Green-Emissive Ferroelectric Optical Thermometer Based on Cyclometalated Dicyanidoplatinate(II) Ions

Multifunctional ferroelectrics combine switchable permanent polarization with other physical functionalities. Among them, luminescent ferroelectrics are coming into the spotlight due to their potential in sensing, data storage, and optoelectronics. This multifunctionality can be improved if their emission is sensitive to external stimuli, such as temperature (T), which is utilized in optical thermometry. Aiming at ferroelectric luminescent thermometers, we decided to explore photoluminescent cyanido metal complexes. We report a hybrid salt of strongly emissive dicyanido(2-phenylpyridinato)platinate(II) complexes and N-methyl-quinuclidinium cations, (Meabco)[Pt^II(CN)₂(ppy)] (1). It crystallizes in a polar Ima2 space group, as confirmed by the second harmonic generation activity reaching 44% of the KDP reference. Piezoresponse force microscopy (PFM) studies indicate ferroelectricity at room temperature (RT). Due to [Pt^II(CN)₂(ppy)]^- complexes, 1 shows T-dependent charge-transfer photoluminescence (PL), which was applied for ratiometric optical thermometry that was proven experimentally and through TD-DFT calculations. Its best performance is located at cryogenic temperatures, but optical thermometry is preserved in the around-RT region with a convenient linear response of the thermometric parameter to the T variation. This electro-optical multifunctionality was demonstrated for a crystalline sample. However, we prove the processability of 1 into a thin film of several nanometers in thickness, opening the applications-oriented exploration of related materials.