Oxidization enhances type I ROS generation of AIE-active zwitterionic photosensitizers for photodynamic killing of drug-resistant bacteria

Type I photosensitizers (PSs) with an aggregation-induced emission (AIE) feature have received sustained attention for their excellent theranostic performance in the treatment of clinical diseases. However, the development of AIE-active type I PSs with strong reactive oxygen species (ROS) production capacity remains a challenge due to the lack of in-depth theoretical studies on the aggregate behavior of PSs and rational design strategies. Herein, we proposed a facile oxidization strategy to enhance the ROS generation efficiency of AIE-active type I PSs. Two AIE luminogens, MPD and its oxidized product MPD-O were synthesized. Compared with MPD, the zwitterionic MPD-O showed higher ROS generation efficiency. The introduction of electron-withdrawing oxygen atoms results in the formation of intermolecular hydrogen bonds in the molecular stacking of MPD-O, which endowed MPD-O with more tightly packed arrangement in the aggregate state. Theoretical calculations demonstrated that more accessible intersystem crossing (ISC) channels and larger spin-orbit coupling (SOC) constants provide further explanation for the superior ROS generation efficiency of MPD-O, which evidenced the effectiveness of enhancing the ROS production ability by the oxidization strategy. Moreover, DAPD-O, a cationic derivative of MPD-O, was further synthesized to improve the antibacterial activity of MPD-O, showing excellent photodynamic antibacterial performance against methicillin-resistant S. aureus both in vitro and in vivo. This work elucidates the mechanism of the oxidization strategy for enhancing the ROS production ability of PSs and offers a new guideline for the exploitation of AIE-active type I PSs.