Chiral Trigonal Pyramidal Germanium Halides for Enhanced Second Harmonic Generation

ABSTRACT 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 Ge 2+ 4 s 2 lone pair, we synthesized chiral germanium halides, ( R/S ‐THNA)GeX 3 (X = Br, I). Crystallographic and topological analyses confirm the complete cleavage of secondary Ge···X interactions, yielding isolated [GeX 3 ] − chromophores. Furthermore, guided by transition dipole moment calculations, ( R ‐THNA)GeI 1.75 Br 1.25 was constructed via a mixed‐halide strategy. Single‐crystal SHG measurements reveal that ( R ‐THNA)GeI 1.75 Br 1.25 achieves an exceptional response, being 1.66 and 9.44 times those of ( R ‐THNA)GeI 3 and ( R ‐THNA)GeBr 3 crystals, respectively. Standard Kurtz–Perry measurements of ( R ‐THNA)GeI 1.75 Br 1.25 at a 125–150 µm particle size reveal a powder SHG efficiency of 3.06 times that of KH 2 PO 4 , 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.