Synthesis, Photophysical and TD-DFT Evaluation of Triphenylphosphonium-Labeled Ru(II) and Ir(III) Luminophores

The chemistry of 5-acetamido derivatized 1,10-phenanthroline was developed to yield a series of pro-ligands (L^1-4) and related triphenylphosphonium (TPP⁺) appended cationic variants (L^5-8). The resulting heteroleptic complexes [Ru(bipy)₂(L^1-8)]ⁿ⁺ (where bipy = 2,2'-bipyridine) and cyclometalated species [Ir(tmq)₂(L^1-8)]ⁿ⁺ (where tmq = 2,6,7-trimethyl-3-phenylquinoxaline) were isolated and fully characterized using a range of analytical and spectroscopic methods, including electrochemistry and time-resolved photophysics. Multinuclear NMR spectroscopies were used to characterize the compounds, including ³¹P NMR showing δ_P 21.3-24.4 ppm for the phosphonium species. Two X-ray crystal structures were successfully obtained on TPP⁺ functionalized Ru(II) and Ir(III) species: key features include the distorted octahedral coordination spheres, and the defined spatial relationships between the complex core and the TPP⁺ unit. All Ru(II) and Ir(III) complexes were phosphorescent in the red region with ³MLCT or ³MLCT/³LLCT character, respectively. Comparison across the series suggest the presence of the TPP⁺ unit induced moderate quenching of the complex phosphorescence. A comparison with quaternary ammonium analogues suggests this may be due to differences in ion pairing and solvation phenomena in the TPP⁺ complexes.