Endohedral Fullerenes: Electron Transfer and Spin

The chapter gives an introduction into the field of endohedral fullerenes and describes development in the synthesis and molecular structure determination of these compounds. An overview of elements capable of being encapsulated within the fullerene cage is given. Different types of endohedral metallofullerenes and clusterfullerenes as well as peculiarities in the carbon cage isomerism caused by encapsulated species are discussed.

Fullerenes exhibit rich redox activity and are able to accommodate up to 6 surplus electrons or give away 1–2 electrons in solution. EMFs inherit this property from empty fullerenes, and also add a new dimension to the redox behavior because endohedral clusters can exhibit their own redox activity despite their shielding by the carbon cage. This chapter provides a systematic overview of electrochemical properties of different classes of endohedral metallofullerenes. In particular, the balance between fullerene- and cluster-based redox activity in complex endohedral metallofullerenes is discussed using frontier molecular orbitals as a guide.

Development of non-chromatographic separation for endohedral metallofullerenes has been put forward by many research groups with the goal of more straightforward and less expensive alternatives to HPLC. This chapter describes the progress in non-chromatographic separation approaches utilizing redox properties of EMFs, including electrolysis-assisted separation, separation with the use of redox-active solvents and redox reagents, or the use of complexation of fullerenes with Lewis acids.

This chapter describes the studies of the endometallofullerene (EMF) ions in the gas phase by various mass spectrometric methods. Historically, mass spectrometry was the first method to indicate the possible existence of EMFs, which happened several years before the discovery of their bulk arc-discharge synthesis. The studies of the ions of EMFs in the gas phase often preceded studies by other techniques that require substantial quantities of EMFs and provided a lot of information on their electronic properties and reactivity. In this chapter, particular attention is devoted to the gas-phase reactions of EMF ions with small molecules, and to the studies of the electron affinity of EMFs.

Fullerenes are popular electron acceptors for their high electron affinity and low reorganization energy. Photoexcitation in endohedral fullerenes (EMFs)-based donor–acceptor dyads have been comprehensively studied in the past decade. Different donor moieties such as ferrocene, exTTF, zinc tetraphenylporphyrine, triphenylamine were successfully used to construct EMF-based donor–acceptor dyads, and the charge/energy transfer mechanisms between versatile donors and EMF acceptors were extensively investigated. Besides, the charge carrier mobility of solid EMFs and applications of EMFs in organic photovoltaics (OPVs) and photoelectrochemical (PEC) cells were also reviewed.

In this chapter, we discuss two topics concerning the inner space of hollow molecules such as fullerenes, nanotubes, and even potentially materials like metal-organic frameworks. The first topic describes the special properties of electronic states, whose orbitals are not bound to specific atoms, but rather confined to vacuum region within the hollow molecules or materials. The second topic describes the dynamics of endohedral clusters within hollow molecules, whose motion can be manipulated by inelastic electron scattering, in order to realize a single-molecule switch.

The electron spin resonances of metallofullerenes are helpful to disclose their geometry, electronic structures, and internal dynamic modes though analyzing the hyperfine couplings of their ESR spectra. The chapter describes the electron spin resonance and the tunable paramagnetic properties of M@C₈₂ (M = Sc, Y, La), Sc₃C₂@C₈₀, Y₂@C₇₉N, TiSc₂N@C₈₀, and others. The electron spin manipulation based on these paramagnetic species has been realized by changing temperatures and exohedral modification.

The chapter describes electron spin resonance (ESR) spectroscopic studies of lanthanide endohedral metallofullerenes. A summary of recent investigations of La@C₈₂, La₂@C₈₀, and their reduced anion and radical species is presented. Also discussed are ESR studies of lanthanide metallofullerene with high spin state, including Eu@C₇₄, Eu@C₈₂, Gd@C₈₂, and Gd₂@C₇₉N.

