We report the synthesis and single-crystal X-ray diffraction, magnetic, and luminescence measurements of a novel family of luminescent cage-like tetranuclear silsesquioxanes (PhSiO15)8(LnO15)4(O)(C5H8O2)6(EtOH)2(CH3CN)2-2CH3CN (where Ln = Tb, 1; Tb/Eu, 2; and Gd, 3), featuring seven-coordinated lanthanide ions arranged in a one-capped trigonal prism geometry. Compounds 1 and 2 exhibit characteristic Tb[sup 3+] and Tb[sup 3+]/Eu[sup 3+]-related emissions, respectively, sensitized by the chelating antenna acetylacetonate (acac) ligands upon excitation in the UV and visible spectral regions. Compound 3 is used to assess the energies of the triplet states of the acac ligand. For compound 1, theoretical calculations on the intramolecular energy transfer and multiphonon rates indicate a thermal balance between the [sup 5]D4 Stark components, while the mixed Tb[sup 3+]/Eu[sup 3+] analog 2, with a Tb:Eu ratio of 3:1, showcases intra-cluster Tb[sup 3+]-to-Eu[sup 3+] energy transfer, calculated theoretically as a function of temperature. By utilizing the intensity ratio between the [sup 5]D4^[sup 7]F5 (Tb[sup 3+]) and [sup 5]D0^[sup 7]F2 (Eu[sup 3+]) transitions in the range 11-373 K, we demonstrate the realization of a ratiometric luminescent thermometer with compound 2, operating in the range 11-373 K with a maximum relative sensitivity of 2.0% K[sup -1] at 373 K. These findings highlight the potential of cage-like silsesquioxanes as versatile materials for optical sensing-enabled applications. [ABSTRACT FROM AUTHOR]