Merging
Supramolecular and Covalent Helical Polymers:
Four Helices Within a Single Scaffold
Supramolecular and covalent polymers share multiple structural effects such as chiral amplification, helical inversion, sergeants and soldiers, or majority rules, among others. These features are related to the axial helical structure found in both types of materials, which are responsible for their properties. Herein a novel material combining information and characteristics from both fields of helical polymers, supramolecular (oligo(p-phenyleneethynylene) (OPE)) and covalent (poly(acetylene) (PA)), is presented. To achieve this goal, the poly(acetylene) must adopt a dihedral angle between conjugated double bonds (ω1) higher than 165°. In such cases, the tilting degree (Θ) between the OPE units used as pendant groups is close to 11°, like that observed in supramolecular helical arrays of these molecules. Polymerization of oligo[(p-phenyleneethynylene)n]phenylacetylene monomers (n = 1, 2) bearing L-decyl alaninate as the pendant group yielded the desired scaffolds. These polymers adopt a stretched and almost planar polyene helix, where the OPE units are arranged describing a helical structure. As a result, a novel multihelix material was prepared, the ECD spectra of which are dominated by the OPE axial array.
Introduction<!><!>Introduction<!><!>Introduction<!>Results and Discussion<!><!>Results and Discussion<!><!>Results and Discussion<!><!>Results and Discussion<!>Conclusions<!>
<p>Helices
are abundant structural motifs present in nature in many
macromolecules such as peptides, proteins, DNA, and polysaccharides
and are directly related to the biological functions of these biomolecules.1−4 This structure–function relationship led the scientific community
to look for novel materials that adopt helical structures, such as
covalent and supramolecular helical polymers.5−20 Nowadays, it is possible to understand how these polymers are folded
and which of the structural features of the building blocks induce
covalent or supramolecular polymers to adopt a helical structure.
Moreover, these studies led to the development of dynamic helical
polymers—covalent and supramolecular—whose helical sense
(plus (P) or minus (M)),21−26 elongation (compressed or stretched),27−31 or aggregate shape (J-aggregate, H-aggregate, etc.)32−36 can be altered by the presence of different external stimuli (e.g.,
solvent, pH, temperature, metal ions, chiral additives, or light).
This dynamics revealed different communication mechanisms between
components in both covalent and supramolecular copolymers. Therefore,
different chiral amplification or chiral enhancement effects such
as sergeants and soldiers, majority rules, chiral coalition, chiral
accord, and chiral conflict have been observed.37−50 As a consequence, the common properties presented by these polymers
have allowed the development of materials combining these two structural
motifs.51−65 Regarding the helical sense induction mechanism, some research groups
have demonstrated the efficient long-distance transmission of chiral
information for poly(isocyanide)s,66,67 poly(vinylterphenylene)s,68,69 and poly(acetylene)s.70−73 Recently, our group has reported on poly[oligo(p-phenyleneethynylene)acetylene]s (POPEPAs), a novel family
of helical polymers.74 In these polymers,
even though the chiral center is placed at a remote position from
the backbone, a helix induction occurs due to the chiral arrangement
of the achiral spacers, which is harvested by the polyene backbone.
