posted on 2023-12-12, 05:04authored byZijian Zhao, Jili Li, Wei Yuan, Dajiao Cheng, Suze Ma, Ye-Fei Li, Zhang-Jie Shi, Ke Hu
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, 3NMB+*, 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.