Density functional theory analysis of a mixed-ligand iridium compound for multi-color organic light-emitting diodes

Electronic states and their energies are calculated for a mixed-ligand Ir(III) compound, (5-chloro-8-hydroxyquinoline) bis(2-phenylpyridyl) iridium (called IrQ(ppy)(2)-5CI) using time-dependent density functional theory (TDDFT) calculations and are compared with the experimental result. A good agreement is obtained between the calculated and measured absorption spectra: The d-pi(Q)* molecular orbital transition gives the lowest-energy triplet state absorption band. Its energy is estimated as 1.84 eV (671 nm), which is close to the absorption band position of 1.86 eV (666 nm) observed for IrQ(ppy)(2)-5CI doped in 4,4'-NN'-dicarbazole-biphenyl (CBP) host and of 1.88 eV (660 nm) observed for IrQ(ppy)(2)-5CI doped in polystyrene (PS). The second triplet state absorption band is caused by d-pi(ppy) transition. Its position is calculated as 2.51 eV (494 nm). The dipole moment is estimated as 3.45 D, which is lower than the dipole moment of fac-Ir(ppy)(3). This is understood by a reduced charge transfer between Ir(III) and quinoline ligand. Copyright (C) 2008 John Wiley & Sons, Ltd.