Two Metastable Endohedral Metallofullerenes Sc2C2@C1(39656)-C82 and Sc2C2@C1(51383)-C84: Direct-C2-Insertion Products from Their Most Stable Precursors.

Endohedral metallofullerenes (EMFs) are sub-nano carbon materials with diverse applications, yet their formation mechanism, particularly for metastable isomers, remains ambiguous. The current theoretical methods focus mainly on the most stable isomers, leading to limited predictability of metastable ones due to their low stabilities and yields. Herein, we report the successful isolation and characterization of two metastable EMFs, Sc2C2@C1(39656)-C82 and Sc2C2@C1(51383)-C84, which violate the isolated pentagon rule (IPR). These two non-IPR EMFs exhibit a rare case of planar and pennant-like Sc2C2 clusters, which can be considered hybrids of the common butterfly-shaped and linear configurations. More importantly, the theoretical results reveal that despite being metastable, these two non-IPR EMFs survived as the products from their most stable precursors, Sc2C2@C2v(5)-C80 and Sc2C2@Cs(6)-C82, via a C2 insertion during the post-formation annealing stages. We propose a systematic theoretical method for predicting metastable EMFs during the post-formation stages. The unambiguous molecular-level structural evidence, combined with the theoretical calculation results, provides valuable insights into the formation mechanisms of EMFs, shedding light on the potential of post-formation mechanisms as a promising approach for EMF synthesis.

Self-driven carbon atom implantation into fullerene embedding metal-carbon cluster.

Hundreds of members have been synthesized and versatile applications have been promised for endofullerenes (EFs) in the past 30 y. However, the formation mechanism of EFs is still a long-standing puzzle to chemists, especially the mechanism of embedding clusters into charged carbon cages. Here, based on synthesis and structures of two representative vanadium-scandium-carbido/carbide EFs, VSc2C@Ih (7)-C80 and VSc2C2@Ih (7)-C80, a reasonable mechanism-C1 implantation (a carbon atom is implanted into carbon cage)-is proposed to interpret the evolution from VSc2C carbido to VSc2C2 carbide cluster. Supported by theoretical calculations together with crystallographic characterization, the single electron on vanadium (V) in VSc2C@Ih (7)-C80 is proved to facilitate the C1 implantation. While the V=C double bond is identified for VSc2C@Ih (7)-C80, after C1 implantation the distance between V and C atoms in VSc2C2@Ih (7)-C80 falls into the range of single bond lengths as previously shown in typical V-based organometallic complexes. This work exemplifies in situ self-driven implantation of an outer carbon atom into a charged carbon cage, which is different from previous heterogeneous implantation of nonmetal atoms (Group-V or -VIII atoms) driven by high-energy ion bombardment or high-pressure offline, and the proposed C1 implantation mechanism represents a heretofore unknown metal-carbon cluster encapsulation mechanism and can be the fundamental basis for EF family genesis.

Carbene Addition Isomers of C70 formed in the Flame of Low-Pressure Combustion.

In the flames during low-pressure combustion, not only a rich variety of fullerenes but also many reactive intermediates can be produced (e.g., carbene, CH2) that are short-lived and cannot be stabilized directly under normal circumstances. These intermediates can be captured by fullerene carbon cages for stabilization. In this paper, three C71H2 isomers were synthesized in situ in low-pressure benzene-acetylene-oxygen diffusion flame combustion. The results, which were unambiguously characterized by single-crystal X-ray diffraction, show that the three isomers are carbene addition products of D5h-C70 on different sites. The relative energies and stability of different C71H2 isomers are revealed by Ultraviolet-Visible (UV-Vis) absorption spectroscopy, in combination with theoretical calculations, in this work. Both the in situ capture and theoretical study of these C71H2 isomers in low-pressure combustion will provide more information regarding carbene additions to other fullerenes or other carbon clusters at high temperatures.

Capturing nonclassical C70 with double heptagons in low-pressure combustion.

A double-heptagon-containing C70H6 (dihept-C70H6) was isolated and unambiguously characterized in the soot of low-pressure combustion, which shares the identical heptagonal cage as dihept-C70Cl6 previously identified in the products of carbon arc, and thus represents the first nonclassical fullerene isolable in both carbon arc and combustion.

Surface Re-Engineering of Perovskites with Buckybowls to Boost the Inverted-Type Photovoltaics.

