Time-dependent density functional calculations of phosphorescence parameters for fac-tris(2-phenylpyridine) iridium

fac-Tris(2-phenylpyridine) iridium [fac-Ir(ppy)₃] produces strong phosphorescence and has therefore been used as materials in organic light emitting diodes to overcome the efficiency limit imposed by the formation of triplet excitons. Accounting for this circumstance we present in this paper a theoretical study of phosphorescence in the Ir(ppy)₃ complex. The spin-orbit coupling effects and the radiative lifetime in the high temperature limit (τ) are calculated by time-dependent density functional theory using quadratic response technology in order to elucidate the main mechanism of the phosphorescence. It is found that the orbital structure of the T₁ state has a localized character and that the T₁ → S₀ transition is determined mostly by charge transfer from one of the ligands to the metal. At the vertical S₀ → T₁ excitation the triplet state is highly delocalized among all three ligands and has a mixed ππ^* and metal-to-ligand charge transfer character. The intensity borrowing from the S₀ to S₅ transitions is mostly responsible for the strong phosphorescence emission from the x and y spin sublevels. Our results concord with the experimental data on temperature and magnetic field dependence of the phosphorescence kinetics. The calculated radiative lifetime in the high temperature limit agrees well with the measured decay times (2 μs) accounting for negligible non-radiative quenching of the lowest triplet state.