Structural Relationships and Theoretical Study of Electron Transfer Properties of 1,3,2-Dithiazolyl Radicals with Fullerenes in Nanostructure [1,3,2-DTA(s)]@Cn Supramolecular Complexes

Various empty carbon fullerenes with different carbon atoms have been obtained and investigated. The dithiazolyl radicals have shown important electron-transfer properties. Topological indices are digital values that are assigned based on chemical composition. These values are purported to correlate chemical structures with various chemical and physical properties. They have been successfully used to construct effective and useful mathematical methods to establish clear relationships between structural data and the physical properties of these materials. In this study, the number of carbon atoms in the fullerenes was used as an index to establish a relationship between the structures of 2,3-naphthalene-1,3,2-dithiazolyl (NDTA), 2,3-quinoxaline-1,3,2-dithiazolyl (QDTA), and 1,2,5-thiazolo[3,4-b]-1,3,2-dithiazolo[3,4-b]pyridazin-2-yl (TDP-DTA), radicals, 1–3 as molecular conductor radicals and fullerenes C_n (n = 60, 70, 76, 82, and 86), which create [1,3,2-DTA(s)]@C_n, A-1 to A-5 (NDTA]@C_n), B-1 to B-5 ([QDTA]@C_n), and C-1 to C-5 ([TDP-DTA]@C_n). The relationship between the number of carbon atoms and the free energies of electron transfer (ΔG_et(1) to ΔG_et(4) ) are assessed using the Rehm–Weller equation for A-1 to A-5, B1 to B-5, and C-1 to C-5 supramolecular [1,3,2-DTA(s)]@C_n complexes. Calculations are presented for the four reduction potentials ( ^Red.E₁ to ^Red.E₄ ) of fullerenes C_n . The results were used to calculate the four free energies of electron transfer (ΔG_et(1) to ΔG_et(4) ) of supramolecular complexes A-1 to A-18, B-1 to B-18, and C-1 to C-18 (5–60) for fullerenes C₆₀ to C₃₀₀.     

Synthesis and Isolation of the Titanium–Scandium Endohedral Fullerenes—Sc2TiC@Ih‐C80, Sc2TiC@D5h‐C80 and Sc2TiC2@Ih‐C80: Metal Size Tuning of the TiIV/TiIII Redox Potentials

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed‐metal Sc–Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH₄ and Ti/CH₄ systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH₄ system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH₄ system are Sc₄C₂@C₈₀ (the most abundant EMF from this synthesis), Sc₃C₂@C₈₀, isomers of Sc₂C₂@C₈₂, and the family Sc₂C_{2 n} (2 n=74, 76, 82, 86, 90, etc.), as well as Sc₃CH@C₈₀. The Sc–Ti/CH₄ system produces the mixed‐metal Sc₂TiC@C_{2 n} (2 n=68, 78, 80) and Sc₂TiC₂@C_{2 n} (2 n=80) clusterfullerene families. The molecular structures of the new, transition‐metal‐containing endohedral fullerenes, Sc₂TiC@I_h‐C₈₀, Sc₂TiC@D_5h‐C₈₀, and Sc₂TiC₂@I_h‐C₈₀, were characterized by NMR spectroscopy. The structure of Sc₂TiC@I_h‐C₈₀ was also determined by single‐crystal X‐ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc₂TiC‐ and Sc₂TiC₂‐containing clusterfullerenes have Ti‐localized LUMOs. Encapsulation of the redox‐active Ti ion inside the fullerene cage enables analysis of the cluster–cage strain in the endohedral fullerenes through electrochemical measurements.     

