Visible light irradiation of a ruthenium(II) quinone-containing complex, [(phen)(2)Ru(phendione)](2+) (1(2+)), where phendione = 1,10-phenanthroline-5,6-dione, leads to DNA cleavage in an oxygen independent manner. A combination of NMR analyses, transient absorption spectroscopy, and fluorescence measurements in water and acetonitrile reveal that complex 1(2+) must be hydrated at the quinone functionality, giving [(phen)(2)Ru-(phenH(2)O)](2+) (1H(2)O(2+), where phenH(2)O = 1,10-phenanthroline-6-one-5-diol), in order to access a long-lived (MLCThydrate)-M-3 state (tau similar to 360 ns in H2O) which is responsible for DNA cleavage. In effect, hydration at one of the carbonyl functions effectively eliminates the low-energy (MLCTSQ)-M-3 state (Ru-III phen-semiquinone radical anion) as the predominant nonradiative decay pathway. This (MLCTSQ)-M-3 state is very short-lived (<1 ns) as expected from the energy gap law for nonradiative decay,(1) and too short-lived to be the photoactive species. The resulting excited state in 1H(2)O(2+*) has photophysical properties similar to the (MLCT)-M-3 state in [Ru(phen)(3)](2+*) with the added functionality of basic sites at the ligand periphery. Whereas [Ru(phen)(3)](2+*) does not show direct DNA cleavage, the deprotonated form of 1H(2)O(2+*) does via a proton-coupled electron transfer (PCET) mechanism where the peripheral basic oxygen sites act as the proton acceptor. Analysis of the small molecule byproducts of DNA scission supports the conclusion that cleavage occurs via H-atom abstraction from the sugar moieties, consistent with a PCET mechanism. Complex 1(2+) is a rare example of a ruthenium complex which 'turns on' both reactivity and luminescence upon switching to a hydrated state.