Unique Unilateral-Chelated Mode-Induced d–p−π Interaction Enhances Second-Harmonic Generation Response in New Ln₃LiMS₇ Family

Multiple chelated configurations of anionic groups offer a great chance to design new nonlinear optical (NLO) materials with excellent performances for infrared (IR) application. In this work, the new polar Ln3LiMS7 (Ln = rare-earth; M = Ge, Sn) family as promising IR NLO candidates were successfully designed and synthesized by the unique close-ring combination between LnS8 and MS4 ligands. Note that La3LiGeS7 and La3LiSnS7 exhibit the good NLO effects (0.7 vs 1.2 × benchmark AgGaS2) with phase-matching behavior and high laser damage thresholds (6.0 vs 2.5 × AgGaS2), respectively. First-principles calculation indicates that their NLO effects originate from the synergistic effect of intrinsic dipole moments and d–p delocalized-π electron-induced dipole oscillations in the Ln3LiMS7 family. This result also verifies that unique unilateral-chelated configuration induced d–p−π interactions are beneficial to improve the NLO performances, which offers us one feasible way (incorporation of Ln-centered ligands into crystal structure) to design new promising IR NLO materials.