Concyclic CH-π arrays for single-axis rotations of a bowl in a tube

Confinement effects on the structure and reactivity of encapsulated buckybowls in cycloparaphenylene

We theoretically investigated the host-guest chemistry between belt-like cycloparaphenylenes (CPPs) and entrapped bowl-shaped sumanene and corannulene. Density functional theory calculations show that the buckybowls can be stabilized in a CPP host with an appropriately sized cavity (e.g., [10]CPP) through multi-site CH-π interactions. Arising from the confined intermolecular interactions within the cavity, the restrictive buckybowls display novel reactivity distinct from that in their free state.

Noncovalent interaction with a spirobipyridine ligand enables efficient iridium-catalyzed C-H activation

Exploitation of noncovalent interactions for recognition of an organic substrate has received much attention for the design of metal catalysts in organic synthesis. The CH-π interaction is especially of interest for molecular recognition because both the C-H bonds and the π electrons are fundamental properties of organic molecules. However, because of their weak nature, these interactions have been less utilized for the control of organic reactions. We show here that the CH-π interaction can be used to kinetically accelerate catalytic C-H activation of arenes by directly recognizing the π-electrons of the arene substrates with a spirobipyridine ligand. Computation and a ligand kinetic isotope effect study provide evidence for the CH-π interaction between the ligand backbone and the arene substrate. The rational exploitation of weak noncovalent interactions between the ligand and the substrate will open new avenues for ligand design in catalysis.

Stoichiometry validation of supramolecular complexes with a hydrocarbon cage host by van 't Hoff analyses

Defining chemical processes with equations is the first important step in characterizing equilibria for the assembly of supramolecular complexes, and the stoichiometry of the assembled components must be defined to generate the equation. Recently, this subject has attracted renewed interest, and statistical and/or information-theoretic measures were introduced to examine the validities of the equilibrium models used during curve fitting analyses of titration. The present study shows that these measures may not always be appropriate for credibility examinations and that further reformation of the protocols used to determine the overall stoichiometry is necessary. Hydrocarbon cage hosts and their chloroform complexes formed via weak CH-π hydrogen bonds were studied, which allowed us to introduce van 't Hoff analyses for effective validation of the stoichiometries of supramolecular complexes. This study shows that the stoichiometries of supramolecular complexes should be carefully examined by adopting multiple measures with different origins.

Host-Guest Chemistry in Binary and Ternary Complexes Utilizing π-Conjugated Carbon Nanorings

The carbon nanorings, possessing a radial π system, have garnered significant attention primarily due to their size-dependent photophysical properties and the presence of a unique curved π-conjugated cavity. This is evidenced by the rapid proliferation of publications. Furthermore, the integration of building blocks into CPP skeletons can confer [n]CPPs with novel and exceptional photophysical and electronic characteristics, as well as chiral properties and host-guest interactions, thereby augmenting the diversity of [n]CPPs. Notably, the curved π surface structures and concave cavity of carbon nanorings enable them to host aromatic or non-aromatic guests with a complementarily curved surface, resulting in interesting binary or ternary complexes. This review provides a comprehensive treatment of literature reports on binary and ternary complexes, focusing on both their host-guest interactions and properties. It is important to note that the scope of this review is limited to host-guest chemistry in binary and ternary complexes based on π-conjugated carbon nanorings.

Polymorphism and Light-Driven Forward Movement of TPE Derivative Micro-Crystal due to ArH-pi Interactions Difference

Aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) are two classes of opposite luminescence phenomena. It is almost impossible to show both AIE and ACQ effect simultaneously by the same molecule. However, here we report that a simple TPE derivative TAP-TPE grows into both AIE crystals and ACQ ones. It is found that equatorial, contact distance-longer and weak ArH-π interactions exist in AIE crystals while vertical, contact distance-shorter and strong ArH-π interactions appear in ACQ crystals. Theoretical calculation of electron density on the interaction atoms unveils that ACQ crystals have much larger change in electron density than AIE ones, suggesting that the intermolecular electron transfer aroused by the strong ArH-π hydrogen bonds leads to ACQ phenomenon. This result provides a new insight into the emission mechanism in aggregation state. Interestingly, due to the ArH-pi interactions difference, only one of five kinds of crystals shows rapid photochromism, and can act as multimode anti-counterfeiting materials. Very exceptionally, for the first time we find that the photochromic micrometric rod-like crystal even makes forward rolling movement as it repeatedly bends and straightens by responding to on and off of the ultraviolet light irradiation, displaying potential for photo-actuator and light-driven micro-vehicle.