Bonding in Endohedral Metallofullerenes as Studied by Quantum Theory of Atoms in Molecules

Metal–cage and intracluster bonding was studied in detail by quantum theory of atoms in molecules (QTAIM) for the four major classes of endohedral metallofullerenes (EMFs), including monometallofullerenes Ca@C₇₂, La@C₇₂, M@C₈₂ (M=Ca, Sc, Y, La), dimetallofullerenes Sc₂@C₇₆, Y₂@C₈₂, Y₂@C₇₉N, La₂@C₇₈, La₂@C₈₀, metal nitride clusterfullerenes Sc₃N@C_2n (2n=68, 70, 78, 80), Y₃N@C_2n (2n=78, 80, 82, 84, 86, 88), La₃N@C_2n (2n=88, 92, 96), metal carbide clusterfullerenes Sc₂C₂@C₆₈, Sc₂C₂@C₈₂, Sc₂C₂@C₈₄, Ti₂C₂@C₇₈, Y₂C₂@C₈₂, Sc₃C₂@C₈₀, as well as Sc₃CH@C₈₀ and Sc₄O_x@C₈₀ (x=2, 3), that is, 42 EMF molecules and ions in total. Analysis of the delocalization indices and bond critical point (BCP) indicators such as G_bcp/ρ_bcp, H_bcp/ρ_bcp, and |V_bcp|/G_bcp, revealed that all types of bonding in EMFs exhibit a high degree of covalency, and the ionic model is reasonable only for the Ca‐based EMFs. Metal–metal bonds with negative values of the electron‐density Laplacian were found in Y₂@C₈₂, Y₂@C₇₉N, Sc₄O₂@C₈₀, and anionic forms of La₂@C₈₀. A delocalized nature of the metal–cage bonding results in a topological instability of the electron density in EMFs with an unpredictable number of metal–cage bond paths and large elipticity values.     

Structure,Chirality, and Formation of Giant Icosahedral Fullerenes and Spherical Graphitic Onions

We describe the topology, structure, and stability of giant fullerenes exhibiting various symmetries (I, I _h , D _2h , T). Our results demonstrate that it is possible to create two new families of nested chiral icosahedral (I) fullerenes namely C₂₆₀@C₅₆₀@C₉₈₀@C₁₅₂₀@, ...,and C₁₄₀@C₃₈₀@C₇₄₀@C₁₂₂₀@ ..., which exhibit interlayer separations of ca. 3.4 Å. These chiral fullerenes are thought to possess metalliclike conduction properties. We discuss in detail the transformation of polyhedral graphitic particles into quasispherical nested giant fullerenes by reorganization of carbon atoms, which result in the formation of additional pentagonal and heptagonal carbon rings. These spherical structures are metastable and we believe they could be formed under extreme conditions, such as those produced by high-energy electron irradiation. There is circumstantial experimental evidence for the presence of heptagonal rings within these spherical fullerenes.     

Bingel–Hirsch Addition of Diethyl Bromomalonate to Ion-Encapsulated Fullerenes M@C60 (M=Ø, Li+, Na+, K+, Mg2+, Ca2+, and Cl−)

In the last 30 years, fullerene-based materials have become popular building blocks for devices with a broad range of applications. Among fullerene derivatives, endohedral metallofullerenes (EMFs, M@C_x) have been widely studied owing to their unique properties and reactivity. For real applications, fullerenes and EMFs must be exohedrally functionalized. It has been shown that encapsulated metal cations facilitate the Diels–Alder reaction in fullerenes. Herein, the Bingel–Hirsch (BH) addition of ethyl bromomalonate over a series of ion-encapsulated M@C₆₀ (M=Ø, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cl⁻; Ø@C₆₀ stands for C₆₀ without any endohedral metal) is quantum mechanically explored to analyze the effect of these ions on the BH addition. The results show that the incarcerated ion has a very important effect on the kinetics and thermodynamics of this reaction. Among the systems studied, K⁺@C₆₀ is the one that leads to the fastest BH reaction, whereas the slowest reaction is given by Cl⁻@C₆₀.     

A Supramolecular [10]CPP Junction Enables Efficient Electron Transfer in Modular Porphyrin–[10]CPP⊃Fullerene Complexes

Efficient photoinduced electron transfer was observed across a [10]cycloparaphenylene ([10]CPP) moiety that serves as a rigid non‐covalent bridge between a zinc porphyrin and a range of fullerenes. The preparation of iodo‐[10]CPP is the key to the synthesis of a porphyrin–[10]CPP conjugate, which binds C₆₀, C₇₀, (C₆₀)₂, and other fullerenes (K_A>10⁵ m ^?1). Fluorescence and pump–probe spectroscopy revealed intramolecular energy transfer between CPP and porphyrin and also efficient charge separation between porphyrin and fullerenes, affording up to 0.5 μs lifetime charge‐separated states. The advantage of this approach towards electron donor–acceptor dyads is evident in the case of dumbbell‐shaped (C₆₀)₂, which gave intricate charge‐transfer behavior in 1:1 and 2:1 complexes. These results suggest that [10]CPP and its cross‐coupled derivatives could act as supramolecular mediators of charge transport in organic electronic devices.     

