Influence of Hydrated Environment on Luminescent Properties of Pr(III) and Eu(III) Heterolanthanide Complexes with Cucurbituril

Supramolecular lanthanide complexes with cucurbituril [{Eu_xPr_1–x(H₂O)₅(NO₃)}₂CB[6]](NO₃)₄·HNO₃ 6H₂O (x = 0.25, 0.5, 0.75, structural type I) are prepared. Keeping these compounds over CaCl₂ results in a phase transition which is caused by partial loss of water molecules and leads to the formation of [{Eu_{x Pr1–x}(H₂O)₃(NO₃)₂}₂CB[6]](NO₃)₂HNO₃ (structural type II). The I to II transition takes 24 h and is reversible: when kept in water vapors for 2 h, II transforms completely into I. Luminescent properties of I and II are studied. Differences between the intensities of bands assigned to hypersensitive Eu-centered ⁵D₀ → ⁷F₂ transitions are due to the fact that coordination environments of the central atom are different and contain five (for I) and three (for II) aqualigands. Significantly higher quantum yield of II is due to the suppression of luminescence quenching by O–H vibrations.