Mechanistic Insights into the Photoconversion of Tire Rubber Antioxidant 6PPD at the Liquid-like Layer of Ice Crystals

The rubber antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) is used globally to retard tire oxidative aging. While its photochemical fate in aqueous systems is known, little is understood about its behavior under freezing conditions across the cryosphere. Here, we show that ice acts as an efficient photochemical medium that converts 6PPD to the toxic 6PPD-quinone (6PPDQ) via a dual-oxygen mechanism involving both O₂ and H₂O. Mechanistically, the freeze-induced concentration of 6PPD within the liquid-like layer of ice induces J-aggregation of 6PPD, which facilitates intersystem crossing and prolongs the triplet exciton lifetime, thereby enhancing the quantum yield of superoxide radicals from O₂. The proton enrichment (i.e., pH decrease) concurrently shifts 6PPD to its 6PPDH₂²⁺ form and activates a nucleophilic attack by H₂O that is thermodynamically unfavorable in bulk water. These combined effects yield a 3.9-fold higher molar yield of 6PPDQ in ice than in aqueous solution, with the photodegradation products of 6PPD in ice causing substantially increased mortality of E. coli and zebrafish embryos. Our findings thereby uncover a previously unrecognized route and mechanisms of 6PPDQ formation and raise critical concerns regarding the fate and ecological risks of 6PPDQ in seasonally frozen eco-environmental systems.