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

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 degrees 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 degrees C, with an increase to 7.0 % C-1 at 95 degrees 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.