Hence, in a first step the stereogenic centers of the monomer repeating
units (mru) command the achiral rigid oligo(p-phenyleneethynylene)
(OPE) spacers to arrange with a specific tilting degree (Θ),
the value of which depends on the absolute configuration of the chiral
center. This chiral arrangement of the OPE units, which are stabilized
through π–π interactions between them, is then
harvested by the polyene backbone, adopting a specific P or M helical sense (Figure 1a). This chiral arrangement of OPE units
in POPEPAs drove us to study the supramolecular self-assembly of the
monomer units. Remarkably, it was demonstrated that these monomers
arrange into long supramolecular helical polymers, which are stabilized
by hydrogen bond interactions between the amide groups of the chiral
moieties and π–π interactions among the OPE units
(Figure 1b).75 The presence of a chiral OPE arrangement in
both covalent and supramolecular helical polymer systems attracted
our attention, and we thought about the possibility of combining both
families of helical polymers within a single helical structure to
create a multihelix material.</p><!><p>Chiral OPE arrays in (a) covalent and (b) supramolecular
helical
polymers.</p><!><p>To produce these multihelix materials
based on POPEPAs, modeling
studies varying the main dihedral angles (ω1, ω2, ω3, and ω4) (Figure 2a) were first carried
out. To this end, different helical structures were built that comprised
the requirements of both materials, namely the poly(acetylene) (PA)
helix and the supramolecular OPE helix. From these studies it was
extracted that it is necessary to control the dihedral angle between
conjugated double bonds (ω1), which is directly related
to the tilting degree adopted by the OPE units, to generate the desired
POPEPA. Hence, if POPEPA adopts a cis–cisoidal polyene scaffold (ω1 < 90°),76−78 the classical helix is formed and constituted in turn by two coaxial
helices (Figure 2b).
In such helical scaffolds, the internal helix is determined by the
polyene backbone (helix 1), while the external rotation sense is defined
by the helical array of the pendant groups (helix 2) (Figure 2b).79,80</p><!><p>(a)
Main dihedral angles involved in the helical structures of
PPAs and their derivatives. (b) POPEPA adopting a classical helical
structure. (c) POPEPA showing a multihelix scaffold.</p><!><p>The tilting degree (Θ) between OPE units within these
helical
structures is high, describing a hand-fan-shaped array. As a result,
no evidence of a multihelix material should be found in cis–cisoidal POPEPAs. Nevertheless, it was found that if POPEPA adopts an extended cis–transoidal helical structure with a ω1 dihedral angle higher than 160°, the two classical internal
and external helices found in PAs (helix 1 and helix 2, respectively)
coexist with two other helical scaffolds. These novel helices (helix
3 and 4) correspond to the helical array of the achiral OPE units
used as spacers between the chiral pendant and the polyene backbone,
and the rotation sense of the helices is coincident with that observed
for the outer helix (helix 2) (Figure 2c). It should be pointed out that the helical structures
described by the OPE units within cis–transoidal helical polymers are coincident with those found
in an OPE supramolecular helix (Figure 1b). Therefore, using this approach we decided to seek
the stabilization of a supramolecular helix within a covalent helical
polymer.</p><!><p>To perform these studies, it
is necessary to design and prepare
a POPEPA that adopts a cis–transoidal helical
scaffold. From the literature, it is known that poly(phenylacetylene)s
(PPAs) bearing benzamide connectors between the backbone and the pendant
groups promote the formation of cis–transoidal structures.81 Thus, as a model compound
we used the PPA that had the benzamide of L-decyl
alaninate (poly-1, Figure 3b), which adopts a stretched cis–transoidal (2/1) helix.82,83 OPE monomers
(n = 1, 2) containing the benzamide of the L-decyl alaninate, namely decyl (4-((4-ethynylphenyl)ethynyl)benzoyl)-L-alaninate and decyl (4-((4-((4-ethynylphenyl)ethynyl)phenyl)ethynyl)benzoyl)-L-alaninate (m-[2] and m-[3],
respectively) (Figure 3a), were prepared and further polymerized with a Rh(I) catalyst,
which generated poly-[2] and poly-[3] in
good yields and low polydispersity and with a high content of cis-double bonds (Figure 3b and Tables S1 and S2).
