S11). Although no clear red-
shift was observed, aggregation was visible for P1 by the
appearance of an additional shoulder near 480 nm, which
can be attributed to self-assembly and the presence of long-
range order in the material (Fig. 4).35 This shoulder is only
found for the linear polymer and to a small extent also for
P2. However, we should note that for P2 not all chains are
branched and this signal is possibly due to the stacking of
those nonbranched chains.36 The other polymers do not
show this peak, implying that they do not possess long-
range order.
b
% branching5 a=
2
and is tabulated in Table 1. Apart from P5, the amount of
branching is nicely reflected in the amount of 4 present in
the feed. The deviation in case of P5 can be explained by
crosslinking and the fact that only the soluble fraction is
measured.
The DP can be calculated by dividing the total amount of
units present by the number of chains. While the former
equals the amount of inner units (either linear or branching
units) and the amount of end-groups, the latter can be found
from the number of end-groups as follows: a linear chain
has two end-groups; therefore, the number of chains equals
the amount of end-groups divided by 2. Since every branch-
ing unit increase the number of end-groups by one, the DP
equals:
a=21b1f 1ðg1iÞ=3
DP5
1=2ððg1iÞ=31f 2bÞ
From Table 1, it is clear that all polymers have a similar DP
(11–12), a little lower than targeted.
UV–vis
FIGURE 3 Assignment of the aromatic region of the 1H NMR
The self-assembly behavior of P1-P5 was first studied by
UV–vis. For each polymer 10 different solutions were made
spectrum of P5.
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FIGURE 4 A: UV–vis spectra of P1-P5, (B) fluorescence spectra, and (C) CD spectra in 100% CHCl3 and 90% in MeOH/10% in CHCl3,
the fluorescene spectra are corrected for absorption.
Fluorescence
methanol (90/10; Fig. 4). When the spectra in the mixture
chloroform and methanol are compared to those in pure
chloroform a decrease in intensity of a factor 10 is meas-
ured. In addition, an additional peak appears near 490 nm
for P1, which is not found for the other polymers. This peak
A further investigation of the aggregation behavior was done
by fluorescence spectroscopy with excitation at 440 nm. The
materials are measured in pure chloroform, in which they
are molecularly dissolved, and a mixture of chloroform/
4
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has already been attributed in other (linear) PPEs to aggre-
gation.37 In the other polymers, this peak is absent or less
pronounced. When we study the spectra in mixtures of chlo-
roform/methanol more closely, a systematic decrease in
intensity for lower degrees of branching is observed. Clearly,
branching shields the polymer chains from each other, reduc-
ing the quenching of the fluorescence. Hence, these results
confirm the results from UV–vis spectroscopy in the sense
that branching complicates the aggregation and that even
very small amounts of branching disrupts the long-range
order.
of branching. These polymers were made by a copolymeriza-
tion of A2 and B2 monomers with an AB2 monomer via
Sonogashira couplings. Polymers with 2.5, 5, 10, and 20%
branching monomer were synthesized. Subsequently, the
self-assembly of these materials, correlated to their supramo-
lecular structure, was studied by UV–vis -, CD-, fluorescence
spectroscopy, and DSC. It was shown that in all cases self-
assembly occurs upon addition of a poor solvent, even for a
branching degree up to 20%. However, the introduction of
the slightest amount of branching destroyed the long-range
order in the supramolecular structures.
For the chiral expression studied by CD-spectroscopy it is
found that indeed branching destroys long-range order, but
that it does not suppress the chiral expression: the highest
chiral response is even observed for the most branched
material. This result is in agreement with earlier observa-
tions that the introduction of a certain amount of defects in
a chiral material can improve the chiral expression.
CD-Spectroscopy
The presence of the chiral alkoxy side-chains allows to inves-
tigate the chiral expression.38,39 As for UV–vis, for each poly-
mer 10 different solutions were made in chloroform adding
more and more methanol (Supporting Information Figs. S7–
S11). Although aggregation upon addition of a poor solvent
was not visible in the UV–vis spectra, the CD-spectra of P1-
P5 (Fig. 4) show clear bisignate CD-signals for all polymers.
Bisignate Cotton effects result from chiral exciton coupling
originating from chirally oriented polymer chains. When the
CD-spectra for P1 are observed more into detail, a bisignate
Cotton effect originating from chiral exciton coupling at the
kmax of the UV–vis spectrum with an additional monosignate
Cotton-effect, at the wavelength of the shoulder in UV–vis.
The latter, pointing at long-range order, is absent or much
less pronounced in the other polymers, which is in agree-
ment with the UV–vis spectra. By comparing the intensity of
the bisignate Cotton effects in 90% MeOH for P1-P5, it is
clear that branching in general does not impede chiral
expression, but rather increases it. Maximum De values are
calculated for P3 and P5. Although there is no clear trend
between the branching degree and chiral expression, we
nevertheless can conclude that chiral expression does not
require long-range order. To the contrary, the introduction of
branching, leading to the disruption of long-range order, can
seemingly cause an increase of the chiral expression of the
material. We should note that the chiral signal we analyze
does not require long-range order as it is a bisignate Cotton-
effect arising from exciton coupling between chiral oriented
chains. While perhaps strange at first sight, it has already
been found in other systems that molecular imperfections
(regio-irregularity, achiral units, chiral additive), in chiral
materials, can lead to an increase of the chiral
expression.40–43
ACKNOWLEDGMENTS
The authors are grateful to the Onderzoeksfonds K.U.Leuven/
Research Fund K.U.Leuven and the Fund for Scientific Research
(FWO-Vlaanderen) for financial support. JS is grateful to IWT
for a doctoral fellowship.
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