Electrogenerated chemiluminescence from newly synthesized α-diimine-ligated heteroleptic iridium(III) complexes

A series of α-diimine ligands including alkyl-, bromo-, silyl- or silylethynyl-substituted 2,2′-bipyridines (bpy) for auxiliary neutral ligands on iridium(III) complexes were prepared. Using those ligands, α-diimine-ligated heteroleptic iridium(III) complexes with 2,4-difluorophenylpyridine as the primary cyclometalating ligand were synthesized by a conventional two-step synthesis followed by the anion exchange reaction using NH₄PF₆. The physical features such as spectroscopic, electrochemical and electrogenerated chemiluminescence (ECL) properties for the newly prepared complexes were characterized and compared to that of well-known tris(2,2′-bipyridine)rutheniun(II), [Ru(bpy)₃](PF₆)₂, and previously reported (2,2′-bipyridine)bis[2-(2,4-difluorophenyl)pyridine]iridium(III), [Ir(dfppy)₂(bpy)](PF₆) (2), complex as the reference. All the iridium(III) complexes synthesized showed strong green emission and higher ECL intensities compared to the well-known Ru(bpy)₃^{2 +}. The iridium(III) complexes containing α-diimine ligands with electron donating dimethyl, trimethylsilyl, and triphenylsilyl groups on bpy showed the increased ECL intensities compared to that of the reference (2). Among those iridium(III) complexes, the complex ligated with 5-(trimethylsilyl)-2,2′-bipyridine and 2,4-difluorophenylpyridine showed the most intense ECL responses and exhibited ECL intensities as high as 7.6 times greater than that of the reference (2). However, the iridium(III) complexes containing α-diimine ligands containing more conjugated carbon-carbon triple bond showed the decreased ECL intensities compared to that of the complexes with no triple bond conjugation.