The Current Advances in Design Strategy (Indirect Strategy and Direct Strategy) for Type-I Photosensitizers

Type-I photosensitizers (PSs) are among the most potential candidates for photodynamic therapy (PDT), as their low dependence on oxygen endow them with many advantages for treating hypoxic tumor. However, most of the reported type-I PSs have a contingency of molecular design, because electron transfer (ET) reaction is more difficult to achieve than energy transfer (EET) process. Therefore, it is urgent to understand molecular design mechanisms for type-I PSs. In this review, the two ways to achieve the type-I PSs, i.e., inhibiting EET process (type-II) or enhancing ET process (type-I), are detailly explained. In response, the current design strategies of type-I PSs are summarized from two perspectives: indirect strategy (inhibiting EET process: reducing the energy of the lowest triplet excited state (T₁) to lower than the energy required for the excitation energy transfer to produce singlet oxygen) and direct strategy (enhancing ET process: promoting the ET efficiency of PSs to generate superoxide radicals). The construction of direct strategy can be realized by forming an electron-rich microenvironment, providing an electron-deficient intermediate transmitter, and introducing an enhanced electron transfer capacity primitive.