Despite their multifaceted advantages, inverted perovskite solar cells (PSCs) still suffer from lower power conversion efficiencies (PCEs) than their regular counterparts, which is largely due to recombination energy losses (Eloss) that arise from the chemical, physical, and energy level mismatches, especially at the interfaces between perovskites and fullerene electron transport layers (ETLs). To address this problem, we herein introduce an aminium iodide derivative of a buckybowl (aminocorannulene) that is molecularly layered at the perovskite-ETL interface. Strikingly, besides passivating the PbI2-rich perovskite surface, the aminocorannulene enforces a vertical dipole and enhances the surface n-type character that is more compatible with the ETL, thus boosting the electron extraction and transport dynamics and suppressing interfacial Eloss. As a result, the champion PSC achieves an excellent PCE of over 22%, which is superior compared to that of the control device (∼20%). Furthermore, the device stability is significantly enhanced, owing to a lock-and-key-like grip on the mobile iodides by the buckybowls and the resultant increase of the interfacial ion-migration barrier. This work highlights the potential of buckybowls for the multifunctional surface engineering of perovskite toward high-performance and stable PSCs.

Synthesis and Fluorescent Properties of Multi-Functionalized C70 Derivatives of C70(OCH3)10[C(COOEt)2] and C70(OCH3)10[C(COOEt)2]2.

Due to the partially reduced π-conjugation of the fullerene cage, multi-functionalized fullerene derivatives exhibit remarkable fluorescent properties compared to pristine fullerenes, which have high potential for application in organic light-emitting diodes (OLEDs). In this study two multi-functionalized C70 derivatives, C70(OCH3)10[C(COOEt)2] and C70(OCH3)10[C(COOEt)2]2, with excellent fluorescence properties, were designed and synthesized. Compared with C70(OCH3)10 containing a single kind of functional group, both the C70(OCH3)10[C(COOEt)2] and C70(OCH3)10[C(COOEt)2]2 exhibited enhanced fluorescence properties with blue fluorescence emission. The fluorescence quantum yields of the C70(OCH3)10[C(COOEt)2] and C70(OCH3)10[C(COOEt)2]2 were 1.94% and 2.30%, respectively, which were about ten times higher than that of C70(OCH3)10. The theoretical calculations revealed that the multi-functionalization of the C70 increased the S1-T1 energy gap, reducing the intersystem crossing efficiency, resulting in the higher fluorescence quantum yield of the C70 derivatives. The results indicate that multi-functionalization is a viable strategy to improve the fluorescence of fullerene derivatives.

Ambient-pressure synthesis of ethylene glycol catalyzed by C60-buffered Cu/SiO2.

Bulk chemicals such as ethylene glycol (EG) can be industrially synthesized from either ethylene or syngas, but the latter undergoes a bottleneck reaction and requires high hydrogen pressures. We show that fullerene (exemplified by C60) can act as an electron buffer for a copper-silica catalyst (Cu/SiO2). Hydrogenation of dimethyl oxalate over a C60-Cu/SiO2 catalyst at ambient pressure and temperatures of 180° to 190°C had an EG yield of up to 98 ± 1%. In a kilogram-scale reaction, no deactivation of the catalyst was seen after 1000 hours. This mild route for the final step toward EG can be combined with the already-industrialized ambient reaction from syngas to the intermediate of dimethyl oxalate.

Dissection of bicapped octahedral copper hydride cluster to form two chiral tetrahedral copper hydride cluster series exhibiting auto deracemization and photoluminescence.

Three series of copper hydride clusters [Cu8H6L6]2+ (1), [Cu4HX2L4]+ where X- = Cl- (2a), Br- (2b), I- (2c), N3- (2d) and SCN- (2e), and [Cu4HX3L3] where X- = Br- (3b) and I- (3c) (L = 2-(diphenylphosphino)pyridine, dppy) were synthesized and characterized by single-crystal X-Ray crystallography and standard spectroscopic techniques. The metal core of 1, Cu8, can be described as a bicapped octahedron, while those of 2 and 3 series adopt tetrahedral structures. The hydride positions were deduced from difference electron density maps and corroborated by NMR and DFT calculations. For 1, there are two µ4-H-, one each in the two tetrahedral cavities of the two capping atoms and four µ3-H- on the six triangular faces around the waist of the octahedron. For [Cu4HX2L4]+ and [Cu4HX3L3] series, the single µ4-H- resides in the center of the Cu4 tetrahedron. It was found that these three series of copper clusters are intimately connected and can convert from one to another under specific reaction conditions. Their transformation pathways were investigated in detail. Spontaneous resolution to form optically pure enantiomeric single crystals was observed for [Cu4H(SCN)2L4]+ (2e) and [Cu4HBr3L3] (3b). Photoluminescence was observed for [Cu4HX2L4]+, as well as [Cu4HX3L3] with strong emissions from green to yellow regions.

