A 2:1 Receptor/C60 Complex as a Nanosized Universal Joint

Buckycatcher II, a C₅₁H₂₄ hydrocarbon with two corannulene pincers on a dibenzonorbornadiene tether, exhibits an affinity toward C₆₀ in organic solvents that is dramatically higher than the original buckycatcher C₆₀H₂₈ and other corannulene‐based molecular receptors for fullerenes. In addition to the formation of an usual 1:1 C₆₀@catcher inclusion complex, a trimeric C₆₀@(catcher)₂ assembly is detected in solutions and in the solid state. X‐ray structure determination reveals a remarkable “universal joint” solvent‐free crystal arrangement of the trimer, with a single fullerene cage wrapped by four corannulene subunits of two cooperating catcher receptors.     

Fluorinated and Trifluoromethylated Corannulenes

The syntheses and properties of corannulenes carrying electron‐withdrawing groups (F, CF₃, C₆F₅) are reported. Direct fluorination of corannulene (C₂₀H₁₀) was carried out with xenon difluoride, and the crystal structure of the product was confirmed by the X‐ray analysis. Novel trifluoromethylated corannulenes, including the versatile 4,9‐dibromo‐1,2‐bis(trifluoromethyl)corannulene, were obtained by various established ring‐closing reactions. Besides the use of hexafluorobutyne for the construction of fluoranthenes by Diels–Alder reaction as precursor molecules to form 1,2‐disubstituted corannulenes, bis(pentafluorophenyl)acetylene was employed as dienophile. The molecular structure and crystal packing of a trifluoromethylated corannulene was determined by single‐crystal X‐ray analysis and compared with those known brominated and trifluoromethylated corannulenes. The general electron‐acceptor properties of corannulenes bearing substituents introduced in particular positions by liquid‐phase synthesis are discussed together with published computational results.     

Bowl-Assisted Ball Assembly for Solvent-Processing the C60 Electron Transport Layer of High-Performance Inverted Perovskite Solar Cell

Pristine fullerene C₆₀ is an excellent electron transport material for state-of-the-art inverted structure perovskite solar cells (PSCs), but its low solubility leaves thermal evaporation as the only method for depositing it into a high-quality electron transport layer (ETL). To address this problem, we herein introduce a highly soluble bowl-shaped additive, corannulene, to assist in C₆₀-assembly into a smooth and compact film through the favorable bowl-ball interaction. Our results show that not only corannulene can dramatically enhance the film formability of C₆₀, it also plays a critical role in forming C₆₀-corannulene (CC) supramolecular species and in boosting intermolecular electron transport dynamics in the ETL. This strategy has allowed CC devices to deliver high power conversion efficiencies up to 21.69 %, which is the highest value among the PSCs based on the solution-processed-C₆₀ (SP-C₆₀) ETL. Moreover, the stability of the CC device is far superior to that of the C₆₀-only device because corannulene can retard and curb the spontaneous aggregation of C₆₀. This work establishes the bowl-assisted ball assembly strategy for developing low-cost and efficient SP-C₆₀ ETLs with high promise for fully-SP PSCs.     

Probing the binding sites and coordination limits of buckybowls in a solvent-free environment: Experimental and theoretical assessment

This work overviews the coordination properties of fullerene fragments or buckybowls, a new class of open geodesic polyaromatic hydrocarbons that map onto the surface of fullerenes but lack their full closure. In contrast to fullerenes, the bowl-shaped polyarenes have both the convex and concave unsaturated carbon surfaces open and available for coordination, which makes them unique and interesting π-ligands for metal binding reactions. Variable synthetic methods based on solution and solid-state reactions have been developed to access transition metal complexes of buckybowls and to reveal their coordination properties. These studies have been mainly focused on the smallest fragments of the C₆₀-fullerene, corannulene (C₂₀H₁₀) and sumanene (C₂₁H₁₂). In order to utilize directional metal–π-arene interactions effectively and to differentiate π-bonding sites of buckybowls, we have introduced a micro-scale gas-phase deposition method. We have proven this technique to be very effective for the preparation of metal complexes of buckybowls in a single crystalline form. Plus, it allowed us to expand the coordination studies to larger bowls, including dibenzo[a,g]corannulene (C₂₈H₁₄), monoindenocorannulene (C₂₆H₁₂), and the C₃-symmetric hemifullerene (C₃₀H₁₂). Specifically, several dimetal core complexes of varied electrophilicity have been used in the gas-phase coordination reactions to identify the preferential binding sites and to test the coordination limits of π-bowls in a solvent-free environment. The coordination preferences of several buckybowls having different surface area and bowl depth, as well as different curvature and strain are compared and discussed here based on the results of X-ray diffraction and DFT calculation studies.     

