Tetranuclear lanthanide-based silsesquioxanes: towards a combination of a slow relaxation of the magnetization and a luminescent thermometry.

Lanthanide-based silsesquioxanes constitute an emerging family of cage-like metallasilsesquioxanes with exciting optical and magnetic properties. We report here the synthesis, structures and luminescence properties of a series of tetranuclear lanthanide-silsesquioxane compounds of general formula [NEt 4 ] 2 [(Ph 4 Si 4 O 8 ) 2 (Ln/Ln') 4 (NO 3 ) 6 (EtOH) 2 (MeCN) 2 ]·4(MeCN) with different lanthanide ions (where Ln/Ln' = Dy 3+ /Eu 3+ (1), Dy 3+ /Tb 3+ (2) and Eu 3+ /Tb 3+ /Y 3+ (3)) and investigate the impact of the lanthanide ions combination on magnetic and photo-luminescent properties. Compound 1 behaves as a field-induced Single Molecule Magnet (SMM) and presents temperature-dependent luminescence characteristics of Eu 3+ making it an emissive thermometer working in the temperature range 293-373 K with the maximum relative sensitivity of 1.15% K -1 achieved at 293 K. Compounds 2 and 3 are paramagnets, which demonstrate a characteristic photoluminescence with Dy 3+ to Tb 3+ and Tb 3+ to Eu 3+ energy transfers, respectively.