Photoreduction induced by electron transfer from di- and trialkylamines to the triplet state of nitronaphthalenes in polar media

Abstract

The photoinduced reactions of 1- and 2-nitronaphthalene (1- and 2-NN) and 1-methoxy-4-nitronaphthalene (MNN) in the presence of DABCO, triethylamine (TEA) and diethylamine (DEA) in neat acetonitrile and acetonitrile–water were studied by UV–vis spectroscopy and conductometry upon excitation by ns laser pulses at 354 nm. The rate constant of quenching of the nitronaphthalene triplet state by the amines is close to the diffusion-controlled limit in deoxygenated acetonitrile and smaller (k_q ≊ 2 × 10⁹ M⁻¹ s⁻¹) in the presence of water (5 M). Electron transfer yields the nitronaphthalene radical anions which were observed as secondary transients with maxima at 390 nm. At higher TEA or DEA concentrations (>1 mM) nitronaphthalene radicals are formed in a second, delayed event by reduction of the nitronaphthalene ground state involving the α-aminoalkyl radicals. The nitronaphthalene radicals react via second-order termination yielding nitrosonaphthalenes and further products. The photoinduced electron transfer and subsequent radical disproportionation reactions are supported by time-resolved conductometric measurements. The mechanism of the two-step reduction reactions in the presence of TEA or DEA and the effects of amine concentration, oxygen and water are discussed.