Fullerene‐Catalyzed Reduction of Azo Derivatives in Water under UV Irradiation

Metal‐free fullerene (C₆₀) was found to be an effective catalyst for the reduction of azo groups in basic aqueous solution under UV irradiation in the presence of NaBH₄. Use of NaBH₄ by itself is not sufficient to reduce the azo dyes without the assistance of a metal catalyst such as Pd and Ag. Experimental and theoretical results suggest that C₆₀ catalyzes this reaction by using its vacant orbital to accept the electron in the bonding orbital of azo dyes, which leads to the activation of the N?N bond. UV irradiation increases the ability of C₆₀ to interact with electron‐donor moieties in azo dyes.     

Isolation and Structural X‐ray Investigation of Perfluoroalkyl Derivatives of Six Cage Isomers of C84

Perfluoroalkylation of a higher fullerene mixture with CF₃I or C₂F₅I, followed by HPLC separation of CF₃ and C₂F₅ derivatives, resulted in the isolation of several C₈₄(R^F)_n (n=12, 16) compounds. Single‐crystal X‐ray crystallography with the use of synchrotron radiation allowed structure elucidation of eight C₈₄(R^F)_n compounds containing six different C₈₄ cages (the number of the C₈₄ isomer is given in parentheses): C₈₄ (23)(C₂F₅)₁₂ ( I ), C₈₄ (22)(CF₃)₁₆ ( II ), C₈₄ (22)(C₂F₅)₁₂ ( III ), C₈₄ (11)(C₂F₅)₁₂ ( IV ), C₈₄ (16)(C₂F₅)₁₂ ( V ), C₈₄ (4)(CF₃)₁₂ ( VI with toluene and VII with hexane as solvate molecules), and C₈₄ (18)(C₂F₅)₁₂ ( VIII ). Whereas some connectivity patterns of C₈₄ isomers (22, 23, 11) had previously been unambiguously confirmed by different methods, derivatives of C₈₄ isomers numbers 4, 16, and 18 have been investigated crystallographically for the first time, thus providing direct proof of the connectivity patterns of rare C₈₄ isomers. General aspects of the addition of R^F groups to C₈₄ cages are discussed in terms of the preferred positions in the pentagons under the formation of chains, pairs, and isolated R^F groups.     

The Exohedral Diels–Alder Reactivity of the Titanium Carbide Endohedral Metallofullerene Ti2C2@D3h‐C78: Comparison with D3h‐C78 and M3N@D3h‐C78 (M=Sc and Y) Reactivity

The chemical functionalization of endohedral (metallo)fullerenes has become a main focus of research in the last few years. It has been found that the reactivity of endohedral (metallo)fullerenes may be quite different from that of the empty fullerenes. Encapsulated species have an enormous influence on the thermodynamics, kinetics, and regiochemistry of the exohedral addition reactions undergone by these species. A detailed understanding of the changes in chemical reactivity due to incarceration of atoms or clusters of atoms is essential to assist the synthesis of new functionalized endohedral fullerenes with specific properties. Herein, we report the study of the Diels–Alder cycloaddition between 1,3‐butadiene and all nonequivalent bonds of the Ti₂C₂@D_3h‐C₇₈ metallic carbide endohedral metallofullerene (EMF) at the BP86/TZP//BP86/DZP level of theory. The results obtained are compared with those found by some of us at the same level of theory for the D_3h‐C₇₈ free cage and the M₃N@D_3h‐C₇₈ (M=Sc and Y) metallic nitride EMFs. It is found that the free cage is more reactive than the Ti₂C₂@D_3h‐C₇₈ EMF and this, in turn, has a higher reactivity than M₃N@D_3h‐C₇₈. The results indicate that, for Ti₂C₂@D_3h‐C₇₈, the corannulene‐type [5, 6] bonds c and f , and the type B [6, 6] bond 3 are those thermodynamically and kinetically preferred. In contrast, the D_3h‐C₇₈ free cage has a preference for addition to the [6, 6] 1 and 6 bonds and the [5, 6] b bond, whereas M₃N@D_3h‐C₇₈ favors additions to the [6, 6] 6 (M=Sc) and [5, 6] d (M=Y) bonds. The reasons for the regioselectivity found in Ti₂C₂@D_3h‐C₇₈ are discussed.     

