Shape-Persistent Triptycene-Derived Pillar[6]arenes: Synthesis, Host-Guest Complexation, and Enantioselective Recognitions of Chiral Ammonium Salts

Construction of macrocyclic hosts with a novel structure and excellent property has emerged as an intriguing undertaking for the past few years. Here, we reported the synthesis of shape-persistent triptycene-derived pillar[6]arene (TP[6]). The single crystal structure analysis revealed that the macrocyclic molecule adopts a hexagonal structure, featuring a helical and electron-rich cavity capable of encapsulating electron-deficient guests. In order to obtain chiral TP[6] from an enantiomerically pure triptycene building block, an efficient resolution of chiral triptycene was successfully developed through introducing chiral auxiliaries into triptycene skeletons. The ¹H NMR and isothermal titration calorimetry investigations demonstrated that chiral TP[6] exhibited enantioselectivity toward four pairs of chiral guests containing a trimethylamino group, implying a significant promising application in area of enantioselective recognition.

Dual-Way-Switchable Ester Rotaxanes Constructed Using the Recognition of Malonate Diesters

Malonate diesters can thread into the cavity of a di(ethylene glycol)-containing macrocycle under the templating effect of a Na⁺ ion; the corresponding rotaxanes can be synthesized with good efficiency by applying several stoppering reactions. A molecular switch, in which the interlocked macrocycle was moved between two rarely used stations (i.e., malonate and TAA) through the addition of acid/base and the presence/absence of Na⁺ ions, was constructed using this new recognition system.

Mechanically interlocked polymers based on rotaxanes

The nature of mechanically interlocked molecules (MIMs) has continued to encourage researchers to design and construct a variety of high-performance materials. Introducing mechanically interlocked structures into polymers has led to novel polymeric materials, called mechanically interlocked polymers (MIPs). Rotaxane-based MIPs are an important class, where the mechanically interlocked characteristic retains a high degree of structural freedom and mobility of their components, such as the rotation and sliding motions of rotaxane units. Therefore, these MIP materials are known to possess a unique set of properties, including mechanical robustness, adaptability and responsiveness, which endow them with potential applications in many emerging fields, such as protective materials, intelligent actuators, and mechanisorption. In this review, we outline the synthetic strategies, structure-property relationships, and application explorations of various polyrotaxanes, including linear polyrotaxanes, polyrotaxane networks, and rotaxane dendrimers.

A cyclic bis[2]catenane metallacage

Catenated cages represent chemistry’s challenging synthetic targets because a three-dimensional assembly is necessary for their formation. Herein, a cyclic bis[2]catenane is constructed through the coordination-driven self-assembly of the interlocked bis-metallacage, by the 90° Pt(II) heteroligation of the endo-functionalized double-bridged tweezer bearing pyridyl moieties and the tetra-carboxylated linker. NMR spectrometry, X-ray crystallography and mass spectrometry confirm the formation of a cyclic bis[2]catenane with “∞”-shaped topology via a 14-component self-assembly. Particularly, reversibly responsive transformation between the bis[2]catenane and the bis-metallacage can be realized by guest exchange, concentration effect and solvent effect. This work represents a novel example of a cyclic cage-based [2]catenane oligomer.

Catenated cages are challenging synthetic targets in chemistry. Here, the authors employ a multi-component coordination strategy using a Pt(II) heteroligation to construct a cyclic bis[2]catenane metallacage, which could be reversibly transformed between the catenated structure and the bis-metallacage.

Exploring and Exploiting the Symmetry-Breaking Effect of Cyclodextrins in Mechanomolecules

Cyclodextrins (CDs) are cone-shaped molecular rings that have been widely employed in supramolecular/host–guest chemistry because of their low cost, high biocompatibility, stability, wide availability in multiple sizes, and their promiscuity for binding a range of molecular guests in water. Consequently, CD-based host–guest complexes are often employed as templates for the synthesis of mechanically bonded molecules (mechanomolecules) such as catenanes, rotaxanes, and polyrotaxanes in particular. The conical shape and cyclodirectionality of the CD “bead” gives rise to a symmetry-breaking effect when it is threaded onto a molecular “string”; even symmetrical guests are rendered asymmetric by the presence of an encircling CD host. This review focuses on the stereochemical implications of this symmetry-breaking effect in mechanomolecules, including orientational isomerism, mechanically planar chirality, and topological chirality, as well as how they support applications in regioselective and stereoselective chemical synthesis, the design of molecular machine prototypes, and the development of advanced materials.

