Large-Amplitude Conformational Changes in Self-Assembled Multi-Stranded Aromatic Sheets

Single-crystal polymers (SCPs): from 1D to 3D architectures

Single-crystal polymers (SCPs) with unambiguous chemical structures at atomic-level resolutions have attracted great attention. Obtaining precise structural information of these materials is critical as it enables a deeper understanding of the potential driving forces for specific packing and long-range order, secondary interactions, and kinetic and thermodynamic factors. Such information can ultimately lead to success in controlling the synthesis or engineering of their crystal structures for targeted applications, which could have far-reaching impact. Successful synthesis of SCPs with atomic level control of the structures, especially for those with 2D and 3D architectures, is rare. In this review, we summarize the recent progress in the synthesis of SCPs, including 1D, 2D, and 3D architectures. Solution synthesis, topochemical synthesis, and extreme condition synthesis are summarized and compared. Around 70 examples of SCPs with unambiguous structure information are presented, and their synthesis methods and structural analysis are discussed. This review offers critical insights into the structure-property relationships, providing guidance for the future rational design and bottom-up synthesis of a variety of highly ordered polymers with unprecedented functions and properties.

Sulfate-induced large amplitude conformational change in a Solomon link

A doubly-interlocked [2]catenane - or Solomon link - undergoes a complex conformational change upon addition of sulfate in methanol. This transformation generates a single pocket where two SO42- anions bind through multiple hydrogen bonds and electrostatic interactions. Despite the close proximity of the two anions, binding is highly cooperative.

Reversible conformational switching of a photo-responsive ortho-azobenzene/2,6-pyridyldicarboxamide heterofoldamer

We report on a convenient synthetic route to rapidly access a new photo-responsive ortho-azobenzene/2,6-pyridyldicarboxamide heterofoldamer. The adoption of a stable helical conformation has been established for this scaffold in both the solid state and in solution using single crystal X-ray diffraction and circular dichroism (CD) spectroscopy respectively. Reversible control over the stimuli-driven structural re-ordering of the supramolecular scaffold, from a stable helical conformation under non-irradiative conditions, to a less well-ordered state under irradiative conditions, has been identified. The robust nature of the responsive, conformational, molecular switching behaviour has been determined using UV/Vis, 1H NMR and CD spectroscopy. Minimal loss in the efficiency of the stimuli-driven, structural re-ordering processes of the foldamer scaffold is observed, even upon multiple cyclic treatments with irradiative/non-irradiative conditions.

Foldaxanes: Rotaxane-like Architectures from Foldamers

Mechanically interlocked molecules such as rotaxanes and catenanes contain free-moving components that cannot dissociate and have enabled the investigation and control of various translational and rotational molecular motions. The architecture of pseudo-rotaxanes and of some kinetically labile rotaxanes is comparable to that of rotaxanes but their components are reversibly associated and not irreversibly interlocked. In other words, pseudo-rotaxanes may fall apart. This Account focuses on a peculiar family of rotaxane-like architectures termed foldaxanes.Foldaxanes consist of a helically folded oligomer wound around a rod-like dumbbell-shaped guest. Winding of the helix around the rod thus entails an unwinding-rewinding process that creates a kinetic barrier. It follows that foldaxanes, albeit reversibly assembled, have significant lifetimes and may not fall apart while defined molecular motions are triggered. Foldaxanes based on helically folded aromatic oligoamide hosts and oligo(alkyl carbamate) guests can be designed rationally through the inclusion of complementary binding motifs on the rod and at the inner rim of the helix so that helix length and rod length match. Single helical foldaxanes (bimolecular species) and double helical foldaxanes (trimolecular species) have thus been produced as well as poly[n]foldaxanes, in which several helices bind to long rods with multiple binding stations. When the binding stations differ and are organized in a certain sequence, a complementary sequence of different stacked helices, each matching with their binding station, can be assembled, thus reproducing in an artificial system a sort of translation process.Foldaxane helix handedness may be controlled by stereogenic centers on the rod-like guest. Handedness can also be transmitted from helix to helix in polyfoldaxanes. Foldaxane formation has drastic consequences for the rod properties, including its stiffening and the restriction of the mobility of a macrocycle already interlocked on the rod. Fast translation (without dissociation) of helices along rod-like guests has been demonstrated. Because of the helical nature of the hosts, translation may be accompanied by rotation in various sorts of screw-like motions. The possibility, on longer time scales, for the helix to dissociate from and reassociate to the rod has allowed for the design of complex, kinetically controlled supramolecular pathways of a helix on a rod. Furthermore, the design of helices with a directionality, that is, with two distinct termini, that bind to nonsymmetrical rod-like guests in a defined orientation makes it possible to also control the orientation of molecular motion. Altogether, foldaxanes constitute a distinct and full-of-potential family of rotaxane-like architectures that possess designer structures and allow orchestration of the time scales of various supramolecular events.

