β-Strand inspired bifacial π-conjugated polymers

Article abstract of DOI:10.1039/c9sc01724k

Access to diverse, relatively high molecular weight soluble linear polymers without pendant solubilizing chains is the key to solution state synthesis of structurally diverse nanoribbons of conjugated materials. However, realizing soluble 1D-π-conjugated polymers without pendant solubilizing chains is a daunting task. Herein, inspired from the polypeptide β-strand architecture, we have designed and developed novel bifacial π-conjugated polymers (M_n: ca. 24 kDa) that are soluble (ca. 70 to >250 mM) despite the absence of pendant solubilizing chains. The impact of varying the bifacial monomer height on polymer solubility, optical properties, and interactions with small molecules is reported.

Full text of DOI:10.1039/c9sc01724k

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EDGE ARTICLE
β-Strand Inspired Bifacial π-Conjugated Polymers
Saikat Chaudhuri, Manikandan Mohanan†, Andreas V. Willems†, Jeffery A. Bertke,
Nagarjuna Gavvalapalli*a
Received 00th January 20xx,
Accepted 00th January 20xx
Access to diverse, relatively high molecular weight soluble linear polymers without pendant solubilizing chains is the key to
solution state synthesis of structurally diverse nanoribbons of conjugated materials. However, realizing soluble 1D-π-
conjugated polymers without pendant solubilizing chains is a daunting task. Herein, inspired from the polypeptide β-strand
architecture, we have designed and developed novel bifacial π-conjugated polymers (Mn: ca. 24 kDa) that are soluble (ca.
70 to >250 mM) despite the absence of pendant solubilizing chains. The impact of varying the bifacial monomer height on
polymer solubility, optical properties, and interactions with small molecules is reported.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
Scheme 1. β-Strand inspired bifacial polymer architecture.
Extended π-conjugation in conformationally rigid planar
polymers including ladder polymers, nanoribbons, 2D-grids,
and 2D-π-conjugated polymers (2D-π-CoPs) provides
intriguing optical and electronic properties compared to 1D-π-
conjugated polymers (1D-π-CoPs).1-17 However, lack of
solution state synthesis, assembly, and solution processability
of the 2D-π-conjugated materials limits their structural
11, 18-22
diversification and use in various applications.10,
Although appending pendant solubilizing chains is a successful
strategy in overcoming the above-mentioned challenges in
the conformationally flexible 1D-π-CoPs, it is not effective for
the 2D-π-conjugated materials of larger width due to strong
van der Waals interactions.10, 11, 18-22
Solution state synthesis of nanoribbons requires linear
polymers that are soluble without pendant solubilizing chains
so that monomers can be added laterally to extend the ribbon
width (Scheme 1). In addition, for the resultant nanoribbons
to be soluble, the π-surface of the ribbons should be masked
efficiently to disrupt inter-ribbon interactions.12-16 Realizing
these critical requirements, Schluter has shown two decades
20, 23, 24
molecular weight 2.5 kDa (Scheme 1).11,
Soluble
polymers of high molecular weight are needed to generate
nanoribbons of larger dimensions and harness the advantages
of electronic conjugation. Thus, access to diverse, relatively
high molecular weight soluble linear polymers without
pendant solubilizing chains is the key to realizing solution
state synthesis of structurally diverse nanoribbons.
ago in
a seminal paper that incorporating macrocycle
containing monomers without pendant solubilizing chains,
known as ansa-monomers, disrupts interchain (π-π)
interactions and enables soluble low-molecular weight linear
polymers as well as the corresponding ladder polymers of
Nature routinely forms 2D-sheets i.e., β-sheets through
lateral polymerization of polypeptide β-strands.25 The peptide
β-strands are designed such that the side-chains on alternate
repeat units are positioned above and below the strand to
allow the strands to come close and polymerize laterally
through hydrogen-bonding interactions. Since the β-strand
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the π- conjugation plane, similar to the side chains in the β-
strands, mask the π-surface and control face-to-face
intersheet interactions, and hence their solubility and
assembly (Scheme 1 and Scheme 2).
Scheme 2. Chemical structures of bifacial monomers and polymers synthesized
in this study.
Importantly, the bifacial monomer design allows to easily
vary the monomer height (vide infra) by changing the
cycloalkyl group (e.g., from cyclohexyl to adamantyl) without
significantly altering the monomer width (Scheme 2). The
bifacial polymer solubility increased by changing the
monomer height as well as incorporating a co-monomer of
different height. The solubility of the bifacial polymers (BiPs)
(ca. 70 to >240 mM) exceeded the solubility limit (ca. 70 mM)
of the conventional poly(p-phenylenebutadiynylenes) (PPB)
based π-conjugated polymer with pendant solubilizing chains.
