Structural, Electronic, and Nonlinear Optical Properties of C₆₆H₄ and C₇₀Cl₆ Encapsulating Li and F Atoms

Recently, nonclassical fullerene derivatives C₆₆H₄ and C₇₀Cl₆, which both contain two negatively curved moieties of heptagons, have been successfully synthesized. Inspired by these experimental achievements, the structural and electronic properties of C₆₆H₄, C₇₀Cl₆, Li@C₆₆H₄, F@C₆₆H₄, Li@C₇₀Cl₆, and F@C₇₀Cl₆ were systematical studied through density functional theory calculations in this work. Our results show that the reduction of the front molecular orbital gap of fullerene derivatives occurs with the introduction of Li and F atoms. After quantitative analysis of back-donations of charge between an encapsulated atom and an external carbon cage, it is found that C₆₆H₄ and C₇₀Cl₆ prefer to act as electron acceptors. It is interesting to note that the strong covalent nature of the interactions between a F atom and a carbon cage is observed, whereas the weak covalent and strong ionic interactions occur between a Li atom and a carbon cage. On the other hand, according to the first hyperpolarizability results, the encapsulation of the Li atom enhances the nonlinear optical response of fullerene derivatives. This work provides a strategy to improve nonlinear optical properties of C₆₆H₄ and C₇₀Cl₆, reveals the internal mechanism of the contribution from Li and F atoms to endohedral fullerene derivatives, and will contribute to the designation of endohedral fullerene derivative devices.