Photochemistry of substituted 4-halogenophenols: effect of a CN substituent
The photochemistry of 5-chloro-2-hydroxybenzonitrile 1 was studied in aqueous solution using transient absorption spectroscopy and product analysis. The triplet carbene 3-cyano-4-oxocyclohexa-2,5-dienylidene 2 (λmax/nm 385, 368) was successfully detected and identified on the basis of its characteristic reactivity. This transient species is converted into cyanobenzo-1,4-quinone-O-oxide (λmax/nm 470) by reaction with oxygen and is reduced into 2-cyanophenoxyl radical (λmax/nm 402, 387) by propan-2-ol. The product studies confirm the intermediary formation of carbene 2. 2,5-Dihydroxybenzonitrile 3 and the biphenyls 4 and 5 are primary photoproducts in deoxygenated solutions whereas 2-hydroxybenzonitrile 9 and 5-bromo-2-hydroxybenzonitrile 10 are cleanly produced upon addition of propan-2-ol (0.13 M) and bromide ions (10–2 M), respectively. In oxygen-saturated solutions, cyanobenzo-1,4-quinone 8 is the main photoproduct. The quantum yield of carbene formation (0.062) is reduced by 40% in the presence of oxygen and is increased up to a value of 0.20 upon addition of bromide or iodide ions. These results can be interpreted in terms of triplet quenching and heavy-atom enhancement and support the assumption that the carbene 2 is formed from the triplet excited state of 1; this assumption is supported by a detailed study of the phototransformation of 1 in ethanol. Mono- and biphotonic formations of solvated electrons and 4-chloro-2-cyanophenoxyl radicals (λmax/nm 427, 408) are also observed from neutral 1. The effects of CN substitution can be traced to deprotonation of the lowest excited singlet state on the one hand (pK* = 0.12 ± 0.04) and to an increase of the triplet lifetime on the other.