Laser photolysis studies of electron transfer between triplet naphthoquinones and amines

Electron transfer between triplet naphthoquinones ((3)Nap*; 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone and 2,3-dimethyl-1,4-naphthoquinone in the triplet state) and N,N-dimethylaniline (DMA) or triethylamine (TEA) in acetonitrile (MeCN) and a mixture of MeCN and H2O (4:1, v/v) was studied by laser photolysis techniques at 295 K. By analysis of the transient absorption spectrum observed after the completion of electron transfer, the absorption spectra and coefficients of the naphthoquinone anion radicals (Nap(.)-) were determined on the basis of those of the DMA cation radical (DMA(.+)) in MeCN and MeCN-H2O (4:1, v/v). The values of the rate constants (k(ct)) for electron transfer between (3)Nap* and DMA were found to be close to those for diffusion-controlled processes as predicted by the RehmWeller theory on electron transfer. Using the determined molar absorption coefficients of Nap(.-), the efficiencies (Gamma(rad)) for the formation of Nap(.-) and DMA(.+) were shown to be unity since the radical ion pair produced ((3)[Nap(.-) + DMA(.+)]) readily dissociates into Nap(.-) and DMA(.+) due to the triplet spin multiplicity.