Fullerene size controls the selective complexation of [11]CPP with pristine and endohedral fullerenes

Noncovalent interactions and properties of host-guest systems based on C82/C82Gd bucky-balls and symmetry broken nanohoop TP-[11]CPP

The nanoscale host-guest interactions between a symmetry broken carbonaceous nanohoop TP-[11]cycloparaphenylene (TP-[11]CPP) and endohedral metallofullerene (EMF) C82Gd were explored by using density functional theory calculations. The geometry mutual-matching between TP-[11]CPP and C82Gd is perfect, and the two main configurations of TP-[11]CPP@C82Gd host-guest complexes could be formed spontaneously with high binding energies. Interestingly, the position of the Gd atom in the C82 cage can be adjusted by its external host molecule. The binding strength depends on the structure of the host, but the binding thermodynamics is decided by the structure of the fullerene cage. The selective binding of empty cage C82 from its mixture with EMF C82Gd is discussed by using a standard Boltzmann expression of statistical thermodynamics. In addition, the FT-IR and UV-visible spectra are simulated, host-guest noncovalent interaction regions are investigated based on the electron density and reduced density gradient, and magnetic susceptibility is preliminarily investigated, which may be helpful for a deep understanding of the present host-guest systems in the future. It is anticipated that such a theoretical calculation regarding to carbonaceous nanosize host-guest structures would be a driven force for the developments of novel nanohoop@EMF systems in functional materials, nonchromatographic separation and even nano single molecular electret devices.

Dual molecular tweezers extending from a nanohoop

The field of nanohoops is mature enough that synthetic protocols exists to tune their size, composition (incorporation of heteroaromatic building blocks), connectivity (para versus meta linkages), and solubility in different media (hydrophobic versus hydrophilic). Here, we report an additional dimension incorporating the concept of fullerene tweezers into a nanohoop. The resulting hybrid nanohoop is highly strained at 77 kcal mol-1, possesses a quantum yield of 0.12, emits at 584 nm, and displays a positive cooperative binding for C60 (4K2 ≫ K1).

Stability of [10-12]cycloparaphenylene complexes with pristine fullerenes C76,78,84 and endohedral metallofullerenes M3N@C78,80

[n]Cycloparaphenylenes ([n]CPPs) are strained macrocycles, comprising only sp2-hybridized carbon atoms. In recent years, [n]CPPs have become of great research interest in the field of supramolecular chemistry since their special structure enables the formation of novel host-guest complexes. In this work, we investigate the gas-phase chemistry of noncovalent complexes of [10-12]CPP with the pristine fullerenes C76/78/84 and the endohedral metallofullerenes (EMFs) Sc3N@D3h-C78, Sc3N@D5h-C80 and M3N@Ih-C80 (M = Sc, Y, Lu, Gd). The [1 : 1] complexes with [10-12]CPP are detected as radical cations. The stability and charge distributions of these complexes are studied using energy-resolved collision-induced dissociation (ER-CID). Our results assess the size complementarity, the influence of fullerene symmetry and size as well as the role of the metal size inside the EMF on the binding affinity and complex stability. Two main trends in complex stability have been found: First, [10-12]CPP form more stable complexes with EMFs than with pristine fullerenes and second, all complexes of EMFs with the C80 skeleton show similar stability despite the different metal clusters encapsulated. Another major finding is the fact that [11]CPP is generally the most suitable host for fullerenes with a C76/78/80/84 skeleton. Considering the charge distributions, we observe the existence of two different fragmentation channels for complexes with EMFs where the radical cation is either located at the CPP or at the EMF: (1) [n]CPP+˙ + EMF and (2) [n]CPP + EMF+˙. This behavior allows a clear distinction of the cage isomers ([11]CPP⊃Sc3N@Ih-C80)+˙ and ([11]CPP⊃Sc3N@D5h-C80)+˙ in the MS2 experiment. The experimental results are accompanied by density functional theory (DFT) calculations of ionization potentials (IPs) and fragmentation energies. The computational results fully confirm the measured order of complex stabilities and explain the prevalence of EMF or CPP signals in the spectra by the trend in ionization potentials.

