Hydrogen‐Bond‐Directed Assembly of Steric Networks in Two‐Dimensional Germanium, Tin, and Lead Halide Perovskites: Highly Efficient, Ultranarrow Emission and Nonlinear Optical Properties

In two‐dimensional (2D) metal halide perovskites, the structural and photophysical properties are critically governed by interlayer organic cations. While steric effects and weak van der Waals interactions typically dominate spacer assembly, we exploit geometrically defined intercarboxyl hydrogen bonds to achieve precise lattice control. Using protonated 4‐aminomethylcyclohexanecarboxylic acid (4‐AMCA + ) as the spacer, we synthesized ten Ruddlesden–Popper perovskites ( B = Pb, Sn, Ge; X = Br, I; n = 1–3). Single‐crystal analysis reveals two distinct hydrogen‐bonded assemblies: rigid, extended helical chains (dominant) and isolated dimers (exclusive to germanium bromide compounds). The helical chains suppress out‐of‐plane octahedral tilting, amplify lattice rigidity, and enable exceptional optoelectronic performance including free exciton emission in (4‐AMCA) 2 GeI 4 (rare for Ge‐based halide perovskites), efficient blue emission in (4‐AMCA) 2 PbBr 4 (PLQY = 48%), and ultranarrow emission of FWHM = 14 nm in (4‐AMCA) 2 PbI 4 . In contrast, dimer‐based germanium bromides exhibit minimal bandgaps yet nonemissive behavior. We further correlated the effective coordination number of individual metal halide octahedron to bandgaps in 2D Ge‐based halide perovskites. (4‐AMCA) 2 GeI 4 exhibited nonlinear optical properties, contrasting with inactive (4‐AMCA) 2 (MA)Ge 2 Br 7 . We validate the hydrogen bonding, not merely steric constraints, is the essential ingredient for confining lone‐pair electron activity and enhancing emission for halide perovskite materials.