3. Synthesis and Characterization of Lithium-Ion-Containing Fullerene

The use of lithium plasma as a lithium ion source improved the efficiency of Li encapsulation in the C₆₀ cage. The black deposit obtained by this production process was investigated for isolation and purification of lithium-containing [60]fullerene. At first, extraction with 1-chloronapthalene was performed to increase the content of Li@C₆₀. However, inductively coupled plasma optical emission spectrometry in addition to routine laser desorption ionization time-of-flight mass spectrometry revealed that purity of Li@C₆₀ was only 5–8%, with the remainder being unreacted C₆₀. The molar ratio of Li@C₆₀ to C₆₀ ranged between 1:12 and 1:20, which was explained by charge transfer interaction among these molecules. To suppress this undesired interaction, the mixture containing Li@C₆₀ and C₆₀ was oxidized with an ammoniumyl oxidant. This treatment afforded lithium-ion-containing [60]fullerene salt, [Li⁺@C₆₀]X⁻ as a pure PF₆ salt by HPLC separation through the use of an additional electrolyte in the mobile phase. Li⁺@C₆₀ was fully characterized by mass spectrometry, NMR, UV–Vis absorption and IR spectroscopy, and electrochemical measurements. The data conclusively revealed the properties of Li⁺@C₆₀, showing that it had almost the same orbital energy gaps as C₆₀ and strong electron-accepting character.