Theoretical investigations on the structural, spectroscopic and photophysical properties of iridium (III) complexes with non-conjugated ligands toward blue phosphor in OLEDs

Abstract

To achieve true blue-emitting iridium(III) complexes for organic light-emitting diodes, we theoretically designed a complex (dfbpy)2Ir(PˆN) (2, PˆN = 5-(diphenylphosphinomethyl)-3-(trifluoromethyl)pyrazole), dfbpy = 2-(2,4-difluorobenzyl)-pyridine), which derived from its prototype complex (dfppy)2Ir(PˆN) (1, dfppy: 2-(4,6-difluorophenyl)pyridine) by the insertion of methylene group between the pyridyl and phenyl segments. Then, their geometrical and electronic structure, absorption and emission were systematically and comparatively investigated using density functional theory calculations. It is found that the highest occupied molecular orbital (HOMO) (π component) of the functionalized complex 2 resides mainly on metal iridium atom and dfbpy ligand, whereas the lowest unoccupied molecular orbital (LUMO) is predominantly centered on π* orbital of cyclometalated dfbpy chelates. Our calculated results reveal the absorption at the 359 nm for 2 can be attributed to the mixture of MdπLπ*(dfbpy)CT (metal-ligand charge transition, MLCT) and ILπ(dfbpy)→π*(dfbpy)CT (intra-ligand charge transition, ILCT) transitions. In particular, the maxima emission for 2 is at 450 nm, which presents a blue shift up to 80 nm as compared to prototype complex 1, indicating the introduction of methylene group is a feasible strategy to archive true blue-emitting materials.