Charge delocalization and aromaticity of doubly reduced double-walled carbon nanohoops

Cycloparaphenylenes (CPPs) exhibit selective host capabilities, featuring the ability to incorporate smaller CPPs to form double-walled host-guest complexes. Moreover, CPPs can also be stabilized by global aromaticity under twofold oxidation or reduction, involving electronic conjugation along with the overall structural backbone. Herein we explore the structural modifications, bonding, electron delocalization and magnetic properties of doubly reduced double-walled CPP complexes with DFT methods, in the isolated and aggregate [n + 5]CPP⊃[n]CPP^2- (n = 5-8) species. Our results show that the hosts undergo structural, bonding and delocalization deformations towards quinoidal configurations and exhibit global long-ranged shielding cones similar to global aromatic free dianionic CPPs, accounting for charge delocalization on the outer nanohoops, whereas the guests preserve local aromatic benzenoid configurations, resulting in global and local aromatic circuits within the host-guest aggregate. This observation suggests that in multi-layered related species electronic delocalization will be retained at the outer structural surface. The aromaticity of the hosts is manifested in the strong upfield shifts of the guests ¹H-NMR signals. Hence, CPP complexes can be extended to doubly reduced species stabilized by global host aromaticity expanding our understanding of doubled-walled nanotubes at the nanoscale regime.

Time-Dependent Solvent-Driven Solid-State Fluorescence-based Numeric Coding

Controllable solid-state transformations can provide a basis for novel functional materials. Herein, we report a series of solid-state systems that can be readily transformed between amorphous, co-crystalline, and mixed crystalline states via grinding or exposure to solvent vapors. The present solid materials were constructed using an all-hydrocarbon macrocycle, cyclo[8](1,3-(4,6-dimethyl)benzene) (D_4d-CDMB-8) (host), and neutral aggregation-caused quenching dyes (guests), including 9,10-dibromoanthracene (1), 1,8-naphtholactam (2), diisobutyl perylene-3,9-dicarboxylate (3), 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (4), 4,7-di(2-thienyl)-benzo[2,1,3]thiadiazole (5), and 4-imino-3-(pyridin-2-yl)-4H-quinolizine-1-carbonitrile (6). Seven co-crystals and six amorphous materials were obtained via host-guest complexation. Most of these materials displayed turn-on fluorescence emission (up to 20-fold enhancement relative to the corresponding solid-state guests). The interconversion between amorphous, co-crystalline states, and crystalline mixtures could be induced by exposure to solvent vapors or by subjecting to grinding. The transformations could be monitored readily by means of single-crystal and powder X-ray diffraction analyses, as well as solid-state fluorescent emission spectroscopy. The externally induced structural interconversions resulted in time-dependent fluorescence changes. This allowed sets of privileged number array codes to be generated.

Bowl-Shaped Bispyrrole-Fused Perylene-diimide and Its Anions

Incorporating two pyrrole subunits at the bay positions of perylene-diimide has been a long-pursued goal since 2009, but it has not been achieved due to high strain. Herein, via one step Buchwald-Hartwig reaction, PDI-2N was successfully generated with a bowl depth of 1.52 Å. Though with electron-rich pyrrole embedding, PDI-2N's radical anion and dianion were facilely prepared and were investigated both experimentally and theoretically. Moreover, PDI-2N crystallized in different manners under distinct conditions, and it formed tubular crystals with infinite two-directional columnar stacking under DMF conditions. This finding develops a dream bowl-shaped PDI derivative that holds great promise in organoelectronics.

