Excitation-Selective Hydrochromic Upconversion in Zero-Dimensional Metal Halides

Smart luminescent materials that dynamically respond to external stimuli are of considerable interest for optical readout and sensing applications. Here, we show that, in Cs₃GdCl₆-based zero-dimensional metal halides, the occurrence of hydrochromic switching is determined by the excitation condition itself. In particular, Cs₃Gd_0.8Er_0.2Cl₆ 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.