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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 benzophenone, 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 x lo5 I mol-l s-l w a s determined for this process. Nitrogen-flushed acetonitrile solutions of triphenylamine gave, on flash photolysis, an intermediate (Amsx. 61 0 nm) identified, from previous work, as a 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.

Triplet benzophenone does not react efficientlywith aniline, o-toluidine, and diphenylamine to give isolable products. However, flash-photolysis studies showed that the ketone quite efficiently abstracts hydrogen from the amines.From a consideration of the kinetics of decay of the a-hydroxydiphenylmethyl radical, pathways were suggested whereby the radicals may react so as to produce the starting compounds. These results lead to a questioning of the assumption that primary and secondary aromatic amines act purely as physical quenchers for triplet ketones and the conclusion is reached that deactivation may be a result of either or both electron transfer and hydrogen-atom transfer. 9.1 O-Dihvdroacridine reacts with excited benzophenone, benzylideneacetophenone, and benzylideneacetone to give isolable reduction products.AN intriguing aspect of the photoreactions of aromatic carbonyl cornpodnds with aromatic tertiary amines l a p is that they take place by an electron-transfer mechanism. The nature of the intermediates produced by the electron transfer process, i.e., an exciplex or radical ions (see Scheme l), depends upon the polarity of the solvent. Kinetic measurements have shown that deactivation of the triplet ketone by the electron-transfer process is very efficient lb92 and flash-photolysis experiments have verified that radical-ion formation takes place in polar solvents.la9Backstrom and Sandros have reported3 upon the quenching of biacetyl phosphorescence by primary andsecondary aromatic amines and have shown that the rate constants for quenching approach the diff usion-controlled limit. In contrast, triphenylamine was found to be a less efficient quencher. Turro and Engel confirmed these results 4 and interpreted the quenching as occurring by electron transfer from the amine to the triplet ketone. If this mechanism is correct the efficiency of quenching should be related to the ionisation potential of the amine and consequently triphenylamine should be as good a quencher as aniline. In a related study, Cohen and Davis have reported that aniline and diphenylamine quench excited fluorenone much more readily than tri-~henylamine.~" However these workers did find that the quenching ability of a series of anilines was related to their ionisat ion PO t entialssb 1 (a) R.

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