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

A series of heteroleptic Ir( iii ) complexes composed of 2-(2,4-difluoro-3-(trifluoromethyl)phenyl)-4-methylpyridine (dfCF 3 ) as the main ligand and such ancillary ligands as acetylacetonate [Ir(dfCF 3 ) 2 (acac)] ( acac ), picolinate [Ir(dfCF 3 ) 2 (pic)] ( pic ), and tetrakis-pyrazolyl borate [Ir(dfCF 3 ) 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 M s) of the emission decrease in the order bor (0.97) > acac (0.87) > pic (0.76), while the nonradiative rate constants ( k nr /10 5 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 ( 3 MLCT) to a nonemissive metal-centered state ( 3 MC). The activation energies for the crossing from 3 MLCT to 3 MC 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.