Theoretical Exploration of Electronic and Second-Order Nonlinear Optical Properties of Superalkali-(Superhalogen Endohedral) Fullerene Complexes: Ta@Si ₁₆ –C ₆₀ and Ta@Si ₁₆ –BO ₂ @C ₆₀

To develop novel high-performance deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials, two series of complexes (Ta@Si16–C60 and Ta@Si16–BO2@C60) were designed by integrating the superalkali Ta@Si16, C60 fullerenes, and the superhalogen BO2. The results indicate that all complexes possess high structural stability, as reflected by significantly negative interaction energies. Compared to Ta@Si16–C60, the corresponding Ta@Si16–BO2@C60 complexes exhibit significant enhancements in both linear and nonlinear optical properties, which are reflected by their isotropic polarizabilities and first hyperpolarizabilities, respectively. Taking the most stable systems (A for Ta@Si16–C60; D-BO2, and E-BO2 for Ta@Si16–BO2@C60) as representative cases, the spatial contribution and structural origin of the hyperpolarizability were clarified through analyses of hyperpolarizability tensor and hyperpolarizability density, respectively. Molecular orbital and hole–electron analyses of crucial excited states further deepen the understanding of the electronic excitation nature of these key complexes. UV–vis absorption spectra confirm that all complexes have a deep-UV transparent region (≤200 nm), highlighting their potential as new deep-UV NLO molecular candidates. This work provides valuable insights for the rational design of high-performance deep-UV NLO materials based on superatom-based complexes.