Dipyridylketone as a versatile ligand precursor for new cationic heteroleptic cyclometalated iridium complexes

Abstract

Three new bis-cyclometalated iridium(III) complexes, of general formula [Ir(2-phenylpyridine)₂(L)]⁺, are reported. The compounds contain a dipyridine-type ligand (L) derived from di-2-pyridylketone (dipyridin-2-ylmethanol, 2,2′-(hydrazonomethylene)dipyridine and 3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile) and were synthesized through two different reaction pathways. The alternative synthetic pathway herein proposed, namely the direct reactions on the complex [Ir(2-phenylpyridine)₂(2,2′-dipyridylketone)]⁺, overcame the inconveniences encountered with the standard reaction between the dimeric precursor [Ir(2-phenylpyridine)₂(μ-Cl)]₂ and the ancillary ligands (L). The photophysical characterization of the iridium complexes reveals that modifications on the ancillary ligand introduce large changes in the photophysical behaviour of the complexes. High emission quantum yield is associated with the presence of a saturated carbon between the two pyridyl moieties: [Ir(2-phenylpyridine)₂(2,2′-dipyridylketone)]⁺ and [Ir(2-phenylpyridine)₂(2,2′-(hydrazonomethylene)dipyridine)]⁺ are extremely low emissive, while [Ir(2-phenylpyridine)₂(dipyridin-2-ylmethanol)]⁺ and [Ir(2-phenylpyridine)₂(3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile)]⁺ are good photoemitters. DFT and TD-DFT calculations confirmed the mixed LC/MLCT character of the excited states involved in the absorption and emission processes and highlighted the role of the π-conjugation between the two subunits of the ancillary ligand in determining the nature of the LUMO.