Monochromatic red electroluminescence from a homodinuclear europium(iii) complex of a β-diketone tethered by 2,2′-bipyrimidine

Abstract

A new homodinuclear complex [Eu(btfa)₃]₂bpm (1) was synthesized incorporating 4,4,4-trifluoro-1-phenyl-1,3-butanedione (btfa) as the primary and 2,2′-bipyrimidine (bpm) as the auxiliary ligand. The complex was characterized by analytical and spectroscopic techniques, with special emphasis on the crystal structure and photoluminescence (PL) properties, both experimentally and theoretically. Single crystal X-ray diffraction (SCXRD) analysis reveals that the two Eu(III) ions in 1 are eight coordinate, where the bpm behaves as a N₄ donor linking the two Eu(III) ions symmetrically. Each Eu(III) ion displays a distorted square antiprismatic (DSAP, D_4d) coordination geometry. Complex 1 displayed bright red emission with (CIE)_x,y color coordinates = 0.670; 0.330 and an absolute photoluminescence quantum yield (PLQY) of 54.4%. The theoretical intensity parameters (Ω₂ and Ω₄), radiative (A_R) and non-radiative (A_NR) decay rates, intrinsic quantum yield (Q^Eu_Eu), sensitization efficiency (η_sen) and PLQY were assessed using the LUMPAC software and showed very good agreement with the experimental values. Based on these, an energy transfer (ET) mechanism is proposed and discussed. Moreover, 1 was used as an emitter to fabricate single- and double-emitting layer (EML) red-emitting electroluminescent devices. Double-EML device at the optimum doping concentration of 4.0 wt% displayed impressive EL performances, brightness (B) = 812 cd m⁻², maximum current efficiency (η_c) = 3.97 cd A⁻¹, maximum power efficiency (η_p) = 3.89 lm W⁻¹, and external quantum efficiency (EQE) = 2.8% at very low V_turn-on = 3.2 V with (CIE)_x,y = 0.662, 0.321 which is close to the standard red color recommended by NTSC (0.67, 0.33).