The photochemistry of dibenzo-1,4-dioxin (7) and 2,3,7,8-tetramethyldibenzo-1,4-dioxin (15) — both of which have the parent ring system of the well-known enrimental contaminant dioxin— has been studied in aqueous solution and in selected organic solvents. It is shown that a novel intramolecular photorearrangement is the major mode of reaction for 7 and the exclusive reaction for 15, giving rise to observable (by UV—Vis spectrophotometry) intermediate 2,2′-biphenylquinones. Subsequent reduction by the organic co-solvent gives rise to 2,2′-biphenols as major or exclusive product. Thus, photolysis of these compounds generates oxidizing agents in the biphenylquinones. Photolysis in the presence of added NaBH4 resulted is greatly enhanced yields of biphenols along with more than 90% material balance. These findings have relevance to dioxin photodecomposition, which can be used as a method for its destruction.

The photochemical dechlorination of Aroclor 1232, 1242, 1254, and 1260 was examined in the presence and absence of sodium borohydride. In both cases the photoreduction proceeded to give dechlorinated biphenyls by sequential loss of chlorines, but the photoreactions in the presence of borohydride were much faster. In the absence of the borohydride the total removal of chlorines was impractically slow, but its presence made it possible to proceed to essentially total destruction of the PCB's.

The photochemical debromination of a series of bromobiphenyls was examined in the presence and absence of sodium borohydride. In both cases the photoreduction proceeded cleanly to give the corresponding debrominated biphenyls, but the photolyses in the presence of borohydride were accelerated up to a 100-fold. A free radical chain mechanism is involved.