Investigation into the Phosphorescence of a Series of Regioisomeric Iridium(III) Complexes

A series of heteroleptic cyclometalated Ir(III) complexes with the general structure [Ir(piq-X)2(acac)] (where piq = 1-phenylisoquinolato, X = bromine, 9,9-dioctyl-2-fluorenyl, poly(9,9-dioctyl-2,7-fluorene), acac = acetyl acetonate) have been prepared. The complexes are regioisomers where the X substituents occupy positions 2, 3, or 4 on the phenyl ring. The isomers all show red phosphorescence but have varying wavelengths and quantum yields. The nature and site of substitution influence the energy and localization of the frontier molecular orbitals, and this is investigated using electrochemistry, absorption and emission spectroscopy, and density functional theory calculations. Substitution in the 3-phenyl site leads to complexes with the highest quantum yields and results in an increase in the highest occupied molecular orbital (HOMO) energy. Conversely, substitution at the 4-phenyl position lowers the lowest unoccupied orbital energy (LUMO). Some of the complexes are applied in single-layer-polymer light-emitting devices (PLEDs), which show red electrophosphorescence.