Electronic Structure of Carbazole-Based Phosphine Oxides as Ambipolar Host Materials for Deep Blue Electrophosphorescence: A Density Functional Theory Study

We report the results of Density Functional Theory calculations on a series of carbazole-based phosphine oxides that experimental data have shown to be promising ambipolar host molecules for deep blue electrophosphorescence. The hosts under investigation contain either 1, 2, or 3 carbazole subunits attached to the phenyl rings of a triphenylphosphoryl group, with the carbazoles acting as hole transporters/acceptors and the triphenylphosphoryl groups as electron transporters/acceptors. The results underline that, in addition to the strong inductive effect of the phosphoryl groups, the LUMO of these hosts is further stabilized by the molecular orbital interactions among the phenyl rings of the triphenylphosphoryl group, which is modulated by the electron-withdrawing inductive effects of the carbazole subunits. The lowest triplet state of the hosts correspond to localized transitions within the carbazole units, which leads to a high triplet energy on the order of 3 eV. We describe the important buffer role of the phenyl rings in preventing the phosphoryl moiety from negatively affecting the hole-accepting characteristics and high triplet energies of the carbazole units.