Kyriakos Porfyrakis

A Cyclic Porphyrin Trimer as a Receptor for Fullerenes

A cyclic porphyrin trimer has been synthesized which has a high affinity for fullerenes. It forms 1:1 complexes with C(60) and C(70) with association constants of 2 x 10(6) and 2 x 10(8) M(-1), respectively, in toluene. Its affinities for C(86) and La@C(82) are too strong to measure by fluorescence titration. The solvent dependence of the association constants shows that solvation of both the guest and the host influence the binding strength.

Electron spin relaxation of N@C60 in CS2

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S> or =1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60 , indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170 K.

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.

Pairs and heptamers of C70 molecules ordered via PTCDI-melamine supramolecular networks

In this paper, we report on the use of two PTCDI-melamine supramolecular networks on Au(111) to trap C70 molecules. The different supramolecular networks were formed by changing the postannealing temperature after molecular deposition. We observed, using scanning tunneling microscopy, that the deposition of C70 onto a PTCDI melamine network with parallelogram cavities results in the long-range ordering of paired C70, whereas the deposition of C70 molecules onto a PTCDI-melamine honeycomb network results in the trapping of C70 heptamers.

Rotating fullerene chains in carbon nanopeapods.

The rotation of fullerene chains in SWNT peapods is studied using low-voltage high resolution transmission electron microscopy. Anisotropic fullerene chain structures (i.e., C300) are formed in situ in carbon nanopeapods via electron beam induced coalescence of individual fullerenes (i.e., C60). A low electron accelerating voltage of 80 kV is used to prevent damage to the SWNT. The large asymmetric C300 fullerene structure exhibits translational motion inside the SWNT and unique corkscrew like rotation motion. Another asymmetric fullerene chain containing mixed fullerene species is prepared by fusing smaller C60 fullerenes to a larger Sc@C82 fullerene, and this also exhibits corkscrew rotational motion. Chains of Sc3C2@C80 in SWNT peapods adopt a zigzag packing structure, and the entire zigzag chain rotates inside the SWNT to induce structural modifications to the SWNT diameter and cross-sectional shape of the SWNT. The expansion and contraction of the diameter of the SWNT is measured as 17%, demonstrating nanoactuation behavior in carbon nanopeapods.

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.

Dynamics of Paramagnetic Metallofullerenes in Carbon Nanotube Peapods

We filled SWNTs with the paramagnetic fullerene Sc@C82 to form peapods. The interfullerene 1D packing distance measured using TEM is d = 1.1 +/- 0.02 nm. The Sc@C82 in SWNT peapods continuously rotated during the 2 s TEM exposure time, and we did not see the Sc atoms. However, Sc@C82 metallofullerenes in MWNT peapods have periods of fixed orientation, indicated by the brief observation of Sc atoms. La@C82 peapods were also prepared and their rotational behavior examined. The interfullerene 1D packing of both La@C82 and Sc@C82 peapods is identical and thus independent of the charge transfer state for these paramagnetic fullerenes. The La@C82 metallofullerenes in the peapods have fixed orientations for extended periods of time, up to 50 s in some cases. The La@C82 spontaneously rotates rapidly between fixed orientations.

Chemical reactions inside single-walled carbon nano test-tubes

We report the application of SWNTs as templates for forming covalent polymeric chains from C(60)O reacting inside SWNTs; the resulting peapod polymer topology is different from the bulk polymer in that it is linear and unbranched.

Redox-Dependent Franck-Condon Blockade and Avalanche Transport in a Graphene-Fullerene Single-Molecule Transistor.

We report transport measurements on a graphene-fullerene single-molecule transistor. The device architecture where a functionalized C60 binds to graphene nanoelectrodes results in strong electron-vibron coupling and weak vibron relaxation. Using a combined approach of transport spectroscopy, Raman spectroscopy, and DFT calculations, we demonstrate center-of-mass oscillations, redox-dependent Franck-Condon blockade, and a transport regime characterized by avalanche tunnelling in a single-molecule transistor.

