Crystallization Dynamics Governs Dimensionality Switching and Emission Pathways in 1,9-Nonanediaminium Lead Iodide...

Hybrid organic-inorganic perovskites present a diverse structural landscape in which dimensionality governs optoelectronic properties. Although chemical modification of organic spacers is commonly employed to tailor connectivity, the influence of crystallization kinetics on dimensional topology selection remains largely unexplored. This study demonstrates that the cooling rate during solution growth serves as a decisive control parameter, directing the self-assembly of nonanediaminium lead iodide perovskite (H3NC9H18NH3)PbI4 into either a thermodynamically stable two-dimensional (2D) layered phase or a kinetically trapped zero-dimensional (0D) cluster-based polymorph. Slow evaporation at elevated temperature produces the 2D layered structure, whereas rapid cooling to 50 °C or quenching in liquid nitrogen selectively yields the 0D phase, which consists of isolated face-sharing [Pb3I12]6- trimers. Single-crystal and powder X-ray diffraction confirm the structural purity of both polymorphs. Optical absorption measurements indicate a dimensionality-driven bandgap shift from 2.47 eV (2D) to 2.78 eV (0D). Low-temperature photoluminescence spectroscopy reveals fundamentally distinct emission mechanisms: the 2D polymorph exhibits narrow free-exciton emission, while the 0D polymorph displays intense broadband luminescence attributed to radiative recombination at anion-related defects, as evidenced by its correlation with lattice disorder. These results establish thermal history as an independent synthetic dimension, orthogonal to chemical design, enabling on-demand switching between excitonic and defect-mediated emission within a single composition. This approach provides a scalable pathway for engineering low-dimensional hybrid materials for tunable solid-state lighting, scintillators, and quantum-light sources.