Redox‐Triggered Chirality Switching and Guest‐Capture/Release with a Pillar[6]arene‐Based Molecular Universal Joint

Triphenylamine[3]arenes: Streamlining Synthesis of a Versatile Macrocyclic Platform for Supramolecular Architectures and Functionalities

Triphenylamine[3]arenes (TPA[3]s), featuring [16]paracyclophane backbone with alternating carbon and nitrogen bridging atoms, were synthesized through a BF3 ⋅ Et2O-catalyzed cyclization reaction using triphenylamine derivatized monomers and paraformaldehyde. This molecular design yielded a series of TPA[3] macrocycles with high efficiency, with their facile derivatizations also successfully demonstrated. On account of the strong electron-donating properties of the TPA moieties, these TPA[3]s exhibit remarkable delayed fluorescence, and possess a significant affinity for iodine. Furthermore, their inherent three-fold symmetry rendered TPA[3]s as novel building blocks for the construction of extended frameworks and molecular cages. This advancement expands the versatility of discrete macrocycles into complex architectures, enhancing their applicability across a broad spectrum of applications.

"Rim-Differentiated" Pillar[6]arenes

A "rim-differentiated" pillar[6]arene (RD-P[6]) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid-state conformation of this macrocycle could switch from the 1,3,5-alternate to a truly rim-differentiated one upon guest inclusion. This highly symmetric RD-P[6] not only hosts metal-containing molecules inside its cavity, but also can form a pillar[6]arene-C₆₀ adduct through co-crystallization on account of donor-acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.

Symmetrically Tetra-functionalized Pillar[6]arenes Prepared by Fragment Coupling

Due to the highly symmetrical structures generated from one-pot syntheses, the partial functionalization of macrocycles is usually beset with low yields and onerous purifications of the target multifunctional macrocycles. To improve this circumstance, taking pillar[6]arenes as an example, a two-step fragment coupling method is developed for synthesizing symmetrically tetra-functionalized pillar[6]arenes, namely X-pillar[6]arenes. This method is simple and versatile, which makes hetero-fragment coupling and pre-functionalization available. Nine new macrocycles and a pillar[6]arene-based cage are prepared. In addition, one of the newly synthesized macrocycles, COOEtEtXP[6] , exhibits a strong cyan luminescence in the solid state under irradiation by 365 nm UV light. This emission originates from intramolecular through-space conjugation. Meanwhile, formation of a supramolecular polymer by multiple non-covalent intra/intermolecular interactions help rigidify the structure and make COOEtEtXP[6] an efficient solid-state emitter. It is believed that this fragment coupling can also be used to realize the multi-functionalization of other macrocycles.

Stimuli-Responsive Chiroptical Switching

"Chirality" governs many fundamental properties in chemistry and biochemistry. While early investigations on stereochemistry are primarily dedicated to static chirality, there is an increasing interest in the field of dynamic chirality (chiral switches). These chiral switches are essential in controlling the directionality in molecular motors. Dynamic chiralities are equally crucial in switchable stereoselectivity, switchable asymmetric catalysis and enantioselective separation. Herein, we limit our discussion to recent advances on stimuli-induced chiroptical switching of axial, helical, and planar chirality in response to external stimuli. We also discuss a few examples of applications of the switchable chirality.

Electrochemical Polymerization Induced Chirality Fixation of Crystalline Pillararene-Based Polymer and Its Application in Interfacial Chiral Sensing

A new strategy has been developed for the direct chirality fixation, which is induced by electrochemical polymerization, of macrocyclic hosts pillar[5]arene. Taking advantage of electrochemical polymerization, thiophene-modified pillar[5]arene monomers (Th-P[5]A) have been regularly arranged under the action of an electric field to form chiral nanofiber-like crystalline pillar[5]arene-based polymers (poly-Th-P[5]A), showing a significant circular dichroism (CD) signal. With the active photochemical properties, poly-Th-P[5]A is first used as a photoelectrochemical (PEC) chiral sensor for the identification and determination of l- and d-ascorbic acid (l-AA, d-AA) without adding any extra photoactive probes. Importantly, the chiral recognition between poly-Th-P[5]A and l-AA also triggers a polarity conversion for the photocurrent of the polymer, and it greatly results in a broad chiral detection range for l-AA, crossing 6 orders of magnitude. This work provides a promotional strategy for building a PEC chiral recognition platform based on pillararenes.

A Multiple Chirality Switching Device for Spatial Light Modulators

A new and simple strategy towards electric-field-driven multiple chirality switching device has been designed and fabricated by combining a newly synthesized base-responsive chiroptical polymer switch (R-FLMA) and p-benzoquinone (p-BQ) via proton-coupled electron transfer (PCET) mechanism. Clear and stable triple chirality states (silence, positive, negative) of this device in visible band can be regulated reversibly (>1000 cycles) by adjusting voltage programs. Furthermore, such chiral switching phenomena are also accompanied by apparent changes of color and fluorescence. More importantly, the potential application of this device for a spatial light modulator has also been demonstrated.

Crown ether-pillararene hybrid macrocyclic systems

A combination of Nobel macrocycle-crown ether and star macrocycle-pillararenes together in organic synthesis and material science is significant in obtaining hybrid systems, with rigid/flexible structural architecture, induced planar chirality, a negative cooperative effect and multiple fused cyclic hosts. In this review, we will discuss the synthesis/preparation of crown ether-pillararene hybrid macrocyclic systems by covalent bonds, supramolecular interactions and mechanical bonds, leading to hybrid compounds, supramolecular assemblies and mechanically interlocked molecules. The practical applications of crown ether-containing pillararenes will also be discussed in diverse areas, such as molecular recognition via fused multiple macrocycles and ion channels as well as external stimuli-responsive smart materials. We also call the attention of related researchers towards academic and technical issues about topological structures and applied functions in this fresh new fused macrocyclic field.

Synergetic effects in the enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylic acid mediated by β-cyclodextrin-pillar[5]arene-hybridized hosts

Tri-cavity hosts consisting of one pillar[5]arene (P5) sandwiched by two β-cyclodextrins (CDs) were synthesized, and their diastereoseparation was successfully accomplished. Photocyclodimerization of 2-anthracenecarboxylate with these hybrid hosts demonstrated the critical dependence of stereoselectivity on the absolute configuration of the central P5 and the conjugating positions on the β-CD, and gave the non-classical HT photodimers in up to 87% ee.