Luminescent Ln3+-based silsesquioxanes with a β-diketonate antenna ligand: toward the design of efficient temperature sensors

We report the synthesis and single-crystal X-ray diffraction, magnetic, and luminescence measurements of a novel family of luminescent cage-like tetranuclear silsesquioxanes (PhSiO<jats:sub>1.5</jats:sub>)<jats:sub>8</jats:sub>(LnO<jats:sub>1.5</jats:sub>)<jats:sub>4</jats:sub>(O)(C<jats:sub>5</jats:sub>H<jats:sub>8</jats:sub>O<jats:sub>2</jats:sub>)<jats:sub>6</jats:sub>(EtOH)<jats:sub>2</jats:sub>(CH<jats:sub>3</jats:sub>CN)<jats:sub>2</jats:sub>⋅2CH<jats:sub>3</jats:sub>CN (where Ln = Tb, <jats:bold>1</jats:bold>; Tb/Eu, <jats:bold>2</jats:bold>; and Gd, <jats:bold>3</jats:bold>), featuring seven-coordinated lanthanide ions arranged in a one-capped trigonal prism geometry. Compounds <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold> exhibit characteristic Tb<jats:sup>3+</jats:sup> and Tb<jats:sup>3+</jats:sup>/Eu<jats:sup>3+</jats:sup>-related emissions, respectively, sensitized by the chelating antenna acetylacetonate (acac) ligands upon excitation in the UV and visible spectral regions. Compound <jats:bold>3</jats:bold> is used to assess the energies of the triplet states of the <jats:italic>acac</jats:italic> ligand. For compound <jats:bold>1</jats:bold>, theoretical calculations on the intramolecular energy transfer and multiphonon rates indicate a thermal balance between the <jats:sup>5</jats:sup>D<jats:sub>4</jats:sub> Stark components, while the mixed Tb<jats:sup>3+</jats:sup>/Eu<jats:sup>3+</jats:sup> analog <jats:bold>2</jats:bold>, with a Tb:Eu ratio of 3:1, showcases intra-cluster Tb<jats:sup>3+</jats:sup>-to-Eu<jats:sup>3+</jats:sup> energy transfer, calculated theoretically as a function of temperature. By utilizing the intensity ratio between the <jats:sup>5</jats:sup>D<jats:sub>4</jats:sub>→<jats:sup>7</jats:sup>F<jats:sub>5</jats:sub> (Tb<jats:sup>3+</jats:sup>) and <jats:sup>5</jats:sup>D<jats:sub>0</jats:sub>→<jats:sup>7</jats:sup>F<jats:sub>2</jats:sub> (Eu<jats:sup>3+</jats:sup>) transitions in the range 11–373 K, we demonstrate the realization of a ratiometric luminescent thermometer with compound <jats:bold>2</jats:bold>, operating in the range 11–373 K with a maximum relative sensitivity of 2.0% K<jats:sup>−1</jats:sup> at 373 K. These findings highlight the potential of cage-like silsesquioxanes as versatile materials for optical sensing-enabled applications.