Fullertubes: Cylindrical Carbon with Half-Fullerene End-Caps and Tubular Graphene Belts, Their Chemical Enrichment, Crystallography of Pristine C90-D5h(1) and C100-D5d(1) Fullertubes, and Isolation of C108, C120, C132, and C156 Cages of Unknown Structures.

We report a chemical separation method to isolate fullertubes: a new and soluble allotrope of carbon whose structure merges nanotube, graphene, and fullerene subunits. Fullertubes possess single-walled carbon nanotube belts resembling a rolled graphene midsection, but with half-fullerene end-caps. Unlike nanotubes, fullertubes are reproducible in structure, possess a defined molecular weight, and are soluble in pristine form. The high reactivity of amines with spheroidal fullerene cages enables their removal and allows a facile isolation of C96-D3d(3), C90-D5h(1), and C100-D5d(1) fullertubes. A nonchromatographic step (Stage 1) uses a selective reaction of carbon cages with aminopropanol to permit a highly enriched sample of fullertubes. Spheroidal fullerenes are reacted and removed by attaching water-soluble groups onto their cage surfaces. With this enriched (100-1000 times) fullertube mixture, Stage 2 becomes a simple HPLC collection with a single column. This two-stage separation approach permits fullertubes in scalable quantities. Characterization of purified C100-D5d(1) fullertubes is done with samples isolated in pristine and unfunctionalized form. Surprisingly, C60 and C100-D5d(1) are both purplish in solution. For X-ray crystallographic analysis, we used decapyrrylcorannulene (DPC). Isomerically purified C90 and C100 fullertubes were mixed with DPC to obtain black cocrystals of 2DPC{C90-D5h(1)}·4(toluene) and 2DPC{C100-D5d(1)}·4(toluene), respectively. A serendipitous outcome of this chemical separation approach is the enrichment and purification of several unreported larger carbon species, e.g., C120, C132, and C156. Isolation of these higher cage species represents a significant advance in the unknown experimental arena of C100-C200 structures. Our findings represent seminal experimental evidence for the existence of two mathematically predicted families of fullertubes: one family with an axial hexagon with the other series based on an axial pentagon ring. Fullertubes have been predicted theoretically, and herein is their experimental evidence, isolation, and initial characterization.

Hybrid Fullerene-Based Electron Transport Layers Improving the Thermal Stability of Perovskite Solar Cells.

The structure-dependent thermal stability of fullerene electron transport layers (ETLs) and its impact on device stability have been underrated for years. Based on cocrystallographic understanding, herein, we develop a thermally stable ETL comprising a hybrid layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and [6,6]-phenyl-C61-propylbenzene (PCPB). By tuning the weight ratios of PCBM and PCPB to influence the noncovalent intermolecular interactions and packing of fullerene derivatives, we obtained a champion device based on the 20PCPB (20 wt % addition of PCPB into the mixture of PCBM/PCPB) ETL and excellent thermal stability of 500 h under 85 °C thermal aging in a N2 atmosphere in the dark. The present work exemplifies that cocrystallography can be a precise tool to probe the interaction and aggregation of fullerene derivatives in ETLs, and mixed fullerene derivatives can be sought as promising ETLs to enhance the long-term stability of perovskite solar cells under high-temperature working environments.

Rational synthesis of an atomically precise carboncone under mild conditions.

Carboncones, a special family of all-carbon allotropes, are predicted to have unique properties that distinguish them from fullerenes, carbon nanotubes, and graphenes. Owing to the absence of methods to synthesize atomically well-defined carboncones, however, experimental insight into the nature of pure carboncones has been inaccessible. Herein, we describe a facile synthesis of an atomically well-defined carboncone[1,2] (C70H20) and its soluble penta-mesityl derivative. Identified by x-ray crystallography, the carbon skeleton is a carboncone with the largest possible apex angle. Much of the structural strain is overcome in the final step of converting the bowl-shaped precursor into the rigid carboncone under mild reaction conditions. This work provides a research opportunity for investigations of atomically precise single-layered carboncones having even higher cone walls and/or smaller apex angles.

