Ru(II) complexes of new tridentate ligands: unexpected high yield of sensitized 1O2.

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) (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) (n = 0, 1) complexes is a ligand-centered (3)pipi* localized on the pydppn ligand with lifetimes of approximately 20 mus. The Ru(tpy)(n)(pydppn)(2-n) (n = 0, 1) complexes are able to generate (1)O(2) with approximately 100% efficiency. Both Ru(tpy)(pydppn) and Ru(pydppn)(2) bind to DNA, however, the former exhibits a approximately 10-fold greater DNA binding constant than the latter. Efficient DNA photocleavage is observed for Ru(tpy)(pydppn), owing to its ability to photosensitize the production of (1)O(2), which can mediate the reactivity. Such high quantum yields of (1)O(2) photosensitization of transition metal complexes may be useful in the design of new systems with long-lived excited states for photodynamic therapy.