Exfoliatable Layered 2D Honeycomb Crystals of Host-guest Complexes Networked by CH-π Hydrogen Bonds.

Studies of graphene show that robust chemical bonds such as covalent bonds with trigonal-planar atoms afford layered atomic 2D crystals possessing unique properties. Although layered molecular crystals are of interest to diversify elements and structures of 2D materials, the structural diversity of molecules as well as weak intermolecular interactions inevitably makes the design to be one-off and individual. We herein report a versatile method to assemble layered molecular crystals. By developing a D3-symmetry host at vertices to form a honeycomb layer, a diverse range of layered 2D host-guest crystals were obtained. Substituents on the host, elements/structures of the guest, the stereochemistry of the host and types of intercalants were diversified, which should allow for 6×32×3×2 combinations for structural diversification.

Concise Synthesis of Molecular Hyperboloids by Oligomeric Macrocyclization of Octagonal Molecules.

Molecular hyperboloids were designed and synthesized. The synthesis was achieved by development of oligomeric macrocyclization of an octagonal molecule with a saddle shape. The saddle-shaped molecule, that is, [8]cyclo-meta-phenylene ([8]CMP), was decorated with two linkers for the oligomeric macrocyclization and was synthetically assembled by Ni-mediated Yamamoto coupling. Three congeners of the molecular hyperboloids (2mer-4mer) were obtained, and 2mer and 3mer were subjected to X-ray crystallographic analysis. Crystal structures revealed nanometer-sized hyperboloidal structures with 96π and 144π electrons, which also possessed nanopores on the curved molecular structures. Structures of [8]CMP cores of the molecular hyperboloids were compared with those of saddle-shaped phenine [8]circulene with a negative Gauss curvature to confirm their structural resemblance, which suggests further explorations of expanded networks of molecular hyperboloids.

Singly and Triply Linked Magnetic Porphyrin Lanthanide Arrays.

The introduction of paramagnetic metal centers into a conjugated π-system is a promising approach toward engineering spintronic materials. Here, we report an investigation of two types of spin-bearing dysprosium(III) and gadolinium(III) porphyrin dimers: singly meso-meso-linked dimers with twisted conformations and planar edge-fused ß,meso,ß-linked tapes. The rare-earth spin centers sit out of the plane of the porphyrin, so that the singly linked dimers are chiral, and their enantiomers can be resolved, whereas the edge-fused tape complexes can be separated into syn and anti stereoisomers. We compare the crystal structures, UV-vis-NIR absorption spectra, electrochemistry, EPR spectroscopy, and magnetic behavior of these complexes. Low-temperature SQUID magnetometry measurements reveal intramolecular antiferromagnetic exchange coupling between the GdIII centers in the edge-fused dimers (syn isomer: J = -51 ± 2 MHz; anti isomer: J = -19 ± 3 MHz), whereas no exchange coupling is detected in the singly linked twisted complex. The phase-memory times, Tm, are in the range of 8-10 µs at 3 K, which is long enough to test quantum computational schemes using microwave pulses. Both the syn and anti Dy2 edge-fused tapes exhibit single-molecule magnetic hysteresis cycles at temperatures below 0.5 K with slow magnetization dynamics.

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 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.

Target-oriented design of helical nanotube molecules for rolled incommensurate bilayers.

Incommensurate double-wall carbon nanotubes give rise to unique stereochemistry originating from twisted stacks of hexagon arrays. However, atomic-level studies on such unique systems have rarely been performed, even though syntheses of molecular segments of carbon nanotubes have been extensively explored. The design of cylindrical molecules with chirality, particularly, in pairs provides synthetic challenges, because relationships between diameters specified with chiral indices and structures of arylene panels have not been investigated in a systematic manner. Here we show that a molecular version of incommensurate double-wall carbon nanotubes can be designed through the development of an atlas for the top-down design of cylindrical molecules. A large-bore cylindrical molecule with a diameter of 1.77 nm was synthesized using a readily available pigment and encapsulated a small-bore cylindrical molecule with a diameter of 1.04 nm. The large- and small-bore molecules possessed helicity in atomic arrangements, and their coaxial assembly proceeded in nonstereoselective manner to give both heterohelical and homohelical combinations.

