TRIPLET AND SINGLET PROCESSES IN THE PHOTOINDUCED ELECTRON-TRANSFER FROM N,N'-DIMETHYLANILINE TO CHLOROBIPHENYLS

N,N'-Dimethylaniline (DMA) forms a fluorescent exciplex with biphenyl in low polarity solvents. With monochlorobiphenyls (ClBi) exciplex emission is only observed for the isomers substituted in position 3 and 4, but not with 2-chlorobiphenyl. The quenching of the excited singlet of DMA by the three ClBi was studied in methanol and ethyl acetate. In the alcohol the exciplex emission is not observed. In ethyl acetate the exciplex emission quantum yield is 0.037 for 3-ClBi and 0.024 for 4-ClBi. In both solvents DMA sensitized the photodechlorination of the three isomers. At 0.1 M the HCl quantum yields are 0.34; 0.07 and 0.30 for 2-, 3- and 4-ClBi respectively in ethyl acetate, and 0.42; 0.10 and 0.44 for 2-, 3- and 4-ClBi respectively in methanol. A mechanism involving an electron transfer process from both excited singlet and triplet DMA to the ClBi is proposed. From the dependence of the quantum yields on the ClBi concentration the dechlorination efficiencies for the singlet and triplet mediated processes are obtained. For the 2 and 4 isomer the singlet process is the most important in the concentration range investigated. For the 3 isomer, at concentrations lower than 0.1 M, the dechlorination arises mainly in the triplet state.