Distributed Polarizability Model for Covalently Bonded Fullerene Nanoaggregates: Origins of Polarizability Exaltation

Polarizability exaltation is typical for (C₆₀)_n nanostructures. It relates to the ratio between the mean polarizabilities of (C₆₀)_n and C₆₀: the first one is higher than the n-fold mean polarizability of the original fullerene. This phenomenon is used in the design of novel fullerene compounds and the understanding of its properties but still has no chemical rationalization. In the present work, we studied the distributed polarizability of (C₆₀)₂ and isomeric (C₆₀)₃ nanoaggregates with the density functional theory method. We found that polarizability exaltation increases with the size of the nanostructure and originates from the response of the sp²-hybridized carbon atoms to the external electric field. The highest contributions to the dipole polarizability of (C₆₀)₂ and (C₆₀)₃ come from the most remote atoms of the marginal fullerene cores. The sp³-hybridized carbon atoms of cyclobutane bridges negligibly contribute to the molecular property. A similar major contribution to the molecular polarizability from the marginal atoms is observed for related carbon nanostructures isomeric to (C₆₀)₂ (tubular fullerene and nanopeanut). Additionally, we discuss the analogy between the polarizability exaltation of covalently bonded (C₆₀)_n and the increase in the polarizability found in experiments on fullerene nanoclusters/films as compared with the isolated molecules.

Creation of Mo/Tc@C60 and Au@C60 and molecular-dynamics simulations

The formation of middle- and/or high-weight atom (Mo, Au)-incorporated fullerenes was investigated using radionuclides produced by nuclear reactions. From the trace radioactivities of ⁹⁹Mo/^99mTc or ¹⁹⁴Au after high-performance liquid chromatography, it was found that the formation of endohedral and/or heterofullerene fullerenes in ⁹⁹Mo/^99mTc and ¹⁹⁴Au atoms could occur by a recoil process following the nuclear reactions. Furthermore, the ^99mTc (and ¹⁹⁴Au) atoms recoiled against β-decay remained present inside these cages. To confirm the produced materials experimentally, ab initio molecular dynamics (MD) simulations based on an all-electron mixed-basis approach were performed. The possibility of the formation of endohedral fullerenes containing Mo/Tc and Au atoms is verified; here, the formation of heterofullerenes is excluded by MD simulations. These findings suggest that radionuclides stably encapsulated by fullerenes could potentially play a valuable role in diagnostic nuclear medicine.

The formation of Mo, Au-incorporated fullerenes was investigated using radionuclides produced by nuclear reactions and using AIMD simulations. The possibility of the formation of endohedral fullerenes containing Mo/Tc and Au atoms is verified.

Construction of a short metallofullerene-peapod with a spin probe

A short metallofullere-peapod of Y2@C79N⊂[4]CHBC was constructed. The strong confinement effect from the large π-extended [4]CHBC nanoring induces molecular orientation of the wrapped Y2@C79N, which can be sensed by a Y2@C79N spin probe. The low susceptibility of the spin phase memory time (Tm) for the Y2@C79N spin was also found in a confined space.

Close Encounters of the Weak Kind: Investigations of Electron-Electron Interactions between Dissimilar Spins in Hybrid Rotaxanes

