Proton Conductive Luminescent Thermometer Based on Near-Infrared Emissive {YbCo₂} Molecular Nanomagnets

Lanthanide(III)-based coordination complexes have been explored as a source of bifunctional molecular materials combining Single-Molecule Magnet (SMM) behavior with visible-to-near-infrared photoluminescence. In pursuit of more advanced multifunctionality, the next target is to functionalize crystalline solids based on emissive molecular nanomagnets toward high proton conductivity and an efficient luminescent thermometric effect. Here, a unique multifunctional molecule-based material, (H₅O₂)₂(H)[Yb^III(hmpa)₄][Co^III(CN)₆]₂·0.2H₂O (1, hmpa = hexamethylphosphoramide), composed of molecular {YbCo₂}^3- anions noncovalently bonded to acidic H₅O₂⁺ and H⁺ ions, is reported. The resulting Yb^III complexes present a slow magnetic relaxation below 6 K and room temperature NIR 4f-centered photoluminescence sensitized by [Co(CN)₆]^3- ions. The microporous framework, built on these emissive magnetic molecules, exhibits a high proton conductivity of the H-hopping mechanism reaching σ of 1.7 × 10^-4 S·cm^-1 at 97% relative humidity, which classifies 1 as a superionic conductor. Moreover, the emission pattern is strongly temperature-dependent which was utilized in achieving a highly sensitive single-center luminescent thermometer with a relative thermal sensitivity, S_r > 1% K^-1 in the 50-175 K range. This work shows an unprecedented combination of magnetic, optical, and electrical functionalities in a single phase working as a proton conductive NIR-emissive thermometer based on Single-Molecule Magnets.