1‐Aryl‐4‐Silylmethyl[60]fullerenes: Synthesis,Properties, and Photovoltaic Performance

The efficient nucleophilic addition of aryl Grignard reagents (aryl=4‐MeOC₆H₄, 4‐Me₂NC₆H₄, Ph, 4‐CF₃C₆H₄, and thienyl) to C₆₀ in the presence of DMSO produced 1,2‐arylhydro[60]fullerenes after acid treatment. The reactions of the anions of these arylhydro[60]fullerenes with either dimethylphenylsilylmethyl iodide or dimethyl(2‐isopropoxyphenyl)silylmethyl iodide yielded the target compounds, 1‐aryl‐4‐silylmethyl[60]fullerenes. The properties and structures of these 1‐aryl‐4‐silylmethyl[60]fullerenes (aryl=4‐MeOC₆H₄, thienyl) were examined by electrochemical studies, X‐ray crystallography, flash‐photolysis time‐resolved microwave‐conductivity (FP‐TRMC) measurements, and electron‐mobility measurements by using a space‐charge‐limited current (SCLC) model. Organic photovoltaic devices with a polymer‐based bulk heterojunction structure and small‐molecule‐based p–n and p–i–n heterojunction configurations were fabricated by using 1‐aryl‐4‐silylmethyl[60]fullerenes as an electron acceptor. The most efficient device exhibited a power‐conversion efficiency of 3.4 % (short‐circuit current density: 8.1 mA/ cm², open‐circuit voltage: 0.69 V, fill factor: 0.59).     

Do Carbon Nano-onions Behave as Nanoscopic Faraday Cages? A Comparison of the Reactivity of C60, C240, C60@C240, Li+@C60, Li+@C240, and Li+@C60@C240

From the analysis of the polarizability of carbon nano-onions (CNOs), it was concluded that CNOs behave as near perfect nanoscopic Faraday cages. If CNOs behave as ideal Faraday cages, the reactivity of the C₂₄₀ cage should be the same in Li⁺@C₂₄₀ and Li⁺@C₆₀@C₂₄₀. In this work, the Diels–Alder reaction of cyclopentadiene to the free C₂₄₀ cage and the C₆₀@C₂₄₀ CNO together with their Li⁺-doped counterparts were analyzed using DFT. It was found that in all cases the preferred cycloaddition is on bond [6,6] of type B of C₂₄₀. Encapsulation of Li⁺ results in lower enthalpy barriers due to the decrease of the energy of the LUMO orbital of the C₂₄₀ cage. When the Li⁺ is placed inside the CNO C₆₀@C₂₄₀, the decrease in enthalpy barrier is similar to that of Li⁺@C₂₄₀. However, the location of Li⁺ in Li⁺@C₂₄₀ (off-centered) and Li⁺@C₆₀@C₂₄₀ (centered) is quite different. When Li⁺ was placed in the center of the C₂₄₀ cage in Li⁺@C₂₄₀, the barriers increased significantly. Taking into account this effect, the barriers in Li⁺@C₂₄₀ and Li⁺@C₆₀@C₂₄₀ differ by about 4 kcal mol⁻¹. This result can be attributed to the shielding effect of C₆₀ in Li⁺@C₆₀@C₂₄₀. As a result, we conclude that this CNO does not act as a perfect Faraday cage.     

Structures,electronic and magnetic properties of the FemOn@Cx (m = 1–3, n = 1–4, x = 50, 60) clusters

The structures, electronic and magnetic properties of the Fe_mO_n@C_x (m?=?1–3, n?=?1–4, x?=?50, 60) clusters have been investigated by using PBE functional. The C₅₀, C₆₀ can significantly increase the structural stabilities of the Fe_mO_n molecules. Fe₂O₃@C₅₀ and Fe₃O₄@C₅₀ are more chemically stable than the Fe₂O₃@C₆₀ and Fe₃O₄@C₆₀ while FeO@C₆₀ is more chemically stable than the FeO@C₅₀. The spin densities of the Fe_mO_n fragments degenerate to zero. Carbon encapsulation leads to the internal charges of the Fe_mO_n fragments transfer from 4 s to 4p orbital.

