Laser photolysis studies of transient processes in the photoreduction of naphthalimides by aliphatic amines

Abstract

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 a-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 AH2+ + AH2 -A H + AH3+ (where AH2 and AH are the amine and a-aminoalkyl radical, respectively) is a key reaction in which the a-aminoalkyl radical is formed. Its rate constant is found to decrease by more than 4 orders of magnitudes when AH2 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 a-position to the nitrogen. The electron transfer between the a-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.

Photoreduction of 9,10-Anthraquinone Derivatives: Transient Spectroscopy and Effects of Alcohols and Amines on Reactivity in Solution¶

“…Selected excitation of quinones before electron transfer from N,N ‐diethylaniline (DEA), triethylamine (TEA) and 1,4‐diazabicyclo[2.2.2]octane (DABCO) is therefore possible. The photoreduction of AQ and 1‐Cl (ClAQ) and 1,8‐Cl 2 (Cl 2 AQ) derivatives (13–18), other quinones (4,7) and related aromatic compounds (39–42) by amines have been the subject of several investigations. A complication arises because QH 2 , in contrast to other hydroquinones, is not stable in solution, and addition of oxygen to QH 2 causes a complete recovery into AQ (10,20).…”

Photoreduction of p-Benzoquinones: Effects of Alcohols and Amines on the Intermediates and Reactivities in Solution¶

“…For TEA in acetonitrile (40–42) or water (45), however, equilibrium 11 with efficient formation of the α‐aminoethyl radical ( • DH) is known from related systems.…”

“…Benzoquinones, because of their n,π transition, can be excited at 308 or 350 nm in relatively low concentration and are therefore accessible for spectroscopy in the presence of additives absorbing at 250–300 nm. The photoreduction of quinones (10–14,26–33) and related aromatic compounds (40–43) by amines, such as N,N ‐dialkylamines, triethylamine (TEA) or 1,4‐diazabicyclo[2.2.2]octane (DABCO), has been intensively studied. Fourier‐transform infrared spectroscopy of quinones provides access toward structural information.…”

Photochemistry of Naphthalene Diimides: EPR Study of Free Radical Formation via Photoredox Process¶

“…Saito et al (23) reported that photoreduction of IIB can proceed through an intramolecular electron transfer from the monothioacetal group on the side chain to the triplet state of the 1,8‐naphthalene imide chromophore. Earlier, formation of a radical anion from a related NI (R: ‐Ph) has been observed by EPR during its irradiation in acetonitrile in the presence of aliphatic amines as electron donors (17). The lack of an EPR spectrum from the IIB radical anion in our system could be due to its inherent low stability and, possibly, its rapid quenching by traces of oxygen, which affords superoxide.…”

“…Laser flash photolysis studies revealed that on irradiation, NI and NDI compounds, including IA , participate in photoredox processes because on photoexcitation they abstract electrons from certain donors such as aliphatic amines, DABCO and DNA bases (9, 11–13, 17, 18). In the absence of other donors, the NDI triplet was quenched by NDI molecules in the ground state (9, 12, 13).…”