Unusual photophysical properties of a ruthenium(II) complex related to [Ru(bpy)2(dppz)]2+.

A new ruthenium polypyridyl complex, Ru(bpy)(2)(dpqp) (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) (dppz = dipyrido[3,2-a:2'-3'-c]phenazine) under similar conditions. Variation of the absorption and emission spectra of Ru(bpy)(2)(dpqp) 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) 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). 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) than Ru(bpy)(2)(dppz).