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 (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) 4 acac (0.87) 4 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) 4 pic (1.17) 4 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 ((MLCT)-M-3) to a nonemissive metal-centered state ((MC)-M-3). The activation energies for the crossing from (MLCT)-M-3 to (MC)-M-3 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.