New Cyclometalating Ligands for Emissive Iridium(III) Complexes for OLED Applications

Abstract

The literature surrounding the design of iridium(III) complexes for OLED applications is summarised, with a particular focus on tuning the colour of the emission via structural modification.
In this thesis a new family of sulfone-containing ligands was synthesised, with the aim of blue-shifting the emission of the resultant iridium complexes compared to the parent complex, FIrpic. Introduction of a sulfone unit to key positions on dfppy resulted in a blue-shift of 10 nm compared to FIrpic, due to the lowering of the HOMO with respect to the LUMO. This series of complexes was then extended to include complexes which contain fewer aromatic fluorines, including a ‘fluorine-free’ complex, as these are known to make the complexes unstable in devices. To blue-shift the emission further a new ancillary ligand was synthesised and combined with some of the sulfone ligands to produce complexes. Devices fabricated in the Department of Physics retained the blue emission observed in solution and performed comparably with FIrpic.
Donor-acceptor chromophoric ligands were also synthesised to investigate the solvent dependent dual emission properties of the resultant complexes. The donor and acceptor components were varied, producing complexes with differing degrees of dual emission character in solution, which is tentatively assigned to the prescence of a ligand based CT state. While the complexes did not display dual emission properties in the solid state, devices fabricated in the Department of Physics showed favourable performances as blue emitters compared to a FIrpic device.

New complexes with oxazoline- and oxazole- based ancillary ligands were also synthesised which showed an unusual HOMO-LUMO distribution. The emission colour of the complexes was tuned via introduction of substituents on to both the ancillary and the cyclometalating ligand, and the complexes’ performance in OLED devices was investigated.
Finally, hexadentate tripodal ligands were synthesised based on known cyclometalating ligands, and the tripodal complexes were synthesised with the aim of enhancing complex stability and reducing quenching relative to their tris-bidentate analogues. The synthesis and photophysical properties of these materials are discussed. The complexes displayed comparable or superior PLQYs and lifetimes to the parent/related complexes, and the 1H NMR data suggested some limited motion in solution.