Compressed Corannulene in a Molecular Cage

Self‐assembled coordination cages can be employed as a molecular press, where the bowl‐shaped guest corannulene (C₂₀H₁₀) is significantly flattened upon inclusion within the hydrophobic cavity. This is demonstrated by the pairwise inclusion of corannulene with naphthalene diimide as well as by the dimer inclusion of bromocorannulene inside the box‐like host. The compressed corannulene structures are unambiguously revealed by single‐crystal X‐ray analysis.     

Quantum molecular dynamics simulations of fullerenes and graphitic microtubules

We describe the results of extensiveab initio molecular dynamics calculations of the properties of fullerenes and microtubules. Our finite temperature quantum MD simulations for solid C₆₀ are in excellent agreement with NMR, photoemission and neutron scattering data. The C₆₀ isomer containing two pairs of adjacent five-fold rings has a binding energy only 1.6 eV smaller than that of perfect C₆₀, but the transformation between these two structures is hindered by a 5.4 eV barrier. It thus requires high temperatures and long annealing times. High temperatures are also needed for the transformation of the lowest energy C₂₀ isomer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C₆₀. The corannulene structure is a natural precursor for the formation of C₆₀. Simulations of reactions show that C₂ can insert into C₅₈, perfect C₆₀, and defect C₆₀ fullerenes without an activation barrier, while C₃ attaches only to their surfaces. Evaporative fragmentation of carbon clusters during annealing is unlikely, but atom and fragment exchange during collision favor "locally" most stable structures, such as C₆₀. These results may explain the large increase in the abundance of C₆₀ and C₇₀ when carbon clusters are annealed at high density. We have also carried out calculations for paradigmatic microtubules, both reflection-symmetric and chiral. We find that the optimized geometries of the tubules are close to the ideal ones. It is possible to fabricate tubules with direct band gaps away from the Γ point by exploiting the similarities between the projected band structure of graphite and that of the tubule. The semiconducting tubules can be doped n- and p-type by substitutional N and B, respectively.     

Electrochemical behavior of heterometallic chalcogenide clusters

The electrochemical properties of sixteen tri- and tetranuclear chalcogenide-bridged heterometallic clusters in comparison with the tetrahedral clusters Cp′₄M₄S₄ (M = Cr, V; Cp′ = CH₃C₅H₄) simulating ferredoxins were studied. For complexes with μ₄-coordination of the chalcogen, only reduction processes involving the metal-heterometal bonds are reversible. For complexes with μ₃-coordination of the chalcogen, the oxidation processes are reversible except for triiron-chalcogen-pnicogenide clusters having an easily and reversibly oxidizable E-Fe(CO)₂C₅H₄Bu-t bond (E = Sb, Bi) at the pnicogen atom. The electron-compensating role of the lone pair at the bridging chalcogen atom in the stabilization of the oxidation products of the clusters is discussed.     

A Hybrid of Corannulene and Azacorannulene: Synthesis of a Highly Curved Nitrogen‐Containing Buckybowl

A highly curved nitrogen‐containing buckybowl, which can be considered as a corannulene/azacorannulene hybrid, was synthesized and characterized. This molecule has a polycyclic aromatic C₄₀N core, corresponding to a partial azafullerene structure C_80−xN_x (x=1,2,3…), and exhibits interesting properties that arise from its large and highly curved π surface and the embedded nitrogen atom, which include association with C₆₀, a lower LUMO level relative to azapentabenzocorannulene, and the formation of a radical cationic species upon oxidation.     

Monoreduced 1,2‐dihydrocorannulene versus the parent corannulene

The monoanion of dihydrogenated corannulene isolated in the form of its potassium salt, namely tris(diglyme‐κ³O,O′,O′′)potassium hexacyclo[11.5.2.0^4,17.0^7,16.0^10,15.0^14,18]icosa‐1,3,5,7(16),8,10(15),11,13,17‐nonaenide, [K(C₆H₁₄O₃)₃](C₂₀H₁₂), has been structurally characterized for the first time. The X‐ray study confirms the previous NMR spectroscopic prediction that the two H atoms are attached to the same six‐membered ring to form 1,2‐dihydrocorannulene, thus destroying the aromaticity of only one arene ring of the corannulene core. The direct comparison of (C₂₀H₁₂)⁻ with the parent corannulene anion, (C₂₀H₁₀)⁻, is provided to illustrate the geometry perturbations caused by rim hydrogenation.     

