Mechanistic insights into the photodegradation of cyclotriphosphazene flame retardants from computational and experimental evidence.

Cyclotriphosphazene (CTP) flame retardants are extensively utilized in lithium-ion batteries, and their release into the environment during battery production, usage, and disposal poses risks as emerging contaminants; however, the photodegradation pathways and transformation mechanisms of CTPs remain poorly understood. This study integrates experimental photolysis with quantum chemical calculations to elucidate these processes, using environmentally prevalent hexaphenoxycyclotriphosphazene (HPCTP, CAS 1184-10-7) as the target compound. Photodegradation experiments identified primary products via high-resolution mass spectrometry, while density functional theory (DFT) calculations provided the energy barriers and bond dissociation energies required for the photodegradation. Cross-validation between experimental and computational data revealed dual photodegradation mechanisms: cleavage of peripheral groups (O-Ph bonds: 4.76-4.92 eV; P-O bonds: 3.83-3.97 eV) and ring-opening via P-N bond cleavage (4.32-4.33 eV). DFT further predicted photodegradation pathways for six additional CTPs, demonstrating that degradation of most peripheral structures (e.g., O-Ph, O-methyl, O-ethyl) and core P-N bonds require radiation beyond UV-B (<280 nm). Exceptions include the P-N bonds in hexachlorocyclotriphosphazene (HCCTP, 940-71-6) and pentachlorine (phenoxy) cyclotriphosphazene (3028-10-2), as well as the O-OPh bonds in HPCTP, which can be cleaved under UV-A (3.10-3.94 eV). Deep UV radiation (<200 nm) is necessary to cleave P-Cl and P-F bonds in HCCTP and hexafluorocyclotriphosphazene (15599-91-4). These results indicate limited solar photodegradability for most valence bonds in CTPs, suggesting their potential to persist as environmental pollutants. This study provides the first systematic elucidation of the photodegradation mechanism of CTPs and the proposal of the potential for UV-C treatment.