The quintessential cancer treatment would target only the cancer cells and leave the healthy alone, minimizing the negative effects of treatment. PDT (Photodynamic Therapy) is known to target tumor tissue selectively, but classic PDT methods are dependent on the availability of oxygen. The ideal PDT agent is strictly non-toxic in the dark and when irradiated, becomes very toxic. Furthermore, PDT often causes apoptosis (programmed cell death) rather than necrosis of the cancerous tissue. The advantage of the former is that healthy human tissue is not affected by the treatment procedure, whereas the latter is often the cause for serious complications, such as inflammation and allergic shock reactions. Unfortunately malignant and especially drug-resistant cells are often hypoxic (very low in oxygen content) and thus render PDT useless. A special class of ruthenium-complexes, which are good PDT-agents because they can cut DNA under irradiation even in the absence of oxygen, could be the answer to this PDT problem provided that they can be introduced into the cancer cells.

Currently, we are developing prototypes for these new PDT-drugs, which are catenanes and diads formed by chemically and mechanically linked ruthenium(II)-polypyridyl complexes and electron-relays of the 4,4'-viologen-type and their cyclophane (Stoddart-relay). These compounds combine suitable photophysical properties (excitation by multi-photon-processes, long lifetimes of their charge-separated states and high reactivity towards DNA) with the ability to be excreted by mammalian kidneys.