Optical and electronic properties of phosphorescent iridium(III) complexes with phenylpyrazole and ancillary ligands

Phosphorescent Ir(III) complexes Ir(ppz)3, Ir(ppz)2(acac), Ir(ppz)2(pic) and Ir(ppz)2(dbm) (here ppz = phenylpyrazole, acac = acetylacetonate, dbm = dibenzoylmethane, and pic = picolinate) have been synthesized and investigated by optical spectroscopy, electrochemistry as well as density functional theory (DFT) calculations. These complexes show either no emission or medium intensity emission in solution or solid state at room temperature, but exhibit very strong emission from blue (422 nm) to orange-red (587 nm) at low temperature (77 K). Combined experimental and theoretical study, we reveal that replacing one of ppz ligand by ancillary ligand acac, dbm, and pic is a feasible way to alter the electronic structures of complexes resulting in changing emission colors. Both the cyclic voltammetry and DFT study testify that ancillary ligands have little influence on the highest occupied molecular orbital (HOMO) but great effect on lowest unoccupied molecular orbital (LUMO) by lowering the LUMO levels dramatically. The decreased LUMO level induced by the ancillary ligand makes the excited state of the complex far from the non-radiative states, which might enhance the quantum efficiency.