Photoinduced demethylation of 4-nitro-N,N-dimethylaniline

Abstract

The title compound (3) upon photoexcitation becomes demethylated by either an external acceptor in benzene or the methoxide ion in methanol. The triplet state of 4-nitroaniline (1), 4-nitro-N-methylaniline (2), 3 and the 3-nitro analogue (4) in solution at room temperature was characterized by time-resolved UV-vis spectroscopy. The triplet state of 3 in benzene reacts with other nitroarenes, such as nitrobenzene and nitronaphthalenes, by demethylation, yielding eventually 2 and further products. This quenching reaction is close to the diffusion-controlled limit. The conversion of 3 into 2 is accompanied by a permanent bleaching at 370–400 nm within 10 μs and the quantum yield is up to 0.3, this being in agreement with the quantum yield of intersystem crossing. An intermediate with peaks at 420 and 700 nm is attributed to a C-centered radical: H₂˙C(CH₃)N–C₆H₄–NO₂, which is also involved with 3 in methanol in the presence of methoxide. The overall H-atom transfer takes place within a few μs. Another electron transfer reaction, involving the non-absorbing α-aminoethyl radical, occurs from triethylamine to the triplet state of 3. The observed transient with a maximum at 330 nm in benzene or acetonitrile is assigned to the conjugate acid of the radical anion of 3 which subsequently disproportionates in the ms range. The photochemical properties of 1–4 and the mechanisms of the demethylation and reduction reactions are discussed.