Abstract:
Three new europium complexes, [Eu(PBI)(3)(.)3H(2)O] (1), [Eu(PBI)(3)(.)2TOPO] (2), and [Eu(PBI)(3)(.)2TPPO(.)H(2)O] (3) (where HPBI, TOPO, and TPPO stand for 3-phenyl-4-benzoyl-5-isoxazolone, tri-n-octylphosphine oxide, and triphenylphosphine oxide, respectively), with different neutral ligands were synthesized and characterized by elemental analysis, Fourier transform infrared, H-1 NMR, thermogravimetric analysis, and photoluminescence (PL) spectroscopy. The coordination geometries of the complexes were calculated using the Sparkle/AM1 (Sparkle Model for the Calculation of Lanthanide Complexes within the Austin Model 1) model. The ligand-Eu3+ energy-transfer rates were calculated in terms of a model of the intramolecular energy-transfer process in lanthanide coordination compounds reported in the literature. The room-temperature PL spectra of the europium(ill) complexes are composed of the typical Eu3+ red emission, assigned to transitions between the first excited state (D-5(0)) and the multiplet (F-7(0-4)). On the basis of emission spectra and lifetimes of the D-5(0)-emitting level, the emission quantum efficiency (eta) was determined. The results clearly show that the substitution of water molecules by TOPO leads to greatly enhanced quantum efficiency (i.e., 26% vs 92%) and longer D-5(0) lifetimes (250 vs 1160 us). This can be ascribed to a more efficient ligand-to-metal energy transfer and a less nonradiative D-5(0) relaxation process. Judd-Ofelt intensity parameters Omega(2) and Omega(4)) were determined from the emission spectra for the Eu3+ ion based on the D-5(0) -> F-7(2) and D-5(0) -> F-7(4) electronic transitions, respectively, and the D-5(0) -> F-7(1) and magnetic-dipole-allowed transition was taken as the reference. A point to be noted in these results is the relatively high value of the Omega(2) intensity parameter for all of the complexes. This may be interpreted as being a consequence of the hypersensitive behavior of the D-5(0) -> F-7(2) transition. The dynamic coupling mechanism is, therefore, dominant, indicating that the Eu3+ ion is in a highly polarizable chemical environment.
