External electric field-induced remarkable enhancement of the hyperpolarizability in aza-12-crown-4-based alkaline earthide complexes.

This study explores the structural and electronic properties of a new series of alkaline earthides using aza-12-crown-4 (abbreviated as a-12c-4), where alkali metals (Li-K) serve as electron donors to exohedrally positioned alkaline earth metals (Be-Ca), forming M+(a-12c-4)M-. The energetic favorability and electronic stabilization of the designed materials were evaluated through ionization potential, chemical potential, hardness, softness, and interaction energy analyses at the applied level of theory. Further investigations, including frontier molecular orbital, molecular electrostatic potential, electron density difference, and natural bond orbital, reveal the localization of the HOMO, excess electrons and negative charge over alkaline earth metals (AEMs), hence confirming their earthide nature. Additionally, the partial density of the state spectra was observed, which clearly shows the position of the HOMO over the AEMs. A non-covalent interaction analysis is performed, which confirms the presence of van der Waals forces between the doped metals in the complexes. The designed complexes exhibit a small energy gap (E g) between their HOMO and LUMO (2.91 to 3.50 eV) compared with the pristine cage (9.93 eV). Two-level model analysis indicates that the complexes exhibit low excitation energy, i.e., 0.87 to 3.44 eV. These low energies correlate with the enhanced hyperpolarizability of the K+(a-12c-4)Ca- complex up to 1.86 × 105 a.u. UV-vis absorption analysis indicates that the complexes are transparent to ultraviolet light. Additionally, after applying an external electric field (EEF) of 10 × 10-4 a.u. strength, a 100-fold increase was noted in the hyperpolarizability of Li+(a-12c-4)Be-, i.e., from 8.65 × 103 to 1.69 × 105 a.u. These findings suggest that the studied complexes may serve as promising candidates for the design of advanced NLO materials with enhanced optoelectronic performance.