Luminescence Change from Orange to Blue for Zero‐Dimensional Cs 2 InCl 5 (H 2 O) Metal Halides in Water and a New Post‐doping Method

Zero‐dimensional metal halides have attracted much attention due to their attractive photoelectric properties. Here, we propose a new strategy of synthesizing metal halides crystals by recrystallization in water. The as‐synthesized Cs 2 InCl 5 (H 2 O)‐orange crystals are dissolved and recrystallized in water ( Cs 2 InCl 5 (H 2 O)‐blue ), with its photoluminescence (PL) changing from orange to blue, both of which are derived from self‐trapping excitons (STEs). The time‐resolved photoluminescence (TRPL) spectrum of Cs 2 InCl 5 (H 2 O)‐blue shows that it has an ultralong lifetime up to milliseconds (τ=52.98 ms), which is expected to be applied in biological sensors. The photoluminescence quantum yield (PLQY) increases from 2.25% to 11.61% in the self‐assembly process. By using a post‐doping method, the PL of crystals turns into red when we introduce Mn 2+ as dopant while there is no obvious change upon using a traditional solvent‐thermal method. Recrystallization in water and post‐doping provide a new perspective for the synthesis and doping of metal halides.