Dinuclear Ruthenium Complexes as Photosensitizers

Ruthenium bipyridyl complexes are well known photosensitizers. They absorb visible light in the 400 – 600 nm region due to metal-to-ligand-charge-transfer (MLCT) transitions and form exceptionally stable [Ru(II)-bpy]* redox-active photo-excited states. Remarkably, the [Ru(II)-bpy]* excited states act as both electron donors and acceptors. The oxidativee quenching of the excited states is applied in the photocatalytic generation of hydrogen from water, and the reductive quenching is a critical step in the application of these systems in dye sensitized solar cells (DSSC). We have synthesized two diruthenium(II,II)-bipyridyl complexes which also contain a -conjugated bridging ligand. Trans-{[Ru(bpy)2]2(µ-L2)}(PF6)4 and trans-{[Ru(bpy)2]2(µ-Me4L2)}(PF6)4, where, L2 = N, N'-Bis((2'-pyridyl)methylene)benzene-1,4-diamine and Me4L2 = N, N'-Bis((2'-pyridyl)methylene)-2,3,5,6-tetramethylbenzene-1,4-diamine. The bridging ligands feature an extended -conjugated benzene-1,4-diimine system, and we hypothesize that the ligands will further enhance the photochemical redox behavior of the Ru(bpy)2 chromophore present in the complexes. A mononuclear analog, namely, [Ru(bpy)2(dmpma)](ClO4)2, where, dmpma = N-(2'-Pyridyl)methylene-2,6-di-methylaniline is also synthesized towards establishing any possible electron exchange and electron transfer phenomena performed by the -conjugated L2 and Me4L2 ligands. The new complexes are characterized by 1H NMR, UV-Vis, and fluorescence spectra. Two of the complexes are also characterized by single crystal X-ray diffraction data.