Salts of Lanthanide(III) Hexafluoroacetylacetonates [Ln = Sm(III), Eu(III) and Tb(III)] with Dipyridylammonium Cations: Synthesis, Characterization, Photophysical Properties and OLED Fabrication

A series of tetrakis lanthanide complexes with the general formula [Ln(hfaa) ₄] ^-(DpaH) ⁺ [Ln = (Sm-1), (Eu-1) and (Tb-1)], hfaa = hexafluroacetylacetonate and Dpa −2,2′-dipyridylamine] has been synthesized by the reaction of LnCl ₃, hfaa and Dpa in the presence of ammonia solution (25%). The complexes have been characterized by analytical and spectroscopic methods. The solution molecular structure of the complexes was elucidated by one- and two-dimensional NMR spectroscopy which shows that the DpaH ⁺ cation retains a close interaction with the lanthanide anion in solution. The crystal structure of Eu-1, determined by single crystal X-ray diffraction, confirms this intermolecular interaction in the solid-state through a N–H⋯O hydrogen bond of 2.187 Å. In the [Eu(hfaa) ₄] ^- anion the EuO ₈ coordination polyhedron has a distorted triangular dodecahedron geometry with approximate D _2d-symmetry around the metal centre. Photophysical, thermal, and electroluminescent properties of the complexes have been investigated. The Sm-1 and Eu-1 complexes displayed efficient typical red emission with a sizeable photoluminescence quantum yield (PLQY) while Tb-1 displayed near-white light emission. The complexes have been used as dopants to fabricate single- and double-emitting layer (EML) OLEDs through the thermal evaporation method. At the optimum doping concentration, double-EML Eu-1 based device displayed orange electroluminescence (EL) with a brightness (B) of 417 cd/m ² and very low V _turn-on = 3.4 V. Interestingly, the Sm-1 based single-EML device exhibited pure red emission with the Commission internationale de l'éclairage [(CIE) _x,y = 0.613, 0.321], which is rare. The Sm-1 based device performance [B = 145 cd/m ², current efficiency (η _c) = 0.35 cd/A, power efficiency (η _p) = 0.15 lm/W with an external quantum efficiency (EQE) = 0.3% and V _turn-on = 7.1 V] surpassed that of the only reported Sm-based single red-OLED (R-OLED).