Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino and dimethylamino derivatives of benzophenone

Abstract

The electron pulse radiolysis technique has been used to study the spectral, kinetic, and radiation chemical properties of the triplet excited states as well as the reduced and oxidized radicals of 2-amino, 4-amino-, 4-(dimethylamino)-, and 4,4'-bis(dimethylamino)benzophenones. Hydrated electron (e_aq^-) has been found to react with these benzophenone derivatives to form the anion radical species with the rate constants varying from (4 to 10)×10⁹ dm³mol^-1s^-1, which are slower than that for the parent ketone, benzophenone. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them at suitable pH. The absorption peaks of the ketyl radicals appear in the region 565-630 nm and those of the anion radicals are red shifted in the wavelength region 565-630 nm. The pK_a values of the ketyl ⇔ anion radical equilibria vary from 8.8 to 10.7 for various substituted aminobenzophenones. The cation radicals of these derivatives have been generated by pulse radiolysis in 1,2-dichloroethane and tetrachloromethane solvents along with other transient species, such as ion pairs and radical adducts. The triplet states of these compounds have been generated in benzene. The probable mechanism of the self-quenching interaction between the triplet and the ground state is the simple hydrogen atom abstraction reaction by the nπ^∗ triplet, which is energetically close to the lower lying ππ^∗ triplet state. Mechanisms of interaction between the ground states of the amino-substituted benzophenone derivatives and the benzophenone triplet have been found to depend on the driving force for the charge- or electron-transfer reaction between the members of a particular pair. nπ^∗ Triplet state of benzophenone interacts with 2-aminobenzophenone via energy transfer to the latter, with 4-aminobenzophenone via hydrogen atom abstraction and with 4-(dimethylamino)- and 4,4'-bis(dimethylamino)benzophenones by electron-transfer coupled with proton-transfer reactions. Only in the case of the interaction between triplet benzophenone and 4-(dimethylamino)benzophenone, evidence for exciplex formation has been obtained.