A Europium(III) Complex with an Unusual Anion–Cation Interaction: A Luminescent Molecular Thermometer for Ratiometric Temperature Sensing

Abstract An unusual thermally sensitive anion–cation interaction, which is characteristic of the anion [Eu(FOD)4]−, occurs in the complex [CHOL][Eu(FOD)4] (1; CHOL=choline; FOD=1,1,1,2,2,3,3‐heptafluoro‐7,7‐dimethyl‐4,6‐octanedionate) and affects both quantum yield and thermochromic behavior. This prompted the design of an Eu3+‐based ratiometric thermometer that functions at temperatures up to 95 °C through a thermally excited state absorption of the Eu3+ ion. The reusable temperature‐sensitive luminescent complex showed a range of relative sensitivity between 0.45 % C−1 at 25 °C, with an increase to 7.0 % C−1 at 95 °C. Confinement of compound 1 in a transparent film of polysulfone resulted in a higher thermal stability of 1 while its luminescence showed a strong temperature dependence.