Novel oxacalix[2]arene[2]triazines with thermally activated delayed fluorescence and aggregation-induced emission properties

New kinds of oxacalixarenes based on the triazine moiety have been synthesized, which showed highly twisted conformations and small ΔEST, as well as temperature-dependent transient PL decay curves, indicating the excellent thermally activated delayed fluorescence properties. Moreover, the oxacalixarenes also exhibited apparent aggregation-induced emission properties in THF/H2O mixtures.

Synthesis, Structure, Property, and Dinuclear Cu(II) Complexation of Tetraoxacalix[2]arene[2]phenanthrolines

A number of novel tetraoxacalix[2]arene[2]phenanthrolines 7-11 containing phenanthroline and diverse aromatic linkages were conveniently synthesized by a one-pot protocol between a series of dihydroxy arenes and 1,10-phenanthroline derivatives. Single-crystal diffraction analysis revealed that the resulting macrocycles possess diverse conformational and cavity structures which are regulated by the different aromatic linkages. In line with the length of the aromatic linkages, the distance between the two phenanthroline moieties ( d_N-N) gradually increases from 6.92 to 13.30 Å, respectively. The physicochemical properties of these macrocyclic compounds were investigated by spectroscopic, CV, and DPV measurements. Owing to the coordination ability of the phenanthroline moieties and the tunable conformational structure, the macrocyclic hosts can form distinct dinuclear complexation with Cu²⁺. Typically, with a short aromatic linkage the 7b-2Cu(II) complex gives an O-bridged dicopper structure, while with long linkage the 11b-2Cu(II) complex possesses two discrete copper centers. The spectroscopic structure and the redox property of the dicopper complexes were investigated by XPS, CV, and DPV techniques. This work hence provides a platform to access biomimetic copper-containing small-molecule models with well-defined structures.

Triptycene-Derived Macrocyclic Arenes: From Calixarenes to Helicarenes

The development of new types of synthetic macrocyclic hosts is always one of the most important and attractive topics in macrocyclic and supramolecular chemistry. Calixarenes, resorcinarenes, cyclotriveratrylenes, pillararenes, and their analogues are all composed of hydroxy-substituted aromatic rings bridged by methylene or methenyl groups and thus can be considered a type of macrocyclic arenes. Because of their unique structural features, easy functionalization, and wide applications in many research areas, such macrocyclic arenes have become some of the most important and studied synthetic macrocyclic hosts during the last decades. Triptycene and its derivatives are a class of organic molecules having unique three-dimensional rigid structures, and they have proved to be useful building blocks for constructing new synthetic macrocyclic hosts with specific structures and properties. Dihydroxy-substituted triptycene derivatives are readily available compounds, which encouraged us to conduct studies of triptycene-derived macrocyclic arenes about 10 years ago. Consequently, a series of triptycene-derived calixarenes and analogues containing 1,8-dihydroxy-substituted triptycene subunits were conveniently synthesized. With 2,7-dihydroxy-substituted triptycene as a precursor, new types of calixarene, oxacalixarene, and homooxacalixarene analogues were also obtained. These triptycene-derived macrocyclic hosts all showed fixed conformations in solution and exhibited expanded cavities compared with the corresponding typical calixarenes and analogues. The special structural features also make these triptycene-derived macrocycles show wide potential applications in molecular recognition and self-assembly. In particular, it was found that the threading direction and the orientation based on macrocycles with nonsymmetric structures could be finely controlled by adjusting the electrostatic and steric effects of the guests, which could form the oriented [2]rotaxane by unidirectional threading. We recently developed a new kind of chiral macrocyclic arenes named helicarenes that are composed of chiral 2,6-dihydroxy-substituted triptycene subunits bridged by methylene groups. It was found that the helicarenes not only exhibited convenient synthesis, high stability, good solubility, fixed conformations, and easy functionalization but also showed complexation abilities with various chiral and achiral organic guests. In particular, the switchable complexation based on these macrocycles could be efficiently controlled by multiple stimuli, including acid/base, redox, anion, or light stimuli under a photoacid. Moreover, the helicarenes have also found applications in the construction of interlocked molecules and molecular machines. This Account summarizes our recent research results on the synthesis and structures of the triptycene-derived macrocyclic arenes and analogues and their applications in host-guest chemistry and molecular assembly. We believe that these macrocyclic arenes, especially helicarenes, could be utilized as new synthetic hosts and find wide potential applications in macrocyclic and supramolecular chemistry.

