The emission enhancement behavior and photocleavage activity of a ruthenium(II) arene complex, [(η⁶-p-cymene)Ru(dppn)(py)]²⁺ (1) (dppn = 4,5,9,16-tetraaza-dibenzo[a,c]naphthacene, py = pyridine), towards DNA were compared with [(η⁶-p-cymene)Ru(bpy)(py)]²⁺ (2), [Ru(bpy)₂(dppz)]²⁺ (3) and [Ru(bpy)₂(dppn)]²⁺ (4) (bpy = 2,2′-bipyridine, dppz = dipyrido-[3,2-a:2′,3′-c]phenazine). It was found that 1 emits fluorescence from the dppn-based ligand-centered (LC) singlet excited state and generates singlet oxygen (¹O₂) from the dppn-based LC triplet excited state. As a result, 1 displays emission enhancement behavior and photocleavage activity towards DNA simultaneously. In contrast, 3 is the most classical DNA light switch but shows poor DNA photocleavage activity, while 4 is an efficient DNA photocleaver but cannot report DNA binding by luminescence enhancement. An increased cytotoxicity against human lung carcinoma cells A549 by about 10-fold was also observed for 1 upon visible light activation. These intriguing properties result from the unique combination of the Ru(II) arene and dppn subunits.