Theoretical Studies of the Ground and Excited Electronic States in Cyclometalated Phenylpyridine Ir(III) Complexes Using Density Functional Theory

The ground state and low-lying excited electronic states in the Ir(III) complex Ir(ppy)3, and in the related complexes Ir(ppy)2(acac) and Ir(ppy)2(bza), are studied using density functional theory techniques where ppy = 2-phenylpyridine, acac = acetoylacetonate, and bza = benzyolacetonate. Ir complexes of ppy have been the subject of numerous photophysical absorption and luminescence experiments and have been examined as potential donors in organic light emitting diodes (OLEDs). The electronic properties of the neutral molecules, in addition to the positive and negative ions, are studied using the B3LYP functional. Optimized geometries are compared to experimentally observed structures. Excited triplet and singlet states are examined using time-dependent density functional theory (TDDFT). The calculated energies of the lowest triplet state (2.4−2.6 eV) and lowest singlet state (2.6−2.7 eV) in the three complexes are in good agreement with experimental absorption spectra and luminescence studies. All of the low-lying transitions are categorized as metal-to-ligand charge-transfer (MLCT) transitions. The metal orbitals involved in the transitions have a significant admixture of ligand π character, as shown by the amount of metal 5d character which varies from 45 to 65%. The nature of the lowest unoccupied orbital changes from ppy-localized to bza-localized for the series of three molecules.