[Ir(C^N)2(N^N)]+ emitters containing a naphthalene unit within a linker between the two cyclometallating ligands

Abstract

The synthesis of four cyclometallated [Ir(C^N)₂(N^N)][PF₆] compounds in which N^N is a substituted 2,2′-bipyridine (bpy) ligand and the naphthyl-centred ligand 2,7-bis(2-(2-(4-(pyridin-2-yl)phenoxy)ethoxy)ethoxy)naphthalene provides the two cyclometallating C^N units is reported. The iridium(III) complexes have been characterized by ¹H and ¹³C NMR spectroscopies, mass spectrometry and elemental analysis, and their electrochemical and photophysical properties are described. Comparisons are made with a model [Ir(ppy)₂(N^N)][PF₆] compound (Hppy = 2-phenylpyridine). The complexes containing the naphthyl-unit exhibit similar absorption spectra and excitation at 280 nm leads to an orange emission. The incorporation of the naphthalene unit does not lead to a desirable blue contribution to the emission. Density functional theory calculations were performed to investigate the geometries of the complexes in their ground and first triplet excited states, as well as the energies and compositions of the highest-occupied and lowest unoccupied molecular orbital (HOMO and LUMO) manifolds. Trends in the HOMO–LUMO gaps agree with those observed electrochemically. The energy difference between the LUMO and the lowest unoccupied MO located on the naphthyl unit (LUMO+7) is large enough to explain why there is no contribution from the naphthyl-centred triplet excited state to the phosphorescence emission. Singlet excited states were also investigated. Light-emitting electrochemical cells (LECs) using the [Ir(C^N)₂(N^N)][PF₆] and [Ir(ppy)₂(N^N)][PF₆] complexes in the emissive layer were made and evaluated. The presence of the naphthyl-bridge between the cyclometallating units does not significantly alter the device response.