Buckycatcher polymer versus fullerene‐buckycatcher complex: Which is stronger?

The C₆₀H₂₈ buckycatcher (BC) is an excellent host for fullerenes. This receptor features two corannulene pincers which trap C₆₀/C₇₀ via π stacking interactions. Although, the formation of C₆₀@C₆₀H₂₈ complexes is readily observed, the dimerization of C₆₀H₂₈ is not a competitive process, even at high concentrations. By means of first principle calculations, we have studied the thermodynamics of the polymerization of BCs and the formation of fullerene complexes. The results obtained with the M06‐2X, B97‐D, B3LYP‐D3BJ, PBE‐D2, and PBE‐D3 functionals indicated that the interaction energy of (C₆₀H₂₈)₂ is larger than the one computed for C₆₀@C₆₀H₂₈, by 8–10 kcal/mol. Because of the greater number of atoms, and due to the presence of more hydrogens, the inclusion of free energy corrections lowers the energetic separation between (C₆₀H₂₈)₂ and C₆₀@C₆₀H₂₈, even though the dimer maintains its position as being slightly more bound than that of the C₆₀@C₆₀H₂₈. Our calculations indicated that up to the C₆₀H₂₈ trimer could be formed with a free energy change larger than that corresponding to the dimerization and fullerene complexation processes. Finally, we found that the inversion of the corannulene pincers attached to the cyclooctatetraene core is 2–3 kcal/mol lower than that corresponding to free corannulene. We expect that this work can motivate new investigations that may lead to the observation of C₆₀H₂₈ polymers. © 2015 Wiley Periodicals, Inc.     

IR,Raman, and UV/Vis Spectra of Corannulene for Use in Possible Interstellar Identification

The spectroscopic characterization of corannulene (C₂₀H₁₀) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography‐mass spectrometry (GC‐MS). During a high‐performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time‐dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium.     

Synthesis and Properties of 2,3‐Dihydro‐1H‐corannuleno[2,3‐cd]pyridine (=2,3‐Dihydro‐1H‐dibenzo[1,10 : 6,7]fluorantheno[3,4‐cd]pyridine) Derivatives: Heterocyclic peri‐Anellated Corannulenes

The anellation of a 6‐membered ring to the 2,3‐position of corannulene (=dibenzo[ghi,mno]fluoranthene; 1 ) leads to curved aromatic compounds with a significantly higher bowl‐inversion barrier than corannulene (see Fig. 1). If the bridge is −CH₂−NR−CH₂−, a variety of linkers can be introduced at the N(2) atom, and the corresponding curved aromatics act as versatile building blocks for larger structures (see Scheme). The locked bowl, in combination with an amide bond (see 9 and 10 ), gives rise to corannulene derivatives with chiral ground‐state conformations, which possess the ability to adapt to their chiral environment by shifting their enantiomer equilibrium slightly in favor of one enantiomeric conformer. Rim annulation of corannulene seems to display a significantly lower electron‐withdrawing effect than facial anellation on [5,6]fullerene‐C₆₀‐I_h, as determined by an investigation of the basicity at the N‐atom of CH₂−NR−CH₂ (see 4 vs. 15 in Fig. 2).     

Synthesis of an Octacyclic C60 Fragment

Fragments of buckminsterfullerene (C₆₀) include the monumental three compounds corannulene, sumanene, and truxene. These three have served as leading molecules in ongoing research for curved, fused, and π-extended polyaromatic materials. Achieving more structural variations that join the ranks of these three archetypes remains challenging. Herein we report synthesis of an octacyclic hydrocarbon that is an unexplored C₆₀-fragment, namely, a 4,11-dihydrodiindeno[7,1,2-ghi:7′,1′,2′-pqr]chrysene (C₂₈H₁₆, which we named Metelykene). The key to success was solution-compatible synthesis in which double pentagonal rings flank hexagonal ones. This solution-phase approach, coupled with the resulting non-planar π-conjugation, is so straightforward that it offers an entry to a derivative such as a cardo aromatic monomer.     

