Fig. 3
From: The role of photodynamic therapy on multidrug resistant breast cancer
The photosensitization process of PDT. When PS absorbs photon energy, it transits from ground singlet state (PS) to an excited singlet state (1PS*) which then undergoes internal conversion and changes to a triplet state (3PS*). The triplet PS reacts with either tissue substrate (Type I mechanism) to form a superoxide anion radicals or with molecular oxygen to form a reactive oxygen species (Type II mechanism)