Two Rings Around One Ball: Stability and Charge Localization of [1 : 1] and [2 : 1] Complex Ions of [10]CPP and C60/70 [ *]

We investigate the gas-phase chemistry of noncovalent complexes of [10]cycloparaphenylene ([10]CPP) with C₆₀ and C₇₀ by means of atmospheric pressure photoionization and electrospray ionization mass spectrometry. The literature-known [1 : 1] complexes, namely [10]CPP⊃C₆₀ and [10]CPP⊃C₇₀ , are observed as radical cations and anions. Their stability and charge distribution are studied using energy-resolved collision-induced dissociation (ER-CID). These measurements reveal that complexes with a C₇₀ core exhibit a greater stability and, on the other hand, that the radical cations are more stable than the respective radical anions. Regarding the charge distribution, in anionic complexes charges are exclusively located on C₆₀ or C₇₀ , while the charges reside on [10]CPP in the case of cationic complexes. [2 : 1] complexes of the ([10]CPP₂ ⊃C_60/70 )⁺ ⋅^/- ⋅ type are observed for the first time as isolated solitary gas-phase species. Here, C₆₀ -based [2 : 1] complexes are less stable than the respective C₇₀ analogues. By virtue of the high stability of cationic [1 : 1] complexes, [2 : 1] complexes show a strongly reduced stability of the radical cations. DFT analyses of the minimum geometries as well as molecular dynamics calculations support the experimental data. Furthermore, our novel gas-phase [2 : 1] complexes are also found in 1,2-dichlorobenzene. Insights into the thermodynamic parameters of the binding process as well as the species distribution are derived from isothermal titration calorimetry (ITC) measurements.

Structures and Supramolecular Properties of Inclusion Complexes of Anthracene-Triptycene Nanocages with Fullerene Guests and Their Dynamic Motion as Molecular Gyroscopes

Three derivatives of macrocyclic cage compounds consisting of diarylanthracene and triptycene units were synthesized. These nanocages formed host-guest complexes with C₆₀ and other fullerene guests as confirmed by ¹ H NMR and fluorescence spectroscopy. The association constant of the mesityl and 2,4,6-tributoxyphenyl derivatives with C₆₀ was determined to be 2.2 × 10⁴  L mol^-1 , which was larger than that of the pentafluorophenyl derivative. Direct experimental evidence of the complexation was obtained by X-ray diffraction analysis: the guest C₆₀ molecule was included in the cavity via multipoint CH⋅⋅⋅π interactions. Dynamic disorders of the included C₆₀ molecule in variable-temperature X-ray analysis indicated uniaxial motion, such as gyroscopic motion. The unique dynamic behavior of the spherical C₆₀ rotor anchored by the cage stator via CH⋅⋅⋅π interactions in the crystal, as well as substituent effects on the association properties, are discussed with the aid of DFT calculations.

A Fourfold Gold(I)-Aryl Macrocycle with Hyperbolic Geometry and its Reductive Elimination to a Carbon Nanoring Host

An ethylene glycol-decorated [6]cyclo-meta-phenylene (CMP) macrocycle was synthesized and utilized as a subunit to construct a fourfold Au^I ₂ -aryl metallacycle with an overall square arrangement. The corners consist of rigid dinuclear gold(I) complexes previously known to form only triangular metallacycles. The interplay between the conformational flexibility of the [6]CMP macrocycle and the rigid dinuclear gold(I) moieties enable the square geometry, as revealed by single-crystal X-ray diffraction. The formation of the gold complex shows size-selectivity compared to an alternative route using platinum(II) corner motifs. Upon reductive elimination, an all-organic ether-decorated carbon nanoring was obtained. Investigation as a host for the complexation of large guest molecules with a suitable convex π-surfaces was accomplished using isothermal NMR binding titrations. Association constants for [6]cycloparaphenylene ([6]CPP), [7]CPP, C₆₀ , and C₇₀ were determined.

Tier-grown Expansion of Design-of-Experiments Parameter Spaces for Synthesis of a Nanometer-scale Macrocycle

A method to find optimum synthetic conditions was devised by combining a data-driven empirical model with a traditional mechanistic model. In this method, an experimental parameter space was empirically obtained by Design-of-Experiments optimizations with machine-learning supplements and was strategically expanded by examination of the mechanistic model of the reaction paths. An extra tier grown on the original 3×3×3 parameter space succeeded in allocating an optimum reaction condition in the expanded 3×3×4 parameter space. The method was specifically devised for the synthesis of a macrocycle, [n]cyclo-meta-phenylenes ([n]CMP), and the largest congener with n=12 was synthesized and fully characterized for the first time. Crystallographic and photophysical analyses revealed favorable features of [12]CMP for the material applications.

