BowtieArene: A Dual Macrocycle Exhibiting Stimuli‐Responsive Fluorescence

Although highly useful in supramolecular chemistry, pillararenes lack a fluorophore in their skeleton. Here we present BowtieArene, a novel fluorescent dual macrocycle, featuring a central tetraphenylethylene‐derived fluorophore and two pillar‐like, pentagon‐shaped cavities which are comparable to pillar[5]arene. This concisely prepared, figure‐of‐eight molecule exhibits vapor absorption and host–guest capabilities, as well as intriguing switchable fluorescence. The fluorochromism of BowtieArene can be triggered by multiple external stimuli including solvent, vapor, and mechanical force, with excellent reversibility and stability. Experimental and theoretical evidence indicate that the fluorochromism should be closely related to molecular packing.     

卤代烷亲卤取代反应的理论基础与影响因素

基于卤代烷亲核取代(S_N2,本文记作S_N2C)反应,介绍关于卤代烷亲卤取代反应(S_N2X)的理论基础、影响因素;还介绍了几类常见亲核试剂参与的S_N2X反应以及关于不对称催化S_N2X反应的最新研究进展。本文内容是对现行教科书相关内容的深度延伸和补充。     

Mechanistic In Situ and Ex Situ Studies of Phase Transformations in Molecular Co-Crystals

Co-crystallisation is widely explored as a route to improve the physical properties of pharmaceutical active ingredients, but little is known about the fundamental mechanisms of the process. Herein, we apply a hyphenated differential scanning calorimetry—X-ray diffraction technique to mimic the commercial hot melt extrusion process, and explore the heat-induced synthesis of a series of new co-crystals containing isonicotinamide. These comprise a 1:1 co-crystal with 4-hydroxybenzoic acid, 2:1 and 1:2 systems with 4-hydroxyphenylacetic acid and a 1:1 crystal with 3,4-dihydroxyphenylactic acid. The formation of co-crystals during heating is complex mechanistically. In addition to co-crystallisation, conversions between polymorphs of the co-former starting materials and co-crystal products are also observed. A subsequent study exploring the use of inkjet printing and milling to generate co-crystals revealed that the synthetic approach has a major effect on the co-crystal species and polymorphs produced.     

A Macrocycle Based on a Heptagon-Containing Hexa-peri-hexabenzocoronene

A cyclophane is reported incorporating two units of a heptagon-containing extended polycyclic aromatic hydrocarbon (PAH) analogue of the hexa-peri-hexabenzocoronene (HBC) moiety (hept-HBC). This cyclophane represents a new class of macrocyclic structures that incorporate for the first time seven-membered rings within extended PAH frameworks. The saddle curvature of the hept-HBC macrocycle units induced by the presence of the nonhexagonal ring along with the flexible alkyl linkers generate a cavity with shape complementarity and appropriate size to enable π interactions with fullerenes. Therefore, the cyclophane forms host–guest complexes with C₆₀ and C₇₀ with estimated binding constants of K_a=420±2 m ⁻¹ and K_a=(6.49±0.23)×10³ m ⁻¹, respectively. As a result, the macrocycle can selectively bind C₇₀ in the presence of an excess of a mixture of C₆₀ and C₇₀.     

1,4-双正丙氧基柱[7]芳烃的合成及主客体化学

我们以1,4-二正丙氧基-2,5-双甲氧甲基苯为原料用对甲苯磺酸为催化剂在二氯甲烷中制备了1,4-双正丙氧基柱[5]芳烃, 1,4-双正丙氧基柱[6]芳烃和1,4-双正丙氧基柱[7]芳烃. 我们用氢谱, 碳谱和质谱对它们进行了表征. 它们有不同的氢谱却有相似的碳谱. 对比它们的空腔尺寸, 柱[5]的内径大约是4.6 Å, 与葫芦脲[6]及α-环糊精类似. 柱[6]的内径大约是6.7 Å, 与葫芦脲[7]及β-环糊精类似. 柱[7]的内径大约是8.7 Å, 与葫芦脲[8]及γ-环糊精类似. 我们用正辛基三乙基六氟磷酸铵盐作为模型客体研究了它们之间的主客体络合. 柱[5]与之有微弱的络合, 柱[6]显示了良好的络合, 而柱[7]与之没有络合.     

