Ru(II) complexes possessing new tridentate ligands with extended pi 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 (3)pi pi* localized on the pydppn ligand with lifetimes of similar to 20 mu s. The [Ru(tpy)(n)(pydppn)(2-n)](2+) (n = 0, 1) complexes are able to generate O-1(2) with similar to 100% efficiency. Both [Ru(tpy)(pydppn)](2+) and [Ru(pydppn)(2)](2+) bind to DNA, however, the former exhibits a similar to 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 O-1(2), which can mediate the reactivity. Such high quantum yields Of O-1(2) photosensitization of transition metal complexes may be useful in the design of new systems with long-lived excited states for photodynamic therapy.