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

Abstract 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 Pt II porphyrin complexes is demonstrated. The maximum external QEs of phosphorescent OLEDs doped with the facially non‐encumbered Pt II porphyrin complex 1 [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 Pt II porphyrin complex 2 [5,15‐bis(2,6‐dimethoxyphenyl)‐2,8,12,18‐tetrahexyl‐3,7,13,17‐tetramethylporphyrinato platinum( II )], the Pt II porphyrin complex 3 that 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” Pt II porphyrin complex 4 were 1, 4.2, 7.3, and 8.2 %, respectively. The trend of increasing QE values in the order of 1 < 2 < 3 < 4 may be related to facial encumbrance and steric bulkiness of meso ‐aryl substituted Pt II porphyrin complexes. Especially, in the case of the Pt II porphyrin 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 Pt II porphyrins 1 – 4 in OLEDs at an injection current density of 0.55 mA cm –2 were 80, 103, 140, and 152 μs, respectively. We believe that the trend of increasing triplet lifetime in going from 1 to 4 is correlated with suppressing non‐radiative decay.