Nature-Inspired Photocatalytic Azo Bond Cleavage with Red Light
Zijian Zhao, Jili Li, Wei Yuan, Dajiao Cheng, Suze Ma, Yefei Li, Zhang‐Jie Shi, Ke Hu
Abstract
The emerging field of photoredox catalysis in mammalian cells enables spatiotemporal regulation of a wealth of biological processes. However, the selective cleavage of stable covalent bonds driven by low-energy visible light remains a great challenge. Herein, we report that red light excitation of a commercially available dye, abbreviated NMB +, leads to catalytic cleavage of stable azo bonds in both aqueous solutions and hypoxic cells and hence a means to photodeliver drugs or functional molecules. Detailed mechanistic studies reveal that azo bond cleavage is triggered by a previously unknown consecutive two-photon process. The first photon generates a triplet excited state, 3 NMB + *, that is reductively quenched by an electron donor to generate a protonated NMBH • + . The NMBH • + undergoes a disproportionation reaction that yields the initial NMB + and two-electron-reduced NMBH (i.e., leuco-NMB, abbreviated as LNMB ). Interestingly, LNMB forms a charge transfer complex with all four azo substrates that possess an intense absorption band in the red region. A second red photon induces electron transfer from LNMB to the azo substrate, resulting in azo bond cleavage. The charge transfer complex mediated two-photon catalytic mechanism reported herein is reminiscent of the flavin-dependent natural photoenzyme that catalyzes bond cleavage reactions with high-energy photons. The red-light-driven photocatalytic strategy offers a new approach to bioorthogonal azo bond cleavage for photodelivery of drugs or functional molecules.