Complexation between a triptycene-derived oxacalixarene and π-extended viologens: linker-length-dependent orientation of the macrocycles in pseudo[3]rotaxanes

With an expanded electron-rich cavity and a fixed conformation, macrocycle H was found to encapsulate π-extended viologens G1-G4 to form the first case of pseudo[3]rotaxanes based on oxacalixarenes. The complexation was investigated in detail both in solution and in the solid state using NMR spectroscopy and X-ray crystallography. Due to the three-dimensional nonsymmetric structure of H, three orientational isomers of the pseudorotaxanes could be expected theoretically. However, as the crystal structure analysis revealed, only one of the three isomers was obtained with either G1 or G3. Moreover, with regard to the different lengths of the linkers in the guest molecules, completely opposite orientations of the macrocycles on the axles were observed, which could be explained by different complexation modes between the components in the pseudo[3]rotaxanes. Additionally, the complexation between the host and the guest could be reversibly switched on and off using a suitable acid and base. These results will provide us with the opportunity to design and elaborately regulate high-order molecular devices.

Synthesis and Structures of Triptycene-Derived Oxacalixarenes with Expanded Cavities: Tunable and Switchable Complexation towards Bipyridinium Salts

New triptycene-derived oxacalixarene H1 was efficiently synthesized by a template cyclization step, and anilino-substituted macrocycle H2 was subsequently afforded through straightforward nucleophilic displacement of the active chlorine atom in H1. Oxacalixarene H1 adopts a fixed boat-like 1,3-alternate conformation and shows moderate complexation abilities towards various bipyridinium salts. However, the affinities of H2 towards the guests were found to be substantially stronger, which could be tentatively attributed to the additional hydrogen-bonding site, π-π stacking site, and especially the increased electron richness of the host. Furthermore, the acid-base switchable complexation process between H1 and the bipyridinium salt was also realized, which could potentially facilitate the construction of high-level stimuli-responsive supramolecular structures based on the newly synthesized oxacalixarene.

Azacalix[2]arene[2]carbazoles: synthesis, structure and properties

A new type of –NH– bridged azacalix[2]arene[2]carbazoles has been synthesized by aromatic nucleophilic substitution reactionviaone pot and/or fragment coupling strategy.

Guest-dependent directional complexation based on triptycene derived oxacalixarene: formation of oriented rotaxanes

Structural changes in the guest cause inversion of the dominant threading direction in triptycene derived oxacalixarene with different semi-cavities.

The manipulation of supramolecular devices to carry out sophisticated and programmed tasks is bound up with the spatial allocation of their components, especially the threading direction of the guest, which controls the host–guest orientation in the device. However, insights are needed to probe more possibilities for steering the threading direction. We have developed a new system consisting of a three-dimensional nonsymmetric oxacalixarene (H) with a fixed comformation and (bi)pyridinium salts (G1–G3), in which we found that based on the intrinsic discrepancies between the two semi-cavities of H, the electron densities of the axles greatly affect the threading direction. This was unequivocally demonstrated by NMR spectra and single crystal structures. With elaborate design, unidirectional threading was achieved, resulting in an oriented rotaxane. Therefore, we describe a new approach in which the threading direction and final orientation may be finely controlled by adjustment of the structure of the guest.

Unique binding behaviour of water-soluble polycationic oxacalix[4]arene tweezers towards the paraquat dication

The first water-soluble polycationic oxacalix[4]arene molecular tweezers able to recognize - under pH control - the paraquat dication as a result of a delicate balance between electrostatic repulsion, Coulombic shielding and attractive π-stacking interactions are reported.

Solvent-driven selective π-cation templating in dynamic assembly of interlocked molecules

The different solvent responses for bipyridinium and trispyridinium-based dynamic imine [2]rotaxanes allow their interconversion with high selectivity.