Mono- and Mixed Metal Complexes of Eu(3+), Gd(3+), and Tb(3+) with a Diketone, Bearing Pyrazole Moiety and CHF(2)-Group: Structure, Color Tuning, and Kinetics of Energy Transfer between Lanthanide Ions

Three novel lanthanide complexes with the ligand 4,4-difluoro-1-(1,5-dimethyl-1H-pyrazol-4-yl)butane-1,3-dione (HL), namely [LnL(3)(H(2)O)(2)], Ln = Eu, Gd and Tb, were synthesized, and, according to single-crystal X-ray diffraction, are isostructural. The photoluminescent properties of these compounds, as well as of three series of mixed metal complexes [Eu(x)Tb(1-x)L(3)(H(2)O)(2)] (Eu(x)Tb(1-x)L(3)), [EuxGd(1-x)L(3)(H(2)O)(2)] (Eu(x)Gd(1-x)L(3)), and [Gd(x)Tb(1-x)L(3)(H(2)O)(2)] (Gd(x)Tb(1-x)L(3)), were studied. The Eu(x)Tb(1-x)L(3) complexes exhibit the simultaneous emission of both Eu(3+) and Tb(3+) ions, and the luminescence color rapidly changes from green to red upon introducing even a small fraction of Eu(3+). A detailed analysis of the luminescence decay made it possible to determine the observed radiative lifetimes of Tb(3+) and Eu(3+) and estimate the rate of excitation energy transfer between these ions. For this task, a simple approximation function was proposed. The values of the energy transfer rates determined independently from the luminescence decays of terbium(III) and europium(III) ions show a good correlation.