Redox-switchable host-guest complexes of metallocenes and [8]cycloparaphenylene

The cycloparaphenylene (CPP) nanocarbons are an appealing family of macrocyclic organic semiconductors with size-tunable structures and unique optoelectronic properties, which can be further modulated by complexation with guest molecules. While many π-π-stabilized CPP-fullerene host-guest complexes are known, CPPs can also host polycyclic guests stabilized by aromatic CH-π interactions. Here we combine experimental and computational results to report that CH-π interactions can also be tapped to include redox-active metallocene guests in [8]cycloparaphenylene ([8]CPP). Oxidation of a metallocene guest is accompanied by an increase in binding affinity and tilt angle. Crystallographically determined solid-state structures reveal CH-π interactions in the ferrocene complex (Fc⊂[8]CPP) and additional π-π interactions in the cobaltocenium complex (CoCp₂⁺⊂[8]CPP). Functionalizing Fc with oxygen-bearing side chains also improves complex stability to a similar extent as oxidation, due to the formation of CH-O hydrogen bonds with the host's p-phenylene units. This work shows that CH-π bonding can be generalized as a driving force for CPP host-guest complexes and combined with other supramolecular forces to enhance stability. Owing to their semiconducting nature, amenability to functionalization, and reversible redox-dependent behavior, the [8]CPP-metallocene host-guest complexes may expand the library of synthons available for designing bespoke nanoelectronics and artificial molecular machines.

Dynamic Au-C σ-Bonds Leading to an Efficient Synthesis of [n]Cycloparaphenylenes (n = 9-15) by Self-Assembly

The transmetalation of the digold(I) complex [Au₂Cl₂(dcpm)] (1) (dcpm = bis(dicyclohexylphosphino)methane) with oligophenylene diboronic acids gave the triangular macrocyclic complexes [Au₂(C₆H₄) _x (dcpm)]₃ (x = 3, 4, 5) with yields of over 70%. On the other hand, when the other digold(I) complex [Au₂Cl₂(dppm)] (1') (dppm = bis(diphenylphosphino)methane) was used, only a negligible amount of the triangular complex was obtained. The control experiments revealed that the dcpm ligand accelerated an intermolecular Au(I)-C σ-bond-exchange reaction and that this high reversibility is the origin of the selective formation of the triangular complexes. Structural analyses and theoretical calculations indicate that the dcpm ligand increases the electrophilicity of the Au atom in the complex, thus facilitating the exchange reaction, although the cyclohexyl group is an electron-donating group. Furthermore, the oxidative chlorination of the macrocyclic gold complexes afforded a series of [n]cycloparaphenylenes (n = 9, 12, 15) in 78-88% isolated yields. The reorganization of two different macrocyclic Au complexes gave a mixture of macrocyclic complexes incorporating different oligophenylene linkers, from which a mixture of [n]cycloparaphenylenes with various numbers of phenylene units was obtained in good yields.

Activation of positive cooperativity by size-mismatch assembly via inclination of guests in a single-site receptor

A halogenated bowl-shaped guest, corannulene, was encapsulated in a cylindrical host, [4]cyclochrysenylene, to form a bowl-in-tube complex, which mimicked supramolecular complexes between bowl guests and carbon nanotubes. As was the case with carbon nanotubes, the cylindrical space of [4]cyclochrysenylene trapped multiple corannulene molecules in an array, and 1:2 complexes were commonly obtained with the corannulene guests with various halogen substituents (F, Cl, Br and I). Careful statistical analyses of isothermal titration calorimetry titration data succeeded in revealing the stoichiometry, and the molecular structures of the 1:2 complexes were further clarified by X-ray crystallographic analyses. Two fluorinated corannulene guests were stacked perpendicular to the cylinder axis, while two chlorinated guests were stacked with inclined orientations. The structural difference resulted in a large difference in the cooperativity of the two-stage association in solution: fluorinated corannulene guests showed negative cooperativity for the 1:2 complexation, and the other, larger halogenated corannulene guests showed positive cooperativity.

A Defective Nanotube Molecule of C552 H496 N24 with Pyridinic and Pyrrolic Nitrogen Atoms

A 3-nm molecule comprising a cylindrical core and cross-shaped rims was designed and synthesized by developing a modular synthetic route. By using a cyclic precursor from previous studies as a starting material, multiple carbazole units were installed at the rims of the defective cylinder. The defective cylinder was synthetically doped with two types of nitrogen atoms, that is, pyridinic and pyrrolic nitrogen atoms, which resulted in solvatochromic shifts in fluorescence by charge-transfer interactions. The structure of the large, C₅₅₂ H₄₉₆ N₂₄ molecule was fully disclosed by crystallographic analyses, and the unique helical arrangement of nitrogen-doped cylinders in the crystal was revealed.

