Novel inorganic aromatic mixed-valent superalkali electride CaN3Ca: an alkaline-earth-based high-sensitivity multi-state nonlinear optical molecular switch

Abstract

Focusing on innovative high-performance single-pole double-throw nonlinear optical (NLO) molecular switches, two C_3v configurations (1 and 3) and one D_3h configuration (2) of bipyramidal CaN₃Ca have been obtained by using quantum mechanical methods. Not only are 1, 2, and 3 alkaline-earth-based aromatic superalkalis, but they are also interesting electrides. The salt-like electronic structures of e⁻⋯Ca²⁺N₃³⁻Ca²⁺ (1) and Ca²⁺N₃³⁻Ca²⁺⋯e⁻ (3) with localized redox centres are rare inorganic Robin–Day class II-type structures, and e^0.5−⋯Ca²⁺N₃³⁻Ca²⁺⋯e^0.5− (2) with a delocalized structure is a class III-type mixed-valent superalkali electride. Under a small external electric field of ±0.0110 a.u. (0.565 V Å⁻¹), the short-distance hopping of Ca atoms in CaN₃Ca from the D_3h configuration with in-plane aromaticity to each C_3v configuration with out-of-plane aromaticity brings about the long-range transfer of half an electron from one Ca atom to another. And, subsequently, a large dipole moment (μ₀) and remarkable static first hyperpolarizability (β₀) occur. μ_z and β_zzz range from 0 (D_3h, off form) to −12.1 or 12.1 D (C_3v, on forms) and from 0 (D_3h, off form) to −19 428 or 19 428 a.u. (C_3v, on forms), respectively. These extremely large differences in μ_z and β_zzz values between the D_3h and each of the C_3v configurations confirm the potential of these inorganic aromatic Robin–Day-type superalkali electrides for applications in high-sensitivity multi-state nonlinear optical switches.