Enhancement of External Quantum Efficiency of Red Phosphorescent Organic Light-Emitting Devices with Facially Encumbered and Bulky PtII Porphyrin Complexes

An enhancement in the external quantum efficiency (QE) of red phosphorescent organic light‐emitting devices (OLEDs) by using facially encumbered and bulky meso‐aryl substituted PtIIporphyrin complexes is demonstrated. The maximum external QEs of phosphorescent OLEDs doped with the facially non‐encumbered PtIIporphyrin complex1[5,15‐bis[4‐(4,4‐dimethyl‐2,6‐dioxacyclohexyl)phenyl]‐2,8,12,18‐tetrahexyl‐3,7,13,17‐tetramethylporphyrin platinum(II)], the facially encumbered PtIIporphyrin complex2[5,15‐bis(2,6‐dimethoxyphenyl)‐2,8,12,18‐tetrahexyl‐3,7,13,17‐tetramethylporphyrinato platinum(II)], the PtIIporphyrin complex 3that bears bulkier 3,5‐di‐tert‐butylphenyl substituents [5,15‐bis(3,5‐di‐t‐butylphenyl)‐2,8,12,18‐tetrahexyl‐3,7,13,17‐tetramethylporphyrin platinum(II)], and the “doubly‐decamethylene‐strapped” PtIIporphyrin complex 4were 1, 4.2, 7.3, and 8.2 %, respectively. The trend of increasing QE values in the order of 1< 2< 3< 4may be related to facial encumbrance and steric bulkiness of meso‐aryl substituted PtIIporphyrin complexes. Especially, in the case of the PtIIporphyrin 4, it is considered that the “double straps” play an important role in restricting rotational freedom of the meso‐aryl substituents. The triplet excited‐state lifetimes for PtIIporphyrins 1–4in OLEDs at an injection current density of 0.55 mA cm–2were 80, 103, 140, and 152 μs, respectively. We believe that the trend of increasing triplet lifetime in going from 1to 4is correlated with suppressing non‐radiative decay.