Fluorescence emission enhancement of a T-shaped benzimidazole with a mechanically-interlocked 'suit'

Organic Cage Rotaxanes

Organic cages are a robust class of molecular hosts with a myriad of applications in materials science. Despite this, there has been a paucity of explorations into the modification of their properties via external functionalization. In this work, [n]rotaxanes featuring unoccupied organic cages as stopper components and a small 2,2'-bipyridine macrocycle were constructed using the active metal template (AMT) approach. By exploiting a scrambling methodology, it was possible to synthesize cages with a defined number of interlocked components (n = 2-4). The gas uptake, solubility, and thermal properties of the interlocked systems were compared against those of their constituent, non-interlocked components. In this manner, we were able to demonstrate the potential of exploiting the mechanical bond for modulating the physiochemical properties of these molecular materials.

Allosteric release of cucurbit[6]uril from a rotaxane using a molecular signal

Rotaxanes can be regarded as storage systems for their wheel components, which broadens their application potential as a complement to the supramolecular systems that retain a mechanically interlocked structure. However, utilising rotaxanes in this way requires a method to release the wheel while preserving the integrity of all molecular constituents. Herein, we present simple rotaxanes based on cucurbit[6]uril (CB6), with an axis equipped with an additional binding motif that enables the binding of another macrocycle, cucurbit[7]uril (CB7). We demonstrate that the driving force behind the wheel dethreading originates from the binding of the signalling macrocycle to the allosteric site, leading to an increase in the system's strain. Consequently, the CB6 wheel leaves the rotaxane station overcoming the mechanical barrier. Portal-portal repulsive interactions between the two cucurbituril units play a crucial role in this process. Thus, the repulsive strength and the related rate of slipping off can be finely tuned by the length of the allosteric binding motif. Finally, we show that the CB6 wheel can be utilised within complexes with other guests in the mixture once released from the rotaxane.

A CuI6L4 Cage Dynamically Reconfigures to Form Suit[4]anes and Selectively Bind Fluorinated Steroids

Simple organic ligands can self-assemble with metal ions to generate metal-organic cages, whose cavities bind guests selectively. This binding may enable new methods of chemical separation or sensing, among other useful functions. Here we report the preparation of a CuI6L4 pseudo-octahedral metal-organic cage, the ligands of which self-assemble from simple organic building blocks. Temperature, solvent, and the presence of different guests governed which structure predominated from a dynamic mixture of cage diastereomers with different arrangements of right- or left-handed metal vertices. Dissolution in dimethyl sulfoxide or the binding of tetrahedral guests led to a chiral tetrahedral T-symmetric framework, whereas low temperatures favored the achiral S4-symmetric diastereomer. Tetrahedral guests with long arms were encapsulated to form mechanically bonded suit[4]anes, with guest arms protruding out through host windows. The cage was also observed to bind fluorinated steroids, an important class of drug molecules, but not non-fluorinated steroids, providing the basis for new separation processes.

9,9-Bis[4-(N-aryl)phenyl]methylidene-xanthylidene Derivatives Displaying Mechano-, Crystallo-, and Thermochromism

Tetraphenylethylene (TPE) derivatives bearing a xanthene moiety are of interest because they have novel optical properties. 9,9-Bis[4-(N,N-diphenylamino)phenyl] and 9,9-bis[4-(9-carbazolyl)-phenyl]methylidene-xanthylidenes 3 and 4 were synthesized using Suzuki-Miyaura coupling of 9,9-dibromomethylidene-xanthylidene with the corresponding boronic acids. Diphenylamino derivative 3 exhibits mechanochromism and mechanofluorochromism (MC and MFC) reflected in absorption and fluorescence color changes. In contrast, carbazolyl derivative 4 displays thermo- and crystallo-chromism in addition to MC and MFC in the solid state. Powder X-ray diffraction and single crystal X-ray crystallographic analysis reveal that the solid state photophysical properties of these substances are governed by conformational changes rather by the creation of planar π-conjugation extended geometries.

Docking rings in a solid: reversible assembling of pseudorotaxanes inside a zirconium metal-organic framework

An unprecedented zirconium metal–organic framework featuring a T-shaped benzimidazole strut was constructed and employed as a sponge-like material for selective absorption of macrocyclic guests. The neutral benzimidazole domain of the as-synthesized framework can be readily protonated and fully converted to benzimidazolium. Mechanical threading of [24]crown-8 ether wheels onto recognition sites to form pseudorotaxanes was evidenced by solution nuclear magnetic resonance, solid-state fluorescence, and infrared spectroscopy. Selective absorption of [24]crown-8 ether rather than its dibenzo counterpart was also observed. Further study reveals that this binding process is reversible and acid–base switchable. The success of docking macrocyclic guests in crystals via host–guest interactions provides an alternative route to complex functional materials with interpenetrated structures.

A T-shaped ligand was designed as struts for building a zirconium metal–organic framework. Acid–base switchable docking and releasing a 24-membered crown ether inside crystals was successfully accomplished via post-synthetic modification.

2D and 3D metal-organic frameworks constructed with a mechanically rigidified [3]rotaxane ligand

A mechanically interlocked [3]rotaxane was newly designed, synthesized, and employed as a ligand for constructing metal-organic frameworks (MOFs). The nano-confinement by macrocycles forces the soft bis-isophthalate axle into a pseudo-rigid conformation and coordinates to zinc(II) ions, affording a two- or three-dimensional MOF under controlled conditions. The 2D MOF exhibits a neutral framework with a periodic puckering sheet structure, while an anionic framework with a pts topology was observed for the 3D MOF. The phase purity of both bulk materials was confirmed by powder X-ray diffraction. Thermogravimetric analysis reveals that both materials are stable up to 250 °C. The success of applying mechanical bonds to rigidify flexible ligands provides new insights for the design of metal-organic frameworks.

Synergistic covalent-and-supramolecular polymers connected by [2]pseudorotaxane moieties

Synergistic covalent-and-supramolecular polymers, in which covalent polymers and supramolecular polymers connect with each other through [2]pseudorotaxane moieties, are designed and synthesized. The unique topological structure effectively enhances the synergistic effect between these two polymers, thereby generating a novel class of mechanically adaptive materials.

Synthesis of N-acylbenzimidazoles through [4 + 1] annulation of N-arylpivalimidamides with dioxazolones

Presented herein is a novel and efficient synthesis of N-acylbenzimidazoles through an unprecedented [4 + 1] annulation of N-arylpivalimidamides with dioxazolones.