Important role of ancillary ligand in the emission behaviours of blue-emitting heteroleptic Ir(iii) complexes

Abstract

A series of heteroleptic Ir(III) complexes composed of 2-(2,4-difluoro-3-(trifluoromethyl)phenyl)-4-methylpyridine (dfCF₃) as the main ligand and such ancillary ligands as acetylacetonate [Ir(dfCF₃)₂(acac)] (acac), picolinate [Ir(dfCF₃)₂(pic)] (pic), and tetrakis-pyrazolyl borate [Ir(dfCF₃)₂(bor)] (bor) were prepared, and their emission behaviors depending on the ancillary ligands were systematically investigated. It was found that the Huang–Rhys factors (S_Ms) of the emission decrease in the order bor (0.97) > acac (0.87) > pic (0.76), while the nonradiative rate constants (k_nr/10⁵ s⁻¹) calculated from the quantum yields and lifetimes of emission were in the order acac (4.89) > pic (1.17) > bor (0.28). It was assumed that the large difference of k_nr for the complexes arose from important contributions of the ancillary ligands to the crossing from an emissive state (³MLCT) to a nonemissive metal-centered state (³MC). The activation energies for the crossing from ³MLCT to ³MC were estimated from the temperature dependencies of the emission lifetime and were found to be 46 meV for acac, 61 meV for pic, and >100 meV for bor. The experimental results were in line with the theoretical calculations based on integrating quantum chemical modeling methods. By the excellent emission behavior, bor was applied as a dopant to prototype deep-blue phosphorescent organic light-emitting diode devices, which revealed high emission efficiency and colour purity.