The impact of varying the bifacial monomer height on optical
properties, assembly and polymer interactions with small
molecules is also studied.
The proposed bifacial monomer design differs from
strapped-polymers and insulated molecular wires28-30 in which
the polymer backbone is completely sheathed and therefore
lateral polymerization to generate the 2D-polymers is not
possible. Also, since only the π-surface is masked in the
bifacial polymers, their interchain charge transport is
expected to be comparable or better than the completely
29
insulated molecular wires,28,
making the BiPs useful for
optical and electronic applications. Importantly, the cycloalkyl
groups can be cleaved post-polymerization or post-
processing, if needed, using established desulfurization
protocols to enhance intersheet interactions.31
Results and Discussion
Bifacial molecules have been known since the 1990’s32-34
but surprisingly they have not been used as building blocks for
polymers yet. The bifacial monomers were synthesized using
the modified reported protocols (Scheme
3
and
supplementary information). 2,5-dibromo p-xylene was
brominated using NBS (N-Bromosuccinimide) to generate 2,5-
design
renders
soluble
β-strands,
allows
lateral
polymerization of the strands to generate sheets, and also
controls face-to-face aggregation of the β-sheets,26,
we
27
dibromo p-xylylene dibromide. 1,4-cyclohexyl dimethanol (
was converted into 1,4-cyclohexyl dimethanethioacetate (
1
2
)
)
envisioned designing molecular building blocks that emulate
the β-strand architecture for the solution state synthesis of
ladders, nanoribbons, and 2D-grids.
and reduced to generate 1,4-cyclohexyl dimethanethiol (
2,5-dibromo p- xylylene dibromide and the dithiol compound
) were reacted under dilute conditions to generate 1,4-
dibromo cyclohexanocyclophane ( ). Sonogashira coupling of
) with TMS-acetylene followed by desilylation resulted in
1,4-diethynyl cyclohexanocyclophane monomer ). 1,4-
diethynyl adamantanocyclophane bifacial monomer (12) was
generated from 1,3-adamantane dimethanol ( ) following the
3).
(
3
Herein, inspired from the β-strand design, we have
designed and shown that the bifacial cyclophane monomers,
despite not having pendant solubilizing chains, render soluble,
high molecular weight (24 kDa) π-conjugated linear polymers.
The bifacial polymers are made of bifacial cyclophane building
blocks consisting of a polymerizable aryl group (aryl face) and
a π-surface masking cycloalkyl group (cycloalkyl face) (Scheme
1 and Scheme 2). The aryl group of the bifacial monomer
4
(
4
(
6
7
similar protocols. 1,3-adamantane dimethanol was prepared
by esterifying and reducing 1,3-adamantane dicarboxylic acid.
The height of compounds
4 and 10 i.e., the distance from
generates
a π-conjugated polymer upon polymerization
normal to the phenyl plane to the farthest atom in the
whereas the cycloalkyl groups positioned above and below
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Scheme 3. Synthesis of bifacial polymers.
Reaction conditions: (a) Thioacetic acid, DIAD, PPh3, THF; (b) LiAlH4, THF; (c) 2,5-dibromo p-xylylene dibromide, KOH, C6H6, EtOH; (d) (i) TMS-acetylene, Pd(PPh3)4, CuI,
toluene, triethylamine; (e) TBAF, THF; (f) CuCl, TMEDA, O2, solvent; (g) TMS-acetylene, Pd(PPh3)4, CuI, piperidine.
cycloalkyl group was determined from single crystal X-ray
structures (Figure 1, Figure S1-S2). As expected, the height of
10 (7.22 Å) with an adamantyl cycloalkyl group is greater than
that of
4 (5.90 Å) with a cyclohexyl group, and the heights of
both these monomers are higher than the conventional 1,4-
dihexyl benzene (0 Å), which lacks the cycloalkyl groups on
top of the π-surface.
Monomers
6 and 12 were subjected to Glaser-Hay
polymerization to obtain the BiP polymers (Scheme 2, Scheme
3 and Figure S3). The polymerization mixture was precipitated
in methanol and the crude polymer was subjected to
soxhlation in ethyl acetate and chloroform. The polymer
collected in chloroform was re-precipitated in ether, dried and
used for characterization. Three bifacial homopolymers (BiP-1,
BiP-2a and BiP-2b), one bifacial co-polymer (BiP-3), and a
conventional polymer with dihexyl pendant solubilizing chains
(PPB) were synthesized (Scheme 3). Stereoisomers of the
monomers as synthesized were used for polymerization. The
BiP-1 was synthesized from both the diastereomeric (arising
due to cis and trans cyclohexyl substitution) and racemic
Figure 1. Single crystal X-ray thermal ellipsoid plots of a) compound 4; b) compound
10; and c) compound 15 (dimer) with thermal ellipsoids at 50% probability. Black-C,
Yellow-S, Orange-Br, White- H.
mixture (formed during cyclophane formation) of monomer 6.
