The extended pi-delocalization of dppn (benzo[i]dipyrido[3,2-a:2,3-c]phenazine) results in a (3)pipi* state as the lowest triplet excited state in [Os(bpy)(2)(dppn)](2+) (bpy = 2,2'-bipyridine), which generates a (1)O(2) quantum yield of 0.42. Together with its (3)MLCT absorption, this new osmium complex shows efficient DNA cleavage under irradiation of lambda(irr) > or = 645 nm.