Correlating Experimental Photophysical Properties of Iridium(III) Complexes to Spin Orbit Coupled TDDFT Predictions

Ir(III) complexes are efficient phosphorescent emitters. The transition dipole moment between the triplet and singlet manifolds is formally spin forbidden. However, spin-orbit coupling (SOC), induced by the high angular momentum orbitals in Ir, efficiently mixes the triplet manifold with higher-energy singlets, increasing the transition probability. Spin-orbit coupled time-dependent density functional (TDDFT) calculations within the zero-order relativistic approximation (ZORA) are used to study nine complexes that have a range of emissions from 450 to 630 nm and quantum efficiencies of 0.1-0.9. We find that using the singlet ground-state geometry to calculate radiative rates produces the best correlation with experiment. We also show that the equal thermal population of the three sublevels in the triplet manifold is sufficient to understand rates at 300 K. We find that emission energies and radiative rates are best reproduced at the TD-B3LYP/TZP/DZP//BP86/TZ2P/TZP level of theory, where the larger basis set is supplied for Ir.