Theoretical design of phosphorescence parameters for organic electro-luminescence devices based on iridium complexes

Time-dependent density functional theory with quadratic response methodology is used in order to calculate and compare spin-orbit coupling effects and the main mechanism of phosphorescence of the neutral Ir(ppy)3 and cationic [Ir(bpy)3]3+ tris-iridium compounds, [Ir(ppy)2(bpy)]+ and [Ir(2-phenylpyridine)2(4,4′-tert-butyl-2,2′-bipyridine]+ complexes, including also the recently synthesised [Ir(2-phenylpyridine)2(4,4′-dimethylamino-2,2′-bipyridine]+ and [Ir(2,4-difluorophenylpyridine)2(4,4′-dimethylamino-2,2′-bipyridine]+ dyes, where ppy = 2-phenylpyridine and bpy = 2,2′-bipyridine ligands. Comparison with the symmetric, lighter and more studied [Ru(bpy)3]2+ and [Rh(bpy)3]3+ complexes is also presented. Variations in phosphorescence lifetimes for Ir(ppy)3 and [Ir(bpy)3]3+ dyes as well as for the mixed cationic complexes are well reproduced by the quadratic response method. All the ortho-metalated iridium compounds exhibit strong phosphorescence, which is used in organic light-emitting diodes (OLEDs) to overcome the efficiency limit imposed by the formation of triplet excitons. The results from the first principle theoretical analysis of phosphorescence have helped to clarify the connections between the main features of electronic structure and the photo-physical properties of the studied heavy organometallic OLED materials.