Influences of fluorination on homoleptic iridium complexes with C∧N=N type ligand to material properties, ligand orientation and OLED performances

Abstract

Two new iridium complexes with C∧N=N type ligand (i.e., Ir(BFPPya)3{tris[3,6-bis(4-fluorophenyl)pyridazine]iridium(III)} and Ir(BDFPPya)3{tris[3,6-bis (2,4-di-fluorophenyl)pyridazine]iridium(III)}) attaching with fluorine atoms, were synthesized and the effects of fluorination on the material properties and device performance were investigated. Compared with our previously reported fluorine-free analogue material, that is Ir(BPPya)3{tris[3,6-bis(phenyl)pyridazine]iridium(III)}, blue shifts in the emission spectra as well as in the long wavelength region of the absorptions were observed. The photoluminescence quantum yield (PLQY) (0.44 and 0.84 vs 0.29), phosphoresces lifetime (0.88 and 1.31 vs 0.66 ms), and oxidation potential (1.10 and 1.37 vs 0.95 V) increased obviously after fluorinating the ligand. In contrast, the thermal stability of the iridium complexes decreased slightly (Td: 435 and 402 vs 440 °C). In the density functional theory (DFT) calculations, by comparing the steric shape of the three ligands within one optimized molecule, orientational differences among the complexes were observed. In OLED device studies, bluish green electroluminescence with peak emission of 500 nm, using the electron-transporting host of TPBI [2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl- 1H-benzimidazole)] and the most fluorinated dopant of Ir(BDFPPya)3, was achieved with maximum efficiency of 20.3 cd/A. On one hand this efficiency is not satisfactory considering a high PLQY of 0.84. On the other hand with the similar device structure, that the (HOMO-LUMO)s of all the dopants are wrapped within that of the host TPBI, and all the triplet energies of the dopants are smaller than that of the host TPBI, it is abnormal that the ordering of device efficiencies is contradictory to that of PLQY. Assisting with the phosphorescent spectrum of TPBI and the absorption spectra of the dopant, the contradiction was interpreted reasonably.