Photoreduction of ortho-benzoquinones in the presence of para-substituted N,N-dimethylanilines

Photoreduction of o-benzoquinones irradiated at the wavelengths k(max) appr oximate to 400 and 600 nm corresponding to the S(pi --> pi*) and S(pi --> p i*) electron transitions in the >C=O groups, respectively, in the presence of NN-dimethylaniline and its derivatives was studied. The apparent rate co nstants of the photoreduction (k(H)) of o-quinones are determined by the fr ee energy of electron transfer from the amine molecule to a photoexcited o- quinone molecule (DeltaG(e.t)). The DeltaG(e.t) values are calculated as th e sums of the energies of the 0 -->0 transitions of the lowest triplet exci ted state of o-quinones, the reduction energies of o-quinones, and the oxid ation energies of amines (the last two terms are numerically equal to the c orresponding redox potentials). The maximum rate of photoreduction was foun d for DeltaG(e.t) approximate to 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 wi thin the exiplex.