Nano‐Saturn: Experimental Evidence of Complex Formation of an Anthracene Cyclic Ring with C 60

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.

CH-π Multi-Interaction-Driven Recognition and Isolation of Planar Compounds in a Spheroidal Polyaromatic Cavity

π-π Interactions are established as a powerful supramolecular tool, whereas the usability of CH-π interactions has been rather limited so far. Here we present (i) selective binding of planar polyaromatics and (ii) effective isolation of planar metal complexes by a polyaromatic capsule, utilizing multiple CH-π interactions. In the spheroidal cavity, one molecule of large and medium-sized polyaromatic molecules (i. e., coronene and pyrene) is exclusively bound from mixtures bearing the same number of aromatic CH groups. Theoretical studies reveal that multiple host-guest CH-π interactions (up to 32 interactions) are the predominant driving force for the observed selectivity. In addition, one molecule of planar metal complexes (i. e., porphine and bis(acetylacetonato) Cu(II) complexes) is quantitatively bound by the capsule through aromatic and aliphatic CH-π multi-interactions, respectively. The ESR and theoretical studies demonstrate the isolation capability of the capsular framework and an unusual polar environment in the polyaromatic cavity.

Supramolecular Ensembles Formed via Calix[5]arene-Fullerene Host-Guest Interactions

This minireview introduces the research directions for the synthesis of supramolecular fullerene polymers. First, the discovery of host-guest complexes of pristine fullerenes is briefed. We focus on progress in supramolecular fullerene polymers directed by the use of calix[5]arene-fullerene interactions, which comprise linear, networked, helical arrays of fullerenes in supramolecular ensembles. The unique self-sorting behavior of right-handed and left-handed helical supramolecular fullerene arrays is discussed. Thereafter, an extensive investigation of the calix[5]arene-fullerene interaction for control over the chain structures of covalent polymers is introduced.

On-Surface Synthesis and Collective Spin Excitations of a Triangulene-Based Nanostar

Triangulene nanographenes are open‐shell molecules with predicted high spin state due to the frustration of their conjugated network. Their long‐sought synthesis became recently possible over a metal surface. Here, we present a macrocycle formed by six [3]triangulenes, which was obtained by combining the solution synthesis of a dimethylphenyl‐anthracene cyclic hexamer and the on‐surface cyclodehydrogenation of this precursor over a gold substrate. The resulting triangulene nanostar exhibits a collective spin state generated by the interaction of its 12 unpaired π‐electrons along the conjugated lattice, corresponding to the antiferromagnetic ordering of six S=1 sites (one per triangulene unit). Inelastic electron tunneling spectroscopy resolved three spin excitations connecting the singlet ground state with triplet states. The nanostar behaves close to predictions from the Heisenberg model of an S=1 spin ring, representing a unique system to test collective spin modes in cyclic systems.

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.

A Fullerene-Based Molecular Torsion Balance for Investigating Noncovalent Interactions at the C60 Surface

To investigate the nature and strength of noncovalent interactions at the fullerene surface, molecular torsion balances consisting of C₆₀ and organic moieties connected through a biphenyl linkage were synthesized. NMR and computational studies show that the unimolecular system remains in equilibrium between well-defined folded and unfolded conformers owing to restricted rotation around the biphenyl C-C bond. The energy differences between the two conformers depend on the substituents and is ascribed to differences in the intramolecular noncovalent interactions between the organic moieties and the fullerene surface. Fullerenes favor interacting with the π-faces of benzenes bearing electron-donating substituents. The correlation between the folding free energies and corresponding Hammett constants of the substituents in the arene-containing torsion balances reflects the contributions of the electrostatic interactions and dispersion force to face-to-face arene-fullerene interactions.

Selective Synthesis of Conjugated Chiral Macrocycles: Sidewall Segments of (-)/(+)-(12,4) Carbon Nanotubes with Strong Circularly Polarized Luminescence

Carbon nanotubes (CNTs) have unusual physical properties that are valuable for nanotechnology and electronics, but the chemical synthesis of chirality- and diameter-specific CNTs and π-conjugated CNT segments is still a great challenge. Reported here are the selective syntheses, isolations, characterizations, and photophysical properties of two novel chiral conjugated macrocycles ([4]cyclo-2,6-anthracene; [4]CAn_2,6 ), as (-)/(+)-(12,4) carbon nanotube segments. These conjugated macrocyclic molecules were obtained using a bottom-up assembly approach and subsequent reductive elimination reaction. The hoop-shaped molecules can be directly viewed by a STM technique. In addition, chiral enantiomers with (-)/(+) helicity of the [4]CAn_2,6 were successfully isolated by HPLC. The new tubular CNT segments exhibit large absorption and photoluminescence redshifts compared to the monomer unit. The carbon enantiomers are also observed to show strong circularly polarized luminescence (g_lum ≈0.1). The results reported here expand the scope of materials design for bottom-up synthesis of chiral macrocycles and enrich existing knowledge of their optoelectronic properties.

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.