What are the ideal photoproperties for a sensitizer?

For effective photosensitization of malignant tumours, a sensitizer should exhibit appreciable absorption at red to near-infrared wavelengths and generate cytotoxic species via oxygen-dependent photochemical reactions. These photosensitization mechanisms rely on the excitation of the sensitizer from its electronic ground state to the fluorescent singlet state, which is in turn transformed into the longer-lived triplet state. Efficient formation of this metastable state is required because it is the interaction of the triplet state with tissue components that generates cytotoxic species such as singlet oxygen. Measurement of fluorescence provides a useful means of probing the sensitizer, particularly because detection sensitivities for both the triplet state and the cytotoxic singlet oxygen are much lower. With the development of chemically well-defined sensitizers, in vivo fluorescence detection has the potential to provide quantitative assessment of photoactive sensitizer distributions. The reactive properties of sensitizer triplet states and species such as singlet oxygen can result in significant sensitizer photodegradation, which may appear undesirable but can be exploited in vivo under certain conditions.