Binuclear cyclometalated Ir(iii) complexes with bis-bidentate butterfly-shaped ligands: synthesis, characterization, and application in efficient yellow–orange light-emitting electrochemical cells

Abstract

Two yellow-to-orange–emitting binuclear iridium complexes of the form [Ir₂(ppy)₄(N^N)](ClO₄)₂ (namely IrL1 and IrL2) were designed and synthesized, using phenylpyridine (ppy) as a cyclometalating ligand (C^N) and phenanthroimidazole-based bridging compounds as ancillary ligands (N^N), to explore the effect of sterically hindered bulky bridging ligands and their substituents on the electronic and emission properties of the related ionic transition metal complexes (iTMCs) and the performance of iTMC-based light-emitting electrochemical cells (iTMC-LECs). In CH₂Cl₂ solution, complexes IrL1 and IrL2 afford yellow-to-orange emission centered at 570 and 582 nm with PLQYs of 27% and 36%, respectively. The reversible and quasi-reversible redox behaviors of both complexes are shown to reveal the excellent stability of the emitters in solution. The electrochemical and photophysical data indicate that the substitution at the C2 position of the imidazole moiety does not significantly affect the electronic properties of the complexes. IrL1 and IrL2-based LECs show yellow-to-orange electroluminescence centered at 586 and 598 nm, with a maximum luminance (L_max) of 430 and 570 cd m⁻² and current efficiencies of 3.45 and 4.50 cd A⁻¹, respectively. However, compared with their corresponding archetypecomplex, IrL1, and IrL2-based LECs exhibit several fold higher L_max, which highlights the role of these ligands in the design of emitters.