Confining the Inner Space of Strained Carbon Nanorings

Strained aromatic macrocycles based on cycloparaphenylenes (CPPs) are the shortest repeating units of armchair single-walled carbon nanotubes. Since the development of several new synthetic methodologies for accessing these structures, their properties have been extensively studied. Besides the fundamental interest in these novel molecular scaffolds, their application in the field of materials science is an ongoing topic of research. Most of the reported CPP-type macrocycles display strong binding toward fullerenes, due to the perfect match between the convex and concave π-surfaces of fullerenes and CPPs, respectively. Highly functionalized CPP derivatives capable of supramolecular binding with other molecules are rarely reported. The synthesis of highly functionalized [n]cyclo-2,7-pyrenylenes leads to CPP-type macrocycles with a defined cavity capable of binding non-fullerene guests with high association constants.

Molecular Self-Assembly as a Tool to Construct Transmembrane Supramolecular Ion Channels

Self-assembly has become a powerful tool for building various supramolecular architectures with applications in material science, environmental science, and chemical biology. One such area is the development of artificial transmembrane ion channels that mimic naturally occurring channel-forming proteins to unveil various structural and functional aspects of these complex biological systems, hoping to replace the defective protein channels with these synthetically accessible moieties. This account describes our recent approaches to construct supramolecular ion channels using synthetic molecules and their applications in medicinal chemistry.

Enhanced Thermostability of D-Psicose 3-Epimerase from Clostridium bolteae through Rational Design and Engineering of New Disulfide Bridges

D-psicose 3-epimerase (DPEase) catalyzes the isomerization of D-fructose to D-psicose (aka D-allulose, a low-calorie sweetener), but its industrial application has been restricted by the poor thermostability of the naturally available enzymes. Computational rational design of disulfide bridges was used to select potential sites in the protein structure of DPEase from Clostridium bolteae to engineer new disulfide bridges. Three mutants were engineered successfully with new disulfide bridges in different locations, increasing their optimum catalytic temperature from 55 to 65 °C, greatly improving their thermal stability and extending their half-lives (t_1/2) at 55 °C from 0.37 h to 4−4.5 h, thereby greatly enhancing their potential for industrial application. Molecular dynamics simulation and spatial configuration analysis revealed that introduction of a disulfide bridge modified the protein hydrogen–bond network, rigidified both the local and overall structures of the mutants and decreased the entropy of unfolded protein, thereby enhancing the thermostability of DPEase.

A contorted nanographene shelter

Nanographenes have kindled considerable interest in the fields of materials science and supramolecular chemistry as a result of their unique self-assembling and optoelectronic properties. Encapsulating the contorted nanographenes inside artificial receptors, however, remains challenging. Herein, we report the design and synthesis of a trigonal prismatic hexacationic cage, which has a large cavity and adopts a relatively flexible conformation. It serves as a receptor, not only for planar coronene, but also for contorted nanographene derivatives with diameters of approximately 15 Å and thicknesses of 7 Å. A comprehensive investigation of the host-guest interactions in the solid, solution and gaseous states by experimentation and theoretical calculations reveals collectively an induced-fit binding mechanism with high binding affinities between the cage and the nanographenes. Notably, the photostability of the nanographenes is improved significantly by the ultrafast deactivation of their excited states within the cage. Encapsulating the contorted nanographenes inside the cage provides a noncovalent strategy for regulating their photoreactivity.

A hybrid molecular peapod of sp2- and sp3-nanocarbons enabling ultrafast terahertz rotations

The internal hollow space of carbon nanotubes provides a unique nanometre-sized space to capture various molecular entities. The inner space circumfused by sp²-carbon networks can also encapsulate diamondoid molecules to afford sp²/sp³-hybrid nanocarbon peapods that have recently emerged as unique nanostructures. In this study, the sp²/sp³-hybrid peapods have been mimicked by adopting a cylindrical molecule and the smallest diamondoid, i.e., adamantane, to demonstrate the existence of ultrafast rotational motion. The solid-state rotational frequency is measured by NMR spectroscopy to record 1.06 THz that is, to the best of our knowledge, the largest value recorded for solid-state rotations of molecules. Theoretical calculations reveal that multivalent CH-π hydrogen bonds anchored the diamondoid guest on the π-wall of the cylindrical host. The weak hydrogen bonds are prone not only to cleave but also to regenerate at the interfaces, which give freedom to the guest for ultrafast isotropic rotations in the inertial regime.