Discrete Stacked Dimers of Aromatic Oligoamide Helices

Tight binding was observed between the C‐terminal cross section of aromatic oligoamide helices in aqueous solution, leading to the formation of discrete head‐to‐head dimers in slow exchange on the NMR timescale with the corresponding monomers. The nature and structure of the dimers was evidenced by 2D NOESY and DOSY spectroscopy, mass spectrometry and X‐ray crystallography. The binding interface involves a large hydrophobic aromatic surface and hydrogen bonding. Dimerization requires that helices have the same handedness and the presence of a C‐terminal carboxy function. The protonation state of the carboxy group plays a crucial role, resulting in pH dependence of the association. Dimerization is also influenced by neighboring side chains and can be programmed to selectively produce heteromeric aggregates.

Subtle Modification of Imine-linked Helical Receptors to Significantly Alter their Binding Affinities and Selectivities for Chiral Guests

Aromatic helical receptors P-1 and P-2 were slightly modified by aerobic oxidation to afford new receptors P-7 and P-8 with right-handed helical cavities. This subtle modification induced significant changes in the binding properties for chiral guests. Specifically, P-1 was reported to bind d-tartaric acid (K_a =35500 M^-1 ), used as a template, much strongly than l-tartaric acid (326 M^-1 ). In contrast, its modified receptor P-7 exhibited significantly reduced affinities for d-tartaric acid (3600 M^-1 ) and l-tartaric acid (125 M^-1 ). More dramatic changes in the affinities and selectivities were observed for P-2 and P-8 upon binding of polyol guests. P-2 was determined to selectively bind d-sorbitol (52000 M^-1 ) over analogous guests, but P-8 showed no binding selectivity: d-sorbitol (1890 M^-1 ), l-sorbitol (3330 M^-1 ), d-arabitol (959 M^-1 ), l-arabitol (4970 M^-1 ) and xylitol (4960 M^-1 ) in 5% (v/v) DMSO/CH₂ Cl₂ at 25±1 °C. These results clearly demonstrate that even subtle post-modifications of synthetic receptors may significantly alter their binding affinities and selectivities, in particular for guests of long and flexible chains.

Crystal structures capture multiple stoichiometric states of an aqueous self-assembling oligourea foldamer

We report here an oligourea foldamer able to self-assemble in aqueous conditions into helix bundles of multiple stoichiometries. Importantly, we report crystal structures of several of these stoichiometries, providing a series of high-resolution snap-shots of the structural polymorphism of this foldamer and uncovering a novel self-assembly.

Large-Amplitude Conformational Changes in Self-Assembled Multi-Stranded Aromatic Sheets

The orchestration of ever larger conformational changes is made possible by the development of increasingly complex foldamers. Aromatic sheets, a rare motif in synthetic foldamer structures, have been designed so as to form discrete stacks of intercalated aromatic strands through the self‐assembly of two identical subunits. Ion‐mobility ESI‐MS confirms the formation of compact dimers. X‐ray crystallography reveals the existence of two distinct conformational dimeric states that require large changes to interconvert. Molecular dynamics simulation validates the stability of the two conformations and the possibility of their interconversion.

Switchable supramolecular ensemble for anion binding with ditopic hydrogen-bonded macrocycles

A novel supramolecular strategy has been proposed by using a ditopic H-bonded amide macrocycle that is capable of controlling the binding process in response to external stimulus due to its assembly-and-disassembly-induced anion binding.