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The multiplicity of the photo-excited state responsible for the dechlorination reaction of monochlorobiphenyls was determined in 2-propanol. Linear Stern-Volmer plots were obtained by the use of 1,3-cyclohexadiene for the dechlorination reactions of 2- and 3-chlorobiphenyl; the slopes were 35.7 and 11.0 dm3 mol−1 respectively. Fluorescence emissions were also quenched, and the quenching constants for 2- and 3-chlorobiphenyls were in accord with the respective values of the slope obtained from the Stern-Volmer plot for the reaction. The reaction of 4-chlorobiphenyl was quenched by cis-1,3-pentadiene, but was accelerated by 1,3-cyclohexadiene. Photosensitized dechlorinations of monochlorobiphenyls took place with several ketones, such as acetone and acetophenonc. These sensitized reactions were, however, very inefficient compared with the reactions in the direct excitations, except in the case of 4-chlorobiphenyl. It was, then, concluded that the dechlorinations of 2- and 3-chlorobiphenyl occurred only from the excited singlet state, while that of 4-chlorobiphenyl took place predominantly from the excited triplet state.

The photodechlorinations of 2,3-dichlorobiphenyl (2,3-DCB) and 3,4-dichlorobiphenyl (3,4-DCB) in 2-propanol gave 3-chlorobiphenyl and 4-chlorobiphenyl respectively. The quantum yields of monochlorobiphenyl formation were 0.2 for 2,3-DCB and 0.06 for 3,4-DCB, independent of the substrate concentration and the wavelength of the exciting light. On the basis of the results obtained in quenching experiments using cis-1,3-pentadiene, it was concluded that 2,3-DCB was photodechlorinated exclusively in the excited singlet state, while 3,4-DCB was photodechlorinated in both excited singlet and triplet states. In the case of 3,4-DCB, the relative contributions of excited singlet and triplet states to the total dechlorination were evaluated as 0.8 and 0.2 respectively. The participation of the excited triplet state in the photodechlorination of 3,4-DCB was ascertained both by quenching reactions with biacetyl as a quencher and by photosensitized dechlorinations with acetone and acetophenone as sensitizers.