Nevertheless, in these polymers the cis-content of
the double bonds cannot be quantified with precision due to the large
differences in the T2 NMR values for the
different protons of the polymers (see section 3.2 of the Supporting Information).84 Next, structural and dynamic behavior studies were carried out for
poly-2 and poly-3.</p><!><p>Chemical structures of
(a) monomers m-[1–3] and (b) polymers
poly-1–3.</p><!><p>CD studies of poly-2 in different solvents such as
CHCl3, DCM, THF, and DMF ([poly-2] = 0.44
mM) showed the classical ECD trace of a helical polymer with three
alternating Cotton effects (Figures 4a and S21). The first positive
Cotton band, which corresponds to the polyene, indicates the presence
of a P internal helix (Figure 4a). To obtain information related to the
orientation of the external helix, AFM studies were performed. A 2D
crystal of poly-2 was prepared from a CHCl3 solution by the Langmuir–Schaefer technique85 and employing highly oriented pyrolytic graphite (HOPG)
as the substrate. The AFM analysis revealed the presence of well-ordered
monolayers. From these high-resolution images it was possible to extract
the orientation of the external helix, namely the M helix, and different structural parameters, such as the helical
pitch (4.6 nm) and the packing angle (80°) (Figures 4b and S27).</p><!><p>(a) Chemical structure of poly-2 and ECD
and UV–vis
spectra (THF, [poly-2] = 0.44 mM, 25 °C) compared
to the simulated ECD (fwhm = 20 nm) and UV–vis traces. (b)
High-resolution AFM images and 3D model of poly-2 in
polar solvents. (c) Comparison of the UV–vis spectra obtained
for poly-2 in CHCl3 and toluene. (d) ECD and
UV–vis spectra of poly-2 in toluene ([poly-2] = 0.44 mM, 25 °C). (e) 3D model of poly-2 displaying a multihelix material.</p><!><p>To corroborate the information obtained from the experimental data,
computational studies (TD-DFT(CAM-B3LYP)/3-21)] were performed on
the M helix (n = 8) of poly-2, namely the cis–transoidal skeleton (ω1 = 165° and ω3 = 80°). In this model, the chiral moiety was introduced in
an antiperiplanar conformation by placing both carbonyl
groups in opposite orientations and, to reduce computational demands,
the long alkyl chain was replaced by a methyl group. The simulated
ECD spectrum (Figure 4a and the SI for detailed information)
is in good agreement with the one obtained experimentally, indicating
that the proposed model is a good approximation to the structure described
for poly-2. Interestingly, when poly-2 is
dissolved in low-polar solvents such as CCl4 or toluene,
a yellow to red color change occurs that is indicative of helical
stretching. UV–vis studies confirmed the elongation of the
polyene chain due to a 100 nm bathochromic shift of the vinylic band
from 425 (CHCl3) to 525 nm (toluene) (Figure 4c). Moreover, the solubility
of the polymer decreased in these solvents due to the presence of
a highly stretched, almost planar, helix. ECD studies of poly-2 in CCl4 and toluene revealed the disappearance
of the classical ECD trace with three alternating Cotton effects,
now depicting a large bisignated (∓) signal centered at 323
nm (Figures 4d and S21). Intriguingly, the ECD trace obtained for
poly-2 in CCl4 or toluene is coincident with
the CD signature of an OPE supramolecular helix, where the ∓
sign of the CD trace is indicative of a M helical
array of the OPE units within the POPEPA scaffold.75 From this information, a molecular model was built for
poly-2 dissolved in low-polarity solvents using a large
ω1 value (ca., 175°), which corresponded to
an almost planar structure (Figure 4e). By looking at this 3D model, it is possible to
visualize the two coaxial helices of a PPA, one described by the polyene
backbone (P1, internal helix, ω1= +175°) and the other described by the
pendant groups (M2, external helix). As
expected, the presence of a cis–transoid polyene
skeleton causes the inner and outer helices to rotate in opposite
directions (P1/M2). In addition to these two coaxial helices, two other helices
can be observed in this 3D model structure, namely M3 and M4. They are described
by the OPE units used as spacers to separate the chiral pendant group
from the polyene backbone (Θ = 11°) and show
helical senses that coincide with that described by the outer helix M2. Interestingly, although this structure is
made up of four helices going inside out, namely P1, M3, M4, and M2, the ECD spectrum is
governed by the helical array of the OPE units, and the chiroptical
information on the polyene chain becomes negligible (ECD null at ca.
