Photoreduction of trans-4-R-4′-nitrostilbenes with R: OMe, H and NO2 by di- and trialkylamines in polar media

Abstract

The photoinduced reactions of trans isomers of 4-nitrostilbene (1), 4,4′-dinitrostilbene (2), and 4-methoxy-4′-nitrostilbene (3) as well as a nitrostilbazolium salt (4) by DABCO, diethylamine (DEA) and triethylamine (TEA) in acetonitrile and mixtures with water were studied by pulsed (λ_exc = 354 nm) and steady-state (λ_irr = 366 nm) photolysis. The rate constant for quenching of the triplet states of 1–3 in acetonitrile by TEA is k_q = 1 × 10⁸ M⁻¹ s⁻¹ and is significantly smaller in the presence of water. Quenching the triplet states of 1–3 by DABCO is more efficient and k_q of 4 in acetonitrile is close to the diffusion-controlled limit. The subsequent transient of 1–3 in the presence of the tertiary aliphatic amines is ascribed to the radical anion. With TEA (or DEA) the conjugate acid of the radical anions with maxima at 400–500 nm are generated (via the α-aminoethyl radical) in a secondary step. Only for 2 do the spectra of the two radicals absorb differently. The radical intermediates subsequently undergo a termination reaction yielding (partly) the corresponding nitroso compounds and eventually arylhydroxylamines. The maximum quantum yield of decomposition of 1–4 in the presence of TEA (0.3 M) is 0.1–0.4. Solvent separated ion pairs for 1–3, in contrast to 4, were observed by conductivity measurements. The charges of 1–3 remain separated for times which are comparable or longer than the half-lives of the radicals. The mechanisms of the two subsequent reduction reactions are discussed.