Nuclephilic Photoreaction of Chlorobenzene in Methanol as Studied by Emission Spectroscopy

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Abstract
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 °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(π,σ*) 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(π,σ*)→S0 process and the homolytic dissociative process concerning the C–Cl bond in the 3(π,σ*) 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(π,π*) state as well as in the 3(π,σ*) state.

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