Steric hindrance at metal centre quenches green phosphorescence of cationic iridium(III) complexes with 1-(2-pyridyl)-pyrazoles

Cationic cyclometalated iridium(III) complexes [(N,C2′-2-phenylpyridyl)2Ir(N^N)](PF6) with alkyl-substituted 1-(2′-pyridyl)-pyrazole N^N ligands exhibit green phosphorescence from a cyclometalating-ligand-centered excited state with structured emission spectrum and with quantum yield and excited-state lifetime of up to 26% and 1.4 μs in argon-saturated dichloromethane solution. The alkyl groups do not change the energy or the nature of the emissive state; however, those alkyl groups that hinder coordination of the N^N ligand to iridium(III) strongly quench phosphorescence of the complex.