Germanium in Carbon Fullerenes: Quantum-Chemical Insights into Substitution, Adsorption, and Encapsulation Phenomena

Germanium (Ge) incorporation profoundly modifies the structural and electronic characteristics of carbon fullerenes, giving rise to a diverse landscape of substitutional, exohedral, and endohedral Ge-fullerene architectures. Although experimental studies demonstrate that Ge can be introduced into fullerene matrices through nuclear recoil implantation and arc-discharge synthesis, only exohedral germylated derivatives have been structurally confirmed to date. Substitutional germanium-doped fullerene (Ge-C₆₀) species remain experimentally elusive, with available evidence relying largely on radiochemical signatures and indirect spectroscopic data. In contrast, computational investigations provide a detailed and coherent picture of germanium doping across fullerene sizes, showing that Ge induces significant cage distortion, breaks local symmetry, narrows the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, and enhances charge localization at the dopant site. These electronic perturbations strongly increase the affinity of Ge-doped fullerenes for external guest molecules, leading to enhanced adsorption energies and distinct optical and transport responses in exohedral complexes. Theoretical studies of endohedral systems further indicate that Ge atoms or small clusters could form stable encapsulated species with unique electronic properties. Collectively, current evidence positions germanium-doped fullerenes as electronically versatile nanostructures with potential applications in sensing, optoelectronics, catalysis, and nanomedicine, while highlighting the need for definitive experimental synthesis and structural validation of substitutional Ge-fullerene derivatives.