Influence of substituents on the energy and nature of the lowest excited states of heteroleptic phosphorescent Ir(III) complexes: a joint theoretical and experimental study

A series of Ir(III)-based heteroleptic complexes with phenylpyridine (ppy) and 2-(5-phenyl-4H-1,2,4]triazol-3-yl)-pyridine (ptpy) derivatives as coordinating ligands has been characterized by a number of experimental and theoretical techniques. Density functional theory (DFT) calculations were able to reproduce and rationalize the experimental redox and excited-states properties of the Ir complexes under study. The introduction of fluorine and trifluoromethyl substituents is found not only to modulate the emission energy but also often to change the ordering of the lowest excited triplet states and hence their localization. The lowest triplet states are best characterized as local excitations of one of the chromophoric ligands (ppy or ptpy). The admixture of metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) character is small and strongly depends on the nature of the excited state; their role is, however, primordial in defining the radiative decay rate of the complexes. The extent of charge-transfer contributions depends on the energy gaps between the relevant molecular orbitals, which can be modified by the substitution pattern.