Mechanical motions in hybrid sp²/sp³ -hybrid nanocarbon peapods might lead to promising materials applications, but have been insufficiently explored. Here the authors demonstrate that a diamondoid molecule trapped inside a carbonaceous cylinder undergoes solid-state rotations at terahertz frequencies.

Confined Spaces in [n]Cyclo-2,7-pyrenylenes

A set of strained aromatic macrocycles based on [n]cyclo-2,7-(4,5,9,10-tetrahydro)pyrenylenes is presented with size-dependent photophysical properties. The K-region of pyrene was functionalized with ethylene glycol groups to decorate the outer rim and thereby confine the space inside the macrocycle. This confined space is especially pronounced for n=5, which leads to an internal binding of up to 8.0×104  m-1 between the ether-decorated [5]cyclo-2,7-pyrenylene and shape-complementary crown ether-cation complexes. Both the ether-decorated [n]cyclo-pyrenylenes as well as one of their host-guest complexes have been structurally characterized by single-crystal X-ray analysis. In combination with computational methods the structural and thermodynamic reasons for the exceptionally strong binding have been elucidated. The presented rim confinement strategy makes cycloparaphenylenes an attractive supramolecular host family with a favorable, size-independent read-out signature and binding capabilities extending beyond fullerene guests.

A Molecular Transformer: A π-Conjugated Macrocycle as an Adaptable Host

Here, we report a facile method to synthesize a series of macrocycles with different conformations. The planar macrocycle dimer (1), twisted macrocycle trimer (2) and "figure-eight" tetramer (3) are clearly elucidated by X-ray single-crystal analysis, in which the electron-rich phenanthrene units offer the possibility of supramolecular assembly. As expected, in the solid state, 1 and 3 assemble into a columnar stack and an interlocking dimer, respectively, via π-π interactions between the phenanthrene units. Compared to the rigid conformation of dimer 1, the structure of tetramer 3 is more flexible due to its enlarged ring size. 3 can deform from a figure-eight into a boat-shaped geometry to host a planar electron-deficient guest using its electron-rich phenanthrene units. When assembled with spherical electron-deficient C₆₀ , interestingly, 3 further undergoes a conformational transformation from a figure-eight to a belt shape in order to host C₆₀ . These supramolecular assembly behaviors of 3 demonstrate that it is an adaptable macrocyclic host for both planar molecules and fullerenes.

Stereoselectivity in spontaneous assembly of rolled incommensurate carbon bilayers

The periodicity of two-dimensional entities can be manipulated by their stacking assembly, and incommensurate stacks of bilayers are attracting considerable interest in materials science. Stereoisomerism in incommensurate bilayers was first noted with incommensurate double-wall carbon nanotubes composed of helical carbon networks, but the lack of structural information hampered the chemical understanding such as the stereoselectivity during bilayer formation. In this study, we construct a finite molecular version of incommensurate carbon bilayers by assembling two helical cylindrical molecules in solution. An outer cylindrical molecule is designed to encapsulate a small-bore helical cylindrical molecule, and the spontaneous assembly of coaxial complexes proceeds in a stereoselective manner in solution with a preference for heterohelical combinations over diastereomeric, homohelical combinations. The rational design of incommensurate bilayers for material applications may be facilitated by the design and development of molecular versions with discrete structures with atomic precision.

Complexation of an Anthracene-Triptycene Nanocage Host with Fullerene Guests through CH⋅⋅⋅π Contacts

A bicyclic anthracene macrocycle containing two triptycene units at the bridgehead positions was synthesized by Ni-mediated coupling of the corresponding precursor as a cage-shaped aromatic hydrocarbon host. This cage host formed an inclusion complex with C₆₀ or C₇₀ guest in 1 : 1 ratio in solution. The association constants (K_a ) determined by the fluorescence titration method were 1.3×10⁴ and 3.3×10⁵  L mol ^-1 for the C₆₀ and C₇₀ complexes, respectively, at 298 K in toluene. DFT calculations revealed that the guest molecules were included in the middle of the cavity with several CH⋅⋅⋅π contacts. The strong affinity of the cage host for the fullerene guests and the high selectivity toward C₇₀ are discussed on the basis of spectroscopic and structural data.

