Enhanced Helical Folding of ortho-Phenylenes through the Control of Aromatic Stacking Interactions

Abstract

The ortho-phenylenes are a simple class of foldamers, with the formation of helices driven by offset aromatic stacking interactions parallel to the helical axis. For the majority of reported o-phenylene oligomers, the perfectly folded conformer comprises perhaps 50-75% of the total population. Given the hundreds or thousands of possible conformers for even short oligomers, this distribution represents a substantial bias toward the folded state. However, "next-generation" o-phenylenes with better folding properties are needed if these structures are to be exploited as functional units within more complex architectures. Here, we report several new series of o-phenylene oligomers, varying both the nature and orientation of the substituents on every repeat unit. The conformational behavior was probed using a combination of NMR spectroscopy, DFT calculations, and X-ray crystallography. We find that increasing the electron-withdrawing character of the substituents gives oligomers with substantially improved folding properties. With moderately electron-withdrawing groups (acetoxy), we observe >90% of the perfectly folded conformer, and stronger electron withdrawing groups (triflate, cyano) give oligomers for which misfolded states are undetectable by NMR. The folding of these oligomers is only weakly solvent-dependent. General guidelines for the assessment of o-phenylene folding by NMR and UV-vis spectroscopy are also discussed.

Tetraphenylethylene Foldamers with Double Hairpin‐Turn Linkers, TNT‐Binding Mode and Detection of Highly Diluted TNT Vapor

“…These artificial foldamers can not only serve as protein–protein interaction mediators and molecular receptors like biopolymers, but also exhibit great potential in molecular materials, such as molecular containers and tubes, sensors, molecular shuttles and machines, and so on. However, although numerous artificial foldamers have been reported, with confines ranging from flexible to rigid, folded types forming a secondary folded motif are still comparatively rare . While most foldamers adopt a secondary structure in helices, other types of folding, such as parallel folding of the chemical units, are very rare .…”

Helical Poly(5-alkyl-2,3-thiophene)s: Controlled Synthesis and Structure Characterization

“…It has been reported recently that oligo(o-arylene)s intrinsically possess a compressed helical structure with a helical pitch of 0.3−0.5 nm in the solid state as revealed by single crystal X-ray analysis, including oligo(o-phenylene)s, oligo(o-thiophene)s, oligo(o-thiazole)s, and oligo(o-furan)s. 6−10 Moreover, Hartley and co-workers disclosed that this compressed helical structure of oligo(o-arylene)s is stabilized by aromatic stacking interactions, 11,12 which is different from traditional helical polymers commonly stabilized by steric effects or hydrogen bonding. 13−15 Thus, due to their helical structures, oligo(o-arylene)s are reported to possess certain unique properties with potential applications different from their meta-or para-counterparts.…”

Scanning tunnelling microscopy analysis of octameric o-phenylenes on Au(111)

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Due to their heavily angled aromatic connection, oligomeric o-phenylenes are able to adopt a closely packed helical conformation featuring axial chirality. Oligomeric and polymeric o-phenylenes, consisting of multiple phenylene units linked together at their ortho-positions, are an emerging class of such helical molecules.…”

“…9,10 Even though OP 8 Br and OP 8 NO 2 (Fig. Intensive studies by Hartley and coworkers [12][13][14][15][16][17] indicated that OP 8 s exhibit, despite the fact that the perfectly folded helical conformation prevails in the solid state, a complex stereochemistry in solution, involving a large number of partially folded conformers that are similar in energy. Interestingly, upon chemical oxidation of OP 8 NO 2 , the helical inversion of OP 8 NO 2 c + is signicantly suppressed.…”