The photoreaction of aromatic carbonyl compounds with amines: evidence for electron transfer from tertiary aromatic amines to triplet benzophenone

Abstract

Triphenylamine and tri-p-tolylamine quench the photoreduction of benzophenone by diphenylmethanol and in the case of triphenylamine, the quenching effect is substantially increased by a change of solvent from benzene to acetonitrile. Flash photolysis of nitrogen-flushed acetonitrile solutions of tri-p-tolylamine containing benzo-phenone, produced the amine radical cation and the radical anion of the ketone. When oxygenated solutions of either the amine alone of amine plus benzophenone were flash-photolysed, only the radical cation of the amine was observed. The lifetime and concentration of the radical cation is increased as the concentration either of oxygen or of benzophenone is increased. One of the modes of decay of the radical cation appears to be reaction with tri-p-tolylamine and a rate constant of 3·5 × 10³ l mol^–1 s^–1 was determined for this process. Nitrogen-flushed acetonitrile solutions of triphenylamine gave, on flash photolysis, an intermediate (λ_max. 610 nm) identified, from previous work, as cyclisation product, whereas when benzophenone is present in solution, the amine radical cation is produced. These results support the conclusion that tertiary aromatic amines can donate an electron to triplet benzophenone, and lend some credence to the previous postulate that amines react with triplet ketones by an electron-transfer process.