525 nm but UV–vis active).</p><p>In this helical material where
four different helices coexist within
the same structure, it is possible to observe how the supramolecular
helices described by the pendant groups (helices 2, 3, and 4) are
covalently attached to a polyene backbone (the covalent helix (helix
1)) placed in the core of this multihelix material.</p><p>To demonstrate
the versatility and robustness of our hypothesis
and results, similar studies were performed for poly-3, which has an extra OPE unit in the spacer (n =
2).To this end, solutions of poly-3 ([poly-3] = 0.72 mM) were prepared in different solvents such as CCl4, CHCl3, THF, toluene, ODCB, DCM, and 1,2-DCE (Figures 5a and S24). All these poly-3 solutions
showed deep red colors and poor solubilities, which were indicative
of a highly stretched helix with a large hydrophobic surface and a
high aggregation tendency.29 UV–vis
studies of these poly-3 solutions corroborated the stretched
helix due to the presence of the polyene band at ca. 565 nm (Figure 5b and c). In addition,
ECD studies for poly-3 do not show a Cotton band in the
polyene region in any of the tested solvents (Figures 5b, d and S24)
but instead a strong bisignated signal at shorter wavelengths (ca.,
315 nm). The ECD trace obtained is similar to that recorded for the
supramolecular helical arrangement of m-[3] in low polararity
solvents such as MCH (SP-3, Figure 5d and section 7 in the Supporting Information). In this case, the bathochromic shift
of the OPE band, which is visible in the CD spectrum when poly-3 is compared with SP-3, is attributed to its
conjugation with the polyene backbone. These data indicate that poly-3 constitutes a multihelix material where two novel helices
appear due to the chiral arrangement of the OPE units. Hence, four
helical structures coexist in this polymer: the internal helix described
by the polyene backbone (helix 1), the external helix described by
the pendant groups (helix 2), and the helical structures described
by the OPE (n = 2) units employed as spacers (helix
3 and helix 4). These helices are interconnected and it is possible
to extract the orientation of the others by knowing the orientation
of one of them (Figure 5e). Therefore, if poly-3 shows a P orientation
for helices 3 and 4 in DCM (CD (±)), helix 2 rotates in the same P direction, whereas the internal helix 1 must describe
the opposite rotation sense (M helix) according to
the cis–transoidal configuration adopted by
the polyene. Computational studies (TD-DFT(CAM-B3LYP)/3-21) performed
on the M helix (n = 20) for poly-3, namely the cis–transoidal skeleton (ω1 = 175°), confirmed this hypothesis.
To reduce the computational costs, the chiral moiety was removed from
the pendants, keeping only the achiral spacer. This modification in
the model allowed us to create a 3D structure describing a half helix
turn, which is necessary to observe the external helices described
by the OPEs (helix 3 and helix 4). The calculated ECD is in full agreement
with the experimental one obtained in DCM, indicating that the proposed
model is a good approximation to the highly stretched cis–transoidal structure adopted by the polymer.</p><!><p>(a) Chemical
structure of poly-3. (b) Comparison for
the experimental (DCM and THF, [poly-3] = 0.72 mM, 25
°C) and calculated ECD and UV–vis spectra for poly-3; fwhm = 20 nm. (c) UV–vis spectrum of poly-3 in DCM ([poly-3] = 0.72 mM, 25 °C). (d)
Comparison of the ECD and UV–vis spectra obtained for poly-3 (THF, [poly-3] = 0.72 mM, 25 °C) and SP-3 (MCH, [poly-3] = 0.15 mM, 25 °C). (e)
3D model of poly-3 describing a multihelix material.</p><!><p>The helical pitch value is
in good agreement
with the data extracted from the small-angle X-ray scattering (SAXS)
measurements (4.0 nm) (see Figure S28).