     

Atomic structure and electronic properties of nanopeapods: Isomers of endohedral dititanofullerenes Ti2@C80 in carbon nanotubes

The atomic structure, charge and electronic parameters, and energies of formation of new “hybrid” nanostructures-nanopeapods Ti₂@C₈₀@C-NTs, regular linear ensembles of seven isomers of endohedral dititanofullerenes Ti₂@C₈₀ encapsulated into a carbon zigzag (19.0) nanotube—have been calculated by the self-consistent density functional tight-binding method (DFTB). The results are discussed in comparison to the “free” isomers of C₈₀ fullerenes and Ti₂@C₈₀ endofullerenes, as well as to all-carbon nanopeapods, i.e., C₈₀ isomers inside a carbon nanotube (C₈₀@C-NT). It is demonstrated that the type of Ti₂@C₈₀ isomer determines the energy effect of its encapsulation into the tube (exo-or endothermic), the orientational arrangement of end-ofullerenes in the tube, the charge states of the Ti₂@C₈₀ and tube atoms, and the electronic properties of nanopeapods (semiconducting or metallic).     

ThC2@C82versus Th@C84: unexpected formation of triangular thorium carbide cluster inside fullerenes

Synthesis of the first thorium-containing clusterfullerenes, ThC₂@C_s(6)–C₈₂ and ThC₂@C₂(5)–C₈₂, is reported. These two novel actinide fullerene compounds were characterized by mass spectrometry, single-crystal X-ray diffraction crystallography, UV–vis–NIR spectroscopy, and theoretical calculations. Crystallographic studies reveal that the encapsulated ThC₂ clusters in both C_s(6)–C₈₂ and C₂(5)–C₈₂ feature a novel bonding structure with one thorium metal center connected by a C C unit, forming an isosceles triangular configuration, which has not been hitherto observed for endohedral fullerenes or for solid phase thorium carbides. Electronic structure calculations assign a formal electronic structure of [Th⁴⁺(C₂)²⁻]²⁺@[C₈₂]²⁻, with pronounced donation bonding from (C₂)²⁻ to Th⁴⁺, secondary backbonding from the fullerene to thorium and Th–C double bond character in both compounds. This work presents a new family of endohedral fullerenes, MC₂@C_2n−2, being unexpected isomers of MC_2n, and provides broader understanding of thorium bonding.

The first thorium-containing cluster fullerenes, ThC₂@C_s(6)–C₈₂ and ThC₂@C₂(5)–C₈₂, were synthesized and characterized. ThC₂ clusters in both C₈₂ cages feature a novel bonding structure with thorium metal and C C forming an isosceles triangular configuration.     

Synthesis of Long‐Sought C66 with Exohedral Stabilization

Previously reported fused‐pentagon fullerenes stabilized by exohedral derivatization do not share the same cage with those stabilized by endohedral encapsulation. Herein we report the crystallographic identification of ^#4348C₆₆Cl₁₀, which has the same cage as that of previously reported Sc₂@C₆₆. According to the geometrical data of ^#4348C₆₆Cl₁₀, both strain relief (at the fused pentagons) and local aromaticity (on the remaining sp²‐hybrided carbon framework) contribute to the exohedral stabilization of this long‐sought 66 carbon atom cage.     

