Mirror Helices and Helicity Switch at Surfaces Based on Chiral Triangular‐Shape Oligo(phenylene ethynylenes)

Abstract

The self-assembly of triangular-shaped oligo(phenylene ethynylenes) (OPEs), peripherally decorated with chiral and linear paraffinic chains, is investigated in bulk, onto surfaces and in solution. Whilst the X-ray diffraction data for the chiral studied systems display a broad reflection centered at 2θ ∼20° (λ=Cu(Kα)), the higher crystallinity of OPE 3, endowed with three linear decyl chains, results in a diffractrogram with a number of well-resolved reflections that can be accurately indexed as a columnar packing arranged in 2D oblique cells. Compounds (S)-1 a and (R)-1 b-endowed with (S)- and (R)-3,7-dimethyloctyloxy chains-transfer their chirality to the supramolecular structures formed upon their self-assembly, and give rise to helical nanostructures of opposite handedness. A helicity switch is noticeable for the case of chiral (S)-2 decorated with (S)-2-methylnonyloxy chains which forms right-handed helices despite it possesses the same stereoconfiguration for their stereogenic carbons as (S)-1 a that self-assembles into left-handed helices. The stability and the mechanism of the supramolecular polymerization in solution have been investigated by UV/Vis experiments in methylcyclohexane. These studies demonstrate that the larger the distance between the stereogenic carbon and the aromatic framework is, the more stable the aggregate is. Additionally, the self-assembly mechanism is conditioned by the peripheral substituents: whereas compounds (S)-1 a and (R)-1 b self-assemble in a cooperative manner with a low degree of cooperativity, the aggregation of (S)-2 and 3 is well described by an isodesmic model. Therefore, the interaction between the chiral coil chains conditions the handedness of the helical pitch, the stability of the supramolecular structure and the supramolecular polymerization mechanism of the studied OPEs.

C3‐Symmetrical π‐Scaffolds: Useful Building Blocks to Construct Helical Supramolecular Polymers

“…Despite the lack of dichroic response, atomic force microscopy (AFM) imaging of the aggregates formed by enantiomers 5 a and 5 b show isolated, helical structures of opposite handedness (Figures 4e and 4 f). [21] Closely related to these radial OPEs is the octupolar system 6 reported by Casado, López Navarrete and co-workers ( Figure 5a). [22] UV-Vis spectra in solvents of different polarity (CHCl 3 , CH 2 Cl 2 or hexane) show no changes upon varying the concentration, despite that the large aromatic core of 6 should favour the π-stacking into columnar aggregates.…”

“…For this to occur, the presence of specific and directional non‐covalent forces is a prerequisite. Despite the lack of dichroic response, atomic force microscopy (AFM) imaging of the aggregates formed by enantiomers 5 a and 5 b show isolated, helical structures of opposite handedness (Figures e and 4 f) …”

Investigation of the Amide Linkages on Cooperative Supramolecular Polymerization of Organoplatinum(II) Complexes

“…It facilitates the incorporation of intermolecular hydrogen bonds between the neighboring monomers, and thereby induces electronic cooperativity during the supramolecular polymerization process [ 13 , 14 , 15 , 16 , 17 ]. By adopting this principle, a variety of π-conjugated moieties such as arenes, oligo(phenylene ethynylene)s (OPEs), perylene bisimides (PBIs), porphyrins have been reported to form cooperative supramolecular polymers [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ].…”

Chiral information harvesting in helical poly(acetylene) derivatives using oligo(p-phenyleneethynylene)s as spacers

“…These OPE units have been used in the formation of benzene-1,3,5-tricarboxamide (BTA) based supramolecular helical polymers, demonstrating their ability to stack with a certain tilting degree commanded by the chiral center. [32][33][34] Hence, in our design, the chiral moiety will determine the supramolecular chiral orientation of the OPE groups used as spacers, which is further harvested by the polyene backbone. The overall process yields a helix with a preferred screw sense (Fig.…”

“…This step resembles the helical induction mechanism found in supramolecular helical polymers bearing OPE units. [32][33][34] Next, the chiral array induced in the OPE units is harvested by the polyene backbone, resulting in an effective P or M helix induction (Fig. 2).…”