Complexation of Racemic 2,6-Helic[6]arene and Its Hexamethyl-Substituted Derivative with Quaternary Ammonium Salts, N-Heterocyclic Salts, and Tetracyanoquinodimethane

Recent advances in the synthesis and applications of macrocyclic arenes

Macrocyclic arenes including calixarenes, resorcinarenes, cyclotriveratrylene, pillararenes and so on have emerged as highly attractive synthetic macrocyclic hosts due to their unique structures, facile functionalization, and broad range of applications. In recent years, there has been growing interest in the development of novel macrocyclic arenes composed of various aromatic building blocks bridged by methylene groups, which have found applications in various research areas. Consequently, the development of novel macrocyclic arenes has become a frontier and hot topic in supramolecular and macrocyclic chemistry. In this review, we feature the recent advances in the synthesis and applications of novel macrocyclic arenes that have emerged in the last decade. The general synthetic strategies employed for these macrocyclic arenes are systematically summarized, and their wide applications in molecular recognition and assemblies, molecular machines, biomedical science and functional materials are highlighted.

Chiral Nanocluster Complexes Formed by Host-Guest Interaction between Enantiomeric 2,6-Helic[6]arenes and Silver Cluster Ag20: Emission Enhancement and Chirality Transfer

A pair of chiral nanocluster complexes were formed by the host−guest interaction between the enantiomeric 2,6-helic[6]arenes and nanocluster Ag₂₀. The formation and stability of the nanocluster complexes were experimentally and theoretically confirmed. Meanwhile, the chiral nanocluster complexes exhibited enhanced luminescence and induced CD signals at room temperature in the solid state, revealing the stable complexation and chirality transfer from the chiral macrocycles to the nanocluster Ag₂₀.

Macrocyclic host molecules with aromatic building blocks: the state of the art and progress

Macrocyclic host molecules play the central role in host-guest chemistry and supramolecular chemistry. The highly structural symmetry of macrocyclic host molecules can meet people's pursuit of aesthetics in molecular design, and generally means a balance of design, synthesis, properties and applications. For macrocyclic host molecules with highly symmetrical structures, building blocks, which could be described as repeat units as well, are the most fundamental elements for molecular design. The structural features and recognition ability of macrocyclic host molecules are determined by the building blocks and their connection patterns. Using different building blocks, different macrocyclic host molecules could be designed and synthesized. With decades of developments of host-guest chemistry and supramolecular chemistry, diverse macrocyclic host molecules with different building blocks have been designed and synthesized. Aromatic building blocks are a big family among the various building blocks used in constructing macrocyclic host molecules. In this feature article, the recent developments of macrocyclic host molecules with aromatic building blocks were summarized and discussed.

Recent advances on triptycene derivatives in supramolecular and materials chemistry

Triptycene derivatives, a type of specific aromatic compound, have been attracting much attention in many research areas. Over the past several years, triptycene and its derivatives have been described to be useful and efficient building blocks for the design and synthesis of novel supramolecular acceptors, porous materials and luminescent materials with specific structures and properties. In this review, recent researches on triptycene derivatives in supramolecular and materials chemistry are summarized. Especially, the construction of a new type of macrocyclic arenes and organic cages with triptycene and its derivatives as building blocks are focused on, and their applications in molecular recognition, self-assembly and gas selective sorption are highlighted. Moreover, the applications of triptycene and its derivatives in porous organic materials and thermally activated delayed fluorescence (TADF) materials are also discussed.

Synthesis of Chiral Helic[1]triptycene[3]arenes and Their Enantioselective Recognition Towards Chiral Guests Containing Aminoindan Groups

Starting from the enantiopure precursors, a pair of chiral macrocyclic arenes named helic[1]triptycene[3]arenes were conveniently synthesized. The circular dichroism (CD) spectra of the enantiomeric macrocyclic arenes exhibited mirror images, and the X-ray single crystal structures confirmed their absolute conformations as well. Moreover, the macrocyclic arenes showed strong complexation with secondary ammonium and primary ammonium salts containing aminoindan groups. In particular, the chiral macrocyclic arenes exhibited enantioselective recognition ability towards the chiral secondary ammonium salts containing aminoindan groups with an enantioselective ratio up to 3.89.