Functionalized Corannulene Carbocations: A Structural Overview

A detailed structural overview of a family of bowl‐shaped polycyclic aromatic carbocations of the type [C₂₀H₁₀R]⁺ with different R functionalities tethered to the interior surface of corannulene (C₂₀H₁₀) is provided. Changing the identity of the surface‐bound groups through alkyl chains spanning from one to four carbon atoms and incorporating a different degree of halogenation has led to the fine tuning of the bowl structures and properties. The deformation of the corannulene core upon functionalization has been revealed based on X‐ray crystallographic analysis and compared for the series of cations with R=CH₃, CH₂Cl, CHCl₂, CCl₃, CH₂CH₃, CH₂CH₂Cl, and CH₂CH₂Br. The resulting carbocations have been isolated with several metal‐based counterions, varying in size and coordinating abilities ([AlCl₄]^?, [AlBr₄]^?, [(SnCl)(GaCl₄)₂]^?, and [Al(OC(CF₃)₃)₄]^?). A variety of aggregation patterns in the solid state has been revealed based on different intermolecular interactions ranging from cation–anion to π–π stacking and to halogen???π interactions. For the [C₂₀H₁₀CH₂Cl]⁺ ion crystallized with several different counterions, the conformation of the R group attached to the central five‐membered ring of corannulene moiety was found to depend on the solid‐state environment defined by the identity of anions. Solution NMR and UV/Vis investigations have been used to complement the X‐ray diffraction studies for this series of corannulene‐based cations and to demonstrate their different association patterns with the solvent molecules.     

Spontaneous H2 Loss through the Interaction of Squaric Acid Derivatives and BeH2

The most stable complexes between squaric acid and its sulfur‐ and selenium‐containing analogues (C₄X₄H₂; X=O, S, Se) with BeY₂ (Y=H, F) were studied by means of the Gaussian 04 (G4) composite ab initio theory. Squaric acid derivatives are predicted to be very strong acids in the gas phase; their acidity increases with the size of the chalcogen, with C₄Se₄H₂ being the strongest acid of the series and stronger than sulfuric acid. The relative stability of the C₄X₄H₂ ? BeY₂ (X=O, S, Se; Y=H, F) complexes changes with the nature of the chalcogen atom; but more importantly, the formation of the C₄X₄H₂ ? BeF₂ complexes results in a substantial acidity enhancement of the squaric moiety owing to the dramatic electron‐density redistribution undergone by the system when the beryllium bond is formed. The most significant consequence of this acidity enhancement is that when BeF₂ is replaced by BeH₂, a spontaneous exergonic loss of H₂ is observed regardless of the nature of the chalcogen atom. This is another clear piece of evidence of the important role that closed‐shell interactions play in the modulation of physicochemical properties of the Lewis acid and/or the Lewis base.     

Mass spectrometric fragmentation of saturated six membered heterocyclic systems containing two chalcogen family atoms

The mass spectra of six-membered saturated heterocycles containing oxygen, sulphur, selenium and tellurium in the 1,4-positions have been measured. The differing fragmentation modes have been characterized using high resolution, low voltage and metastable ion scan techniques. The important decomposition reactions of the molecular ions involve elimination of C₂H₄ and CH₂X (X is a chalcogen atom) and formation of [C₂H₄X]⁺ and C₂H₅⁺. The propensities of these reactions vary systematically as a function of the ability of the chalcogen to stabilize a positive charge.     

Single and Double Ionization of Corannulene and Coronene

Electron‐transfer processes that involve single and doubly charged cations of corannulene (C₂₀H₁₀) and coronene (C₂₄H₁₂) are examined by three different mass‐spectrometric techniques. Photoionization studies give first‐ionization energies of IE(C₂₀H₁₀)=7.83±0.02 eV and IE(C₂₄H₁₂)=7.21 ±0.02 eV. Photoionizations of the neutrals to the doubly charged cations occur at thresholds of 20.1±0.2 eV and 18.5±0.2 eV for corannulene and coronene, respectively. Energy‐resolved charge‐stripping mass spectrometry yields kinetic energy deficits of Q_min(C₂₀H=13.8±0.3 eV and Q_min(C₂₄H=12.8±0.3 eV for the transitions from the mono‐ to the corresponding dications in keV collisions. Reactivity studies of the C₂₀H and C₂₄H dications in a selected‐ion flow‐tube mass spectrometer are used to determine the onsets for the occurrence of single‐electron transfer from several neutral reagents to the dications, affording two different monocationic products. With decreasing IEs of the neutral reagents, electron transfer to doubly charged corannulene is first observed with hexafluorobenzene (IE=9.91 eV), while neutrals with lower IEs are required in the case of the coronene dication, e.g., NO₂ (IE=9.75 eV). Density‐functional theory is used to support the interpretation of the experimental data. The best estimates of the ionization energies evaluated are IE(C₂₀H₁₀)=7.83±0.02 eV and IE(C₂₄H₁₂)=7.21 ±0.02 eV for the neutral molecules, and IE(C₂₀H)=12.3±0.2 eV and IE(C₂₄H)=11.3±0.2 eV for the monocations.     

