Facile Light-Induced Transformation of [Ru(bpy)(bpyNO)] to [Ru(bpy)].

Ru-based coordination compounds have important applications as photosensitizers and catalysts. [Ru(bpy)(bpyNO)] (bpy = 2,2'-bipyridine and bpyNO = 2,2'-bipyridine--oxide) was reported to be extremely light-sensitive, but its light-induced transformation pathways have not been analyzed. Here, we elucidated a mechanism of the light-induced transformation of [Ru(bpy)(bpyNO)] using UV-vis, EPR, resonance Raman, and NMR spectroscopic techniques. The spectroscopic analysis was augmented with the DFT calculations. We concluded that upon 530-650 nm light excitation, [Ru(bpyNO)(bpy)] is formed similarly to the [Ru(bpy)(bpy)] light-induced state of the well-known photosensitizer [Ru(bpy)]. An electron localization on the bpyNO ligand was confirmed by obtaining a unique EPR signal of reduced [Ru(bpy)(bpyNO)] ( = 2.02, = 1.99, and = 1.87 and N hfs = 12 G, = 34 G, and = 11 G). [Ru(bpyNO)(bpy)] may evolve via breaking of the Ru-O-N fragment at two different positions resulting in [Ru═O(bpy)(bpy)] for breakage at the O-|-N bond and [Ru(HO)(bpy)(bpyNO)] for breakage at the Ru-|-O bond. These pathways were found to have comparable Δ. A reduction of [Ru═O(bpy)(bpy)] may result in water elimination and formation of [Ru(bpy)]. The expected intermediates, [Ru(bpy)(bpyNO)] and [Ru(bpy)], were detected by EPR. In addition, a new signal with = 2.38, = 2.10, and = 1.85 was observed and tentatively assigned to a complex with the dissociated ligand, such as [Ru(HO)(bpy)(bpyNO)]. The spectroscopic signatures of [Ru═O(bpy)(bpy)] were not observed, although DFT analysis and [Ru(bpy)] formation suggest this intermediate. Thus, [Ru(bpy)(bpyNO)] has potential as a light-induced oxidizer.