Endohedral fullerenes represent a unique class of nanostructures where atoms, ions, or clusters are trapped within the hollow interior of a carbon cage. The most prominent examples include metallofullerenes, which encapsulate metal atoms or small metal-containing clusters. These structures exhibit distinct electronic, magnetic, and chemical properties due to the confinement effect, making them valuable for applications in materials science, electronics, and medicine.

The synthesis of endohedral fullerenes primarily involves three methods: arc discharge, laser ablation, and ion implantation. Arc discharge is the most widely used technique, where graphite rods doped with metal oxides or carbides are vaporized in an inert atmosphere. The resulting plasma condenses to form soot containing empty fullerenes and endohedral species. Metallofullerenes such as La@C82 and Sc3N@C80 are commonly produced this way. Laser ablation offers a more controlled approach, where a pulsed laser vaporizes a metal-graphite composite target, yielding endohedral fullerenes with higher selectivity for certain cage sizes. Ion implantation, though less common, involves accelerating ions into pre-formed empty fullerenes, forcing them to penetrate the carbon cage.

Encapsulated species in endohedral fullerenes vary widely, from single atoms to small clusters. Noble gases like helium and argon can be trapped, forming He@C60 or Ar@C60. Metal atoms, particularly lanthanides (e.g., La, Ce, Gd) and group 3 elements (e.g., Sc, Y), are frequently encapsulated due to their ability to transfer electrons to the carbon cage. Cluster fullerenes, such as Sc3N@C80 or Ti2C2@C78, demonstrate the versatility of these structures, where the encapsulated cluster stabilizes the fullerene cage through charge transfer and geometric constraints.

The electronic properties of endohedral fullerenes are heavily influenced by the encapsulated species. For instance, in La@C82, the lanthanum atom donates three electrons to the carbon cage, resulting in a La3+@C823- configuration. This charge transfer alters the electronic structure, creating a hybridized system with unique redox behavior. The energy levels of the fullerene cage shift, leading to modified optical absorption and emission properties. Similarly, Sc3N@C80 exhibits a closed-shell electronic structure due to the transfer of six electrons from the Sc3N cluster to the cage, enhancing its stability and making it less reactive than empty C80.

Magnetic properties arise when paramagnetic species are encapsulated. Gadolinium-based metallofullerenes, such as Gd@C82 or Gd3N@C80, display strong paramagnetism due to the unpaired 4f electrons of Gd3+. These systems are explored for magnetic resonance imaging (MRI) contrast agents, as the shielding effect of the carbon cage reduces toxicity while preserving magnetic moments. The spin dynamics of encapsulated clusters, such as N@C60 (a nitrogen atom with three unpaired electrons), are also of interest for quantum computing due to their long spin coherence times.

The confinement effect within the carbon cage leads to unusual chemical behavior. Encapsulated species are isolated from external environments, preventing aggregation or oxidation. For example, U@C28, a uranium-containing metallofullerene, stabilizes uranium in an unusual +4 oxidation state due to the protective carbon shell. Similarly, reactive species like hydrogen molecules (H2@C60) can be stored and studied without degradation. The internal pressure exerted by the encapsulated species can also distort the fullerene cage, as seen in small-cage metallofullerenes like Ca@C36, where the carbon framework adapts to accommodate the metal ion.

Characterization of endohedral fullerenes relies on advanced techniques. Mass spectrometry identifies the molecular weight and composition, while nuclear magnetic resonance (NMR) spectroscopy reveals the symmetry and dynamics of the encapsulated species. Electron paramagnetic resonance (EPR) is critical for studying paramagnetic metallofullerenes, providing insights into spin density distribution. X-ray crystallography has resolved the structures of many endohedral fullerenes, confirming the positions of encapsulated atoms or clusters within the cage.

Applications of endohedral fullerenes exploit their unique properties. In electronics, they serve as n-type semiconductors or components in molecular transistors due to their tunable electronic structures. Biomedical applications include MRI contrast agents, where gadolinium metallofullerenes offer superior performance compared to traditional chelates. The encapsulation of radioactive isotopes, such as Ho@C82, is explored for targeted radiotheraphy, as the carbon cage minimizes leakage of toxic ions. Energy storage systems benefit from endohedral fullerenes as additives in lithium-ion batteries, where their redox activity enhances charge capacity.

Despite their potential, challenges remain in the production and purification of endohedral fullerenes. Yields are often low, and separation from empty fullerenes requires high-performance liquid chromatography (HPLC) or chemical functionalization strategies. Scalable synthesis methods are needed to meet industrial demands. Additionally, the long-term stability and toxicity of these materials in biological systems require further investigation.

The study of endohedral fullerenes continues to expand, with recent discoveries including multi-metal clusters (e.g., Y2C2@C82) and non-metal encapsulations (e.g., P@C80). Theoretical calculations predict new stable configurations, guiding experimental efforts. As synthetic techniques advance, the range of encapsulated species and their applications will grow, solidifying the role of endohedral fullerenes in nanotechnology. Their ability to combine the properties of the carbon cage with those of the trapped species creates a platform for designing materials with tailored functionalities.