Ultrafast water sensing and thermal imaging by a metal-organic framework with switchable luminescence

TLDR

A rapid, selective water analysis method is highly desirable for industrial and detection processes. The authors constructed a robust microporous Zn‑MOF whose dual‑emissive ligand undergoes reversible ESIPT, enabling single‑crystal‑to‑single‑crystal transformation upon removal or uptake of coordinating water molecules and producing two‑colour photoluminescence switching. This Zn‑MOF delivers a seconds‑scale, highly selective water response, and its ZnO‑based films enable humidity sensing below 1 % RH, detection of sub‑0.05 % water in organic solvents, thermal imaging, and temperature measurement.

Abstract

Abstract A convenient, fast and selective water analysis method is highly desirable in industrial and detection processes. Here a robust microporous Zn-MOF (metal–organic framework, Zn(hpi2cf)(DMF)(H 2 O)) is assembled from a dual-emissive H 2 hpi2cf (5-(2-(5-fluoro-2-hydroxyphenyl)-4,5-bis(4-fluorophenyl)-1 H -imidazol-1-yl)isophthalic acid) ligand that exhibits characteristic excited state intramolecular proton transfer (ESIPT). This Zn-MOF contains amphipathic micropores (<3 Å) and undergoes extremely facile single-crystal-to-single-crystal transformation driven by reversible removal/uptake of coordinating water molecules simply stimulated by dry gas blowing or gentle heating at 70 °C, manifesting an excellent example of dynamic reversible coordination behaviour. The interconversion between the hydrated and dehydrated phases can turn the ligand ESIPT process on or off, resulting in sensitive two-colour photoluminescence switching over cycles. Therefore, this Zn-MOF represents an excellent PL water-sensing material, showing a fast (on the order of seconds) and highly selective response to water on a molecular level. Furthermore, paper or in situ grown ZnO-based sensing films have been fabricated and applied in humidity sensing (RH<1%), detection of traces of water (<0.05% v/v) in various organic solvents, thermal imaging and as a thermometer.