We report a family of hybrid [2]rotaxanes based on inorganic [Cr₇NiF₈(O₂C^tBu)₁₆]⁻ (“{Cr₇Ni}”) rings templated about organic threads that are terminated at one end with pyridyl groups. These rotaxanes can be coordinated to [Cu(hfac)₂] (where Hhfac = 1,1,1,5,5,5-hexafluoroacetylacetone), to give 1:1 or 1:2 Cu:{Cr₇Ni} adducts: {[Cu(hfac)₂](py-CH₂NH₂CH₂CH₂Ph)[Cr₇NiF₈(O₂C^tBu)₁₆]}, {[Cu(hfac)₂][py-CH₂NH₂CH₂CH₃][Cr₇NiF₈(O₂C^tBu)₁₆]}, {[Cu(hfac)₂]([py-CH₂CH₂NH₂CH₂C₆H₄SCH₃][Cr₇NiF₈(O₂C^tBu)₁₆])₂}, {[Cu(hfac)₂]([py-C₆H₄-CH₂NH₂(CH₂)₄Ph][Cr₇NiF₈(O₂C^tBu)₁₆])₂}, and {[Cu(hfac)₂]([3-py-CH₂CH₂NH₂(CH₂)₃SCH₃][Cr₇NiF₈(O₂C^tBu)₁₆])₂}, the structures of which have been determined by X-ray diffraction. The {Cr₇Ni} rings and Cu^II ions both have electronic spin S = 1/2, but with very different g-values. Continuous-wave EPR spectroscopy reveals the exchange interactions between these dissimilar spins, and hence the communication between the different molecular components that comprise these supramolecular systems. The interactions are weak such that we observe AX or AX₂ type spectra. The connectivity between the {Cr₇Ni} ring and thread terminus is varied such that the magnitude of the exchange interaction J can be tuned. The coupling is shown to be dominated by through-bond rather than through-space mechanisms.

Synthesis and Structural Studies of Two Paramagnetic Metallofullerenes with Isomeric C72 Cage

We synthesized and isolated two paramagnetic metallofullerenes of La@C₇₂ and Y@C₇₂ with different fullerene cages, which were characterized by electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. DFT calculations disclosed two possible isomers of La/Y@C₇₂ with C₇₂- C₂ and C₇₂- C_2v cages, both of which have similar thermodynamic stability and one pair of fused pentagons. Their paramagnetic properties were then studied by EPR spectroscopy, and the obtained EPR signals were analyzed with very different hyperfine coupling constants, revealing distinct electron spin distributions for these two species. Furthermore, the experimental coupling constants were compared with those of calculated coupling constants, and comparison results revealed that the produced La@C₇₂ has a C₇₂- C_2v cage and Y@C₇₂ has a C₇₂- C₂ cage. These studies illustrate that the electron spin can be used as a probe to identify metallofullerene structure due to the susceptibility of spin-metal couplings. The successful isolation and characterizations of La@C₇₂ and Y@C₇₂ with such a small C₇₂ cage reveal their stability that is important for application as paramagnetic molecule materials.

Liquid-Crystalline Tris[60]fullerodendrimers

Liquid-crystalline tris[60]fullerodendrimers based on first- and second-generation poly(arylester)dendrons carrying cyanobiphenyl mesogens were synthesized for the first time by the olefin cross-metathesis reaction between type I (terminal) and type II (α,β-unsaturated carbonyl) olefinic precursors, using a second-generation Grubbs or Hoveyda-Grubbs catalyst. The modular synthetic approach developed here also allowed the selective preparation of the [60]fullerene-free, mono[60]fullerodendrimer, and bis[60]fullerodendrimer derivatives from the appropriate precursors. As revealed by polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering, all of the materials displayed liquid-crystalline properties. In agreement with the nature of the dendritic building blocks, the emergence of lamellar mesophases (smectic C and/or smectic A phases), with the segregation of the various constitutive parts, was systematically observed. The small variation of the mesomorphic temperature range and of the mesophase stability suggested that the mesomorphism is essentially dominated by the dendrimer itself and is regulated by a subtle adaptive mechanism, in which the proportion of monolayering and bilayering arrangements of the multisegregated lamellar mesophases is modified in order to compensate the space requirements of each of the elementary building blocks, namely, the [60]fullerene units, the cyanobiphenyl mesogens, and the dendritic matrix.

Reactivity and regioselectivity in Diels-Alder reactions of anion encapsulated fullerenes

