Increased Lifetimes of Excited States in Novel Ruthenium Polypyridyl Complexes

Abstract

Ruthenium polypyridyl complexes display excellent properties as photosensitizers, which can be modulated with appropriate changes in complex nuclearity and ligand structure.[1]In this work, we report the synthesis and spectroscopic, electrochemical and photophysical properties of a series of novel mononuclear complexes: [Ru(bpy)2(Mebpy-CN)](PF6)2 (1), [Ru(bpy)(Mebpy-CN)2](PF6)2 (2), [Ru(Mebpy-CN)3](PF6)2 (3), as well as a novel dinuclear complex: [(bpy)2RuII(Mebpy-CN)RuII(NH3)5](PF6)4 (4) (bpy = 2,2?-bipyridine, Mebpy-CN = 4-methyl-2,2?-bipyridine-4?-carbonitrile). The purity of all compounds was checked by chemical analyses, NMR spectra and MS data. By oxidizing (4) with Br2 in CH3CN solution, a new mixed-valent species, [(bpy)2RuII(Mebpy-CN)RuIII(NH3)5]5+ (5), could be obtained.Electrochemical studies disclose a decreasing difference between the first oxidation potential and the first reduction potential with increasing substitution of bpy by Mebpy-CN in the series [Ru(bpy)3-x(Mebpy-CN)x]2+ (x = 0,1,2,3). In agreement with these findings, UV-visible absorption and emission spectra (in CH3CN solution) show a decrease of the energy of the lowest lying MLCT (metal-to-ligand charge transfer) excited state. Interestingly, the emission quantum yields and the lifetimes of the 3MLCT excited states increase in the same direction. Thus, the lifetime of this state for complex (3) in deareated CH3CN, as measured by Laser Flash Photolysis, is t = 1.16 μs, which is almost twice the value for the parent [Ru(bpy)3]2+, and its transient absorption spectrum, shown in the Figure, can be assigned to the charge-separated state *[RuIII(Mebpy-CN)‾?(Mebpy-CN)2]2+. The improvement of the photosensitizing properties can be explained by considering the growing electronic delocalization in the ligand ring when going from x = 0 to x = 3, an effect which can be further enhanced by adding another nitrile substituent.[2]In complex (5), the band appearing at lmax = 790 nm (ɛ = 510 M-1cm-1) can be assigned to a MMCT (metal-to-metal charge transfer). The electronic coupling between both metallic centers is higher than that found in a similar complex, [3] which can be due to increased conjugation.