Synthesis and Properties of a Cyclohexa-2,7-anthrylene Ethynylene Derivative

The synthesis of a cyclohexa-2,7-(4,5-diaryl)anthrylene ethynylene (1) was achieved for the first time by using 1,8-diaryl-3,6-diborylanthracene and 1,8-diaryl-3,6-diiodoanthracene as key synthetic intermediates. Macrocycle 1 possesses a planar conformation of approximately D_6h symmetry, because of the triple-bond linker between the anthracene units at the 2,7-positions. It was confirmed that macrocycle 1, bearing bulky substituents at the outer peripheral positions, behaves as a monomeric form in solution without π-stacking self-association. Macrocycle 1 has an inner-cavity size that allows specific inclusion of [9]cycloparaphenylene ([9]CPP), but not [8]CPP or [10]CPP, through an aromatic edge-to-face CH-π interaction.

Recent advances in probing host–guest interactions with solid state nuclear magnetic resonance

A recent update on how solid state NMR has aided the interpretation and understanding of host–guest interactions in the field of supramolecular assemblies is provided.

Tailoring the cavities of hydrogen-bonded amphidynamic crystals using weak contacts: towards faster molecular machines

This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I. The crystal structures of these rotors, determined by synchrotron radiation experiments at different temperatures, show the presence of such C–H⋯F–C contacts between extended carbazole stators featuring fluorinated phenyl rings and the 1,4-diazabicyclo[2.2.2]octane (DABCO) rotator. According to the ²H NMR results, using deuterated samples, and periodic density functional theory computations, the rotators experience fast angular displacements (preferentially 120° jumps) due to their low rotational activation energies (E_a = 0.8–2.0 kcal mol⁻¹). The higher rotational barrier for 1 (2.0 kcal mol⁻¹) is associated with a larger number of weak C–H⋯F–C contacts generated by the stators. This strategy offers the possibility to explore the correlation among weak intermolecular forces, cavity shape, and internal dynamics, which has strong implications in the design of future fine-tuned amphidynamic crystals.

This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I.

Polarizability and isotope effects on dispersion interactions in water

True understanding of dispersion interaction in solution remains elusive because of difficulty in the precise evaluation of its interaction energy. Here, the effect of substituents with different polarizability on dispersion interactions in water is discussed based on the thermodynamic parameters determined by isothermal titration calorimetry for the formation of discrete aggregates from gear-shaped amphiphiles (GSAs). The substituents with higher polarizability enthalpically more stabilize the nanocube, which is due to stronger dispersion interactions and to the hydrophobic effect. The differences in the thermodynamic parameters for the nanocubes from the GSAs with CH3 and CD3 groups are also discussed to lead to the conclusion that the H/D isotope effect on dispersion interactions is negligibly small, which is due to almost perfect entropy-enthalpy compensation between the two isotopomers.

The Supramolecular Chemistry of Cycloparaphenylenes and Their Analogs

Cycloparaphenylenes (CPPs) and their analogs have recently attracted much attention due to their aesthetical structures and optoelectronic properties with radial π-conjugation systems. The past 10 years have witnessed a remarkable advancement in CPPs research, from synthetic methodology to optoelectronic investigations. In this present minireview, we highlight the supramolecular chemistry of CPPs and their analogs, mainly focusing on the size-selective encapsulation of fullerenes, endohedral metallofullerenes, and small molecules by these hoop-shaped macrocycles. We will also discuss the assembly of molecular bearings using some belt-persistent tubular cycloarylene molecules and fullerenes, photoinduced electron transfer properties in supramolecular systems containing carbon nanohoop hosts and fullerene guests, as well as the shape recognition properties for structure self-sorting by using dumbbell-shaped dimer of [60]fullerene ligand. Besides, the supramolecular complexes with guest molecules other than fullerenes, such as CPPs themselves, iodine, pyridinium cations, and bowl-shaped corannulene, are also discussed.

