NANOSECOND LASER FLASH-PHOTOLYSIS STUDY OF THE KINETICS AND MECHANISM OF PHOTOREDUCTION OF DECAFLUOROBENZOPHENONE

Laser pulse excitation of decafluorobenzophenone (DFB) in acetonitrile yields the triplet 3DFB after rapid intersystem crossing from the singlet 1DFB. The triplet has absorption maxima at 310 and 490 nm. 3DFB abstracts hydrogen from donors such as alkanes and alcohols in a radical-like mechanism to yield ketyl radical. The rate constant k(q) is 10-40 times higher than that of the triplet benzophenone 3BP. Ketyl radical (DFBH) has absorption maxima at 320 and 530 nm. In aqueous solution, the ketyl radical has a pK(a) = 6.9, and the anion has maxima at 335 and 630 nm. The k(q) for reaction between 3DFB and alkenes depend on their redox potentials. This can be interpreted as due to the formation of a triplet charge-transfer complex (CT) or an exciplex. A Rehm-Weller correlation plot log k(q) vs DELTAG reveals that the exciplex has a weak CT contribution. For alkenes with allylic hydrogen, ketyl radical was detected after the decay of 3DFB. Aliphatic amines form ground-state CT complex with DFB. Amine quenches 3DFB with diffusion-controlled rate constant. The transients formed depend on the amine oxidation potential and solvent polarity. In the case of 1,4-diazabicyclo-[2.2.2]octane (DABCO)( E1/2ox = 0.68 V), only dissociated free ions are observed. However, with triethylamine (TEA) (E1/2ox = 0.97 V) both ketyl radical and free ions are observed. The ketyl radical yield decreases with solvent polarity and at a longer delay time the ketyl radical ionizes to yield the anion. The photoreduction of DFB by amine can be interpreted on the basis of initial formation of triplet ion-pair ((IP))-I-3 which subsequently undergoes proton transfer to yield ketyl radical or dissociates to yield free ions.