Transients in the photoreduction of dinitroarenes by triethylamine and N,N-dialkylanilines in benzene

The photochemistry of dinitronaphthalenes (1,3-, 1,4-, 1,5-DNN), 4,4′-dinitrobiphenyl (DNB) and 2,7-dinitrofluorenone (DNF) in benzene at room temperature was studied by time-resolved UV-Vis spectroscopy after ns laser pulses at 354 nm. The triplet states of the five dinitro compounds, 3∗O2NArNO2, react with N,N-dimethylaniline (DMA) and N,N-diethylaniline (DEA), the rate constant of quenching of the triplets is close to the diffusion-controlled limit. An intermediate with peak at 340 nm is attributed to the PhN(R)CHR′ radical (R=CH3, R′=H or R=C2H5, R′=CH3, respectively). On the other hand, quenching of the triplet state of dinitroarenes by triethylamine (TEA) leads to O2NArNO2H, the conjugate acid of the radical anion, which is suggested to yield nitrosoarenes by termination within a few ms. For DNB an H-atom transfer from the α-aminoethyl radical to the O2NArNO2H radical, thereby forming N,N-diethylvinylamine and a quinonoid structure prior to 4-nitroso-4′-nitrobiphenyl, is proposed. Decay of the PhN(R)CHR′ radical, thereby converting the dinitroarenes into nitrosonitroarenes and dealkylation of the dialkylanilines, is faster than that of the O2NArNO2H radical. The mechanisms of demethylation and the reduction reactions in benzene under conditions where no free radical ions are formed, are discussed.