Purification by HPLC and the UV/Vis absorption spectra of the nitrogen-containing incar-fullerenes iNC60, and iNC70

Spin Resonance Clock Transition of the Endohedral Fullerene ^{15}N@C_{60}

The endohedral fullerene ^{15}N@C_{60} has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

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.

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.

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.

X-ray observation of a helium atom and placing a nitrogen atom inside He@C60 and He@C70

Single crystal X-ray analysis has been used as a powerful method to determine the structure of molecules. However, crystallographic data containing helium has not been reported, owing to the difficulty in embedding helium into crystalline materials. Here we report the X-ray diffraction study of He@C60 and the clear observation of a single helium atom inside C60. In addition, the close packing of a helium atom and a nitrogen atom inside fullerenes is realized using two stepwise insertion techniques, that is, molecular surgery to synthesize the fullerenes encapsulating a helium atom, followed by nitrogen radio-frequency plasma methods to generate the fullerenes encapsulating both helium and nitrogen atoms. Electron spin resonance analysis reveals that the encapsulated helium atom has a small but detectable influence on the electronic properties of the highly reactive nitrogen atom coexisting inside the fullerene, suggesting the potential usage of helium for controlling electronic properties of reactive species.

Alignment of N@C60 derivatives in a liquid crystal matrix

The orientation of different N-substituted [N@C60]fulleropyrrolidine derivatives hosted in a nematic liquid crystal matrix has been studied by electron spin resonance spectroscopy. In this study, variations on the zero field splitting parameter of the guest species are employed as a means of determining their degree of orientation. Fulleropyrrolidines with more rigid N-substituents are preferentially oriented in the liquid crystal matrix, whereas those with less rigid substituents are almost randomly distributed. Additionally, the orientation of a C60 fullerene cage bearing a nitroxide radical has also been investigated in comparison.

Quantum control in spintronics

Superposition and entanglement are uniquely quantum phenomena. Superposition incorporates a phase that contains information surpassing any classical mixture. Entanglement offers correlations between measurements in quantum systems that are stronger than any that would be possible classically. These give quantum computing its spectacular potential, but the implications extend far beyond quantum information processing. Early applications may be found in entanglement-enhanced sensing and metrology. Quantum spins in condensed matter offer promising candidates for investigating and exploiting superposition and entanglement, and enormous progress is being made in quantum control of such systems. In gallium arsenide (GaAs), individual electron spins can be manipulated and measured, and singlet-triplet states can be controlled in double-dot structures. In silicon, individual electron spins can be detected by ionization of phosphorus donors, and information can be transferred from electron spins to nuclear spins to provide long memory times. Electron and nuclear spins can be manipulated in nitrogen atoms incarcerated in fullerene molecules, which in turn can be assembled in ordered arrays. Spin states of charged nitrogen vacancy centres in diamond can be manipulated and read optically. Collective spin states in a range of materials systems offer scope for holographic storage of information. Conditions are now excellent for implementing superposition and entanglement in spintronic devices, thereby opening up a new era of quantum technologies.

Efficient dynamic nuclear polarization at high magnetic fields

By applying a new technique for dynamic nuclear polarization involving simultaneous excitation of electronic and nuclear transitions, we have enhanced the nuclear polarization of the nitrogen nuclei in 15N@C60 by a factor of 10(3) at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice the maximum enhancement reported to date. This methodology will allow the initialization of the nuclear qubit in schemes exploiting N@C60 molecules as components of a quantum information processing device.

Davies electron-nuclear double resonance revisited: Enhanced sensitivity and nuclear spin relaxation

Over the past 50 years, electron-nuclear double resonance (ENDOR) has become a fairly ubiquitous spectroscopic technique, allowing the study of spin transitions for nuclei which are coupled to electron spins. However, the low spin number sensitivity of the technique continues to pose serious limitations. Here we demonstrate that signal intensity in a pulsed Davies ENDOR experiment depends strongly on the nuclear relaxation time T(1n), and can be severely reduced for long T(1n). We suggest a development of the original Davies ENDOR sequence that overcomes this limitation, thus offering dramatically enhanced signal intensity and spectral resolution. Finally, we observe that the sensitivity of the original Davies method to T(1n) can be exploited to measure nuclear relaxation, as we demonstrate for phosphorous donors in silicon and for endohedral fullerenes N@C(60) in CS(2).

