Singlets-Driven Photoredox\nCatalysts: Transforming\nNoncatalytic Red Fluorophores to Efficient Catalysts

Red-light absorbing photoredox catalysts offer potential\nadvantages\nfor large-scale reactions, expanding the range of usable substrates\nand facilitating bio-orthogonal applications. While many red-light\nabsorbing/emitting fluorophores have been developed recently, functional\nred-light absorbing photoredox catalysts are scarce. Many photoredox\ncatalysts rely on long-lived triplet excited states (triplets), which\ncan efficiently engage in single electron transfer (SET) reactions\nwith substrates. However, triplets of π-conjugated molecules\nare often significantly lower in energy than photogenerated singlet\nexcited states (singlets). Combined with the inherent low energy of\nred light, this could limit the reductive/oxidative powers. Here,\nwe introduce a series of sustainable heavy atom–free photoredox\ncatalysts based on red-light absorbing dibenzo-fused BODIPY. The catalysts\nconsist of two covalently linked units: a dibenzo-fused BODIPY fluorophore\nand an electron donor, arranged orthogonally. Excitation of the dibenzoBODIPY\nunit induces charge separation (CS) from the donor to the dibenzoBODIPY\nunit, forming a radical pair (RP) state. Unlike the regular BODIPY\ncounterparts, these catalysts do not form triplets. Instead, SET occurs\nfrom the high-energy singlet-born RP states, preventing energy loss\nand effectively utilizing the low-energy red light. The proximity\nof donor molecules allows efficient charge separation despite the\nCS being uphill in energy. The molecules demonstrate efficient catalysis\nof Atom Transfer Radical Addition (ATRA) reaction, yielding products\nwith high yields ranging from 70 to 90%, while the molecule without\na donor group does not exhibit catalytic activity. The mechanistic\nstudies by transient absorption and electron paramagnetic resonance\n(EPR) spectroscopy methods support the proposed mechanism. The study\npresents a new molecular design strategy for converting noncatalytic\nfluorophores to efficient photoredox catalysts operating in the red\nspectral region.