The BiP-2a and BiP-2b were synthesized from the racemic
mixture (formed during cyclophane formation) of monomer
12. Thus, all the synthesized bifacial polymers are atactic in
nature i.e., there is no control over orientation of the repeat
units along the polymer backbone.
The synthesized BiPs are soluble in several typical organic
solvents (e.g., THF, CHCl3, DMF and chlorobenzene) that are
used to solubilize π-conjugated polymers. In order to
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Table 1. GPC, optical properties, and solubility limits of bifacial polymers
Absorption Maximumb
(Δλmax nm)
Molar
Emission
Solubility Limit in
CHCl3
mg/mL
d
Mna
Mwa
Ksv
Extinction
Coefficient
(M-1 cm-1)
Maximumb,c
Polymers
(kDa)
(kDa)
(x 104)
Thin
film
(λmax nm)
mM
Solution
Δλmax
BiP-1
BiP-2a
BiP-2b
11.0
15.0
23.0
29.3
31.8
48.8
421
423
424
433
-
12
-
24500 ±300
27100 ±700
25200 ±100
437
-
29 ±5
>92
90 ±15
>240
2.6 ±0.3
-
433
9
448
26 ±3
70 ±8
1.3 ±0.1
BiP-3
PPB
24.0
16.1
51.4
35.6
424
415
430
443
6
32200 ±300
29900 ±300
445
433
>87
>250
1.7 ±0.3
11 ±1
28
21 ±2
70 ±7
a tetrahydrofuran gel permeation chromatography with polystyrene standards; b recorded in CHCl3; c excited at absorption maximum; d polymer fluorescence quenching studies
with TCNQ
determine the solubilization power of the bifacial
monomer compared to the conventional monomer, solubility
limit of the polymers were determined in chloroform from
multiple trials. The solubility limits were determined by
dissolving polymers in chloroform at elevated temperature
(55 oC) for about 20 minutes and then bringing the solution to
room temperature.
polymer molecular weights (within 0.5 kDa) and
polydispersities (within 0.3) of these three samples, which
indicates that there is no fractionation of the polymer during
the solubility limit tests. It is worth noting that during the
purification stage all the BiPs went through fractionation via
soxhlet extraction. Ethylacetate was used to extract the low
molecular weight polymer while chloroform was used to
obtain the chloroform soluble polymer fraction of the as
synthesized polymers, and only the chloroform soluble
fraction is used for all characterization. Even though the BiP-1
has a higher polydispersity compared to the BiP-2a, the
absence of fractionation confirms that the insoluble polymer
is indeed the excess BiP-1 polymer (beyond the solubility
limit) and not just the high molecular weight polymer fraction
of the BiP-1. The high molecular weight of BiP-2a along with
the absence of fractionation in BiP-1 makes it reasonable to
compare the solubility limits of these two polymers. Within
the adamantyl polymer series, the BiP-2b has higher polymer
molecular weight than the BiP-2a but both have similar
polydispersities. Thus, the solubility limit comparison within
these two polymers clearly highlights the impact of polymer
molecular weight on solubility. Similar to the conventional
polymers, the bifacial polymers with higher molecular weight
have lower solubility limit.
The amount of polymer dissolved in chloroform was
determined using the Beer-Lambert law (see supporting
information Figure S4-S8). The polymer solubility limit follows
the order: PPB (16 kDa) <BiP-1 (11 kDa) <<<BiP-2a (15 kDa)
(see Table 1). During the solubility limit test, an insoluble
polymer was observed in solution in the case of both BiP-1
and PPB indicating that the solutions are saturated. Unlike the
PPB and BiP-1, to our pleasant surprise, the BiP-2a is
completely soluble in chloroform even at three times the
amount of the PPB polymer and the polymer solution turned
highly viscous making it difficult to determine the exact
solubility limit. In order to avoid the discrepancy due to high
molecular weight of the adamantyl repeat unit, the solubility
limits are also determined in mM with respect to the repeat
unit molecular weight, and the trend still holds well (Table 1).
