Europium and terbium ortho-, meta-, and para-methoxybenzoates: Structural peculiarities, luminescence, and energy transfer

A series of methoxybenzoates Ln(MOBA)3·nH2O and Ln(MOBA)3·L (Ln = Eu, Gd, Tb; MOBA – 2-, 3-, and 4-methoxybenzoate anions, L – 1,10-phenanthroline (Phen) and 2,2′-bipyridine (Bpy)) was investigated by methods of optical spectroscopy. The effects of methoxy groups located in different positions of the benzene ring on the structure of Eu3+ luminescence centers, the lifetimes of 5D0 (Eu3+) and 5D4 (Tb3+) states, the energies of the lowest singlet and triplet states of the ligands, and on processes of the excitation energy transfer to Eu3+ and Tb3+ ions are examined. The spectroscopic data for lanthanide methoxybenzoates are in accordance with known structural peculiarities: the lanthanide–oxygen bond lengths and the Ln3+ coordination polyhedron distortions. The low-energy ligand–metal charge transfer state was identified in the compound Eu(4-MOBA)3. It was shown that the distortions of Ln3+ luminescence centers are the smallest for 2-methoxybenzoates. The enhancement of Tb3+ luminescence efficiency by 2–2.5 times for terbium methoxybenzoates with phenanthroline Tb(MOBA)3·Phen in comparison with benzoate Tb(Benz)3·Phen at 295 K is caused by a decrease in the rate of back energy transfer due to an increase in the energy of the lowest Phen triplet state. The highest luminescence efficiency was observed for Tb(4-MOBA)3·Phen. Judging from the results presented, the Tb(4-MOBA)3·Phen can be potentially more preferable for the fabrication of emitter layers in organic light emitting diodes (OLEDs) than the Tb(2-MOBA)3·Phen·H2O, which has been tested before.