Critical discussion of the applications of metal complexes for 2-photon photodynamic therapy

Photodynamic therapy is a light-activated medical technique, which has received increasing attention within the last decade to treat various diseases including bacterial, viral or fungal infections as well as cancer. To overcome the limitations of the currently clinically applied photosensitizers based on a tetrapyrrolic scaffold, metal complexes are sought to be an interesting alternative. While these complexes show attractive photophysical and biological properties, the majority of these compounds are excited using visible light, which is associated with a poor tissue penetration. To circumvent this drawback, increasing attention has been devoted towards the use of 2-Photon instead of 1-Photon irradiation. In this mini-review, the metal complexes prepared in view of 2P-PDT are reviewed and the requirements these compounds need to fulfil to be of interest are critically discussed.