Synthesis, X-ray Structure, Spectroscopic Characterization, and Theoretical Prediction of the Structure and Electronic Spectrum of Eu(btfa)₃·bipy and an Assessment of the Effect of Fluorine as a β-Diketone Substituent on the Ligand−Metal Energy Transfer Process

The 2,2‘-dipyridyl adducts of two europium β-diketonate complexes, Eu(btfa)₃·bipy [btfa = 4,4,4-trifluoro-1-phenyl-2,4-butanedione, bipy = 2,2‘-dipyridyl] and Eu(bzac)₃·bipy [bzac = 1-phenyl-2,4-butanedione], have been prepared. The crystal structure of the former with chemical formula EuC₄₀H₂₆O₆N₂F₉ has been solved by single-crystal X-ray diffraction methods. The complex crystallizes in the monoclinic space group P2₁/n with a = 11.122(5) Å, b = 22.860(8) Å, c = 15.870(6) Å, β = 102.62(3)^o, V = 3937(5) ų, and Z = 4. A single, eight-coordinate environment, which approximates a square antiprism, is found for the europium(III). The UV absorption spectra of both complexes were obtained from ethanol solutions and, in the case of Eu(btfa)₃·bipy, from a thin film. In both cases the absorption spectra are reasonably well predicted by the INDO/S-CI method using, for Eu(btfa)₃·bipy, both the X-ray data and that obtained through the SMLC/AM1 method as input geometry and, for Eu(bzac)₃·bipy, that obtained through the SMLC/AM1 method. There is a blue shift of the calculated spectra relative to the solution spectra and a slightly larger blue shift compared to the spectrum of the thin film. Both complexes are luminescent under near-UV excitation, and the spectra are in accord with the existence of a single emitting site in each. The increased quantum yield in the fluorinated complex is correlated with a decrease in the bipy−europium(III) distance, a closer match of the lowest ligand-centered triplet state (that level which is primarily responsible for the energy transfer from the ligands to the europium(III)), and the lower vibrational energy of the C−F bonds relative to the C−H bonds. In the fluorinated complex the calculations show that the lowest triplet level is primarily localized on the 2,2‘-dipyridyl whereas in the nonfluorinated complex this is the second lowest triplet level.