Solubility limit of the adamantyl bifacial polymer reduced
when the polymer weight increased to 23 kDa (BiP-2b)
indicating that as the molecular weight increases the solubility
limit reduces for a given bifacial repeat unit (Table 1).
Nonetheless, the solubility limit of BiP-2b is still higher than
the conventional polymer (PPB) despite the molecular weight
of BiP-2b (23 kDa) being higher than the PPB (16 kDa).
In order to understand the impact of repeat units of
dissimilar height on polymer solubility and interchain
interactions, a random co-polymer of
6 and 12 (BiP-3) was
synthesized (Scheme 2 and 3). We are thrilled to find that the
solubility limit of the co-polymer (BiP-3, 24 kDa) is more than
three times higher than the adamantyl bifacial homopolymer
(BiP-2b, 23 kDa) of similar molecular weight (Table 1). Even at
three times the amount of BiP-2b, the co-polymer is
completely soluble in chloroform and the polymer solution
turned highly viscous making it difficult to determine the
exact solubility limit. Since the cycloalkyl groups are far from
the ethynes, they have negligible steric and electronic effect
To test whether fractionation of polymers is happening
during the solubility limit tests i.e., if the low molecular weight
polymer preferentially dissolves in chloroform while the high
molecular weight polymer stays insoluble, GPC traces of the
soluble and insoluble portions of the BiP-1 were recorded and
compared with the BiP-1 polymer sample before the solubility
limit test (Figure S3f). There is no significant difference in the
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on the kinetics of the Glaser-Hay coupling. This is evident from
the 1:1 ratio of repeat units in the co-polymers, which reflects
the feed ratio of the monomers as determined from H NMR.
generated bifacial polymers contain stereoisomers associated
with the monomer along the polymer backbone resulting in
atactic polymers. In addition to the presence of the cycloalkyl
groups, the atactic nature of the polymer backbone also
helped to further weaken the interchain interactions and
generate soluble polymers without pendant solubilizing
1
Thus, both the bifacial monomers of different height (1.328 Å
height difference) are randomly positioned along the polymer
backbone, which makes the height profile of the π-surface
rugged along the polymer backbone (Scheme 2). Since both
the BiP-2b and BiP-3 polymers have similar molecular weight
and polydispersities, we attribute the enhanced solubility
limit of the co-polymer compared to the homopolymers to
the ruggedness of the π-surface, which leads to inefficient
polymer chain packing and hence weaker interchain
interactions.
chains. Since all the BiPs are synthesized from
a
stereoisomeric mixture of monomers, improved solubility of
the BiP-2a compared to BiP-1 is attributed to a change in the
height of the monomer. Improved solubility of the BiP-3
compared to BiP-2b is attributed to a random arrangement of
the repeat units of different height along the polymer
backbone.
In the case of ansa-poly(para-phenylene)s made using the
ansa-monomers (Scheme 1), it has been shown that the
isotactic and syndiotactic polymers (polymers synthesized
from enantiomerically pure monomers) are less soluble
compared to the atactic polymer (polymer synthesized from a
racemic mixture).23 Thus, the presence of different
stereoisomers along the polymer backbone also helps to
weaken interchain interactions and improve polymer
solubility. In the case of BiPs also, since a stereoisomeric
mixture of monomers is used to synthesize the polymers, the
In order to understand the impact of the bifacial monomer
height on the strength of interchain interactions, change in
absorption maximum of the polymers from solution to thin
film are compared (Figure 2). The absorption maxima of all the
bifacial homopolymers as well as the BiP-3 co-polymer in
chloroform are ca. 424 nm, which is 9 nm higher than the PPB
(Table 1). Similar absorption maxima of the BiPs and PPB
indicate that the bifacial architecture has minimal effect on
the solution state absorption spectrum. The absorption
maxima of the BiPs in thin films are ca. 430 nm, which is 6-10
nm red-shifted compared to the solution state maxima. On
the other hand, absorption maximum of the PPB in thin film is
442 nm, which is 25 nm red-shifted compared to the solution
state maximum (Table 1). The higher red shift in PPB is due to
strong interchain π-π interactions between the unhindered π-
surface of the polymers. The lower red shift (6-10 nm) in the
case of BiPs support that the cycloalkyl groups weaken
interchain interactions between the polymers. The emission
spectra of the BiPs in chloroform are shown in Figure 3. The
Figure 3. Fluorescence spectra of BiP polymers.
difference in solution state absorption maxima to emission
maxima for the BiPs are between 16-24 nm, which indicates
Figure 2: UV-vis absorption spectra of BiP polymers. (top) in solution state;
(bottom) thin film.
