Tetrakis(naphthoyltrifluoroacetonato)lanthanate(III) complexes (Ln = Eu, Gd) containing the cations tetrabutylammonium, [NBu4]+; 1-butyl-3-methylimidazolium, [C4mim]+; and 1-butyl-3-methylpyridinium, [C4mpyr]+, have been prepared and structurally characterized by single-crystal X-ray diffraction. The {EuO8} coordination sphere in [NBu4][Eu(NTA)4] is best described as a distorted dodecahedron, where the metal ion is located at the 4-fold inversion axis with only one crystallographically independent NTA residue. In [C4mim][Eu(NTA)4] and [C4mpyr][Gd(NTA)4], the central Ln3+ ions are coordinated by eight oxygen atoms from four distinct β-diketonate ligands, in an overall distorted square-antiprismatic geometry. Besides electrostatic interactions, the crystal packing in all three structures is stabilized by offset π−π interactions involving the naphthyl rings of neighboring complexes (and, for [C4mim][Eu(NTA)4] and [C4mpyr][Gd(NTA)4], neighboring naphthyl/imidazolium and naphthyl/pyridinium rings) and C−H···π contacts. The photoluminescence properties of the three EuIII complexes were studied at room temperature and −259 °C by measuring emission and excitation spectra, 5D0 emission decay curves, and absolute emission quantum yields. Under ligand excitation (λex = 290−395 nm), the quantum yields (room temperature) were in the range 0.72−0.77 for the 1-butyl-3-methylimidazolium salt. An immobilized analogue of this complex was prepared by supporting [Eu(NTA)4]− on an ordered mesoporous silica derivatized with 1-propyl-3-methylimidazolium groups. The disappearance of the intra-4f6 lines in the excitation spectrum of the supported material indicated an increase in the ligand’s sensitization process of the Eu3+ ions, relative to direct intra-4f6 excitation. The emission quantum yield measured for the supported material (0.32−0.40, for excitations between 265 and 360 nm) is the highest so far reported for lanthanide-containing ordered mesoporous silicas.