Photochemistry of substituted 4-halogenophenols: effect of a CN substituent

The photochemistry of 5-chloro-2-hydroxybenzonitrile 1 was studied in aqueo us solution using transient absorption spectroscopy and pro duct analysis. The triplet carbene 3-cyano-4-oxocyclohexa-2,5-dienylidene 2 (lambda(max)/n m 385, 368) was successfully detected and identified on the basis of its ch aracteristic reactivity. This transient species is converted into cyanobenz o-1,4-quinone-O-oxide (lambda(max)/nm 470) by reaction with oxygen and is r educed into 2-cyanophenoxyl radical (lambda(max)/nm 402, 387) by propan-2-o l. The product studies confirm the intermediary formation of carbene 2. 2,5 -Dihydroxybenzonitrile 3 and the biphenyls 4 and 5 are primary photoproduct s in deoxygenated solutions whereas 2-hydroxybenzonitrile 9 and 5-bromo-2-h ydroxybenzonitrile 10 are cleanly produced upon addition of propan-2-ol (0. 13 M) and bromide ions (10(-2) M), respectively. In oxygen-saturated soluti ons, cyanobenzo-1,4-quinone 8 is the main photoproduct. The quantum yield o f 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-a tom enhancement and support the assumption that the carbene 2 is formed fro m the triplet excited state of 1; this assumption is supported by a detaile d study of the phototransformation of 1 in ethanol. Mono- and biphotonic fo rmations of solvated electrons and 4-chloro-2-cyanophenoxyl radicals (lambd a(max)/nm 427, 408) are also observed from neutral 1. The effects of CN sub stitution can be traced to deprotonation of the lowest excited singlet stat e on the one hand (pK* = 0.12 +/- 0.04) and to an increase of the triplet l ifetime on the other.