A new type of DNA-intercalating viologen dications, derived from the N,N'-dialkyl-6-(2-pyridyl)phenanthridine structure (in which dialkyl is -CH2CH2-,-CH2CH2CH2-, or (-CH3)2, abbreviated dq2pyp, dq3pyp, and Me2pyp, respectively), are able to produce frank strand breaks in supercoiled plasmid DNA upon irradiation with visible light. The amount of photocleavage is similar for the three drugs. The observed DNA photosensitization appears to follow a single-strand cleavage model, as shown by a kinetic analysis of the reaction with dq2pyp. The photodynamic action of the drugs seems to be initiated by a light-induced electron transfer reaction from the nucleobases, given the singlet excited-state redox potentials (ca. + 2.1 V vs. SHE) and the low quantum yields of singlet molecular oxygen production of the drugs (0.1-0.2 in aerated D2O).