Restricting Bond Rotations by Ring Fusion: A Novel Molecular Design Strategy to Improve Photodynamic Antibacterial Efficacy of AIE Photosensitizers

In recent years, aggregation-induced emission photosensitizers (AIE-PSs) for antibacterial photodynamic therapy (aPDT) have received increasing attention because of their ability to increase reactive oxygen species (ROS) generation in the aggregation state. However, their antibacterial effect still has great room for improvement. Herein, we propose that if the rotation of some bonds in AIE-PSs is restricted, the nonradiative decay could be further suppressed to boost the generation of fluorescence and ROS, so as to improve their antibacterial efficacy. Following this molecular design strategy, we developed a new class of carbazole group-based AIE-PSs (CPVBA, CPVBP, CPVBP2, and CPVBP3), in which the rotation of phenyl-N bonds is restricted in the carbazole ring. Compared with diphenylamine group-based AIE-PSs with free rotation of phenyl-N bonds, carbazole group-based AIE-PSs showed stronger fluorescence, ROS generation, and antibacterial abilities, demonstrating the feasibility of this new design strategy. Notably, CPVBP3 can enter the entire cell of E. coli to exert its antibacterial effect, and there are few reports of photosensitizers with similar functions. Furthermore, to the best of our knowledge, the light dose (1.2 J/cm²) we used for CPVBP2 to kill Staphylococcus aureus is much lower than that of many reported photosensitizers, indicating great prospects for AIE antimicrobial photosensitizers.