Sc2O@Td(19151)‐C76: Hindered Cluster Motion inside a Tetrahedral Carbon Cage Probed by Crystallographic and Computational Studies

A new cluster fullerene, Sc₂O@T_d(19151)‐C₇₆, has been isolated and characterized by mass spectrometry, UV/Vis/NIR absorption, ⁴⁵Sc NMR spectroscopy, cyclic voltammetry, and single‐crystal X‐ray diffraction. The crystallographic analysis unambiguously assigned the cage structure as T_d(19151)‐C₇₆, which is the first tetrahedral fullerene cage characterized by single‐crystal X‐ray diffraction. This study also demonstrated that the Sc₂O cluster has a much smaller Sc?O?Sc angle than that of Sc₂O@C_s(6)‐C₈₂ and the Sc₂O unit is fully ordered inside the T_d(19151)‐C₇₆ cage. Computational studies further revealed that the cluster motion of the Sc₂O is more restrained in the T_d(19151)‐C₇₆ cage than that in the C_s(6)‐C₈₂ cage. These results suggest that cage size affects not only the shapes but also the cluster motion inside fullerene cages.     

Self‐Assembly of Fullerene‐Based Janus Particles in Solution: Effects of Molecular Architecture and Solvent

Two molecular Janus particles based on amphiphilic [60]fullerene (C₆₀) derivatives were designed and synthesized by using the regioselective Bingel–Hirsh reaction and the click reaction. These particles contain carboxylic acid functional groups, a hydrophilic fullerene (AC₆₀), and a hydrophobic C₆₀ in different ratios and have distinct molecular architectures: 1:1 (AC₆₀–C₆₀) and 1:2 (AC₆₀–2C₆₀). These molecular Janus particles can self‐assemble in solution to form aggregates with various types of micellar morphology. Whereas vesicular morphology was observed for both AC₆₀–C₆₀ and AC₆₀–2C₆₀ in tetrahydrofuran, in a mixture of N,N‐dimethylformamide (DMF)/water, spherical micelles and cylindrical micelles were observed for AC₆₀–C₆₀ and AC₆₀–2C₆₀, respectively. A mechanism of formation was tentatively proposed based on the effects of molecular architecture and solvent polarity on self‐assembly.     

Capturing the Most‐Stable C56 Fullerene Cage by In Situ Chlorination

The most‐stable ^#916C₅₆ carbon cage has been captured by in situ chlorination during the radio frequency furnace process. The resulting exohedral ^#916C₅₆Cl₁₂ was separated and unambiguously characterized by single crystal X‐ray structure determination. The discovery of ^#916C₅₆ provides evidence for a thermodynamically controlled mechanism of fullerene formation, and on the other hand shows that the in situ chlorination does not remarkably influence the fullerene formation itself but just results in the capture of preformed cages. A detailed analysis of the chlorination pattern of ^#916C₅₆Cl₁₂ reveals the main factors controlling the reactivity of non‐IPR fullerenes. A high degree of aromatization was observed in the remaining π‐system by considering geometric criteria and nucleus‐independent chemical‐shift analysis (NICS). Along with the well‐known stabilization of pentagon pentagon junctions during chlorination, the formation of aromatic islands plays an important role in the stabilization of the fullerene cage and also in the determination of the chlorination pattern. Based on these empirical rules, the preferable addition patterns for non‐IPR fullerene cages can be easily predicted.     

