Ru(II) Complexes of New Tridentate Ligands: Unexpected High Yield of Sensitized 1O2

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Abstract
Ru(II) complexes possessing new tridentate ligands with extended π systems, pydppx (3-(pyrid-2′-yl)-11,12-dimethyl-dipyrido[3,2-a:2′,3′-c]phenazine) and pydppn (3-(pyrid-2′-yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene), were synthesized and characterized. The investigation of the photophysical properties of the series [Ru(tpy)n(L)2−n]2+ (L = pydppx, pydppn, n = 0−2) reveals markedly different excited state behavior among the complexes. The Ru(II) complexes possessing the pydppx ligand are similar to the pydppz (3-(pyrid-2′-yl)dipyrido[3,2-a:2′,3′-c]phenazine) systems, with a lowest energy metal-to-ligand charge transfer excited state with lifetimes of 1−4 ns. In contrast, the lowest energy excited state in the [Ru(tpy)n(pydppn)2−n]2+ (n = 0, 1) complexes is a ligand-centered 3ππ* localized on the pydppn ligand with lifetimes of ∼20 μs. The [Ru(tpy)n(pydppn)2−n]2+ (n = 0, 1) complexes are able to generate 1O2 with ∼100% efficiency. Both [Ru(tpy)(pydppn)]2+ and [Ru(pydppn)2]2+ bind to DNA, however, the former exhibits a ∼10-fold greater DNA binding constant than the latter. Efficient DNA photocleavage is observed for [Ru(tpy)(pydppn)]2+, owing to its ability to photosensitize the production of 1O2, which can mediate the reactivity. Such high quantum yields of 1O2 photosensitization of transition metal complexes may be useful in the design of new systems with long-lived excited states for photodynamic therapy.

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