Unusual Photophysical Properties of s Ruthenium(Ii) Complex Related to [Ru(Bpy)2(Dppz)]2+

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A new ruthenium polypyridyl complex, [Ru(bpy)2(dpqp)] 2+ (bpy = 2,2′-bipyridne; dpqp = pyrazino[2′,3′:5, 6]pyrazino[2,3-f][1,10]phenanthroline), shows strong luminescence in water at room temperature, a behavior that is strikingly different from that of the nonemissive "DNA light-switch" prototype [Ru(bpy)2(dppz)] 2+ (dppz = dipyrido[3,2-a:2′-3′-c]phenazine) under similar conditions. Variation of the absorption and emission spectra of [Ru(bpy)2(dpqp)]2+ as a function of the pH is consistent with the occurrence of two ground-state protonation steps associated with the dpqp ligand and an apparent pKa* of 2.1. Electrochemistry and theoretical calculations indicate that the lowest unoccupied molecular orbital (LUMO) of [Ru(bpy)2(dpqp)]2+ is localized on the distal portion of the dpqp ligand and lies at a lower energy than the dppz-based LUMO of [Ru(bpy)2(dppz)]2+. The combination of its strong DNA binding affinity and relatively long-lived triplet metal-to-ligand charge-transfer excited state in an aqueous solution results in more efficient DNA photocleavage by [Ru(bpy)2(dpqp)]2+ than [Ru(bpy) 2(dppz)]2+. © 2010 American Chemical Society.

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