ZnS nanocrystallites effectively enhanced photo-reduction of chlorinated benzene derivatives in the presence of triethylamine as a sacrificial electron donor under UV irradiation (l > 300 nm), leading to selective and stepwise dechlorination to give benzene at the final stage. Photochemical detoxification of halogenated compounds, such as PCB, dioxin, and DDT, has been attracting much attention because this can be regarded as a promising process to eliminate C-Cl bonds under environmentally relevant and mild conditions. In particular, TiO 2 -catalyzed photoprocesses involving oxidation and reduction in aqueous systems have been extensively investigated. 1 In such systems, however, photooxidation proceeds through the formation of hydroxyl radical (HO•), leading to unavoidable formation of unknown photoproducts especially from polychlorinated compounds. 2,3 Further, the rate of such photodegradation often slows in the case of polychlorinated compounds because they are electron-deficient molecules, i.e. their oxidation potentials are very positive in nature, showing resistance to the electrophilic attack of HO•. A DMF solution of ZnS nanocrystallites was prepared by the reaction of Zn(ClO 4 ) 2 with H 2 S under cooling with ice and water as described elsewhere. 7 Photoreactions were carried out under cooling with water by UV irradiation of a DMF solution (2 ml) containing a substrate (25 mM), ZnS nanocrystallites (2.5 mM in diatomic concentration as ZnS) and triethylamine (TEA, 1 M) in a Pyrex glass tube using a 500 W high pressure mercury lamp. The reaction mixtures were analyzed by gas chromatography with a fused silica capillary column (HiCap-CBP20, 25 m 3 0.2 mm, Shimadzu) using dodecane as an internal standard. The dechlorination proceeded at a relatively high rate even without ZnS nanocrystallites. The absorption spectrum measured for the mixture of 1,4-dichlorobenzene and TEA showed the appearance of a shoulder at the long wavelength edge of the absorption of 1,4-dichlorobenzene, suggesting the formation of the exciplex between the two compounds. The formation of the exciplex should contribute to the photochemical dechlorination without the catalyst. In our earlier report, ZnS nanocrystallites prepared in DMF act as a photocatalyst for the two-electron reduction of CO 2 to HCOOH or CO in the presence of triethylamine
