A New Heteroleptic Ruthenium(II) Polypyridyl Complex with Long-Wavelength Absorption and High Singlet-Oxygen Quantum Yield

Abstract

Ruthenium(II) polypyridyl complexes with long-wavelength absorption and high singlet-oxygen quantum yield exhibit attractive potential in photodynamic therapy. A new heteroleptic Ru^II polypyridyl complex, [Ru(bpy)(dpb)(dppn)]²⁺ (bpy=2,2′-bipyridine, dpb=2,3-bis(2-pyridyl)benzoquinoxaline, dppn=4,5,9,16-tetraaza-dibenzo[a,c]naphthacene), is reported, which exhibits a ¹MLCT (MLCT: metal-to-ligand charge transfer) maximum as long as 548 nm and a singlet-oxygen quantum yield as high as 0.43. Steady/transient absorption/emission spectra indicate that the lowest-energy MLCT state localizes on the dpb ligand, whereas the high singlet-oxygen quantum yield results from the relatively long ³MLCT(Ru→dpb) lifetime, which in turn is the result of the equilibrium between nearly isoenergetic excited states of ³MLCT(Ru→dpb) and ³ππ*(dppn). The dppn ligand also ensures a high binding affinity of the complex towards DNA. Thus, the combination of dpb and dppn gives the complex promising photodynamic activity, fully demonstrating the modularity and versatility of heteroleptic Ru^II complexes. In contrast, [Ru(bpy)₂(dpb)]²⁺ shows a long-wavelength ¹MLCT maximum (551 nm) but a very low singlet-oxygen quantum yield (0.22), and [Ru(bpy)₂(dppn)]²⁺ shows a high singlet-oxygen quantum yield (0.79) but a very short wavelength ¹MLCT maximum (442 nm).