Polycation Engineering in Polymeric Metal Halides Enables Tunable and Multicolor Emission from Single Transition‐Metals

Zero-dimensional metal halides have emerged as a versatile platform for the development of light-emitting materials, but achieving tunable or even multicolor emission from a material containing a single type of metal has proven highly challenging. Here, we leverage the "structural tolerance" of recently-developed polymeric metal halides to integrate two distinct coordination units of a single metal into a material, thereby achieving highly tunable optical properties in single-phase metal halides. By manipulating the steric hindrance of polycations, facilely adjustable green, red or bicolor emission can be realized in manganese bromides, which originates from controllable transformation from manganese-bromine tetrahedra into octahedra. This design principle is further extended to polymeric copper halides, wherein broad self-trapped exciton emissions derived from distinct copper-iodine polyhedrons allow the emission colors to be linearly tunable from blue to yellow, encompassing pure white light, by tailoring the composition, excitation wavelength, or temperature. This study opens avenues for facile and precise modulation of the optical properties of metal halides by exploiting the intrinsic coordinative diversity of metal elements.