Ligand Effects on the Electronic Structure of Heteroleptic Antimony-Centered Radicals

We report on the structures of three unprecedented heteroleptic Sb-centered radicals [L(Cl)Ga](R)Sb^. ( 2-R , R=B[N(Dip)CH]₂ 2-B , 2,6-Mes₂C₆H₃ 2-C , N(SiMe₃)Dip 2-N ) stabilized by one electropositive metal fragment [L(Cl)Ga] (L=HC[C(Me)N(Dip)]₂, Dip=2,6-i-Pr₂C₆H₃) and one bulky B- ( 2-B ), C- ( 2-C ), or N-based ( 2-N ) substituent. Compounds 2-R are predominantly metal-centered radicals. Their electronic properties are largely influenced by the electronic nature of the ligands R, and significant delocalization of unpaired-spin density onto the ligands was observed in 2-B and 2-N . Cyclic voltammetry (CV) studies showed that 2-B undergoes a quasi-reversible one-electron reduction, which was confirmed by the synthesis of [K([2.2.2]crypt)][L(Cl)GaSbB[N(Dip)CH]₂] ([K([2.2.2]crypt)][ 2-B ]) containing the stibanyl anion [ 2-B ]⁻, which was shown to possess significant Sb−B multiple-bonding character.     

Theoretical study of N20, C20, and B20 clusters “squeezed” inside icosahedral C80 and He80 cages

The equilibrium geometries and spectroscopic and energetic characteristics of model endohedral M₂₀@C ₈₀ ^n? clusters, in which the guest clusters M₂₀ = N₂₀, C₂₀, and B₂₀ are squeezed inside the fullerene C ₈₀ ^n? cages (n = 0, 2, 4, and 6), have been calculated at the density functional theory B3LYP/6-31G and B3LYP/6-31G* levels. Analogous calculations with partial geometry optimization have been performed for their congeners M₂₀@He ₈₀ ^n? with a fixed icosahedral helium cage He₈₀. According to the calculations, all the structures of the N₂₀@C₈₀, C₂₀@C₈₀, and B₂₀@C₈₀ series correspond to local minima of the potential energy surface (all vibrational frequencies are real). In the first cluster, the N₂₀ guest has a structure of a dodecahedron with a diameter of ～4.0 Å. The alternative 10N₂@C₈₀ structure containing ten separated endohedral N₂ molecules is considerably less favorable and transforms without a barrier to the dodecahedral N₂₀@C₈₀ isomer upon geometry optimization. It has been suggested that, under extreme supercompresison conditions, molecular nitrogen can be associated without barriers into highly endothermal chemically bound clusters of the N₂₀ type. In the helium analogues, the relative position of the N₂₀@He₈₀ and 10N₂₀@He₈₀ structures on the energy scale is determined by the degree of compression and can change its sign with a change in the diameter of the external cage D(He₈₀). The mechanism of gradual assembly of the N₂₀ dodecahedron from the 10N₂ set has been traced with a decrease in the diameter D(He₈₀) in the range 7.5–8.6 Å. In the C₂₀@C₈₀ cluster, the C₂₀ guest has a structure of a distorted dodecahedron bound to the C₈₀ cage through four “inner” (endohedral) bonds. In the B₂₀@C₈₀ cluster, the B₂₀ guest is severely squeezed along the C₅ axis. Its equatorial atoms form ten endohedral B-C bonds to C₈₀ cage atoms. In similar systems, the division of the endoclusters into the internal guest and external cage becomes uncertain. Calculations predict that the isolated “salt” molecules of the L _n C₂₀ and L _n B₂₀ type in which the C₂₀ and B₂₀ clusters function as anions surrounded by the L atoms of alkali metals (n = 1–6) should be stable to stepwise dissociation accompanied by elimination of separate L atoms and L₂ molecules.     

Thermodynamics of aqueous carbohydrate surfactant solutions

The paper deals with the application of the micelle formation theory, developed by Nagarajan and Ruckenstein [R. Nagarajan, E. Ruckenstein, Langmuir 7 (1991) 2934–2969] and Nagarajan [R. Nagarajan, in: K. Esumi (Ed.), Structure–Performance Relationships in Surfactants, Dekker, New York, 1997, pp. 1–81; R. Nagarajan, Adv. Colloid Interface Sci. 26 (1986) 205-264] to various n-alkyl-β-d-glucopyranoside surfactants, differing in the surfactant tail length (n-octyl-β-d-glucopyranoside C₈G₁, n-decyl-β-d-glucopyranoside C₁₀G₁ and dodecyl-β-d-glucopyranoside C₁₂G₁). The model predicts that the carbohydrate surfactant molecules assemble for energetic reasons in spherical bilayer vesicles. The critical micellar concentration as function of the temperature shows a minimum value. The formed micellar aggregates exhibit a broad distribution of sizes. It is demonstrated in this study that the thermodynamic theory in combination with phase separation thermodynamics can be used successfully to described the phase separation, which occurs for the system C₁₀G₁+water and C₁₂G₁+water at low surfactant concentrations.     