Encapsulation and surface chemical modification are methodologies to enhance the properties of fullerenes for various applications. Herein, density functional theory calculations are performed to study the Diels-Alder (DA) reactivity of anion encapsulated C₆₀, including [X@C₆₀]^- (X = F, Cl, Br, or I), [S@C₆₀]^2-, and [N@C₆₀]^3-. Computational results reveal that encapsulated Cl^-, Br^-, I^-, or S^2- anions are located close to the center of the C₆₀ molecule; however, encapsulated F^- is displaced from the center. Encapsulated N^3- bonds to the inner surface of the carbon cage, which leads to a negative charge transfer to the C₆₀. In [N@C₆₀]^3-, C-C bonds near to the encapsulated N atom are more reactive. Our calculations reveal that encapsulated halogen or S anions decrease the DA reactivity because of the stronger closed-shell repulsion of the encapsulated anion. However, encapsulated N^3- increases the DA reactivity. The higher distortion energy of the halogen- or S^2--anion encapsulated C₆₀ leads to lower reactivity of the 6-5 bond. Opposite regioselectivity of the DA reaction with [N@C₆₀]^3- is attributed to distortion energy of the cyclopentadiene (CPD) moiety. The asymmetrical transition state geometry leads to a lower distortion energy of the CPD moiety.

Probing the Dipolar Coupling in a Heterospin Endohedral Fullerene-Phthalocyanine Dyad

Paramagnetic endohedral fullerenes and phthalocyanine (Pc) complexes are promising building blocks for molecular quantum information processing, for which tunable dipolar coupling is required. We have linked these two spin qubit candidates together and characterized the resulting electron paramagnetic resonance properties, including the spin dipolar coupling between the fullerene spin and the copper spin. Having interpreted the distance-dependent coupling strength quantitatively and further discussed the antiferromagnetic aggregation effect of the CuPc moieties, we demonstrate two ways of tuning the dipolar coupling in such dyad systems: changing the spacer group and adjusting the solution concentration.

Making hybrid [n]-rotaxanes as supramolecular arrays of molecular electron spin qubits

Quantum information processing (QIP) would require that the individual units involved--qubits--communicate to other qubits while retaining their identity. In many ways this resembles the way supramolecular chemistry brings together individual molecules into interlocked structures, where the assembly has one identity but where the individual components are still recognizable. Here a fully modular supramolecular strategy has been to link hybrid organic-inorganic [2]- and [3]-rotaxanes into still larger [4]-, [5]- and [7]-rotaxanes. The ring components are heterometallic octanuclear [Cr7NiF8(O2C(t)Bu)16](-) coordination cages and the thread components template the formation of the ring about the organic axle, and are further functionalized to act as a ligand, which leads to large supramolecular arrays of these heterometallic rings. As the rings have been proposed as qubits for QIP, the strategy provides a possible route towards scalable molecular electron spin devices for QIP. Double electron-electron resonance experiments demonstrate inter-qubit interactions suitable for mediating two-qubit quantum logic gates.

Synthesis of the first completely spin-compatible N@C60 cyclopropane derivatives by carefully tuning the DBU base catalyst

Two cyclopropane derivatives of N@C60 were synthesised by well-controlled Bingel reactions, in which 94% of the spin centres were retained, as confirmed by a series of quantitative electron spin paramagnetic resonance (EPR) measurements. Further study on the influence of the DBU catalyst base revealed a spin loss mechanism through a fullerene-DBU diradical.

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.

Tuning the interactions between electron spins in fullerene-based triad systems

A series of six fullerene-linker-fullerene triads have been prepared by the stepwise addition of the fullerene cages to bridging moieties thus allowing the systematic variation of fullerene cage (C60 or C70) and linker (oxalate, acetate or terephthalate) and enabling precise control over the inter-fullerene separation. The fullerene triads exhibit good solubility in common organic solvents, have linear geometries and are diastereomerically pure. Cyclic voltammetric measurements demonstrate the excellent electron accepting capacity of all triads, with up to 6 electrons taken up per molecule in the potential range between -2.3 and 0.2 V (vs Fc(+)/Fc). No significant electronic interactions between fullerene cages are observed in the ground state indicating that the individual properties of each C60 or C70 cage are retained within the triads. The electron-electron interactions in the electrochemically generated dianions of these triads, with one electron per fullerene cage were studied by EPR spectroscopy. The nature of electron-electron coupling observed at 77 K can be described as an equilibrium between doublet and triplet state biradicals which depends on the inter-fullerene spacing. The shorter oxalate-bridged triads exhibit stronger spin-spin coupling with triplet character, while in the longer terephthalate-bridged triads the intramolecular spin-spin coupling is significantly reduced.