Photoreduction ofortho-benzoquinones in the presence ofpara-substitutedN,N-dimethylanilines

Photoreduction ofo-benzoquinones irradiated at the wavelengths λmax ≈ 400 and 600 nm corresponding to the S(π → π*) and S(n → π*) electron transitions in the >C=0 groups, respectively, in the presence ofN,N-dimethylaniline and its derivatives was studied. The apparent rate constants of the photoreduction (k H) ofo-quinones are determined by the free energy of electron transfer from the amine molecule to a photoexcitedo-quinone molecule (ΔG e.t). The ΔG e.t. values are calculated as the sums of the energies of the 0→0 transitions of the lowest triplet excited state ofo-quinones, the reduction energies ofo-quinones, and the oxidation energies of amines (the last two terms are numerically equal to the corresponding redox potentials). The maximum rate of photoreduction was found for ΔG e.t≈0. The reaction mechanism is proposed, in which the reversible formation of a triplet exiplex is the rate-determining stage and hydrogen transfer proceeds in parallel with electron transfer within the exiplex.