Positional Isomerism Controls Polarity and Nonlinear Optical Properties in One‐Dimensional Hybrid Germanium Halides

ABSTRACT Organic–inorganic hybrid perovskites (OIHPs) exhibit abundant electronic configurations and structural versatility, rendering them promising candidates for photovoltaic and optoelectronic applications. Despite significant progress in optimizing the structural characteristics of organic cations and inorganic frameworks, the role of cations in determining the electronic structure and nonlinear optical properties has long been underappreciated and remains unclear. We report herein three one‐dimensional germanium‐halide perovskites, AGeI 3 , templated by methylimidazolium cation (A = 1‐Mim/2‐Mim/4‐Mim) positional isomers. Controlling the methyl substitution site on the organic cation can engineer polar structures with distinctly different key optical properties, such as second‐harmonic generation (SHG) and birefringence. (1‐Mim)GeI 3 exhibits the strongest powder SHG response of the OIHP crystals (13 × KH 2 PO 4 @1200 nm) and significant birefringence (0.263 @546 nm). Structural analyses and first‐principles calculations reveal that the SHG response originates from synergism between the [GeI 6 ] and π‐conjugated [C 4 N 2 H 7 ] units, with the unprecedented SHG enhancement in (1‐Mim)GeI 3 being primarily attributed to the asymmetric electron distribution arising from N1‐methyl substitution that enables the favorable ordered alignment of the π‐conjugated 1‐Mim. Our findings not only highlight the critical role of cation positional isomers in controlling physical properties in one‐dimensional hybrid perovskites but also establish one‐dimensional germanium‐iodide perovskites as promising lead‐free candidates for nonlinear optoelectronic applications.