Enhancing Fullerene‐Based Solar Cell Lifetimes by Addition of a Fullerene Dumbbell

Cost‐effective, solution‐processable organic photovoltaics (OPV) present an interesting alternative to inorganic silicon‐based solar cells. However, one of the major remaining challenges of OPV devices is their lack of long‐term operational stability, especially at elevated temperatures. The synthesis of a fullerene dumbbell and its use as an additive in the active layer of a PCDTBT:PCBM‐based OPV device is reported. The addition of only 20 % of this novel fullerene not only leads to improved device efficiencies, but more importantly also to a dramatic increase in morphological stability under simulated operating conditions. Dynamic secondary ion mass spectrometry (DSIMS) and TEM are used, amongst other techniques, to elucidate the origins of the improved morphological stability.     

Encapsulation of Formaldehyde and Hydrogen Cyanide in an Open‐Cage Fullerene

Reaction of C₆₃NO₂(Ph)₂(Py) ( 1 ) with o‐phenylenediamine and pyridine produces a mixture of C₆₃H₄NO₂(Ph)₂(Py)(N₂C₆H₄) ( 2 ) and H₂O@ 2 . Compound 2 is a new open‐cage fullerene containing a 20‐membered heterocyclic orifice, which has been fully characterized by NMR spectroscopy, high‐resolution mass spectrometry, and X‐ray crystallography. The elliptical orifice of 2 spans 7.45 Å along the major axis and 5.62 Å along the minor axis, which is large enough to trap water and small organic molecules. Thus, heating a mixture of 2 and H₂O@ 2 with hydrogen cyanide and formaldehyde in chlorobenzene affords HCN@ 2 and H₂CO@ 2 , respectively. The ¹H NMR spectroscopy reveals substantial upfield shifts for the endohedral species (δ=?1.30 to ?11.30 ppm), owing to the strong shielding effect of the fullerene cage.     

Incarceration of Higher‐Order Fullerenes within Cyclotriveratrylene‐Based Hemicarcerands Allows Selective Isolation of C76, C78, and C84 from a Commercial Fullerene Mixture

Size‐complementary cyclotriveratrylene (CTV)‐based hosts can incarcerate C₇₆, C₇₈, and C₈₄, thus allowing the selective isolation of these higher‐order fullerenes from a commercially available mixture of fullerenes. The hemicarceplexes, formed after the encapsulation of the size‐complementary fullerenes within the hosts, are isolated by column chromatography and released at elevated temperature, thereby leading to the isolation of C₇₆/C₇₈ and C₈₄ in good purities (up to 95 and 88 %, respectively).     

Tightly Bound Double‐Caged [60]Fullerene Derivatives with Enhanced Solubility: Structural Features and Application in Solar Cells

A series of novel highly soluble double‐caged [60]fullerene derivatives were prepared by means of lithium‐salt‐assisted [2+3] cycloaddition. The bispheric molecules feature rigid linking of the fullerene spheres through a four‐membered cycle and a pyrrolizidine bridge with an ester function CO₂R (R=n ‐decyl, n ‐octadecyl, benzyl, and n ‐butyl; compounds 1 a – d , respectively), as demonstrated by NMR spectroscopy and X‐ray diffraction. Cyclic voltammetry studies revealed three closely overlapping pairs of reversible peaks owing to consecutive one‐electron reductions of fullerene cages, as well as an irreversible oxidation peak attributed to abstraction of an electron from the nitrogen lone‐electron pair. Owing to charge delocalization over both carbon cages, compounds 1 a – d are characterized by upshifted energies of frontier molecular orbitals, a narrowed bandgap, and reduced electron‐transfer reorganization energy relative to pristine C₆₀. Neat thin films of the n ‐decyl compound 1 a demonstrated electron mobility of (1.3±0.4)×10⁻³ cm² V⁻¹ s⁻¹, which was comparable to phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and thus potentially advantageous for organic solar cells (OSC). Application of 1 in OSC allowed a twofold increase in the power conversion efficiencies of as‐cast poly(3‐hexylthiophene‐2,5‐diyl) (P3HT)/ 1 devices relative to the as‐cast P3HT/PCBM ones. This is attributed to the good solubility of 1 and their enhanced charge‐transport properties — both intramolecular, owing to tightly linked fullerene cages, and intermolecular, owing to the large number of close contacts between the neighboring double‐caged molecules. Test P3HT/ 1 OSCs demonstrated power‐conversion efficiencies up to 2.6 % ( 1 a ). Surprisingly low optimal content of double‐caged fullerene acceptor 1 in the photoactive layer (≈30 wt %) favored better light harvesting and carrier transport owing to the greater content of P3HT and its higher degree of crystallinity.     