The combined information obtained from the ECD, SAXS, and AFM studies
indicates that poly-2 adopts a cis–transoidal scaffold (ω1 ca., 165°) where internal and
external helices rotate in opposite directions (Pint (CD405 nm= (+)) and Mext (AFM= (−)) (Figure 4b).</p><p>The 1D SAXS profile obtained for
poly-3 revealed two
maxima. These diffraction peaks confirm the proposed 3D model for
the polymer in which the 5.6 nm value is coincident with the helix
width and the 17.0 nm is conincident with the helical pitch (see Figure S28). Further ECD studies indicated that
the helical sense described by the OPE array in poly-3 depends on the dielectric constant of the solvent (ε –
1)/(ε + 1), which denotes that poly-3 is a dynamic
helical polymer that acts as a chiroptical switch triggered by subtle
variations of polarity. Thus, in solvents with (ε – 1)/(ε
+ 1) > 0.8 (i.e., ODCB, DCM, and 1,2-DCE), a bisignated (±)
CD
signature is induced in poly-3, which corresponds to
a P helix of the OPE array (helices 3 and 4), namely
the M1/P3/P4/P2 multihelix
scaffold.</p><p>On the other hand, those solvents with (ε –
1)/(ε
+ 1) < 0.8 (i.e., CCl4, CHCl3, THF, and toluene)
produce an M orientation of the OPE helical array
(helices 3 and 4), yielding a P1/M3/M4/M2 multihelix material (Figures S16c and d).</p><!><p>In
conclusion, we have demonstrated through two different examples,
namely poly-2 and poly-3, that it is possible
to obtain a multihelix material by linking a supramolecular helix
made by OPE units to a covalent helical polymer (PA). From previous
studies in the fields of both covalent and supramolecular polymers,
we envisioned that both structures could fit on a proper multihelix
scaffold. In this special case of PA and OPE systems, the polyene
should adopt an almost planar but twisted helical structure (i.e.,
a cis–transoidal helix with ω1 > 165°). This fact causes the pendant groups of the PA,
in
this case the OPE derivatives, to have a tilting degree between them
(Θ) close to 11°, similar to that present in the supramolecular
OPE polymers. In this way the multihelix material can be prepared
by stabilizing a SP helix in a covalent helix. A perfect example is
poly-2, which bears an OPE with n =
1 (Figure 6). In THF,
this polymer a helical structure with ω1 ca. 165°,
showing a CD signature with three alternating Cotton effects (a classical
PPA helix). On the other hand, when solubilized in toluene or CCl4, poly-2 adopts a more stretched helical scaffold
with ω1 > 170°. This results in a bisignate
CD signature that is governed by the axial orientation of the OPE
units instead of being commanded by the helical orientation of the
PA main chain. Consequently, two new helices emerge within this novel
helical scaffold where four different helices coexist in the helical
material: the two coaxial helices, namely internal (helix 1) and external
(helix 2), and the two helices described by the OPE axial array (helices
3 and 4). These four helices are interconnected and by identifying
the orientation of one of them it is possible to obtain the helical
sense of the others. Therefore, two scenarios are possible: M1/P3/P4/P2 and P1/M3/M4/M2. For its part, the second
design, poly-3, always generates a multihelix material
due to the axial array of its OPE units, with the presence of four
helices within a single polymer. These results open a new horizon
in the design of helical polymers, where the stabilization of supramolecular
helices in covalent polymers will allow the use of these structures
in applications that previously could not be accessed due to the difficulty
of generating SP polymers in polar solvents.</p><!><p>Moving from a classical
helical PPA to a multihelix material by
playing with the dihedral angle between conjugated double bonds.</p><p>Materials and methods,
synthesis of monomers and polymers,
GPC studies, TGA studies, dynamic behavior, formation of SP-3, AFM measurements for poly-2, SAXS experiments,
theoretical calculations, and supporting references (PDF)</p><p>ja1c10327_si_001.pdf</p><p>The authors declare no
competing financial interest.</p>