The affinity of chloride/iodide ions to replace the lattice sites of bromide ions in the CsPbBr3 nanocrystals by anion exchange reactions is investigated. In the presence of an equal number of iodide and chloride ions, the absorption and emission of lead halide perovskites are found to be red‐shifted as a result of the preferential anion exchange with the iodide ions. In order to initiate the anion exchange reactions with the chloride ions from a mixture of chloride and iodide ions, a minimum of 66 % of chloride ions, is essential to observe a significant change. The differential affinity of chloride/iodide ions to substitute bromide ions in CsPbBr3 perovskite is also substantiated using density functional theory (DFT) calculation. Understanding the interactions in the crystal structure of lead halide perovskites can give directions to tune the material properties in the nanoscale for various applications in science and technology.

Photocatalytic solvent-assisted anion exchange of lead halide perovskites are extremely important, as it can tune the redox properties/band positions of the perovskite nanocrystals. Among that, the generation of chloride ions from the solvents chloroform and dichloromethane by accepting an electron from the photoexcited CsPbBr 3 nanocrystals and the resulting anion exchanges are investigated. The redox potentials of the solvent molecules and the band positions of the semiconductor allow for the photoinduced electron transfer process, thermodynamically, in these two solvents. However, the presence of a better electron acceptor N-bromosuccinimide (NBS) can prevent the photocatalytic solvent-assisted chloride ion exchange reactions in CsPbBr 3 . The preferential interaction of Pb 2+ of lead halide perovskites with the bromide ions in preference to Cl − , as a result of Pearson's hard soft acid base theory, stabilizes CsPbBr 3 nanocrystals. This is confirmed by using bromine-containing chloroform as the solvent. Interestingly, in the presence of NBS/ bromine, the anion exchange with chloride ions formed by the reduction of solvents can be prevented. The present investigations can give insights into the mechanistic aspects of the anion exchange reaction and suggest new methodologies to stabilize CsPbBr 3 nanocrystals in chlorine-containing solvents.