Columnar Liquid Crystals from a Giant Macrocycle Mesogen

Columnar liquid crystals composed of a giant macrocyclic mesogen were prepared. The giant macrocyclic mesogen has a square hollow with a 2.5 nm diagonal, which was bounded by diindolo[3,2‐b:2′,3′‐h]carbazole (diindolocarbazole) moieties as the edges and bis(salicylidene)‐o‐phenylenediamine (salphen) moieties as the corners. The shape and size of the macrocycle were directly observed by scanning tunneling microscopy (STM). Each side of the bright square in the STM image corresponds to a diindolocarbazole moiety, and the length of the sides was consistent with the result of the single crystal analysis of diindolocarbazole. Finally, we successfully obtained a giant macrocycle with long and branched side chains, which exhibited a rectangular columnar LC phase over a wide temperature range. To the best of our knowledge, it contained the largest discrete inner space of any thermotropic columnar liquid crystal composed of macrocyclic mesogens.     

Solar Thermal Energy Storage in a Photochromic Macrocycle

The conversion and efficient storage of solar energy is recognized to hold significant potential with regard to future energy solutions. Molecular solar thermal batteries based on photochromic systems exemplify one possible technology able to harness and apply this potential. Herein is described the synthesis of a macrocycle based on a dimer of the dihydroazulene/vinylheptafulvene (DHA/VHF) photo/thermal couple. By taking advantage of conformational strain, this DHA–DHA macrocycle presents an improved ability to absorb and store incident light energy in chemical bonds (VHF–VHF). A stepwise energy release over two sequential ring‐closing reactions (VHF→DHA) combines the advantages of an initially fast discharge, hypothetically addressing immediate energy consumption needs, followed by a slow process for consistent, long‐term use. This exemplifies another step forward in the molecular engineering and design of functional organic materials towards solar thermal energy storage and release.     

A Solvato‐Fluorochromic Macrocycle of Multiple Anthracene Fluorophores in Close Proximity

A fluorescent macrocycle containing four anthracene panels linked by meta‐phenylene spacers and amino hinges was synthesized. The macrocycle adopts a twisted, compressed conformation that places embedded anthracene fluorophores in close contact. Emission from the convoluted macrocycle is highly solvatochromic and significantly enhanced as compared with that of the partial structures.     

Pillar[5]-bis-thiacrown: An Adaptive Tricyclic Host Selectively Recognizing an Organic Guest by Dimetalation

Some biological receptors change their shapes and rigidity by metalation to recognize substrates precisely via adaptive guest binding process. Herein we present a semi-flexible tricyclic host molecule whose conformation is rigidified by dimetalation to uptake organic guests selectively. Considering two metal binding sites and an empty space between them, pillar[5]-bis-thiacrown (L) was synthesized. The tricyclic host L forms a disilver(I) complex [Ag₂L(NO₃)₂], with an Ag⋅⋅⋅Ag separation of 9.976 Å. Binding studies based on ¹H NMR including 2D NOESY and DOSY experiments towards α,ω-dicyanoalkanes [CN(CH₂)_nCN, n=2–6, shortly C2–C6] demonstrated that the dimetalated L, Ag₂L preferentially recognizes C2 over other guests than that of free L. Furthermore, the dimetalated the host only uptakes C2 in the presence of other guests. Crystal structures support the idea that the space between two silver(I) centers plays a decisive role on the selective guest binding forming an Ag-C2-Ag@L arrangement via the length-selective recognition. This work demonstrates the chemical example of the adaptive guest binding and presents a new perspective on the metallosupramolecules of pillararenes.     

Asararenes—A Family of Large Aromatic Macrocycles

We announce the establishment of a new family of macrocycles—the asararenes, which are based on para‐methylene linked “asarol methyl ether” (1,2,4,5‐tetramethoxybenzene) units. Macrocycles with 6–12 aromatic units have been synthesized and isolated in a single step from asarol methyl ether and paraformaldehyde. Even larger rings, with up to 15 asarol methyl ether units, have been observed by high‐resolution mass spectrometry. Single‐crystal X‐ray structures of asar[6]‐, asar[7]‐, asar[8]‐, asar[9]‐, asar[10]‐ and asar[11]arene highlight the diverse structural features of this family of macrocycles. While the cavities of the asar[6–8]arene macrocycles are mostly filled with methoxyl groups, the asar[9]‐ and asar[10]arene rings contain accessible cavities and self‐assemble into infinite channels filled with solvent molecules in the solid state. These solid‐state structures highlight the potential of this family of macrocycles for a wide range of potential applications.     

Metallophilic and Inter-Ligand Interactions in Diargentous Compounds Bound by a Geometrically Flexible Macrocycle

A series of mono- and di-nuclear Ag^I complexes supported by a flexible macrocyclic ligand are reported. The geometric flexibility of the ligand was found to allow for a range of Ag−Ag interactions in the disilver complexes, depending on the identities of both the ancillary ligand and the counterion. Studies of the solution-phase dynamic exchange processes for these latter complexes found rapid interconversion through a mechanism that retained the multi-nuclearity. Quantum Theory of Atoms in Molecules (QTAIM) and Independent Gradient Model based on Hirshfeld partition (IGMH) analyses are used to evaluate the d¹⁰-d¹⁰ interactions between silver centers in the various geometries observed for the solid-state structures of these complexes, revealing nearly identical Ag−Ag interactions, regardless of the relative geometries of the Ag centers. Instead, a weak, but non-negligible, inter-ligand interaction between two isocyanide units may contribute to the folded-ligand geometry observed in the solid state.     

