Red Phosphorescent Organic Light-emitting Diodes

Phosphorescent organic light-emitting diodes (OLEDs) have the great advantage of highlyefficient emission compared with conventional fluorescent OLEDs. Pt(II) porphyrins are among the most promising saturated red phosphorescent dyes, yet tuning of their properties by making structural modifications has only been attempted on a modest scale, despite the large structuraldiversity of synthetic porphyrins. Here we demonstrate enhancement of the external quantum efficiency of red phosphorescent OLEDs by using a novel facially-encumbered and sterically-bulky meso-aryl-substituted Pt(II) porphyrin. This molecule has a spherical shape ("doubly decamethylene-strapped" Pt(II) porphyrin) although conventional Pt(II) porphyrin is flat in shape. We assume that the red phosphorescent emission increases in line with the decrease in the number of non-radiative decay pathways by restricting the rotational freedom of the meso-aryl substituents in the Pt(II) porphyrins. There is a trend whereby the triplet excited-state lifetimes become longer with increasing external quantum efficiencies (QEs) in OLEDs that include Pt(II) porphyrins. This finding suggests that one can obtain higher external QE values in red phosphorescent OLEDs by restricting the rotational freedom of the mesoaryl substituents of the Pt(II) porphyrins, and this concept may serve as a guide to the molecular design of highly-emissive materials.