Photosensitized Dissociation of Di-tert-butyl Peroxide. Energy Transfer to a Repulsive Excited State

Article abstract of DOI:10.1021/ja00393a024

Energy transfer from a variety of aromatic hydrocarbons and ketones to di-tert-butyl peroxide has been examined by using nanosecond laser flash photolysis techniques.Triplet energy transfer to the peroxide leads to its efficient cleavage into two tert-butoxy radicals.Representative rate constants for triplet quenching in benzene at 25 deg C are 7.9*1E6, 3.4*1E6, and 7.0*1E4 M^-1 s^-1 for p-methoxypropiophenone, benzophenone, and benzanthracene, respectively.The rate of transfer for p-methoxypropiophenone (E_T ca. 72.5 kcal/mol) is approximately temperature independent; for lower energy sensitizers ca. 0.17 kcal/mol activation energy is required for each kilocalorie per mole decrease in triplet energy.No evidence indicating exciplex intermediacy was found.A model for energy transfer to a repulsive state of peroxide is proposed in which no activation energy is required if the sensitiser meets the energy requirements at the O-O equilibrium distance.For sensitizers of lower triplet energy, energy transfer to a repulsive state is proposed to occur from a thermally activated ground state having a greater than equilibrium oxygen-oxygen bond lenght.The same mechanism may apply in other systems where the acceptor lacks low-lying excited states.A few rate constants for the quenching of singlet sensitizers have also been determined by using fluorescence techniques.