Homolytic Versus Heterolytic Bond Breaking in Functionalized [R-C20H10]+ Systems

The comprehensive theoretical investigation of stability of functionalized corannulene cations [R-C₂₀H₁₀]⁺ with respect to two alternative bond-breaking mechanisms, namely, homolytic or radical ([R-C₂₀H₁₀]⁺ → R^• + C₂₀H₁₀^+•) and heterolytic or cationic ([R-C₂₀H₁₀]⁺ → R⁺ + C₂₀H₁₀), was accomplished. The special focus was on the influence of the nature of R-group on the energetics of the bond cleavage. Detailed study of energetics of both mechanisms has revealed that the systems with small alkyl groups such as methyl tend to undergo bond breaking in accordance with homolytic mechanism. Subsequent elongation of the chain of the R-group resulted in shifting the paradigm, making heterolytic path more energetically favorable. Subsequent analysis of different components of the bonding between R-group and corannulene polyaromatic core helped to shed light on trends observed. In both mechanisms, the covalent contribution was found to be dominating, whereas ionic part contributes ~25–27%. Two leading components of ΔE_orb, C₂₀H₁₀ → R and R → C₂₀H₁₀, were identified with NOCV-EDA approach. While the homolytic pathway is best described as R → C₂₀H₁₀ process, the heterolytic mechanism shows domination of the C₂₀H₁₀ → R term. Surprisingly, the preparation energy (ΔE_prep) was identified as a key player in stability tendencies found. In other words, the relative stability of corresponding molecular fragments (here R-groups as the corannulene fragment remains the same for all systems) in their cationic or radical forms determine the preference given to a specific bond breaking path and, as consequence, the total stability of target functionalized cations. These conclusions were further confirmed by extending a set of R-groups to conjugated (allyl, phenyl), bulky (iPr, tBu), β-silyl (CH₂SiH₃, CH₂SiMe₃), and benzyl (CH₂Ph) groups. © 2019 Wiley Periodicals, Inc.     

Palladium π-adduct of corannulene

The ternary palladium π-adduct of corannulene and benzene, [Pd₆Cl₁₂·(C₂₀H₁₀)₂·(C₆H₆)₂]·C₆H₆ (1), has been prepared by reacting the cubic Pd₆Cl₁₂-cluster with C₂₀H₁₀ in benzene. It was structurally characterized to reveal η¹-binding of Pd₆Cl₁₂ to a hub C-atom of the convex surface of corannulene (Pd?C, 3.085(3) Å) and its η⁶-complexation to benzene (Pd?C_centroid, 3.431(3) Å). The behavior and persistence of 1 in some aromatic solvents has been revealed by UV-vis and ¹H NMR spectroscopy studies.     

Estimation of the electron affinities of C60, corannulene,and coronene by using the kinetic method

Novel anions that contain one molecule each of C₆₀ and the polycyclic aromatic hydrocarbon coronene are generated in the gas phase by electron attachment desorption chemical ionization. Collision-induced dissociation reveals that these cluster ions are loosely bonded. Fragmentation of the mass-selected cluster anion yields, as the only products, the intact radical anions of the constituent molecules, namely, the C₆₀ radical anion and the coronene radical anion, in almost identical relative abundances. This result is interpreted as evidence that the cluster ion can be considered as the anion radical of one molecule solvated by the other molecule. The known very high electron affinity of C₆₀ (2.66 eV) and the comparable degree to which C₆₀ and the PAH compete for the electron suggests that dissociation may be controlled by the electron affinity of a portion of the C₆₀ surface, that is, in this case the kinetic method yields information on the local electron affinity of C₆₀. The electron affinity of the bowl-shaped compound corannulene is estimated for the first time to be 0.50 ± 0.10 eV by the kinetic method by using a variety of reference compounds. Unlike coronene, corannulene reacts with C _?? ⁶⁰ in the gas phase to form a covalently bonded, denydrogenated cluster ion. Support for the concept of “local” electron affinity of C₆₀ comes from a theoretical calculation on the electronic structure of C₆₀ anions, which shows evidence for localization of the charge in the C₆₀ molecule. The possibility of electron tunneling in the C₆₀-coronene system is discussed as an alternative explanation for the unusual observation of equal abundances of C₆₀ anions and coronene anions upon dissociation of the corresponding cluster ion.