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 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 mu s, respectively. We believe that the trend of increasing triplet lifetime in going from I to 4 is correlated with suppressing non-radiative decay.