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that there is not much difference in the conformational
changes during emission as the repeat unit size is increased by
changing the cycloalkyl group from cyclohexyl to adamantyl.
The powder X-ray diffraction data of all the BiPs shows two
peaks below 25o 2θ (Figure 4). For the BiP-1 the two peaks
correspond to 7.18 Å and 4.12 Å. Since the BiPs lack pendant
solubilizing chains their assembly is expected to be different
from the conventional conjugated polymers with solubilizing
distance from PXRD is found to be greater than the BiP-1 and
corresponds to 7.5 Å. Only a small increase in the interlayer
separation for BiP-2b and BiP-3 indicates strong interdigitation
of the adamantyl cycloalkyl groups in these polymers. Similar
interchain distance for both the BiP-2b and BiP-3 copolymer
confirms that the improved solubility of BiP-3 is due to weaker
interactions in the co-polymer and not due to a difference in
polymer assembly in the solid state.
To further elucidate how the bifacial monomer height
impacts the BiP interactions with other molecules,
fluorescence quenching studies of the BiPs with TCNQ
(tetracyanoquinodimethane) acceptor were carried out and
compared with that of the conventional polymer (PPB) (Figure
5, S9-S11). As the TCNQ concentration increased the
fluorescence intensity of all the polymers reduced resulting in
an upward curvature of the Stern-Volmer (SV) plot. Thus, the
data was fitted to a reported non-linear SV equation36 to
Figure 4. (top) PXRD of BiPs and (bottom) cartoon depicting the assembly of
polymers in solid state.
chains. To gain insights into polymer assembly in the BiPs, a
model-dimer molecule (15) is synthesized (see supplementary
information). The dimer crystal structure (Figure 1 and
supplementary information) suggests existence of two
different layers of polymer chains in the solid state in which
one of the layers is offset with respect to the other and there
is an interdigitation of cycloalkyl groups between the layers.
This type of assembly pattern is reminiscent of the assembly
in polyacetylenes, which also lack the pendant solubilizing
chains.35 The crystal structure of the model dimer molecule
indicates presence of ABA type stacking in the BiPs as shown
in Figure 4. The interlayer separation distance in BiP-1 was
found to be 7.18 Å from PXRD, which corresponds to the peak
at lower 2θ. The interchain separation within the layer along
Figure 5. (top) TCNQ concentration dependent BiP-1 emission spectra in
chloroform, TCNQ to polymer repeat unit concentration is shown in the legend;
(bottom) Stern-Volmer plot and fit of bifacial polymers.
the lateral direction is 8.24
Å but in the PXRD the
corresponding overtone peak at higher 2θ (4.12 Å) is
appearing. In the case of BiP-2b and BiP-3 due to the
increased height of the adamantyl moiety the interlayer
obtain the SV constant (Ksv) (Figure 5, S12). The Ksv
decreased with an increase in the monomer height and
follows the order PPB >> BiP-1 > BiP-3 > BiP-2b. Moreover,
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there is almost an order of magnitude reduction in the Ksv
from PPB to BiP-2b (Table 1). To the best of our knowledge
varying the Ksv of poly(p-phenyleneethynylene),36 which is
similar to the PPB backbone structure, up to an order of
magnitude by varying the pendant side chains has not been
demonstrated so far. All the BiPs have a similar polymer
backbone; thus, the quenching constant primarily depends on
how close a TCNQ molecule can approach the polymer
backbone. In the case of bifacial polymers, the polymer
backbone mimics a cylinder wherein the radius of the cylinder
is determined by the height of the monomer. The larger the
radius of the cylinder (radius of BiP-2b > BiP-3 > BiP-1), the
farther the TCNQ is from the polymer backbone, thereby
resulting in a lower quenching constant. BiPs have lower
quenching constant than the PPB even though the width of
the PPB repeat unit and hence the PPB cylinder diameter is
larger because in the case of BiPs the cycloalkyl groups are
directly positioned above and below the π-surface and thus
are more effective in weakening the BiP-TCNQ interactions
and reducing the Ksv by an order of magnitude.
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Conclusions
In conclusion, novel bifacial π-conjugated polymers that
mimic the β-strand architecture are developed. The lack of
pendant solubilizing chains, and the ability to easily vary
monomer height without significantly altering the monomer
width and to generate relatively high molecular weight
soluble polymers make the bifacial polymers potential
building blocks to control growth, assembly, and solution
processability of the 2D-π-conjugated materials. By proper
design i.e., by incorporating functional moieties that undergo
lateral polymerization, the high molecular weight, soluble
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control over face-to-face interactions in 2D-π-CoPs, which
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