Thermal treatment of water-soluble particles formed by compounds composed of carbon nanobelts and C60 molecules

It was previously shown that spherical particles are self-assembled by compounds composed of C60-(6,6)CNB-C60, where CNB stands for "carbon nanobelt", by mixing two individual solutions of C60 and (6,6)CNB molecules dissolved in 1,2-dichlorobenzene at room temperature. The particles are monodisperse in water thanks to their high absolute value of the zeta potential in water. In this report, we investigate the effect of thermal treatment of the particles on some changes in the physical properties and structures. We find that the particles become electrically conductive after thermal treatment at 600 °C for 1 h. We suppose that the change in the electrical characteristics might have been caused by the structural change of (6,6)CNBs into opened-up ribbons composed of fused benzene rings, which construct networks supported by C60 molecules in the particles, judging by the change in the absorption and mass spectra of the particles after thermal treatment and analysis of a possible change in the structure of C60-(6,6)CNB-C60 based on quantum chemical calculations employing the PM6 method, with which it is known that nanostructures such as carbon nanotubes (CNTs) and (6,6)CNBs can be correctly estimated.

Theoretical Study on a Supramolecular Dimeric Structure Constructed by Metallofullerene Y3N@C80 and Figure-of-Eight Nanoring

A new supramolecular complex with a dimeric structure (2Y₃N@C₈₀⊂OPP) constructed by metallofullerene Y₃N@I_h-C₈₀ and figure-of-eight molecular nanoring of oligoparaphenylene (OPP) was investigated using dispersion-corrected density functional theory (DFT-D3). The interactions between the Y₃N@I_h-C₈₀ guest and the OPP host were studied theoretically at the B3LYP-D3/6-31G(d)∼SDD level. By analyzing geometric characteristics and host-guest binding energies, it is revealed that the OPP is an ideal host molecule for the Y₃N@I_h-C₈₀ guest. Typically, the OPP can well induce the orientation of the endohedral Y₃N cluster on the plane of nanoring. Meanwhile, the configuration of the dimeric structure demonstrates that OPP presents excellent elastic adaptability and shape flexibility during the encapsulation of Y₃N@I_h-C₈₀. Highly accurate binding energy suggests that 2Y₃N@C₈₀⊂OPP (∼-443.82 kJ mol^-1 at the ωB97M-V/def2-QZVPP level of theory) is an extremely stable host-guest complex. Thermodynamic information indicates that the formation of the 2Y₃N@C₈₀⊂OPP dimer is thermodynamically spontaneous. Furthermore, electronic property analysis reveals that this dimeric structure has a strong electron-attracting ability. Energy decomposition and real-space function analyses of host-guest interactions reveal the characteristics and nature of the noncovalent interactions in the supramolecules. These results provide theoretical support for the design of new host-guest systems based on metallofullerene and nanoring.

A molecular Popeye: Li+@C60 and its complexes with [n]cycloparaphenylenes

In this work, we compare for the first time the stability of [n]cycloparaphenylene ([n]CPP)-based host-guest complexes with Li⁺@C₆₀ and C₆₀ in the gas and the solution phase. Our gas-phase experiments reveal a significant increase in stability for the complexes featuring [9-12]CPP with Li⁺@C₆₀. This increased interaction strength is also observed in solution. Isothermal titration calorimetry shows for the formation of [10]CPP⊃Li⁺@C₆₀ a two orders of magnitude larger association constant than that for the C₆₀ analog. Additionally, an increased binding entropy is observed. This study contributes to a better understanding of host-guest complexes between [n]CPPs and endohedral metallofullerenes at a molecular level, which is the prerequisite for future applications.

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.

Theoretical exploration of noncovalent interactions in Sc2C2@C2n (n = 40, 41, and 42)⊂[12]CPP, PF[12]CPP

The encapsulation of fullerenes by carbon nanorings has gained increasing attention because of the unique molecular structure and special properties of the formed complexes. The host-guest interactions between the fullerenes and the carbon nanorings can influence the metal ion orientation and the molecular electronic structure. In this study, we hooped a series of carbide cluster metallofullerenes, namely Sc₂C₂@C_2v(5)-C₈₀, Sc₂C₂@C_3v(8)-C₈₂, and Sc₂C₂@D_2d(23)-C₈₄, with molecular carbon nanorings of [12]cycloparaphenylene ([12]CPP) and perfluoro[12]cycloparaphenylene (PF[12]CPP). The formed complexes were computationally studied via dispersion-corrected density functional theory calculations. The results showed that the deformation rate of PF[12]CPP after the formation of the fullerene-containing complexes was significantly smaller than that of [12]CPP. The binding energy and thermodynamic information showed that PF[12]CPP was more suitable for fullerene encapsulation. Moreover, charge population analysis showed that PF[12]CPP transferred more electrons to Sc₂C₂@C_2n (n = 40, 41, and 42) compared with [12]CPP. Energy decomposition and real-space function analyses of host-guest interactions revealed the characteristics and nature of the noncovalent interactions in the supramolecules. These results provide theoretical support for the study of host-guest systems based on metallofullerenes.

