Optical emission studies of new europium and terbium dinuclear complexes with trifluoroacetylacetone and bridging bipyrimidine. Fast radiation and high emission quantum yield

Abstract

New homodinuclear lanthanide complexes of the type [Ln(tfaa)3]2bpm (Ln=La, Eu and Tb; tfaa=1,1,1-trifluoro-2,4-pentanedione) were synthesized by a one pot-one step method and characterized by elemental analysis, FT-IR, thermogarvimetry and 1H NMR spectroscopy. In these complexes the planar 2,2′-bipyrimidine (bpm) ligand affords a tetradentate coordination mode. The crystal structure of [Tb(tfaa)3]2bpm was determined by single-crystal X-ray diffraction. The intramolecular Tb–Tb distance across the bpm bridging ligand is 6.760(1)Å. The dinuclear complexes are thermally stable up to 180°C, as shown by thermal analysis. The Eu(III) and Tb(III) dinuclear complexes exhibit intense red and green emissions with luminescence lifetimes of 810 and 490μs, respectively. The quantum yields, Фoverall, for the two complexes [Eu(tfaa)3]2bpm and [Tb(tfaa)3]2bpm are 34% and 48%, respectively. Substitution of a water molecule from the coordination sphere of [Ln(tfaa)3H2O] by bpm and the joining of two [Ln(tfaa)3] units through bpm leads to 10- and 2-fold increases, respectively, in the overall quantum yield for the dinuclear Eu and Tb complexes. This enhanced improvement originates mainly due to the (i) better sensitization efficiency of the ancillary ligand (bpm) and (ii) elimination of non-radiative deactivation pathways through harmonics of O–H vibrations.