Abstract
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.
Highlights
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
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]
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)
Summary
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. 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 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. 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). Using this approach we decided to seek the stabilization of a supramolecular helix within a covalent helical polymer
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