facile bandgap tuning. Earlier works on photovoltaics rely on lead iodide and lead bromide based perovskites to achieve devices with different light-absorption profile and open-circuit voltages. [1,17] However, the true benefit of bandgap tuning is realized in lightemitting devices, where strong and narrow line-width emission can be conveniently produced across an extensive ultraviolet to visible to near-infrared spectral region. [6,18] The tailoring of perovskite bandgap and light-emission profile is typically achieved through the variation of halides (i.e., chloride, bromide, or iodide) in the chemical precursors, or via postsynthetic halide exchange. Various strategies for halide exchange have been reported in the literature, [19][20][21][22] and are typically attributed to a direct exchange of anionic halide species. Here, we report a photo-activated halide exchange process between cesium lead halide perovskite nanocrystals and a variety of molecular haloalkanes to achieve full spectral tuning in the visible region. We show, through detailed mechanistic studies, that this process is surface-mediated and involves the photo-activated breakage of covalently linked carbon-halogen bonds. This is consistent with our earlier reports, where we show that perovskite nanocrystals are capable of performing surface-mediated photocatalytic reactions. [23,24] Crucially, we demonstrate that our halide exchange approach could be employed in the micrometer-sized patterning of perovskite films to achieve trichromatic light emission, which has important utility in the manufacturing of functional color displays.We prepared luminescent cesium lead bromide (CsPbBr 3 ) nanocrystals following literature-reported methods, [25] and dispersed the nanocrystals into dichloromethane (DCM) solvent. We added 0.007 m of thiophenol into the solution, which we show in our previous work [23] is capable of enhancing the photo luminescence (PL) of the perovskite nanocrystals. The solution was then illuminated with a coiled white LED strip (1 m, 9 W). The reaction progress was tracked by extracting aliquots of the reaction mixture at timed intervals, followed by the measurement of their PL spectra.As shown in Figure 1a, the PL of the nanocrystal solutions blueshifted monotonically from 523 nm (green) at 0 h to 447 nm (blue) at 8 h. This observation suggests that the bromide ions in CsPbBr 3 were progressively replaced by chloride Lead halide perovskite possesses a semiconductor bandgap that is readily tunable by a variation in its halide composition. Here, a photo-activated halide exchange process between perovskite nanocrystals and molecular haloalkanes is reported, which enables the perovskite luminescence to be controllably shifted across the entire visible spectrum. Mechanistic investigations reveal a mutual exchange of halogens between the perovskite crystal surface and a chemisorbed haloalkane, yielding nanocrystals and haloalkanes with mixed halide contents. Exchange kinetics studies involving primary, secondary, and tertiary haloalkanes sho...