Peptoids are oligomers of N-substituted glycines with predictable folding and strong potentials as guest-binding receptor molecules. In this contribution, we investigate the structural features of a series of designed symmetric cyclic octamer peptoids (with methoxyethyl/propargyl side chains) as free hosts and reveal their morphologic changes in the presence of sodium and alkylammonium guests as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts, reporting the first case of reversible adaptive switching between defined conformational states induced by cationic guests (Na and benzylammonium ion) in the peptoid field. The reported results are based on H NMR data, theoretical models, and single-crystal X-ray diffraction analysis. They represent initial steps toward deciphering the unique conformational states of cyclic octamer peptoids as supramolecular hosts with the aim to fully disclose their functional and dynamic properties.

Formation of stable porous frameworks based on cyclic peptoids can be triggered by strategic choice of appropriate side chains. In this contribution we demonstrate that substitution of distal propargyl side chains with methoxyethyl groups in a fully propargylated cyclic octamer peptoid (cyclo-(Npa)8 1) greatly improves the solid state stability inducing permanent one-dimensional porosity of the compound (cyclo-[(NPa)3(Nme)]2 2, Npa = N-(propargyl)­glycine, Nme = N-(methoxyethyl)­glycine). In both compounds the macrocycles align along the shortest cell axis to form tubes that are filled with guest molecules. In situ hydration and dehydration single crystal X-ray diffraction studies on compound 2 demonstrated the improved stability of the host framework. Hirshfeld surface analysis and lattice energy calculations, also supported by energy frameworks analysis, clarified the determinant packing motifs in the studied compounds explaining the improved stability in terms of architectural robustness. Methoxyethyl side chains act as H-bond acceptor by tightening as wall ties the host framework, at a difference with propargyl side chains that provide CH−π interactions with similar energy, but along a less effective direction.

The investigation of the solid state assembly of propargyl substituted hexa- and octacyclic peptoids highlights the effect of ring size in determining the packing arrangement of the macrocycles. A layered arrangement is obtained in the case of the hexacyclic peptoid 1 and a tubular arrangement in the case of the octacyclic peptoid 2. Guest molecules either intercalate between the layers as in 1 or are located within the peptoid nanotubes as in 2

The confused‐prism[5]arene macrocycle (c‐PrS[5]Me) shows conformational adaptive behavior in the presence of ammonium guests. Upon guest inclusion, the 1,4‐bridged naphthalene flap reverses its planar chirality from pS to pR (with reference to the pS(pR)4 enantiomer). Stereoselective directional threading is also observed in the presence of directional axles, in which up/down stereoisomers of homochiral (pR)5‐c‐PrS[5]Me pseudorotaxanes are formed.

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