Synthesis, Characterization, and Luminescence of β-Cyclodextrin Inclusion Compounds Containing Europium(III) and Gadolinium(III) Tris(β-diketonates)

β-Cyclodextrin (β-CD) inclusion compounds containing the lanthanide complexes Eu(NTA)3·xH2O and Gd(NTA)3·xH2O [NTA = 1-(2-naphthoyl)-3,3,3-trifluoroacetone)] were prepared by treating a saturated aqueous solution of β-CD with a solution of the complex in ethanol. The molar ratio of host to guest in the reaction was either 3:1 or 1:1, leading to products designated as M(NTA)3·3β-CD and M(NTA)3·β-CD (M = Eu, Gd), respectively. The products were characterized in the solid state by elemental analysis, powder X-ray diffraction, thermogravimetric analysis, FTIR, and 13C CP MAS NMR spectroscopy, indicating the formation of true inclusion compounds in which the naphthoyl moieties of the guest species are encapsulated within the β-CD cavities. The photoluminescence results, in particular the determination of the experimental intensity parameters Ω2 and Ω4, show that this encapsulation seems to modify essentially the second Eu3+ coordination shell, namely, the energy and the lifetime of the lowest triplet state of the naphthalene groups. The encapsulation induces emission from the singlet excited state of the ligands with the corresponding presence of an inefficient ligand-to-metal ion energy transfer step.