Frustrated Helicity: Joining the Diverging Ends of a Stable Aromatic Amide Helix to Form a Fluxional Macrocycle

Macrocyclization of a stable two‐turn helical aromatic pentamide, that is, an object with diverging ends that are not prone to cyclization, was made possible by the transient introduction of disruptors of helicity in the form of acid‐labile dimethoxybenzyl tertiary amide substituents. After removal of the helicity disruptors, NMR, X‐ray crystallography, and computational studies show that the macrocycle possesses a strained structure that tries to gain as high a helical content as possible despite being cyclic. Two points of disruption of helicity remain, in particular a cis amide bond. This point of disruption of helicity can propagate along the cycle in a fluxional manner according to defined trajectories to produce ten degenerate conformations.     

Rules of Macrocycle Topology: A [13]‐Macrodilactone Case Study

Shape is an inherent trait of a molecule that dictates how it interacts with other molecules, either in binding events or intermolecular reactions. Large‐ring macrocyclic compounds in particular leverage their shape when they are selectively bound by biomolecules and also when they exhibit macrocyclic diastereoselectivity. Nonetheless, rules that link structural parameters to the conformation of a macrocycle are still rudimentary. Here we use a structural investigation of a family of [13]‐macrodilactones as a case study to develop rules that can be applied generally to macrocycles of different sizes and with a variety of functionality. A characteristic “ribbon” shape is adopted by the [13]‐macrodilactones in the absence of stereogenic centres, which exhibits planar chirality. When one stereogenic centre at key positions on the backbone is incorporated into the structure, the planar chirality is dictated by the configuration of the centre. In cases where two stereogenic centres are present, their relationships can either reinforce the characteristic ribbon shape or induce alternative shapes to be adopted. The rules established in the case study are then applied to the analysis of a structure of the natural product migrastatin. They lay the groundwork for the development of models to understand macrocycle‐biomolecule interactions and for the preparation of macrocycles with designed properties and activities.     

Pinpointing Mechanochemical Bond Rupture by Embedding the Mechanophore into a Macrocycle

Mechanophores contain a mechanically labile bond that can be broken by an external mechanical force. Quantitative measurement and control of the applied force is possible through atomic force microscopy (AFM). A macrocycle was synthesized that contains both the mechanophore and an aliphatic chain that acts as a “safety line” upon bond breaking. This ring‐opening mechanophore unit is linked to poly(ethylene glycol) spacers, which allow investigation by single molecule force spectroscopy. The length increase upon rupture of the mechanophore was measured and compared with quantum chemical calculations.     

Macrocycle Embrace: Encapsulation of Fluoroarenes by m‐Phenylene Ethynylene Host

We report structural characterization of a new member of m‐phenylene ethynylene ring family. This shape‐persistent macrocycle also co‐crystallizes with hexafluoro‐, 1,2,4,5‐tetrafluoro‐, 1,3,5‐trifluoro, and 1,4‐difluorobenzene. The four complexes are almost isostructural, and all show the fluoroarene included into the central cavity of the macrocycle. Characterized by multiple short C?H???F?C contacts, these inclusion complexes further dimerize in the solid state into a 2+2 assembly, in which the two macrocycles embrace each other by their large hydrophobic groups that are rotated by 60° relative to one another.     

Switching between Local and Global Aromaticity in a Conjugated Macrocycle for High-Performance Organic Sodium-Ion Battery Anodes

Aromatic organic compounds can be used as electrode materials in rechargeable batteries and are expected to advance the development of both anode and cathode materials for sodium-ion batteries (SIBs). However, most aromatic organic compounds assessed as anode materials in SIBs to date exhibit significant degradation issues under fast-charge/discharge conditions and unsatisfying long-term cycling performance. Now, a molecular design concept is presented for improving the stability of organic compounds for battery electrodes. The molecular design of the investigated compound, [2.2.2.2]paracyclophane-1,9,17,25-tetraene (PCT), can stabilize the neutral state by local aromaticity and the doubly reduced state by global aromaticity, resulting in an anode material with extraordinarily stable cycling performance and outstanding performance under fast-charge/discharge conditions, demonstrating an exciting new path for the development of electrode materials for SIBs and other types of batteries.