| There is a need to better understand the fate of natural and anthropogenic organic materials being released into terrestrial, aquatic, and atmospheric systems. For halogenated aromatic compounds, environmental degradation via biological pathways is generally ineffective. Hence, abiotic methods of transformation---including photolysis---often play a significant role in the overall environmental fate of these compounds. Among the various potential halogenated aromatic compounds for study, those with a diaryl ether nucleus have been found to be of particular utility in industry, and are known to be the stable products of a wide range of natural and anthropogenic processes. The present work describes photochemical investigations on two representative classes of diaryl ether contaminants---(1) chlorinated dibenzo[1,4]dioxins and representative model analogs, and (2) brominated diphenyl ethers.; In order to better understand the underlying photochemistry of chlorinated dibenzo[1,4]dioxins, photochemical studies on a range of model halogenated, alkoxy, and alkyl dibenzo[1,4]dioxins have been performed in aqueous and organic solutions. The compounds were found to undergo a photochemically initiated aryl-ether bond homolysis that yields reactive 2-spiro-6'-cyclohexa-2 ',4'-dien-1'-one and subsequent 2,2'-biphenylquinone intermediates. Under steady-state irradiation, the 2,2'-biphenylquinones were observed to participate in excited state hydrogen abstraction from the organic solvent to give corresponding 2,2'-dihydroxybiphenyls. In the absence of continued irradiation, 2,2'-biphenylquinones with electron donating substituents thermally rearrange to corresponding oxepino[2,3-b]benzofurans, whereas the unsubstituted 2,2'-biphenylquinone and its derivatives with electron withdrawing groups thermally rearrange to corresponding 1-hydroxydibenzofurans. The findings represent a possible general photochemically initiated mechanism for the degradation of dibenzo[1,4]dioxins, including the highly toxic chlorinated derivatives, that may shed insight into their fate in natural systems and potential mechanisms for toxicological action.; The photochemistry of model brominated diphenyl ethers has been investigated in organic and aqueous solution. These findings suggest that para brominated diphenyl ethers with 1 or 2 bromine substituents will likely undergo exclusive photochemically induced aryl-bromine bond homolysis in aqueous or organic solvents, followed by hydrogen abstraction from organic solvents or similar impurities in natural aquatic systems. No evidence of photochemical aryl-ether bond cleavage was observed with the model para substituted mono- and di-brominated diphenyl ethers. In contrast, the observed formation of brominated dibenzofurans and 2-hydroxybiphenyls from the photolysis of a model hexabrominated diphenyl ether suggests that brominated diphenyl ethers with >6 bromine substituents will undergo both photochemically induced aryl-ether and aryl-bromine bond homolysis in organic solvents. When the brominated diphenyl ether starting material has a bromine substituent in the ortho position relative to the ether linkage, the findings demonstrate that photochemical aryl-bond homolysis can lead to the production of brominated dibenzofurans. |
