Excitation‐Selective Hydrochromic Upconversion in Zero‐Dimensional Metal Halides

ABSTRACT Smart luminescent materials that dynamically respond to external stimuli are of considerable interest for optical readout and sensing applications. Here, we show that, in Cs 3 GdCl 6 ‐based zero‐dimensional metal halides, the occurrence of hydrochromic switching is determined by the excitation condition itself. In particular, Cs 3 Gd 0.8 Er 0.2 Cl 6 shows excitation‐selective hydrochromic color evolution under moisture exposure: it retains its yellow emission under 980 nm excitation, whereas it undergoes a rapid green‐to‐red shift under 1550 nm excitation under the same conditions. Spectroscopic and kinetic analyses reveal that this selective behavior originates from excitation‐pathway‐dependent population of moisture‐sensitive excited states, with the absorption at the excitation wavelength serving as a decisive factor. This excitation‐selective behavior was further exploited to implement excitation‐selective optical readout and a power‐free sticker‐type moisture dosimeter for quantitative analysis of accumulated moisture exposure based on Fick's second law. These findings establish an excitation‐selective switching criterion for hydrochromic upconversion in zero‐dimensional metal halides and provide a design principle for excitation‐selective optical moisture readout.