Cationic iridium(iii) complexes bearing ancillary 2,5-dipyridyl(pyrazine) (2,5-dpp) and 2,2′:5′,2′′-terpyridine (2,5-tpy) ligands: synthesis, optoelectronic characterization and light-emitting electrochemical cells

Abstract

Four cationic iridium(III) complexes of the form [Ir(C^N)₂(N^N)]⁺ bearing either a 2,5-dipyridylpyrazine (2,5-dpp) or a 2,2′:5′,2′′-terpyridine (2,5-tpy) ancillary ligand and either 2-phenylpyridine (ppy) or a 2-(2,4-difluorophenyl)-5-methylpyridine (dFMeppy) cyclometalating ligands were synthesized. The optoelectronic properties of all complexes have been fully characterized by UV-visible absorption, cyclic voltammetry and emission spectroscopy. The conclusions drawn from these studies have been corroborated by DFT and TDDFT calculations. The four complexes were assessed as emitters in light-emitting electrochemical cells. Complex 1a, [Ir(ppy)₂(2,5-dpp)]PF₆, was found to be a deep red emitter (666 nm) both in acetonitrile solution and in the electroluminescent device. Complex 2a, [Ir(ppy)₂(2,5-tpy)]PF₆ was found to be an orange emitter (604 nm) both in solution and in the LEEC. LEECs incorporating both of these complexes were stable over the course of around 4–6 hours. Complex 1b, [Ir(dFMeppy)₂(2,5-dpp)]PF₆, was also determined to emit in the orange (605 nm) but with a photoluminescent quantum yield (Φ_PL) double that of 2a. Complex 2b, [Ir(dFMeppy)₂(2,5-tpy)]PF₆ is an extremely bright green emitter (544 nm, 93%). All four complexes exhibited quasireversible electrochemistry and all four complexes phosphoresce from a mixed charge-transfer excited state.