Nonlinear optical properties of solutions of heavy fullerenes in the near-ultraviolet region

Nonlinear optical properties (particularly optical limiting) are determined for solutions of heavy fullerenes C₇₆ + C₇₈ + C₈₄ + C₉₀ + …, in the near-ultraviolet region (λ ≈ 280 ± 7 nm). It is shown that no optical limiting is observed in solutions of light fullerenes (C₆₀ and C₇₀), but found in solutions of water-soluble fullerenol-d (a mixture of oxypolyalcohols of fullerene C₆₀-C₆₀(OH) _n1O _n2, with their sodium salts) based on light fullerenes.     

Endohedral Metallofullerenes: New Structures and Unseen Phenomena

Endohedral metallofullerenes (EMFs), namely fullerenes with metallic species encapsulated inside, represent an ideal platform to investigate metal–metal or metal–carbon interactions at the sub-nanometer scale by means of single-crystal X-ray diffraction (XRD) crystallography. Herein, recent progress in the identification of new structures and unprecedented properties are discussed according to the categories of monometallofullerenes, dimetallofullerenes, carbide clusterfullerenes, and nitride clusterfullerenes. In particular, the dimerization and the cage-isomer dependent oxidation state of the inner metal atom are summarized in terms of pristine monometallofullerenes. Metal–metal bonds involving lanthanide–lanthanides or actinide–actinides are discussed based on both experimental and theoretical studies. The cluster–cage matching and/or mutual selections, as well as the rarely seen M=C double bonds, are discovered in M₂C₂@C_2n, U₂C@C₈₀, M₂TiC@C₈₀, and Ti₃C₃@C₈₀. Subsequently, the geometries of different M₃N clusters in various cages are discussed, revealing size-matching between the internal M₃N cluster and the outer cage induced by the planarity of the cluster. Finally, an outlook regarding the future developments of the molecular structures and applications of EMFs is presented.     

Synthesis and Characterization of the D5h Isomer of the Endohedral Dimetallofullerene Ce2@C80: Two‐Dimensional Circulation of Encapsulated Metal Atoms Inside a Fullerene Cage

Herein we show the synthesis and characterization of the second known Ce₂@C₈₀ isomer. A ¹³C NMR spectroscopic study revealed that the structure of the second isomer has D_5h symmetry. Paramagnetic NMR spectral analysis and theoretical calculation display that the encapsulated Ce atoms circulate two‐dimensionally along a band of ten contiguous hexagons inside a D_5h‐C₈₀ cage, which is in sharp contrast to the three‐dimensional circulation of two Ce atoms in an I_h‐C₈₀ cage. The electronic properties were revealed by means of electrochemical measurements. The D_5h isomer of Ce₂@C₈₀ has a much smaller HOMO–LUMO gap than cluster fullerenes (M₃N@C₈₀, M=Sc, Tm, and Lu) with the same D_5h‐C₈₀ cages. The chemical reactivity was investigated by using disilirane as a chemical probe. The high thermal reactivity toward 1,1,2,2‐tetramesityl‐1,2‐disilirane is consistent with the trends of the redox potentials and the lower LUMO level of the D_5h isomer of Ce₂@C₈₀ compared with that of C₆₀.     

Kernresonanzspektroskopische Untersuchungen an Pentafluorphenylsubstituierten Silanen: II–Fernkopplungen und Through Space Effekte

Long range ⁵J(FH) and ⁶J(FH) couplings were observed for silanes of the type (C₆F₅)_nSiR_4-n (n = 1, 2, 3; R = alkyl, aryl) and disilanes C₆F₅[Si(CH₃)₂]₂R (R = CH₃, C₆F₅). A characteristic increase in line width for ¹⁹F-NMR spectra of (C₆F₅)_nSiR_4-n(n ≧ 2) is thought to be due to spin systems of the type ([AC]₂B)_n or ([AX]₂M)_n with intra-annular couplings between ortho-fluorine atoms of neighbouring ring systems.