Spectral-luminescent properties of mixed-ligand europium(III) complexes with $p$-methylbenzoic acid

The spectral-luminescent properties of europium(III) complexes with $p$-methylbenzoic acid and nitrogen- and phosphorus-containing neutral ligands of compositions [Eu(p-MBA)$_{3}$ $\cdot$ D]$_{2}$ $\cdot$ $x$H$_{2}$O and [Eu(p-MBA)$_{3}$ $\cdot$ 2H$_{2}$O]$_{n}$, where $p$-MBA is the anion of $p$-methylbenzoic acid and D is 1,10-phenanthroline (phen), 2,2' dipyridyl (dipy), triphenylphosphine oxide (tppo) $(x=0)$, hexamethylphosphortriamide (hmpa) $(x=1)$, or benzotriazole (bt) $(x=2)$ are studied. The electronic absorption spectra, the luminescence excitation spectra, and the Stark structure of $^{}5D_{0}-^{}7F_{j}$ ($j$ = 0–2) electronic transitions in low-temperature luminescence spectra of complex europium(III) $p$-methylbenzoates are analyzed. It is found that the excitation energy transfer to europium ions occurs from the levels of both $p$-methylbenzoic acid and neutral ligands.