Synthesis of heteroleptic iridium complexes with sterically hindered methyl groups on pyrazole ligands for efficient yellow and green light-emitting electrochemical cells

We report the synthesis of four cationic iridium complexes, [Ir(ppy)(2)(mepzpy)]PF6 (1a), [Ir(dfppy)(2)(-mepzpy)]PF6 (1b), [Ir(ppy)(2)(dmpzpy)]PF6 (2a), and [Ir(dfppy)(2)(dmpzpy)]PF6 (2b), containing the methyl substituted pyrazole-based ancillary ligand and the phenylpyridine-based cyclometalating ligands. UV-visible, photoluminescence (PL), and voltammetric measurements were made to study the photo physical and electrochemical properties of complexes 1a-2b. Light-emitting electrochemical cells (LECs) were fabricated, which showed yellow emission for complexes la and 2a and green emission for complexes 1b and 2b. The LEC incorporating 2b exhibited a high luminance of 658 cd m(-2) and a current efficiency of 0.34 cd A(-1). The higher luminance and efficiency of the device based on 2b were due to the smooth surface morphology and the presence of sterically hindered dimethyl groups on the ancillary ligand (dmpzpy), which causes a more balanced charge carrier injection and recombination. (C) 2016 Elsevier Ltd. All rights reserved.