Nucleophilic Photoreaction of Chlorobenzene in Methanol as Studied by Emission Spectroscopy

Article abstract of DOI:10.1246/bcsj.58.2082

The photoreaction of chlorobenzene in methanol has been studied by means of emission spectroscopy.Upon irradiation with UV light, chlorobenzene reacts with methanol to form anisole with a quantum yield of 0.049 at 18 deg C.Contrary to the case of chlorobenzene, the photoreaction of p-dichlorobenzene with methanol is not observed.By combining these results with those of our recent studies on the dual phosphorescence from low-lying triplet states of halogenated benzenes, it is suggested that the photoreaction of chlorobenzene with methanol, which is a nucleophilic substitution reaction, occurs in the ³(?,?*) state.From the temperature dependence of the quantum yield, the activation energy for the nucleophilic photoreaction is estimated to be larger by about 3 - 4 kcal mol^-1 than the apparent activation energy for a combination of the nonradiative ³(?,?*)->S₀ process and the homolytic dissociative process concerning the C-Cl bond in the ³(?,?*) state.It is suggested that the nucleophilic photoreaction contrasts with the radical reaction leading to the dissociation of the C-Cl bond; the latter reaction occurs in the ³(?,?*) state as well as in the ³(?,?*) state.