Synthesis and photoelectric properties of IrIII complexes using fluorobenzylimidazole[2,1-b]thiazole derivatives as primary ligands

Abstract

3-Methyl-6-(3,5-difluorophenyl)imidazo[2,1-b]thiazole (mdfpit) and 3-methyl-6-(3,4,5-trifluorophenyl)imidazo[2,1-b]thiazole (mtfpit) were easily prepared from thiourea, acetone, and 3,5-difluorobenzoyl bromide or 3,4,5-trifluorobenzoyl bromide. These were used as primary ligands to synthesize twelve phosphorescent Ir^III complexes with picolinic acid (pic), isoquinoline-3-carboxylic acid (3-IQA), quinoline-2-carboxylic acid (2-QA), 2-(pyridin-2-yl)phenol (2-ylppy), 2-(2,4-difluorophenyl)pyridine (dfppy), and pyridine-2-sulfonic acid (2-sappy) as auxiliary ligands. Their structures, photoluminescence, and electrochemical properties were investigated. Upon introducing more fluorine atoms into the benzene ring of the primary ligand, the thermal stability, photoluminescence quantum yield (PLQY), LUMO energy level, and luminous efficiency of the resulting Ir^III complexes are significantly improved, and the photoluminescence emission spectra are blue-shifted. Their maximum emission wavelengths are present in the range of 517–618 nm, and the luminous colors span from the green to red light region. Using the synthesized Ir^III complexes as emitters, LED chips based on InGaN chip excitation were developed, which showed good performances. Among all LEDs, the PLQY of the (mtfpmt)₂Ir(pic) based LED is 58.4%, and the luminous efficiency is as high as 17.11 lm W⁻¹; the luminous efficiency of the (mdfpmt)₂Ir(2-QA) based LED is 3.41 lm W⁻¹ with CIE coordinates of 0.60 and 0.38, which are very similar to the saturated standard red light emission. The results demonstrate the potential of the studied Ir^III complexes as candidates for LED materials.