Bright and Efficient, Non-Doped, Phosphorescent Organic Red-Light-Emitting Diodes

Ir^(III) metal complexes with formula [(nazo)₂Ir(Fppz) ] (1), [(nazo)₂Ir(Bppz)] (2), and [(nazo)₂Ir(Fptz)] (3) [(nazo)H = 4-phenyl quinazoline, (Fppz)H = 3-trifluoromethyl-5-(2-pyridyl) pyrazole, (Bppz)H = 3-t-butyl-5-(2-pyridyl) pyrazole, and (Fptz)H = 3-trifluoromethyl-5-(2-pyridyl) triazole] were synthesized, among which the exact configuration of 1 was confirmed using single-crystal X-ray diffraction analysis. These complexes exhibited bright red phosphorescence with relatively short lifetimes of 0.4-1.05 μs in both solution and the solid-state at room temperature. Non-doped organic light-emitting diodes (OLEDs) were fabricated using complexes 1 and 2 in the absence of a host matrix. Saturated red electroluminescence was observed at λ_max = 626 nm (host-emitter complex 1) and 652 nm (host-emitter complex 2), which corresponds to coordinates (0.66,0.34) and (0.69,0.31), respectively, on the 1931 Commission Internationale de 1'Eclairage (CIE) chromaticity diagram. The non-doped devices employing complex 1 showed electroluminance as high as 5780 cd m^-2, an external quantum efficiency of 5.5% at 8 V, and a current density of 20 mA cm^-2. The short phosphorescence lifetime of 1 in the solid state, coupled with its modest π-π stacking interactions, appear to be the determining factors for its unusual success as a non-doped host-emitter.