Lithium salt electride with an excess electron pair - A class of nonlinear optical molecules for extraordinary first hyperpolarizability

A new lithium salt electride with an excess electron pair is designed, for the first time, by means of doping two sodium atoms into the lithium salt of pyridazine. For this series of electride molecules, the structures with all real frequencies and the static first hyperpolarizability (β₀) are obtained at the second-order Møller-Plesset theory (MP2). Pyridazine H₄C₄N₂ becomes the lithium salt of pyridazine Li-H₃C₄N₂ as one H atom is substituted by Li. The lithium salt effect on hyperpolarizability is observed as the β₀ value is increased by about 170 times from 5 to 859 au. For the electride effect, an electride H₄C₄N ₂⋯Na₂ formed by doping two Na atoms into pyridazine, the β₀ value is increased by about 3000 times from 5 to 1.5 × 10⁴ au. Furthermore, combining these two effects, that is, lithium salt effect and electride effect, more significant increase in β₀ is expected. A new lithium salt electride Li-H ₃C₄N₂⋯N₂ is thus designed by doping two Na atoms into Li-H₃C₄N₂. It is found that the new lithium salt electride, Li-H₃C₄N ₂⋯Na₂, has a very large β₀ value (1.412 × 10⁶ au). The β₀ value is 2.8 × 10⁵ times larger than that of H₄C₄N ₂, 1644 times larger than that of Li-H₃C₄N ₂, and still 93 times larger than that of the electride H ₄C₄N₂⋯Na₂. This extraordinary β₀ value is a new record and comes from its small transition energy and large difference in the dipole moments between the ground state and the excited state. The frequency-dependent β is also obtained, and it shows almost the same trends as H₄C₄N₂ ≪ Li-H₃C₄N₂ ≪ H₄C₄N ₂⋯Na₂ ≪ Li-H₃C₄N ₂⋯Na₂. This work proposes a new idea to design potential candidate molecules with high-performance NLO properties.