Chemistry at the Nanoscale: Synthesis of an N@C60–N@C60 Endohedral Fullerene Dimer

Endohedral fullerenes—carbon cages with one or more heteroatoms incarcerated within them—are one of the most exotic classes of molecules. They possess a wealth of fascinating functional properties, including magnetism and photoactivity. While the properties of individual endohedral fullerene molecules are remarkable, the full functional potential of many of these species will likely be realized in systems where two or more of these molecules are connected. One prominent example is the endohedral fullerene radical species N@C60. The highly symmetric central location of the nitrogen atom within the C60 cage, [2] with minimal mixing between the nitrogen atom and fullerene electron wavefunctions, imparts a degree of isolation to the nitrogen radical that is usually only attainable using an ion trap or in an atomic gas. This isolation allows the nitrogen center to retain a S3/2 electron spin ground state and to have an extraordinarily long electron spin coherence time (T2e of 250 ms). [4] This remarkable spin coherence time has lead many research groups to study the potential and feasibility of an N@C60based quantum computer over the past decade. Owing to a number of synthetic challenges associated with N@C60, experimental electron spin resonance studies have so far been restricted to probing the interaction of ensembles of molecules each containing a single N@C60 spin center. These challenges center on the low-yielding production methods available to synthesize N@C60, thus producing at best a 500 ppm N@C60/C60 mixture, [2, 6] and the time consuming processing techniques currently required to enrich the ratio of N@C60 to C60. [7] In addition, compared to C60, chemically functionalized N@C60 derivatives have significantly lower thermal and photo stability. 9] To further explore the potential of N@C60 as a quantum computing element, including the creation of controlled entanglement between electron spins, an array of at least two N@C60 centers with fixed separation is required to understand the interaction of neighboring spin centers with one another and investigate methods of selective manipulation of one of the spin centers. Herein we describe a one-pot method, using a double 1,3dipolar cycloaddition, for the synthesis of a two-center N@C60 molecule with fixed spatial separation (Scheme 1), thus providing a platform for future experiments to probe the nature of the electron interaction between two N@C60 molecules to assess the potential capabilities of a 2 qubit N@C60 quantum computer. To the best of our knowledge, this is the first endohedral fullerene dimer comprising two chemically linked N@C60 spin centers. A Prato 1,3-dipolar cycloaddition reaction was chosen to link the two N@C60 molecules owing to its proven compatibility with the stability of N@C60 and the wide range of amino acid and aldehyde derivatives that it has been reported to be compatible with. A dibenzaldehyde-terminated oligo(pphenylene polyethylene) (OPE) molecule (Ald-3Bz) was synthesized and used as the spacing unit because of its rigidity and modular construction. The incorporation of long alkyl chains on the central phenyl group enhances the inherent poor solubility of the rigid, linear molecule, thus allowing the reaction to be performed at high concentration and making the product fullerene dimer (dimer-3Bz) soluble in common solvents such as chloroform. Previous work has shown that the structure of the amino acid derivative used makes a significant difference to the rate and yield of the 1,3-dipolar cycloaddition reaction. Work within our group has found that the N-(4-(hexyloxy)benzyl)glycine amino acid derivative greatly enhances the dimer formation reaction rate compared to more commonly used amino acid derivatives such as N(ethyl)glycine (see the Supporting Information). In addition, the amino group produces a fullerene dimer that can be purified using standard single-pass HPLC methods. We synthesized three grams of a N@C60/C60 mixture using an ion implantation method, thus yielding an average purity of 50 ppm of N@C60 in C60. Extensive purification of this sample followed. After 19 recycling HPLC runs, a high purity N@C60/C60 peak was isolated. The integrated area of the N@C60/C60 peak indicated the total mass of N@C60/C60 to be 10 mg. This sample then underwent quantitative ESR analysis of the total number of N@C60 spin centers present, thereby giving a lower bound for the mass of N@C60 to be 4.6 mg. Optimized reaction conditions and an efficient purification method for the one-pot dimer synthesis were identified and scaled down to work using only 20 mg of the C60 starting [*] B. J. Farrington, Prof. G. A. D. Briggs, Dr. K. Porfyrakis Department of Materials, University of Oxford Oxford OX1 3PH (UK) E-mail: kyriakos.porfyrakis@materials.ox.ac.uk