Red-light photoredox catalysis with bridged fluorescein derivatives: mechanistic insights and application to fluoride-responsive photopolymerization

We demonstrated bridged fluoresceins (BFLs) as a class of metal-free red-light photocatalysts that feature structurally tunable and switchable activity in aqueous solutions. Mechanistic studies using photoinitiated radical polymerizations under controlled atmospheres revealed that BFLs engage in efficient oxidative and reductive quenching cycles with co-initiators proceeding through excitation, intersystem crossing, single-electron transfer, and regeneration processes. Notably, molecular oxygen plays dual roles: (i) serving as a co-initiator through O₂-derived reactive oxygen species that initiate radical chains, and (ii) functioning as a redox mediator that accelerates regeneration of BFL radical ions. Regarding the structure-performance relationship, bromine substituents on the BFL backbone considerably enhance photocatalytic efficacy by increasing spin–orbit coupling (i.e., the heavy-atom effect), thereby promoting intersystem crossing yield. These findings are supported by thermodynamic and kinetic analyses, including voltammetry, time-resolved photoluminescence under cryogenic conditions, electron paramagnetic resonance (EPR) spectroscopy, and photobleaching studies. We also developed a switchable photoredox catalyst based on a sulfonyl-protected bridged fluorescein. This latent photocatalyst is selectively activated by fluoride ions to restore its photocatalytic activity, enabling red-light photopolymerization. Thus, this work not only offers fundamental mechanistic insights into photoredox catalysis and photoinitiated polymerization, but also establishes design principles for the development of switchable photocatalysts.