Exact exchange and the density functional theory of metal-to-ligand charge-transfer in fac-Ir(ppy)3

The low-energy excitations of iridium(III) complexes, such as fac-Ir(ppy)3, are known to have significant metal-to-ligand charge-transfer character. Hence the degree of exact exchange included in the density functional description of the complex is expected to be important. To investigate this we report both ground state and time-dependent density functional calculations with a range of functionals (with different degrees of exact exchange) and examine the changes in the predictions as we vary the degree of exact exchange within a single functional (while holding the rest of the parameterisation fixed). We find that although the optimal degree of exact exchange (∼20%) gives results in good agreement with experiment for fac-Ir(ppy)3 and the blue emissive fac-Ir(ptz)3, other parameterisations lead to predictions that are both qualitatively and quantitatively inconsistent with experiment. Other differences in the functionals lead to much smaller differences in the predicted properties of the complex. Thus the B3LYP and X3LYP functionals provide the best description of experimental data.