All-metal electride molecules CuAg@Ca7M (M = Be, Mg, and Ca) with multi-excess electrons and all-metal polyanions: molecular structures and bonding modes as well as large infrared nonlinear optical responses

Abstract

All-metal electride molecules, CuAg@Ca₇M (M = Be, Mg and Ca), have been designed and researched in theory for the first time. In these molecules, a pull–push electron relay occurs. Unusually, the all-metal polyanions of fourfold negatively charged [Cu–Ag–Be/Mg]⁴⁻ and [Cu–Ag]⁴⁻ with 4 extra electrons gained from Ca atoms push the remaining valence electrons of the Ca atoms forming the multi-excess electrons (Ne = 10/12). Therefore, these molecules can be described as salt-like [(Ca²⁺)₇(CuAgM)⁴⁻] + 10e⁻ (M = Be and Mg) and [(Ca²⁺)₈(CuAg)⁴⁻] + 12e⁻. In these salt-like molecules, there are extraordinary covalent bonding modes, which include 2c–2e/3c–2e σ-bonding in the polyanions and the Ca²⁺ cations sharing the diffuse multi-excess electrons. For an intriguing nonlinear optical (NLO) response, these all-metal electride molecules display large electronic first hyperpolarizabilities (β₀), thus a new class of NLO molecules, all-metal electride NLO molecules, emerge. Moreover, it is also found that manipulating the atomic number and position of M is a new strategy to enhance β₀. As a result, CuAg@Ca₇Mg(1) exhibits a considerable β₀ (1.43 × 10⁴ au), which is 16 times the β₀ sum of two isolated CuAg and Ca₇Mg(1) subunits, and this deeply reveals the fundamental origin of the considerable β₀, namely, the multi-excess electrons generated by the subunit interaction. These all-metal electride molecules have the infrared (IR) transparent region of 1.3–6 μm, and hence are new IR NLO molecules. In addition the electronic contribution, β₀, the large effects of vibrations on the static first hyperpolarizabilities of these all-metal electride molecules are also estimated. Thus, this study opens the new research field of all-metal electride IR NLO molecules.