General One-step Synthesis of Symmetrical or Unsymmetrical 1,4-Di(organo)fullerenes from Organo(hydro)fullerenes through Direct Oxidative Arylation.

A general one-step synthesis of symmetrical or unsymmetrical 1,4-di(organo)fullerenes from organo(hydro)fullerenes (RC60H) is realized by direct oxidative arylation. The new combination of catalytic trifluoromethanesulfonic acid (TfOH) and stoichiometric o-chloranil is the first to be used to directly generate an R-C60+ intermediate from common RC60H. Unexpectedly, the in situ generated R-C60+ intermediate is shown to be quite stable in whole 13C NMR spectroscopy characterization in the absence of cation quenching reagents. Because the direct oxidation of common RC60H to form the corresponding R-C60+ has never been realized, the present combination of TfOH and o-chloranil solves the challenges associated with the formation of stable RC60+ cations from common RC60H without any coordination of an R group.

Double Negatively Curved C70 Growth through a Heptagon-Involving Pathway.

All previously reported C70 isomers have positive curvature and contain 12 pentagons in addition to hexagons. Herein, we report a new C70 species with two negatively curved heptagon moieties and 14 pentagons. This unconventional heptafullerene[70] containing two symmetric heptagons, referred to as dihept-C70 , grows in the carbon arc by a theoretically supported pathway in which the carbon cluster of a previously reported C66 species undergoes successive C2 insertion via a known heptafullerene[68] intermediate with low energy barriers. As identified by X-ray crystallography, the occurrence of heptagons facilitates a reduction in the angle of the π-orbital axis vector in the fused pentagons to stabilize dihept-C70 . Chlorination at the intersection of a heptagon and two adjacent pentagons can greatly enlarge the HOMO-LUMO gap, which makes dihept-C70 Cl6 isolable by chromatography. The synthesis of dihept-C70 Cl6 offers precious clues with respect to the fullerene formation mechanism in the carbon-clustering process.

An Unconventional Hydrofullerene C66H4 with Symmetric Heptagons Retrieved in Low-Pressure Combustion.

The combustion has long been applied for industrial synthesis of carbon materials such as fullerenes as well as carbon particles (known as carbon black), but the components and structures of the carbon soot are far from being clarified. Herein, we retrieve an unprecedented hydrofullerene C66H4 from a soot of a low-pressure combustion of benzene-acetylene-oxygen. Unambiguously characterized by single-crystal X-ray diffraction, the C66H4 renders a nonclassical geometry incorporating two heptagons and two pairs of fused pentagons in a C2 v symmetry. The common vertexes of the fused pentagons are bonded with four hydrogen atoms to convert the hydrogen-linking carbon atoms from sp2 to sp3 hybridization, which together with the adjacent heptagons essentially releases the sp2-bond strains on the abutting-pentagon sites of the diheptagonal fused pentagon C66 (dihept-C66). DFT computations suggest the possibility for an in situ hydrogenation process leading to stabilization of the dihept-C66. In addition, the experiments have been carried out to study heptagon-dependent properties of dihept-C66H4, indicating the key responsibility of the heptagon for changing hydrocarbon activity and electronic properties. The present work with the unprecedented double-heptagon-containing hydrofullerene successfully isolated and identified as one of the low-pressure combustion products shows that the heptagon is a new building block for constructing fullerene products in addition to pentagons and hexagons in low-pressure combustion systems.

Flexible decapyrrylcorannulene hosts.

The assembly of spherical fullerenes, or buckyballs, into single crystals for crystallographic identification often suffers from disordered arrangement. Here we show a chiral configuration of decapyrrylcorannulene that has a concave 'palm' of corannulene and ten flexible electron-rich pyrryl group 'fingers' to mimic the smart molecular 'hands' for self-adaptably cradling various buckyballs in a (+)hand-ball-hand(-) mode. As exemplified by crystallographic identification of 15 buckyball structures representing pristine, exohedral, endohedral, dimeric and hetero-derivatization, the pyrryl groups twist with varying dihedral angles to adjust the interaction between decapyrrylcorannulene and fullerene. The self-adaptable electron-rich pyrryl groups, susceptible to methylation, are theoretically revealed to contribute more than the bowl-shaped palm of the corannulene in holding buckyball structures. The generality of the present decapyrrylcorannulene host with flexible pyrryl groups facilitates the visualization of numerous unknown/unsolved fullerenes by crystallography and the assembly of the otherwise close-packed spherical fullerenes into two-dimensional layered structures by intercalation.