Photoreduction of nitronaphthalenes in benzene by N,N-dialkylanilines and triethylamine: a time-resolved UV–vis spectroscopic study

Abstract

The triplet state of 1-methoxy-4-nitronaphthalene and 1- and 2-nitronaphthalene in benzene at room temperature was studied by time-resolved UV–vis spectroscopy with ns laser pulses at 354 nm. The triplet state reacts with N,N-dimethylaniline and N,N-diethylaniline; the rate constant is close to the diffusion-controlled limit. A transient with a peak at 340 nm is attributed to the α-aminoalkyl radical, PhN(R)˙CHR′, being formed by H-atom transfer from the dialkylaniline. Decay of this radical takes place within a few μs, as observed by bleaching in the 340–410 nm range. Thereby, a nitronaphthalene is eventually converted into stable products, such as nitrosonaphthalene, and dealkylation of the dialkylaniline occurs. Triplet quenching by triethylamine yields the radical anion, ArNO₂˙⁻, as an intermediate in acetonitrile and the conjugate acid, Ar˙NO₂H, in benzene. These radicals are suggested to ultimately yield nitrosonaphthalenes by slow termination in the ms time range. Decomposition of the three nitronaphthalenes in benzene takes place in the presence of the three amines with maximum quantum yields of 0.1–0.3. The mechanisms of photodemethylation of the dialkylanilines and the photoreduction reactions of the nitronaphthalenes are discussed.