Highly efficient phosphorescent organic light-emitting diodes hosted by 1,2,4-Triazole-Cored triphenylamine derivatives: Relationship between structure and optoelectronic properties

A series of new 1,2,4-triazole-cored triphenylamine derivatives with various linkages between triazole and triphenylamine (TPA) moieties were designed and synthesized through Suzuki cross-coupling reaction. The incorporation of rigid triazole moiety greatly improves their thermal and morphology stability, with T_d and T_g in the ranges 480-531 °C and 106-155 °C, respectively. The o-TPA-linked 7 and 8 show much less intramolecular charge transfer and blue-shifted emission than their p-TPA linked analogues 1 and 3, respectively. The meta- and ortho-structured compounds display higher triplet energy and better electrophophorescent performances than their para-structured congeners. The significant improvement of electrophosphorescent performances can be achieved through subtle change of the host molecular structures, which could be attributed to the well-matched energy levels between the host and hole-transport layer, the high triplet energy of the host, and complete spatial separation of HOMO and LUMO energy levels. Devices hosted by structureoptimized o-TPA-m-PTAZ achieve the best EL performance, with the maximum current efficiencies and maximum external quantum efficiencies as high as 12.4 cd/A and 16.4% for deep red electrophosphorescence, and 50.7 cd/A and 14.2% for green electrophosphorescence.