Construction of Type I Aggregation-Induced Emission Photosensitizers for Photodynamic Therapy via Photoinduced Electron Transfer Mechanism

Photodynamic therapy (PDT) as a non-invasive anticancer modality has received increasing attention due to its advantages of noninvasiveness, high temporospatial selectivity, simple and controllable operation, etc. PDT mainly relies on the generation of toxic reactive oxygen species (ROS) by photosensitizers (PSs) under the light irradiation to cause cancer cell apoptosis and death. However, solid tumors usually exhibit an inherent hypoxic microenvironment, which greatly limits the PDT efficacy of these high oxygen-dependent conventional type II PSs. Therefore, it is of great importance to design and develop efficient type I PSs that are less oxygen-dependent for the treatment of hypoxic tumors. Herein, a new strategy for the preparation of efficient type I PSs by introducing the photoinduced electron transfer (PET) mechanism is reported. DR-NO2 is obtained by introducing 4-nitrobenzyl to (Z)-2-(5-(4-(diethylamino)-2-hydroxybenzylidene)-4-oxo-3-phenylthiazolidin-2-ylidene)malononitrile (DR-OH) with aggregation-induced emission (AIE) feature. The AIE feature ensures their high ROS generation efficiency in aggregate, and the PET process leads to fluorescence quenching of DR-NO2 to promote triplet state formation, which also promotes intramolecular charge separation and electron transfer that is conducive for type I ROS particularly superoxide radicals generation. In addition, DR-NO2 nanoparticles are prepared by nanoprecipitation to possess nanoscaled sizes, high cancer cell uptake, and excellent type I ROS generation ability, which results in an excellent performance in PDT ablation of MCF-7 cancer cells. This PET strategy for the development of type I PSs possesses great potential for PDT applications against hypoxic tumors.