Iridium(III) Complexes of Bifunctional 2‑(2-Pyridyl)imidazole\nLigands: Electrochemiluminescent Emitters in Aqueous Media

A series\nof electrochemiluminescent (ECL) iridium(III) complexes\nwith the general formula [Ir(C<sup>∧</sup>N)<sub>2</sub>(pim)]<sup>+</sup> (where C<sup>∧</sup>N = cyclometalating ligands 2-phenylpyridinato\n(ppy) or 2-(2,4-difluorophenyl)pyridinato (dFppy), and pim = 2-(2-pyridyl)imidazole)\nhave been synthesized. In each case, the 2-(2-pyridyl)imidazole ancillary\nligand has been modified to facilitate bioconjugation and ECL label\ndevelopment. All complexes exhibit blue-shifted optical and electro-generated\nphosphorescence relative to the archetypal complex [Ir(ppy)<sub>2</sub>(bpy)]<sup>+</sup> (bpy = 2,2′-bipyridine). The emission energies\nfor the complexes were unperturbed by functionalization of the imidazole\nunit of the pim ligand, whereas the emission energy was significantly\nblue-shifted when the pyridyl group was modified with an electron-donating\noxyethanol unit. Cyclic voltammetric studies provide results consistent\nwith fluorine substituents on the cyclometalating ligands, or an oxyethanol\nsubstituent on the neutral pim ligand, widening the highest occupied\nmolecular orbital (HOMO)–lowest unoccupied molecular orbital\n(LUMO) gap of these complexes. Most of the complexes have high photoluminescence\nquantum yields (Φ<sub>PL</sub>) in acetonitrile (up to 0.91),\nand some have higher ECL efficiencies than [Ru(bpy)<sub>3</sub>]<sup>2+</sup> in both acetonitrile (up to 177%) and ProCell buffer (up\nto 202%). Theoretical studies provide additional insights into the\nphotophysical and electrochemical properties of this series of compounds.