Quantum Yield of DNA Strand Breaks under Photoexcitation of a Molecular Ruby

Photodynamic therapy (PDT) used for treating cancer relies on the generation of highly reactive oxygen species, for example, singlet oxygen 1O2, by light‐induced excitation of a photosensitizer (PS) in the presence of molecular oxygen, inducing DNA damage in close proximity of the PS. Although many precious metal complexes have been explored as PS for PDT and received clinical approval, only recently, the potential of photoactive complexes of non‐noble metals as PS has been discovered. Using the DNA origami technology that can absolutely quantify DNA strand break cross sections, we assessed the potential of the luminescent transition metal complex [Cr(ddpd)2]3+ (ddpd=N,N′‐dimethyl‐N,N′‐dipyridine‐2‐ylpyridine‐2,6‐diamine) to damage DNA in an air‐saturated aqueous environment upon UV/Vis illumination. The quantum yield for strand breakage, that is, the ratio of DNA strand breaks to the number of absorbed photons, was determined to 1–4 %, indicating efficient transformation of photons into DNA strand breaks by [Cr(ddpd)2]3+. Singlet oxygen sensitized by the molecular ruby [Cr(ddpd)2]3+ under excitation with UV/Vis light induces DNA strand breaks. Utilization of the DNA origami technology together with atomic force microscopy enabled the quantification of the quantum yield of strand breaks to 1–4 %, revealing the promising potential of using [Cr(ddpd)2]3+ as photosensitizer for photodynamic therapy.