From persistence to reactivity: halogen-regulated photosensitization of xanthene chromophores for sustainable water purification.

Halogenated xanthene chromophores act as persistent contaminants yet potent photosensitizers in industrial wastewaters and affected receiving waters. Using fluorescein as the core chromophore, we establish structure-reactivity relationships showing how halogen identity, substitution position, and degree of halogenation regulate intersystem crossing and chromophore-peroxymonosulfate electron transfer, thereby controlling selection between nonradical and radical pathways. Specifically, benzene-ring chlorination stabilizes the framework and suppresses nucleophilic attack, whereas iodination on the xanthene-core extends triplet-state lifetimes and boosts photosensitization. Halogenation induces pronounced red shifts in visible absorption (Δλmax = 67-98 nm) and increases 1O2 quantum yields (ΦΔ) by 20- to 75-fold, consistent with heavy-atom-enhanced spin-orbit coupling and prefiguring a shift toward nonradical pathways. Electron paramagnetic resonance, selective quenching and theoretical calculation identify 1O2 and ·O2- as the dominant intermediates, with SO4·- and ·OH playing secondary roles. Among the xanthene chromophores studied, the catalyst-free erythrosine B system effectively removes various contaminants, even in complex matrices. It achieves high degradation of enrofloxacin, accompanied by concurrent dye photobleaching, across indoor to outdoor continuous-flow reactor, demonstrating its applicability, effectiveness, and sustainability for water purification. This halogenation-based strategy enables self-sensitized, sunlight-driven advanced oxidation processes that minimize reliance on external oxidants and catalysts, facilitating a transition toward more efficient wastewater purification.