Fullerene‐Based Macro‐Heterocycle Prepared through Selective Incorporation of Three N and Two O Atoms into C60

A 14‐membered heterocycle is created on the C₆₀ cage skeleton through a multistep procedure. Key steps involve repeated PCl₅‐induced hydroxylamino N?O bond cleavage leading to insertion of nitrogen atoms, and also piperidine‐induced peroxo O?O bond cleavage leading to insertion of oxygen atoms. The hetero atoms form one pyrrole, two pyran, and one diazepine rings in conjunction with the C₆₀ skeleton carbon atoms. The fullerene‐based macrocycle showed unique reactivities towards fluoride ion and copper salts.     

Advantages and Potential of Lipid‐Membrane‐Incorporating Fullerenes Prepared by the Fullerene‐Exchange Method

Lipid‐membrane‐incorporating C₆₀ and C₇₀ (LMIC₆₀ and LMIC₇₀) were prepared by the fullerene‐exchange reaction from the γ‐cyclodextrin cavity to vesicles (we call this method the “exchange method”). An advantage of this method is that the ratios of [C₆₀]/[lipids] and [C₇₀]/[lipids] can be arbitrarily controlled by adjusting the ratios of the fullerenes and liposome. The maximum ratio (30 mol %) obtained was approximately 14 and 100 times higher than those achieved for LMIC₆₀ and LMIC₇₀, respectively, that were prepared by the classical method, which we call the “premixing method” (dissolving lipids and C₆₀ or C₇₀ in chloroform, followed by concentration and extraction with water). Furthermore, the stabilities and photodynamic activities of the LMIC₆₀ and LMIC₇₀ solutions prepared by the exchange method were shown to be much higher than those prepared by the premixing method. That is, the exchange method was found to be superior to the premixing method as a preparative method of LMIC₆₀ and LMIC₇₀ for applications in photomedical and photomaterials chemistry.     

Dynamics of Photoinduced Electron‐Transfer Processes in Fullerene‐Based Dyads: Effects of Varying the Donor Strength

Two classes of fullerene‐based donor–bridge–acceptor (D–B–A) systems containing donors of varying oxidation potentials have been synthesized. These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p‐anisidine/p‐toluidine). In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents. An increase in the rate constant and quantum yield of charge separation (k_cs and Φ_cs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups. This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve. The long‐lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy. The small reorganization energy (λ) of C₆₀ coupled with large negative free energy changes (‐ΔG°) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products.     

New PN Cage Compounds Generated by Small‐Molecule Activation

The isolation of new small‐cage compounds consisting exclusively of Group 15 elements was achieved by exploiting the reactivity of diphosphadiazanediyls. These biradicaloids were used to activate small molecules containing double bonds, such as diazenes and in situ generated diphosphenes, leading to the formation of hexapnicta‐[2.1.1]bicyclohexanes.     

Design and Applications of Protein‐Cage‐Based Nanomaterials

Materials science is beginning to focus on biotemplation, and in support of that trend, it is realized that protein cages—proteins that assemble from multiple monomers into architectures with hollow interiors—can instill a number of unique advantages to nanomaterials. In addition, the structural and functional plasticity of many protein‐cage systems permits their engineering for specific applications. In this review, the most commonly used viral and non‐viral protein cages, which exhibit a wide diversity of size, functionality, and chemical and thermal stabilities, are described. Moreover, how they have been exploited for nanomaterial and nanotechnology applications is summarized.