Homoleptic Cyclometalated Iridium Complexes with Highly Efficient Red Phosphorescence and Application to Organic Light-Emitting Diode

Phosphorescence studies of a series of facial homoleptic cyclometalated iridium(III) complexes
have been carried out. The complexes studied have the general structure Ir(III)(C−N)3, where (C−N) is a
monoanionic cyclometalating ligand:  2-(5-methylthiophen-2-yl)pyridinato, 2-(thiophen-2-yl)-5-trifluoromethylpyridinato, 2,5-di(thiophen-2-yl)pyridinato, 2,5-di(5-methylthiophen-2-yl)pyridinato, 2-(benzo[b]thiophen-2-yl)pyridinato, 2-(9,9-dimethyl-9H-fluoren-2-yl)pyridinato, 1-phenylisoquinolinato, 1-(thiophen-2-yl)isoquinolinato, or 1-(9,9-dimethyl-9H-fluoren-2-yl)isoquinolinato. Luminescence properties of all the
complexes at 298 K in toluene are as follows:  quantum yields of phosphorescence Φp = 0.08−0.29,
emission peaks λmax = 558−652 nm, and emission lifetimes τ = 0.74−4.7 μs. Bathochromic shifts of the
Ir(thpy)3 family [the complexes with 2-(thiophen-2-yl)pyridine derivatives] are observed by introducing
appropriate substituents, e.g., methyl, trifluoromethyl, or thiophen-2-yl. However, Φp of the red emissive
complexes (λmax > 600 nm) becomes small, caused by a significant decrease of the radiative rate constant,
kr. In contrast, the complexes with the 1-arylisoquinoline ligands are found to have marked red shifts of
λmax and very high Φp (0.19−0.26). These complexes are found to possess dominantly 3MLCT (metal-to-ligand charge transfer) excited states and have kr values approximately 1 order of magnitude larger than
those of the Ir(thpy)3 family. An organic light-emitting diode (OLED) device that uses Ir(1-phenylisoquinolinato)3 as a phosphorescent dopant produces very high efficiency (external quantum efficiency ηex = 10.3%
and power efficiency 8.0 lm/W at 100 cd/m2) and pure-red emission with 1931 CIE (Commission
Internationale de L'Eclairage) chromaticity coordinates (x = 0.68, y = 0.32).