Chiral Trigonal Pyramidal Germanium Halides for Enhanced Second Harmonic Generation

Chiral organic-inorganic hybrid metal halides are highly promising candidates for second-order nonlinear optical (NLO) materials. However, their NLO performance is fundamentally restricted by the limited structural polarization inherent to traditional pseudo-octahedral coordination. In this work, we demonstrate the feasibility of using highly polar tricoordinate trigonal pyramids to overcome the thermodynamic constraints of the octahedral cages, enabling a structural shift from symmetric hexacoordination to discrete structures, thus enhancing second harmonic generation (SHG). By introducing the massive steric hindrance of the chiral R/S-1,2,3,4-tetrahydro-1-naphthylamine (R/S-THNA) cation with the intrinsic stereochemical activity of the Ge²⁺ 4s² lone pair, we synthesized chiral germanium halides, (R/S-THNA)GeX₃ (X = Br, I). Crystallographic and topological analyses confirm the complete cleavage of secondary Ge···X interactions, yielding isolated [GeX₃]^- chromophores. Furthermore, guided by transition dipole moment calculations, (R-THNA)GeI_1.75Br_1.25 was constructed via a mixed-halide strategy. Single-crystal SHG measurements reveal that (R-THNA)GeI_1.75Br_1.25 achieves an exceptional response, being 1.66 and 9.44 times those of (R-THNA)GeI₃ and (R-THNA)GeBr₃ crystals, respectively. Standard Kurtz-Perry measurements of (R-THNA)GeI_1.75Br_1.25 at a 125-150 µm particle size reveal a powder SHG efficiency of 3.06 times that of KH₂PO₄, representing a highly competitive value in chiral germanium halides. This work establishes a rational paradigm for designing high-performance NLO materials through precise coordination tailoring.