Boosting type-I ROS production of molecular photosensitizers using bridge-assisted superexchange coupling

Abstract

Bridge-assisted superexchange coupling capable of long-range electron transfer proves to be effective for charge separation. However, the exploitation of this photochemical process in engineering reactive oxygen species (ROS) production remains unexplored. Herein, piperazine serves as a bridging unit to facilitate a cascade electron transfer from the electron donor site (CO) to the acceptor site (CN) within the COCN molecule, ultimately boosting the generation of superoxide radicals (O₂⁻˙) and hydroxyl radicals (˙OH). Experimental and theoretical studies elucidate that the long-range electron transfer is enabled by a superexchange interaction through the piperazine σ*-bridge, which leads to an effective generation of a radical ion pair CO⁺˙BCN⁻˙. The cationic radical CO⁺˙ can directly catalyze the oxidation of water, while the anionic radical CN⁻˙ transfers one electron to oxygen (O₂). Additionally, COCN has an excited triplet state characterized by a ³(π–π*) electronic configuration, which further promotes sequential electron transfer to O₂. These reactions enable the efficient production of ˙OH and O₂⁻˙, respectively, thus completing a cascade electron cycling process. Based on these findings, nanoparticles of COCN exhibit satisfying O₂⁻˙ and ˙OH production performance even under hypoxic environments and demonstrate potent photodynamic activity in addition to a notably high fluorescence quantum yield of 62.8%, rendering them promising candidates for cellular imaging and ablation assessments. This study contributes to the advancement of photosensitizers proficient in selectively generating ROS, offering valuable insights into the underlying mechanisms that govern ROS production.