Proton Conductive Metal‐Organic Framework Encapsulating Emissive Hexacyanidochromate(III) Ions for Ratiometric and Lifetime‐Based Detection of Humidity and Temperature

Abstract Luminescent metal‐organic frameworks (MOFs) are widely exploited as sensors due to their versatility and sensitivity to external stimuli. Although there are numerous luminescent MOF‐based thermometers or humidity sensors, only a few examples allow efficient detection of both these parameters. We present a family of MOF‐materials based on Sr²⁺ or Ba²⁺ ions, 4,4′‐bipyridine‐N,N’‐dioxide (4,4′‐bpdo) linkers, and [M^III(CN)₆]³⁻ species, described by the formulas {[Sr^II(4,4′‐bpdo)_2.5(NO₃)][Sr^II(4,4′‐bpdo)_2.5(H₂O)]}[M^III(CN)₆]∙n(solvent) (M^III = Cr^III (1), Co^III (2)) and {[Ba^II(4,4′‑bpdo)_1.5(H₂O)₃(NO)₃][Ba^II(4,4′‑bpdo)_1.5(H₂O)₃(MeOH)][M^III(CN)₆]}∙n(solvent) (M^III = Cr^III (3), Co^III (4)). Among them, the air‐stable phase of 1 (1a), deserves main recognition due to reversible water vapor sorption properties, proton conductivity, and dual photoluminescence (PL). This cationic MOF is constructed of blue‐emissive 4,4′‐bpdo linkers while the encapsulated hexacyanidochromate ions exhibit near‐infrared PL ≈800 nm. 1a is a great platform for luminescent sensing of relative humidity (RH) and temperature utilizing the ratiometric approach based on the intensities of dual PL or the NIR‐emission lifetime. The optical thermometry of high performance can be separated from the influence of RH by using the fully dehydrated phase, 1deh, stable at 0% RH. Given that, the obtained MOF encapsulating purely inorganic molecular rubies is a unique example of a multifunctional proton‐conductive luminescent ratiometric sensor of humidity and temperature.