Unusual Photophysical Properties of a Ruthenium(II) Complex Related to [Ru(bpy)2(dppz)]2+

A new ruthenium polypyridyl complex, [Ru(bpy)(2)(dpqp)](2+) (bpy = 2,2'-bipyndne; 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 pK(a)* 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+)