The Supramolecular Chemistry of Strained Carbon Nanohoops

Since 1996, a growing number of strained macrocycles, comprising only sp² - or sp-hybridized carbon atoms within the ring, have become synthetically accessible, with the [n]cycloparaphenyleneacetylenes (CPPAs) and the [n]cycloparaphenylenes (CPPs) being the most prominent examples. Now that robust and relatively general synthetic routes toward a diverse range of nanohoop structures have become available, the research focus is beginning to shift towards the exploration of their properties and applications. From a supramolecular chemistry perspective, these macrocycles offer unique opportunities as a result of their near-perfect circular shape, the unusually high degree of shape-persistence, and the presence of both convex and concave π-faces. In this Minireview, we give an overview on the use of strained carbon-rich nanohoops in host-guest chemistry, the preparation of mechanically interlocked architectures, and crystal engineering.

Retarded Solid-State Rotations of an Oval-Shaped Guest in a Deformed Cylinder with CH-π Arrays

Upon encapsulating an oval-shaped hydrocarbon guest, a cylindrical host deforms its shape to maximize intermolecular contacts. Structure-assembly relationship studies with a series of hydrocarbon guests disclosed the importance of molecular shapes and CH-π contacts. Multiple contacts and weak CH-π hydrogen bonds resulted in an optimal assembly; however, the shape deformation resulted in severe retardation of rotational motions in the crystal. Thus, unlike a circular guest, the oval-shaped guest did not change its orientation in the host. Unexpectedly, the planar guest did not affect the packing structure to form a double helix in intertwined host arrays.

TriQuinoline

The bottom-up synthesis of structurally well-defined motifs of graphitic materials is crucial to understanding their physicochemical properties and to elicit new functions. Herein, we report the design and synthesis of TriQuinoline (TQ) as a molecular model for pyridinic-nitrogen defects in graphene sheets. TQ is a trimer of quinoline units concatenated at the 2- and 8-positions in a head-to-tail fashion, whose structure leads to unusual aromatisation behaviour at the final stage of the synthesis. The central atomic-sized void endows TQ with high proton affinity, which was confirmed empirically and computationally. TQ•H⁺ is a two-dimensional cationic molecule that displays both π-π and CH-π contact modes, culminating in the formation of the ternary complex ([12]cycloparaphenylene(CPP) ⊃ (TQ•H⁺/coronene)) that consists of TQ•H⁺, coronene (flat), and [12]cycloparaphenylene ([12]CPP) (ring). The water-miscibility of TQ•H⁺ allows it to serve as an efficient DNA intercalator for e.g. the inhibition of topoisomerase I activity.

Molecular recognition of planar and non-planar aromatic hydrocarbons through multipoint Ag-π bonding in a dinuclear metallo-macrocycle

Exploration of a novel structural motif of host-guest interactions is one of the most fundamental topics to develop macrocycle-based host-guest/supramolecular systems. Herein, we present an unprecedented mode of inclusion of aromatic hydrocarbons into a macrocyclic cavity via multipoint Ag-π bonding as a driving force. A dinuclear Ag^I-macrocycle encapsulated one molecule of anthracene, a typical planar aromatic hydrocarbon, in solution and in the solid state. Single-crystal X-ray diffraction analysis of the host-guest inclusion complex revealed the binding of anthracene via multipoint Ag-π bonding to both Ag^I ions arranged within the open-ended nano-cavity of the dinuclear Ag^I-macrocycle. Notably, this binding motif based on Ag-π bonding was also applied to the inclusion of triptycene, a non-planar aromatic hydrocarbon with a steric tripodal structure, to evaluate the rotational motion of the molecular paddle-wheel in the Ag^I-macrocycle.

Periphery Design of Macrocyclic Materials for Organic Light-Emitting Devices with a Blue Phosphorescent Emitter

Cyclo- meta-phenylenes were modified with trifluoromethyl groups at their periphery to create host materials suitable for use in blue phosphorescent organic light-emitting devices. The periphery design resulted in molecules with high triplet-state energies, which were required to support the blue emission from Ir phosphors. As a result, an external quantum efficiency of 9.9% was achieved. The most successful host, a pentameric congener, preferred CF-π/CH-π interactions in its crystalline packings, which could be beneficial for the host performance.

Thermally activated isomeric all-hydrocarbon molecular receptors for fullerene separation

A new all-hydrocarbon macrocycle, cyclo[8](1,3-(4,6-dimethyl)benzene) (CDMB-8) has been reported. As prepared, it exists in Cs symmetry and shows no interaction with fullerenes (e.g., C60 or C70). High temperature (573 K) treatment induces thermal conversion of the material to an isomeric conformer with D4d symmetry as a receptor for fullerene separation.