LASER PHOTOLYSIS STUDIES OF TRANSIENT PROCESSES IN THE PHOTOREDUCTION OF NAPHTHALIMIDES BY ALIPHATIC-AMINES

The photoreduction of N-phenyl-1,8-naphthalimide and N-phenyl-2,3-naphthalimide by aliphatic amines has been studied by laser flash photolysis with transient absorption and transient conductivity methods in different solvents. Analysis of transient time profiles establish for most systems the occurrence of a fast primary and a slower secondary reduction process. Primary reduction is ascribed to the reaction between a triplet naphthalimide and an amine, while secondary reduction is assigned to the reaction of an amine-derived alpha-aminoalkyl radical with a ground-state naphthalimide molecule. In polar solvents, with aliphatic amines both primary and secondary reductions proceed by electron transfer. In solvents of intermediate polarity, hydrogen atom transfer (primary reduction) is succeeded by electron transfer (secondary reduction). Finally, in nonpolar solvents, only primary reduction by hydrogen atom transfer is found to occur. Rate constants are obtained for most of these processes by computer modeling of the transient time profiles. In polar solvents, reaction angstromH-2+ + AH-2 --> angstromH + AH-3+ (where AH-2 and angstromH are the amine and alpha-aminoalkyl radical, respectively) is a key reaction in which the alpha-aminoalkyl radical is formed. Its rate constant is found to decrease by more than 4 orders of magnitudes when AH-2 varies from tertiary, through secondary, to primary amine. This is explained by the significant change in the dissociation energy of the C-H bond in the alpha-position to the nitrogen. The electron transfer between the alpha-aminoalkyl radical and the naphthalimide molecule is found to occur in polar solvents with a rate close to the diffusion controlled limit, whatever the type of the aliphatic amines.