Hydrogen-bond-triggered hydrochromic zero-dimensional metal halides for a smart visual luminescent humidity sensor

Hydrogen-bond-triggered reversible hydration/dehydration in 0D metal halides • Mild-condition (50–80 °C) and auxiliary-free hydration/dehydration • Sn 2+ -activated 30 nm emission shift with linear humidity response • Portable visual sensor for electronic packaging humidity monitoring Metal halides are attractive for hydrochromic moisture sensing due to their tunable structures, but the existing systems based on metal-coordinated water suffer from high dehydration temperatures and complex reaction pathways. Herein, we report a distinct hydrogen-bond-triggered hydrochromic mechanism in zero-dimensional (0D) metal halides, realized through a reversible phase transition between hydrated (C 5 H 14 N 2 ·H 2 O)CdBr 4 and dehydrated (C 5 H 14 N 2 )CdBr 4 . In contrast to conventional metal-coordinated water systems, the weak, non-coordinative hydrogen bonds in this system enable a facile and reversible hydration/dehydration cycle at a mild temperature (84 °C) without auxiliary reactants or by-products, while also allowing water molecules to be stored within the lattice during hydration. Doping with Sn 2+ results in a fully reversible 30 nm emission shift, from 630 nm in the dehydrated phase to 660 nm in the hydrated phase, exhibiting a linear response across a broad humidity range (32–82% RH). Leveraging its high stability and recyclability, we fabricate a portable humidity sensor that allows for on-site, visual monitoring of moisture intrusion, offering a promising platform for instrument-free electronic packaging inspection. Download: Download high-res image (328KB) Download: Download full-size image