Kinetic energy release of C70(+) and its endohedral cation N@C70(+): activation energy for N extrusion

Unimolecular decomposition of C70(+) and its endohedral cation N@C70(+) were studied by high-resolution mass-analyzed ion kinetic energy (MIKE) spectrometry. Information on the energetics and dynamics of these reactions was extracted. C70(+) dissociates unimolecularly by loss of a C2 unit, whereas N@C70(+) expels the endohedral N atom. Kinetic energy release distributions (KERDs) in these reactions were measured. By use of finite heat bath theory (FHBT), the binding energy for C2 emission from C70(+) and the activation energy for N elimination from N@C70(+) were deduced from KERDs in the light of a recent finding that fragmentation of fullerene cations proceeds via a very loose transition state. The activation energy measured for N extrusion from N@C70(+) was found to be lower than that for C2 evaporation, higher than the value from its neutral molecule N@C70 obtained on the basis of thermal stability measurements, and coincident with the theoretical value. The results provide confirmation that the proposed extrusion mechanism in which the N atom escapes from the cage via formation of an aza-bridged intermediate is correct.

Atomic–molecular superlattices

In this communication we demonstrate a directly-bonded crystalline fullerene superlattice and show that the incorporation of spin-active N@C60 endohedral fullerenes is readily achieved to give an atomic-molecular hybrid spin-active superlattice material.

Synthesis and reactivity of N@C60O

The endohedral fullerene epoxide N@C60O was synthesised, isolated by High Performance Liquid Chromatography (HPLC), and characterised by Electron Spin Resonance (ESR). This nitrogen radical displays predominantly axial symmetry characteristics as expected for a monoadduct, evidenced by a zero-field splitting D parameter of 6.6 MHz and an E parameter of 0.5 MHz in powder at 77 K. Photo- and thermally-activated silencing of the nitrogen radical were observed, the latter showing the evolution of a new spin signal during heating at 100 degrees C. We suggest that loss of nitrogen spin is due to coupling with a radical formed by opening of the epoxide ring. This implies that the reaction of C60O with C60 in the solid state proceeds via a radical, rather than ionic, intermediate.

High fidelity single qubit operations using pulsed electron paramagnetic resonance

Systematic errors in spin rotation operations using simple rf pulses place severe limitations on the usefulness of the pulsed magnetic resonance methods in quantum computing applications. In particular, the fidelity of quantum logic operations performed on electron spin qubits falls well below the threshold for the application of quantum algorithms. Using three independent techniques, we demonstrate the use of composite pulses to improve this fidelity by several orders of magnitude. The observed high-fidelity operations are limited by pulse phase errors, but nevertheless fall within the limits required for the application of quantum error correction.

Expanding the World of Endohedral Fullerenes?The Tm3N@C2n (39?n?43) Clusterfullerene Family

A new family of endohedral fullerenes, based on an encaged trithulium nitride (Tm(3)N) cluster, was synthesised, isolated and characterised by HPLC, mass spectrometry, and visible-NIR and FTIR spectroscopy. Tm(3)N clusterfullerenes with cages as small as C(76) and as large as C(88) were prepared and six of them were isolated. Tm(3)N@C(78) is a small clusterfullerene. The two isomers of Tm(3)N@C(80) (I and II) were the most abundant structures in the fullerene soot. Tm(3)N@C(82), Tm(3)N@C(84), and Tm(3)N@C(86) represent a new series of higher clusterfullerenes. All six isolated Tm(3)N clusterfullerenes were classified as large energy-gap structures with optical energy gaps between approximately 1.2 and approximately 1.75 eV. Tm(3)N@C(80) (I) and Tm(3)N@C(80) (II) were assigned to the C(80) cages C(80):7 (I(h)) and C(80):6 (D(5h)). For Tm(3)N@C(78), the analysis pointed to an elliptical carbon cage with C(78):1 (D(3)) or C(78):4 (D(3h)) being the probable structures.