Computational design of high triplet energy host materials for phosphorescent blue emitters

Abstract

A series of host molecules for blue electrophosphorescence have been designed using density functional theory by incorporating electron donors (carbazole (cbz) and diphenylamine (tph)) and electron acceptors (benzimidazole (bzi) and diphenylphosphine oxide (pho)) into the p-bis(triphenylsilyl)benzene (UGH2) moiety. Results obtained from the electronic structure calculations show that the triplet energy (E_T), HOMO and LUMO energy levels and HOMO–LUMO gap (E_g) of the designed hosts can be modulated through different linking topologies. Among the designed host molecules benzimidazole with an N-linkage exhibits a higher triplet energy, when compared to the same host with a C-linkage. Asymmetric substitution on an inert host (UGH2) more effectively tunes the charge injection barrier from neighboring layers, HOMO and LUMO energies and E_g than symmetric substitution. The electron injection barrier is substantially diminished when electron transporting units are substituted in the para-position of the core unit compared to the same in the meta-position. Among the newly designed host molecules mcbz–pN–bzi, pcbz–pN–bzi, and ptph–pN–bzi are found to be good host materials for blue emitting phOLEDs.