Complexation‐Mediated Diffusion‐Limited Crystal Growth: A General Framework for Anisotropic Crystal Growth in Cu‐Based Perovskites

The rational synthesis of 1D lead‐free perovskite nanostructures remains challenging due to complex precursor chemistry that defies classical crystal growth models. Here, we establish and experimentally validate a Complexation‐Mediated Diffusion‐Limited Growth (CMDLG) framework that integrates coordination chemistry with mass transport kinetics to direct the growth of Cs 3 Cu 2 I 5 nanowires (NWs). This framework transforms precursor complexation from a synthetic hurdle into a powerful tool for directing anisotropic growth. By engineering the solvent environment to stabilize bulky iodocuprate complexes, we restrict mass transport and introduce a kinetic barrier, complex dissociation, at the growth front, driving the system into a diffusion‐limited regime. This enables tunable synthesis of faceted microwires in polar solvents and ultralong, uniform NWs (aspect ratio > 10 3 ) in low‐polarity 2‐pentanone. In situ optical microscopy captures complex‐rich zones along NW sidewalls and depletion zones at growing tips, providing direct evidence for CMDLG. The resulting single‐crystalline Cs 3 Cu 2 I 5 NWs exhibit excellent structural and optical quality, forming lyotropic liquid crystals after surface passivation and self‐assembly. Shear‐aligned, photopolymerized NW‐polymer composite films show strong perpendicular polarized emission ( p = 0.31) and enhanced chemical stability. The CMDLG framework provides a unified paradigm for understanding and predicting anisotropic growth in complex perovskite and related inorganic systems.