This chapter describes EPR spectroscopic studies of the charged states of dimetallofullerenes, carbide clusterfullerenes, nitride clusterfullerenes, and their derivatives, as well as oxide clusterfullerene Sc₄O₂@C₈₀ and cyano-clusterfullerene Sc₃CN@C₈₀. Spin density distribution in the cation and anion radicals resemble HOMO and LUMO of the neutral molecules. Therefore, EPR spectroscopic studies of the ion radicals of endohedral metallofullerenes provide important information on the electronic structure of pristine molecule. The size of the metal-based hyperfine coupling constants in the ion radicals reveals the extent at which the spin density is localized on the endohedral species or on the fullerene cage. Besides, EPR spectra of ion radicals can provide information on the internal dynamics of the cluster. In particular, freezing rotation of the Sc₃N cluster in the derivative of Sc₃N@C₈₀ is revealed via EPR spectra of their anion radicals.

Paramagnetic NMR spectroscopy of EMFs with paramagnetic metal ions can deliver information on the structural and magnetic properties of EMFs. For a series of Ce-EMFs, analysis of the temperature dependence of chemical shifts in NMR spectra proved that the main contribution to the paramagnetic shifts is the dipolar “pseudocontact” term. As this term includes geometric parameters of the molecule, position and dynamics of endohedral metal atoms can be deduced from variable-temperature NMR spectra. For lanthanide-based nitride clusterfullerenes, pNMR provide information on the sign and size of the magnetic anisotropy of lanthanide ions. The influence of the cluster geometry on the paramagnetic chemical shifts is then discussed for HoM₂N@C₈₀ (M = Sc, Y, Lu) nitride clusterfullerenes.

This chapter summarizes the investigations of the endohedral metallofullerenes (EMFs) with a C₈₀ carbon shell in view of their magnetic properties, where the recently discovered single-molecule magnetism in dysprosium based species are highlighted.

Over the past two decades, nonmetallic endohedral fullerenes containing most of the noble gases and several small molecules have been prepared from C₆₀ and a few other closed- and open-cage fullerenes and isolated in sufficient quantities and purity to be characterized by a variety of spectroscopic and other physical methods. Of particular interest has been determining the effects of encapsulation on the properties both of the cage and of the trapped atom or molecule. Nuclear magnetic resonance, which is independent of the optical properties of the fullerene or the medium, and often insensitive to impurities, has revealed many details of the structure and dynamics of the intracage environment. Low-temperature infrared spectroscopy at both long and short wavelengths, inelastic neutron scattering (INS), and low-temperature solid-state NMR have been used to study the coupled translation–rotation of H₂ and H₂O molecules trapped in C₆₀. Special attention has been paid to detecting, enriching, and monitoring the stability of the para and ortho nuclear spin–rotational isomers of H₂ and H₂O in the endohedral environment with a view toward using the fullerene cage as a “bottle” for storing or releasing the isomers in condensed media under controlled conditions.

Endohedral fullerene N@C₆₀, which is formed by C₆₀ plus an atomic nitrogen located in the centre of the fullerene cage, has attracted interest, due to its exotic properties, such as exceptionally long spin relaxation time. Its preparation and chemical functionalization are discussed in this chapter; those are the most important topics in N@C₆₀ field, as preparation directly affects the availability of this new material, and chemical functionalization enables us to control and tune its properties. Herein, we will firstly introduce different synthesis, purification, and characterization methods of N@C₆₀, then move on to N@C₆₀ stability. At the end, a variety of N@C₆₀-related chemical reactions are reviewed, with emphasis on how the properties of the molecule change after functionalization.

This chapter reviews the present state of the art in using the endohedral fullerenes N@C₆₀ and P@C₆₀ as qubits in a spin quantum computer. After a brief introduction to spin quantum computing (Sect. 14.1), we first discuss (Sect. 14.2) the rich spin structure of these endohedral fullerenes and specific theoretical proposals for architectures and operation models leading to a scalable quantum computer. We then briefly discuss (Sect. 14.3) those aspects of materials science that are needed to realize the proposed architectures. The central part of this chapter (Sect. 14.4) is a review of quantum operations and entanglement realized with endohedral fullerenes. Finally, we review (Sect. 14.5) efforts to realize single spin detection of endohedral fullerenes and conclude (Sect. 14.6) with a brief outlook on outstanding problems to be solved for obtaining a scalable quantum register.