Mixed superalkalis are a better choice than pure superalkalis for B12N12 nanocages to design high-performance nonlinear optical materials

Abstract

Mixed superalkali clusters are a source of excess electrons, as their vertical ionization energies (2.81–3.36 eV) are much lower than those of alkali metals (even cesium (∼3.85 eV)) and the superalkali Li₃O (3.42 eV). In the present work, the geometric, electronic, and nonlinear optical (NLO) properties of mixed superalkali cluster-doped B₁₂N₁₂ nanocages are studied theoretically. All complexes, A–G, have very high interaction energies (−98.02 to −123.13 kcal mol⁻¹) and are thermodynamically stable when compared to previously reported Li₃O@B₁₂N₁₂ (−92.71 kcal mol⁻¹). The designed complexes have smaller HOMO–LUMO energy gaps (3.36–4.27 eV) than pristine B₁₂N₁₂ (11.13 eV). Charge transfer in the complexes is studied through natural population analysis and non-bonding interactions are evaluated through quantum theory of atoms in molecules (QTAIM) and non-covalent interaction analyses. These complexes have absorption maxima (1076–1486 nm) in the near-infrared region (NIR) and they are transparent in the UV region. The first hyperpolarizability of complex C is 1.7 × 10⁷ au, which is much higher than the value of 3.7 × 10⁴ au for a pure Li₃O superalkali-doped B₁₂N₁₂ complex calculated at the same level of theory, as reported by Sun et al. (Dalton Trans., 2016, 45, 7500–7509). The large second hyperpolarizability values also reflect the enhanced nonlinear optical response. The best computed values for the electro-optical Pockels effect, second harmonic generation, and hyper-Rayleigh scattering are 3.29 × 10¹⁰ au, 1.17 × 10¹⁰ au, and 6.71 × 10⁶ au, respectively. Furthermore, the electro-optic dc-Kerr effect and electric-field-induced second harmonic generation have maximum values of 3.96 × 10¹¹ au and 3.46 × 10¹⁰ au at 1064 nm. There are enhancements in the quadratic nonlinear refractive index (n₂) values for complexes A–G, with a highest n₂ value of 3.35 × 10⁻⁸ cm² W⁻¹ at 1064 nm. These results suggest that mixed-superalkali-doped B₁₂N₁₂ nanoclusters are potential candidates when designing high-performance NLO materials.