Onion-Type Complexation Based on Carbon Nanorings and a Buckminsterfullerene

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.

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.

Monocyclic and Dicyclic Dehydro[20]annulenes Integrated with Perylene Diimide

A novel kind of monocyclic and dicyclic dehydro[20]annulenes exhibiting specific sizes and topologies from regioselective unilateral ortho-diethynyl PDI, is developed by Cu-catalyzed Glaser-Hay homo-coupling and cross-coupling. Through the integration of electron-deficient PDI chromophores into the dehydroannulene scaffolding, these macrocycles exhibit intense and characteristic absorption properties and the degenerated LUMO levels. The single-crystal X-ray diffraction analysis unambiguously revealed unique porous supramolecular structures, which display micropore characteristics with surface area of 120.74 m²  g^-1 . A moderate electron mobility of 0.05 cm²  V^-1  s^-1 for chlorine-free dehydro[20]annulene based on micrometer-sized single-crystalline transistors was witnessed. The porous and yet semiconducting features signify the prospects of PDI-integrated dehydroannulenes in organic optoelectronics.

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.

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.

A Saturn-Like Complex Composed of Macrocyclic Oligothiophene and C60 Fullerene: Structure, Stability, and Photophysical Properties in Solution and the Solid State

A Saturn-like 1:1 complex composed of macrocyclic oligothiophene E-8T7A and C₆₀ fullerene (C₆₀ ) was synthesized to investigate the interaction between macrocyclic oligothiophenes and C₆₀ in solution and the solid state. Because the Saturn-like 1:1 complex E-8T7A⋅C₆₀ is mainly stabilized by van der Waals interactions between C₆₀ and the sulfur atoms of the E-8T7A macrocycle, C₆₀ is rather weakly incorporated inside the macro-ring in solution. However, in the solid state the Saturn-like 1:1 complex preferentially formed single crystals or nanostructured polymorphs. Interestingly, X-ray analysis and theoretical calculations exhibited hindered rotation of C₆₀ in the Saturn-like complex due to interactions between C₆₀ and the sulfur atoms. Furthermore, the photoinduced charge transfer (CT) interaction between E-8T7A and C₆₀ in solution was investigated by using femtosecond transient absorption (TA) spectroscopy. The ultrafast TA spectral changes in the photoinduced absorption bands were attributed to the CT process in the Saturn-like structure.

Shortest Double-Walled Carbon Nanotubes Composed of Cycloparaphenylenes

The host-guest chemistry of cycloparaphenylenes (CPPs) of different sizes is described. [n]CPPs (n=5, 6, 7, 8, and 10) selectively interact with [n+5]CPPs, forming complexes [n+5]CPP⊃[n]CPP, which are the shortest double-walled armchair carbon nanotubes. The size selectivity is dictated by the difference in diameters of the CPPs (that is, 0.34-0.35 nm), which maximizes attractive van der Waals interactions. Theoretical calculations suggest that the orbital energies of the CPPs become perturbed upon complex formation, and orbital mixing between the two CPPs is predicted for large CPP pairs. The association constants in 1,1,2,2-[D₂ ]tetrachloroethane, estimated by ¹ H NMR titration, are approximately 10³  mol L^-1 at 50 °C. Van't Hoff plot analysis reveals that complexation is driven mainly by entropy owing to desolvation of the CPPs. [13]CPP also forms a complex with [4]cyclo-2,7-pyrenylene ([4]CPY), which is a π-extended [8]CPP. Theoretical calculations suggest that the formation of [13]CPP⊃[4]CPY is more exothermic than that of [13]CPP⊃[8]CPP. A ternary complex, [15]CPP⊃[10]CPP⊃C₆₀ , is also formed by mixing [15]CPP and [10]CPP⊃C₆₀ .

Aromatic rings in chemical and biological recognition: energetics and structures

This review describes a multidimensional treatment of molecular recognition phenomena involving aromatic rings in chemical and biological systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chemistry, biological affinity assays and biostructural analysis, data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S⋅⋅⋅aromatic, cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramolecular systems, and should inspire further utilization of interactions with aromatic rings to control the stereochemical outcome of synthetic transformations.

Theoretical investigations on the geometric and electronic structures of phenylene-acetylene macrocycles

The geometric and electronic structures of a series of conjugated macrocycles (phenylene-acetylene macrocycles, PAMs) have been studied theoretically with ab initio and semiempirical molecular orbital methods. The ab initio calculations at the HF/6-31G* level demonstrate that the model molecules may have a planar conformation. Bigger macrocycles, for example, 7PAM, 8PAM, and 9PAM, result in several energy minima. The boatlike conformation is the most energetically favored form. Based on the conformational analysis, a novel method for analyzing the ring-strain energy was proposed and used. In view of their potential applications as electronic materials, the electronic structures of a series of PAMs are also investigated. The LUMO-HOMO gaps of the planar PAMs show an odd-even difference behavior. In addition, the HOMOs of the planar species 3PAM, 5PAM, 7PAM, and 9PAM are doubly degenerated.

Two-Dimensional Acetylenic Scaffolding: Extended Donor-Substituted Perethynylated Dehydroannulenes

Starting from (Z)-bis(N,N-diisopropylanilino)-substituted tetraethynylethene (TEE), perethynylated octadehydro[12]- and dodecadehydro[18]annulenes were prepared by oxidative Hay coupling. The dodecadehydro[18]annulene with six peripheral N,N-diisopropylanilino substituents was characterized by X-ray crystallography. Elongation of the Z-bisdeprotected TEE by Cadiot-Chodkiewicz coupling with 1-bromo-2-(triisopropylsilyl)ethyne provided a Z-configured bis(butadiyne), which after alkyne deprotection afforded under Hay coupling conditions N,N-diisopropylanilino-substituted perethynylated hexadecadehydro[20]- and tetracosadehydro[30]annulenes. The diisopropylanilino substituents enhance the properties of these unprecedented all-carbon perimeters in several distinct ways. They ensure their solubility, increase their stability, and importantly, engage in strong intramolecular charge-transfer interactions with the electron-accepting all-carbon cores, resulting in intense, bathochromically shifted charge-transfer bands in the UV/Vis spectra. The charge-transfer character of these bands was confirmed by protonation-neutralization experiments. The redox properties of the new carbon-rich chromophores were investigated by cyclic voltammetry and rotating disk voltammetry, which indicated different redox behavior for aromatic (4n+2 pi electrons) and antiaromatic (4n pi electrons) dehydroannulenes.

Knotting and threading of molecules: chemistry and chirality of molecular knots and their assemblies

How and why do molecules tangle or thread? Investigations of molecular knots (knotanes) may shed some light on the mechanisms of (supra)molecular templation and the folding of molecules that result in intertwining. The topological chirality of these fascinating molecules leads to new types of isomerism and paves the way to nanosized molecular motors. Their preparation and derivatization makes high demands on modern synthetic methods and analytical separation since molecular knots are formed in a more or less planned design based on metal coordination or hydrogen-bonding patterns. This Review describes the development of templation techniques for the synthesis of knotanes and their chiral resolution as well as their selective functionalization and use as building blocks in the synthesis of higher knotane assemblies. Such assemblies can possess linear, branched, or even macrocyclic structures which, on the one hand, introduce unprecedented isomeric compositions that arise from multiple topological stereogenic units and, on the other, define new types of artificial macromolecules beyond polymers and dendritic species.