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.

Synthesis and Chiral Resolution of Twisted Carbon Nanobelts.

Twisted carbon nanobelts could display persistent chirality, which is desirable for material applications, but their synthesis is very challenging. Herein, we report the successful synthesis and chiral resolution of such a kind of molecules (1-H and 1) with a figure-eight configuration. 1-H was synthesized first by macrocyclization through Suzuki coupling reaction followed by benzannulation via Bi(OTf)3-mediated cyclization reaction of vinyl ether. Oxidative dehydrogenation of 1-H gave the fully π-conjugated 1. Their twisted structures were confirmed by X-ray crystallographic analysis and calculations, and they can be resolved by chiral high-performance liquid chromatography. The isolated enantiomers showed persistent chiroptical properties according to the circular dichroism measurements, with moderate |gabs| values (0.0016 for 1-H and 0.005-0.007 for 1). Their photophysical properties were also briefly studied.

Metal-Templated Oligomeric Macrocyclization via Coupling for Metal-Doped π-Systems.

A method for the synthesis of metal-doped aromatic macrocycles has been developed. The method, i.e., metal-templated oligomeric macrocyclization via coupling, adopts Ni as the template and assembles five pyridine units via a Ni-mediated coupling reaction to form aryl-aryl linkages. A pentameric oligopyridyl macrocycle was selectively obtained in good yield, and the reaction was also applicable to a gram-scale synthesis. The pentameric oligopyridyl macrocycle captured d8-Ni(II) at the center to form a paramagnetic pentagonal-bipyramidal complex. The method was applied to the synthesis of a large π-molecule to afford a nanometer-sized, bowl-shaped molecule having a unique combination of 120π and 8d electrons.

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.

Manipulations of Chiroptical Properties in Belt-Persistent Cycloarylenes via Desymmetrization with Heteroatom Doping.

A desymmetrization strategy has been devised in the design of molecular cylinders to maximize the dissymmetry factor relevant to circularly polarized light. Although the highest dissymmetry factor of organic molecules was previously achieved with a chiral belt-persistent cycloarylene having magnetic and electric transition dipole moments in parallel, we noticed that an unbalanced magnitude of two moments was detrimental for higher dissymmetry factors. In this study, a molecular cylinder was desymmetrized by arraying doped and undoped panels via stereoselective cross-coupling macrocyclization. The desymmetrization succeeded in balancing two moments by reducing the electric transition moment at the global minimum but failed to maximize the dissymmetry factor. Structural studies revealed that the dissymmetry factor is sensitive to subtle structural fluctuations, while the rotatory strength is not affected. This study is important for the development of chiroptical materials.

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.

Fused Quinoidal Dithiophene-Based Helicenes: Synthesis by Intramolecular Radical-Radical Coupling Reactions and Dynamics of Interconversion of Enantiomers.

A series of fused quinoidal dithiophene-based double and triple helicenes (1-M, 2-M, 2-M-Cl, 3-M, 3-M-Cl) were synthesized by intramolecular radical-radical coupling followed by oxidative dehydrogenation reaction. These helical molecules show dynamic interconversion of enantiomers in solution as revealed by variable-temperature NMR measurements, and the energy barriers are correlated to the substituents and topological structures. Notably, dynamic high performance liquid chromatography was used to quantitatively investigate the room-temperature racemization process between the (P,P,M)- and (P,M,M)- enantiomers of the triple helical 3-M-Cl, which gave an interconversion energy barrier in consistent with density functional theory calculations. Their optical and electrochemical properties are dependent on the fusion mode. Our studies provide both new synthetic strategy and new dynamic analytical method for helicenes with unique electronic structure.

Crystalline Naphthylene Macrocycles Capturing Gaseous Small Molecules in Chiral Nanopores.

