Study of the aqueous photochemistry of 4-fluorophenol, 4-bromophenol and 4-iodophenol by steady state and nanosecond laser flash photolysis

The mechanisms of the aqueous photoreactions of 4-fluorophenol, 4-bromophenol and 4-iodophenol have been studied by steady state photolysis and by nanosecond laser flash photolysis. The same photoproducts are obtained as were found in the photolysis of aqueous 4-chlorophenol although the photoreaction quantum yields vary considerably across the series of compounds, As for 4-chlorophenol the carbene 4-oxocyclohexa-2,5-dienylidene is formed from the halogenophenol by loss of HX and this species reacts efficiently with oxygen to form 1,4-benzoquinone O-oxide which subsequently yields 1,4-benzoquinone. Consideration of the reaction quantum yields together with the photophysical properties of the four halogenophenols suggests that the carbene is derived from the first excited singlet state rather than a triplet excited state.