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Oligoamide strands containing tertiary amide groups were found to share a basic structural motif that promotes the formation of H-bonded, chain- or tape-like supramolecular assemblies. [ABSTRACT FROM AUTHOR]

As the third-generation rigid macrocycles evolved from progenitor 1 , cyclic aromatic oligoamides 3 , with a backbone of reduced constraint, exhibit extremely strong stacking with an astoundingly high affinity (estimated lower limit of K dimer > 10 13 M -1 in CHCl 3 ), which leads to dispersed tubular stacks that undergo further assembly in solution. Computational study reveals a very large binding energy (-49.77 kcal mol -1 ) and indicates highly cooperative local dipole interactions that account for the observed strength and directionality for the stacking of 3 . In the solid-state, X-ray diffraction (XRD) confirms that the aggregation of 3 results in well-aligned tubular stacks. The persistent tubular assemblies of 3 , with their non-deformable sub-nm pore, are expected to possess many interesting functions. One such function, transmembrane ion transport, is observed for 3 .

This Feature Article gives an account for a host of readily available foldamers and macrocycles with well-defined shapes and non-deformable cavities that appeared over the last decade. Efforts to create porous molecular structures have led to the establishment of an effective strategy for enforcing the folding of unnatural aromatic oligoamide strands based on an especially robust three-center (bifurcated) hydrogen-bonding interaction. Based on such a strategy, aromatic oligoamides adopting crescent and helical conformations that contain non-collapsible cavities of tunable diameters have been created. Extending the same folding principle to the preparation of aromatic polyamides that would adopt pore-containing helical conformation instead led to the discovery of a highly efficient, one-pot macrocyclization process. Such a one-pot macrocyclization process has been successfully applied to the preparation of macrocycles with aromatic amide, hydrazide, urea and other backbones. Mechanistic study indicates that the high efficiencies observed for the formation of these macrocycles are due to the folding of the corresponding uncyclized oligomeric precursors of the corresponding macrocycles. Oligoamide macrocycles, along with their uncyclized, cavity-containing counterparts, i.e., crescent oligoamides, bind guests such as guanidinium (G) and octylguanidinium (OG) ions with tunable selectivity. Recent studies revealed that these rigid macrocycles tend to engage in extraordinarily strong, directional aggregation, leading to nanotubular assemblies containing pores of fixed sizes. Consistent with the presence of self-assembling nanopores, oligoamide macrocycles were found to assemble into transmembrane channels with high conductance.

A class of six-residue, shape-persistent aromatic oligoamide macrocycles bind the guanidinium ion with very high selectivity.

Aromatic tetrasulfonamide macrocycles carrying alkoxy side chains adopt a stable cone conformation in both the solid state and solution.