A series of chiral naphthylene macrocycles, [n]cyclo-epi-naphthylenes ([n]CeNAPs), possessing epi-linkages were synthesized by one-pot macrocyclization. With chiral (R)- or (S)-1,1'-linkages embedded in binaphthyl precursors, the macrocycles were assembled in polygonal structures possessing chiral hinges as corners. Among four chiral [n]CeNAP variants, [8]CeNAP with eight naphthylene panels formed robust columnar assemblies in crystals. The nanoporous crystals maintained a columnar assembly structure even after the removal of encapsulated solvent molecules, and their gas adsorption behavior was thoroughly investigated. Gas adsorption, including state-of-the-art in situ crystallographic analyses, revealed accurate atomic-level structures of the nanopores trapping gaseous N2 molecules in chiral C2 arrangements. With macrocycles as basic frameworks, functional nanopores may be exploited for chiral small-molecule alignments.

Ineffective OH Pinning of the Flipping Dynamics of a Spherical Guest within a Tight-Fitting Tube.

A supramolecular/synthetic method has been devised to affix a sterically hindered substituent onto a fullerene guest encapsulated in a tubular host. A two-wheeled complex of (C59 N)-(C59 N) with a tubular host was oxidatively bisected to afford a C59 N+ cation captured in the tube. The C59 N+ cation in the tube was then trapped by ethanol or water, which led to an oxy substituent pinned on the guest. The guest motions within the tube were modulated by the pinned substituent, and up-and-down flipping motions were halted by an ethoxy substituent. A hydroxy substituent, however, was ineffective in halting the flipping motions, despite the tight-fitting relationship between the tubular host and the spherical guest. Theoretical calculations of the dynamics revealed that the flipping motions were assisted by OH-π hydrogen bonds between the guest and the carbon-rich wall and that sliding motions of the OH group were also facilitated by deformations of the tube.

Author Correction: Concyclic CH-π arrays for single-axis rotations of a bowl in a tube.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Benzidine/Quinoidal-Benzidine-Linked, Superbenzene-Based π-Conjugated Chiral Macrocycles and Cyclophanes.

Synthesis of fully conjugated cyclophanes containing large-size polycyclic aromatics is challenging. Now, three benzidine-linked, hexa-peri-hexabenzocoronene (superbenzene)-based ortho-, para-, and meta-cyclophanes are synthesized through intermolecular Yamamoto coupling reaction of structurally pre-organized precursors. Subsequent oxidative dehydrogenation gave the corresponding quinoidal benzidine-linked cyclophanes. Their geometries were confirmed by X-ray crystallographic analysis and their electronic properties were investigated by electronic absorption, cyclic voltammetry, and DFT calculations. The quinoidal benzidine-linked cyclophanes show thermally populated paramagnetic activity with a relatively large singlet-triplet energy gap. Two enantiomers for the ortho-cyclophanes (1-NH and 1-N) were isolated and their chiral figure-of-eight macrocyclic structures were identified. The cage-like cyclophanes 2-NH and 3-NH with concave surface can selectively encapsulate fullerene C70 .

Regulated Single-Axis Rotations of a Carbonaceous Guest in a van†der Waals Complex with an Entropy Cost.

In a tight host-guest complex assembled solely by nondirectional van der Waals forces, unique motions of the guest, such as solid-state inertial rotations, emerge. The regulation of dynamic motions is an important element to be explored for novel functions of such complexes, which may be seemingly difficult to achieve because of the nondirectionality of the assembling forces. A regulated, single-axis rotation was made possible by choosing an appropriate shape of the guest in the tubular host. Specifically, an ellipsoidal guest was made to stand along a cylinder axis of the host, which consequently resulted in single-axis rotations of the guest in the solid. The rotational frequency was considerably high for solid-state rotations but was suppressed to 10 GHz, which was 1/20 of the isotropic rotation of a spherical guest. In-depth kinetic analyses quantitatively revealed that the entropy cost was a determining factor that regulated the dynamics.

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.