Construction of Chiral Nanoassemblies Based on Host-Guest Complexes and Their Responsive CD and CPL Properties: Chirality Transfer From 2,6-helic[6]arenes to a Stilbazolium Derivative

A couple of water-soluble chiral 2,6-helic[6]arene derivatives P -H1 and M -H1 were synthesized, and they could form 1:1 stable complexes with 4-[(4'-N, N-diphenylamino)-styryl]-N-methylpyridinium iodide (G) in water. Compared with G, the host-guest complexes exhibited enhanced fluorescence, which might be attributed to the spatial confinement of G and restriction of aggregation-caused quenching (ACQ) effects. Based on the host-guest complexation, the first helic[6]arene-based chiral assemblies were then constructed, and they showed rectangular or hexagonal nanostructures by scanning electron microscopy (SEM) images. Interestingly, the assemblies showed clear mirror-image circular dichroism (CD) and circularly polarized luminescence (CPL) spectra in aqueous solution, revealing a consecutive chirality transfer from the chiral macrocyclic cavities of the hosts to G. Moreover, the supramolecular chirality of the assemblies could also show responsiveness to the pH values and temperatures of the system.

Complexation of 2,6-helic[6]arene and its derivatives with 1,1'-dimethyl-4,4'-bipyridinium salts and protonated 4,4'-bipyridinium salts: an acid-base controllable complexation

2,6-Helic[6]arene and its derivatives were synthesized, and their complexation with 1,1'-dimethyl-4,4'-bipyridinium and protonated 4,4'-bipyridinium salts were investigated in detail. It was found that the helic[6]arene and its derivatives could all form 1:1 complexes with both 1,1'-dimethyl-4,4'-bipyridinium salts and protonated 4,4'-bipyridinium salts in solution and in the solid state. Especially, the helic[6]arene and its derivatives containing 2-hydroxyethoxy or 2-methoxyethoxy groups exhibited stronger complexation with the guests than the other helic[6]arene derivatives for the additional multiple hydrogen bonding interactions between the hosts and the guests, which were evidenced by ¹H NMR titrations, X-ray crystal structures and DFT calculations. Moreover, it was also found that the association constants (K _a) of the complexes could be significantly enhanced with larger counteranions of the guests and in less polar solvents. Furthermore, the switchable complexation between the helic[6]arene and protonated 4,4'-bipyridinium salt could be efficiently controlled by acids and bases.

Syntheses of water-soluble acyclic naphthalene oligomers and their applications in water

Water-soluble -CH2- bridged acyclic naphthalene oligomers (WN2 and WN3) were designed and prepared successfully. Interestingly, WN3 can be used in reversible dispersion of carbon nanotubes in water by changing the pH efficiently.

Step-by-step reaction-powered mechanical motion triggered by a chemical fuel pulse

Natural molecular machines perform important tasks in organisms under a reasonable energy supply strategy: a series of step-by-step biochemical reactions after the intake of a fuel such as glucose. As analogues of biomolecular machines, most artificial molecular machines and shuttles are still powered by two opposite stimuli, such as acid and base or oxidation and reduction. This inconvenient stimulus method for artificial molecular machines and shuttles creates great obstacles for realizing more functions of artificial molecular machines and shuttles owing to low energy utilization efficiency and troublesome operation. In this work, we report a new step-by-step reaction system combining two known reactions: (1) the formation of [bis(trifluoroacetoxy)iodo]benzene from the reaction of iodosylbenzene and trifluoroacetic acid; and (2) the catalytic oxidation of alcohols by [bis(trifluoroacetoxy)iodo]benzene in the presence of TEMPO. Owing to the acid absorption features of the first reaction and the acid releasing characteristics of the second reaction, the new step-by-step reaction obtains a pH oscillation feature. Taking advantage of the pH oscillation feature, the new reaction cycle triggered by iodosylbenzene was coupled with an acid-base switchable helicarene-based molecular shuttle. Benefitting from the reaction rate difference of the two known reactions (the reaction rate of the first reaction is faster than that of the second reaction), the movement of the helicarene on the axle could be controlled automatically by the reaction system through adding iodosylbenzene to a solution of alcohol, TEMPO and protonated rotaxane, and the manual addition of another opposite stimulus could be avoided.

