Photoreactivity of linear p-dimethylaminobenzoylated polystyrene and its model compound, 4-dimethylamino-4′-isopropylbenzophenone

Abstract

Various aspects of the photochemical behaviour of linear p-dimethylaminobenzoylated polystyrene, 4-dimethylamino-4′-isopropylbenzophenone and of Michler's ketone have been studied in cyclohexane, benzene and chloroform. At low concentrations, the model compound is efficiently photoreduced in cyclohexane (φ = 0.58), parallel to the behaviour of Michler's ketone (φ = 0.37). Lower values were found in chloroform (φ = 0.12) and in benzene (φ = 0.055). The reduction quantum yield φ decreases as the model compound concentration increases and at concentrations higher than 10⁻⁴ M the quantum yield was found to be dependent on the extent of reaction, whereas it is independent in dilute solutions. A general kinetic scheme consistent with previous contributions to the subject and with our results has been proposed. In this scheme, a bimolecular hydrogen abstraction from the solvent with a second-order rate constant (k_r) competes with the triplet excimer formation (k_e) which is depopulated by two routes: physical deactivation to give the ground state ketones (k_ed) and hydrogen transfer (k_er) to give a pair of ketyl and aminomethyl radicals. The rate constants of hydrogen abstraction (k_r) in the three solvents studied were determined, as well as the ratio k_er/(k_er + k_ed) (approximately equal to 0.02) which measures the importance of hydrogen transfer through excimer formation. It is concluded that the excimer formation process leads principally to the generation of the starting ketone by physical deactivation, and only, to a small extent, to a pair of radicals via hydrogen transfer. Both ketones (the model compound and Michler's ketone) have similar lifetimes at room temperature and transfer their triplet excitaion energy to naphthalene and trans-stilbene with high rates. In contrast with this behaviour, the polymeric sensitizer shows very low reactivity and its energy donor ability is quite limited. These results can be explained by the efficient intramolecular self-quenching which takes place in the functionalized macromolecules owing to the large local concentration of chromophore.