Successive Diels-Alder Cycloadditions of Cyclopentadiene to [10]CPP⊃C60: A Computational Study

Fullerenes have potential applications in many fields. To reach their full potential, fullerenes have to be functionalized. One of the most common reactions used to functionalize fullerenes is the Diels–Alder cycloaddition. In this case, it is important to control the regioselectivity of the cycloaddition during the formation of higher adducts. In C₆₀, successive Diels–Alder cycloadditions lead to the T_h-symmetric hexakisadduct. In this work, we explore computationally using density functional theory (DFT) how the presence of a [10]cycloparaphenylene ring encapsulating C₆₀ ([10]CPP⊃C₆₀) affects the regioselectivity of multiple additions to C₆₀. Our results show that the presence of the [10]CPP ring changes the preferred sites of cycloaddition compared to free C₆₀ and leads to the formation of the tetrakisadduct. Somewhat surprisingly, our calculations predict formation of this particular tetrakisadduct to be more favored in [10]CPP⊃C₆₀ than in free C₆₀.

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.

The relative position of π-π interacting rings notably changes the nature of the substituent effect

The substituent effect in monosubstituted benzene dimers mostly follows changes on electrostatics mainly controlled by the direct interaction of the substituent and the other phenyl ring, whereas the contribution from the interacting rings is smaller. As the substituent is located further away the two contributions become of similar magnitude, so the global result is a combination of both effects. These trends are confirmed in larger systems containing a contact between phenyl rings; at closer distances the interaction of the substituent and the other ring clearly dominates over changes associated with the substituted ring, but as the substituent is located further away its contribution decreases and the contribution from the ring becomes more relevant. Care should be taken in larger systems because the observed energy change can also be affected by interactions with other regions of the molecule not directly involved in the π-π interaction.

Curvature and size effects hinder halogen bonds with extended π systems

The curvature of aromatic systems strengthens the interaction by the concave face while it weakens by the convex one. Parallel structures are favoured over halogen bonded ones.

Newly-designed basket-shaped nanocarbon materials as strong and universal fullerene receptors

DFT calculations were performed to study the host–guest chemistry of a new class of basket-shaped fullerene receptors with strong binding energies and flexible carbon skeletons.

Linear, Non-Conjugated Cyclic and Conjugated Cyclic Paraphenylene under Pressure

The n-paraphenylene family comprises chains of phenylene units linked together by C-C bonds that are between single- and double-bonded, and where n corresponds to the number of phenylene units. In this work, we compare the response of the optical properties of different phenylene arrangements. We study linear chains (LPP), cyclic systems (CPPs), and non-conjugated cyclic systems with two hydrogenated phenylenes (H₄[n]CPP). Particularly, the systems of interest in this work are [6]LPP, [12]- and [6]CPP and H₄[6]CPP. This work combines Raman and infrared spectroscopies with absorption and fluorescence (one- and two-photon excitations) measured as a function of pressure up to maximum of about 25 GPa. Unprecedented crystallographic pressure-dependent results are shown on H₄[n]CPP, revealing intramolecular π-π interactions upon compression. These intramolecular interactions justify the H₄[n]CPP singular optical properties with increasing fluorescence lifetime as a function of pressure.

Planar rings in nano-Saturns and related complexes

Nitrogen substitution into the ortho positions in polyaryl macrocycles creates planar or ribbon structures leading to such structures as this nano-Saturn with a flat ring.

Endohedral alkali cations promote charge transfer transitions in complexes of C60 with [10]cycloparaphenylenes

The endohedral alkali cations in M⁺@C₆₀⋯[10]CPP complexes boost the near infrared absorption bands associated with charge transfer from the nanoring to the fullerene.

Assessment of electronic transitions involving intermolecular charge transfer in complexes formed by fullerenes and donor–acceptor nanohoops

Inserting an anthraquinone or tetracyanoanthraquinone unit in cycloparaphenylene nanohoops facilitates intermolecular electron transfer to a fullerene guest.