A Route to Enantiopure ( O-Methyl)6-2,6-Helic[6]arenes: Synthesis of Hexabromo-Substituted 2,6-Helic[6]arene Derivatives and Their Suzuki-Miyaura Coupling Reactions

A route to enantiopure ( O-methyl)₆-2,6-helic[6]arenes (+)- P-2 and (-)- M-2 has been provided. By the reaction of enantiopure triptycene precursors (+)-1 and (-)-1 in refluxed o-DCB for 12 h in the presence of catalytic amount of FeCl₃, and then followed by treatment of the obtained oligomers under the same conditions, (+)- P-2 and (-)- M-2 could be obtained in 51% and 53% total yield, respectively. It was also found that racemic and enantiopure ( O-methyl)₆-2,6-helic[6]arenes could be easily brominated by Br₂ to give the corresponding hexabromo-substituted helic[6]arene derivatives rac-4, (+)- P-4, and (-)- M-4 in high yields. The crystal structure of (+)- P-4 further confirmed the absolute configuration of the helic[6]arenes and their derivatives. Moreover, a series of hexaaryl-substituted helic[6]arene derivatives 5a-f with deepened cavities could be conveniently synthesized in 55-71% yield by Suzuki-Miyaura coupling reactions of 4 and arylboronic acids. rac-5a could encapsulate chloroform and exhibit self-sorting stacking in solid state. Enantiopure (+)- P-5a-f and (-)- M-5a-f showed mirror images in their CD spectra.

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.

Efficient control of movement in non-photoresponsive molecular machines by a photo-induced proton-transfer strategy

The first successful application of a PIPT strategy executed by the photoacid 1-MEH in controlling the switch of MIM systems has been demonstrated.

Regioselective Synthesis of Methylene-Bridged Naphthalene Oligomers and Their Host-Guest Chemistry

In this research, we report the regioselective synthesis of methylene-bridged naphthalene oligomers from 2,6-dialkoxyl naphthanene and paraformaldehyde by using p-TsOH as the catalyst and CH₂Cl₂ as the solvent. The structures were characterized by NMR spectroscopy and X-ray crystallography. Their host-guest chemistry with organic cations was studied, and optimal naphthalene numbers in the oligomers were revealed for different guests. In addition, the reason for the unsuccessful synthesis of methylene-bridged naphthalene macrocycles was discussed.

Complexation Between (O-Methyl)6 -2,6-Helic[6]arene and Tertiary Ammonium Salts: Acid/Base- or Chloride-Ion-Responsive Host-Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O-methyl)₆ -2,6-helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹ H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base- and chloride-ion-responsive host-guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6-helic[6]arene-based [2]rotaxane was also synthesized from the condensation between the host-guest complex and isocyanate.

Formation of charge-transfer complexes based on a tropylium cation and 2,6-helic[6]arenes: a visible redox stimulus-responsive process

Charge-transfer complexes between 2,6-helic[6]arenes and a tropylium cation formed, and they could undergo visible redox stimulus-responsive and switchable complexation processes.

Synthesis of a water-soluble 2,6-helic[6]arene derivative and its strong binding abilities towards quaternary phosphonium salts: an acid/base controlled switchable complexation process

A water-soluble 2,6-helic[6]arene derivative was synthesized, and it showed strong binding ability and acid/base stimulus-responsive complexation towards quaternary phosphonium salts.