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 LnCl3, 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 Xray diffraction, confirms this intermolecular interaction in the solid-state through a N-HMIDLINE HORIZONTAL ELLIPSISO hydrogen bond of 2.187 A. In the [Eu(hfaa)(4)]- anion the EuO8 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 Vturn-on = 3.4 V. Interestingly, the Sm-1 based single-EML device exhibited pure red emission with the Commission internationale de l'e acute accent clairage [(CIE)(x,y) = 0.613, 0.321], which is rare. The Sm-1 based device performance [B =145 cd/ m(2), current efficiency (eta(c)) = 0.35 cd/A, power efficiency (eta(p)) = 0.15 lm/W with an external quantum efficiency (EQE) = 0.3% and Vturn-on = 7.1 V] surpassed that of the only reported Sm-based single red-OLED (R-OLED).