Optical properties of red, green and blue emitting rare earth benzenetricarboxylate compounds

Red, blue and green emitting rare earth compounds (RE3+=Eu3+, Gd3+ and Tb3+) containing the benzenetricarboxylate ligands (BTC) [hemimellitic (EMA), trimellitic (TLA) and trimesic (TMA)] were synthesized and characterized by elemental analysis, complexometric titration, X-ray diffraction patterns, thermogravimetric analysis and infrared spectroscopy. The complexes presented the following formula: [RE(EMA)(H2O)2], [RE(TLA)(H2O)4] and [RE(TMA)(H2O)6], except for Tb-TMA compound, which was obtained only as anhydrous. Phosphorescence data of Gd3+-(BTC) complexes showed that the triplet states (T) of the BTC3− anions have energy higher than the main emitting states of the Eu3+ (5D0) and Tb3+ (5D4), indicating that BTC ligands can act as intramolecular energy donors for these metal ions. The high values of experimental intensity parameters (Ω2) of Eu3+-(BTC) complexes indicate that the europium ion is in a highly polarizable chemical environment. Based on the luminescence spectra, the energy transfer from the T state of BTC ligands to the excited 5D0 and 5D4 levels of the Eu3+ and Tb3+ ions is discussed. The emission quantum efficiencies (η) of the 5D0 emitting level of the Eu3+ ion have been also determined. In the case of the Tb3+ ion, the photoluminescence data show the high emission intensity of the characteristic transitions 5D4→7FJ (J=0–6), indicating that the BTC ligands are good sensitizers. The RE3+-(BTC) complexes act as efficient light conversion molecular devices (LCMDs) and can be used as tricolor luminescent materials.