Electron spin resonance studies of elementary processes in radiation- and photo-chemistry. Part 14.—Photolysis of solutions containing maleimides

Abstract

The photolysis of maleimide, N-methyl maleimide and N-ethyl maleimide in a number of alcoholic solvents has been examined by e.s.r. spectroscopy using a pulse photolysis–CAT system. The spectra observed show marked effects of chemically induced dynamic electron polarization (CIDEP) arising from both radical-pair and triplet mechanisms.

The reaction mechanism involves abstraction of H from the solvent by triplet maleimide followed very rapidly in neutral solutions by addition of the solvent fragment to another maleimide molecule. Both radicals show strong initial polarization. The H adduct has a half-life < 100 µs at micromolar concentrations in ethanol: the solvent radical adduct has a longer half-life (≈225 µs) under similar conditions. When base is present (triethylamine or OH^–) base catalysed electron transfer occurs and the only radical observed is the radical-anion of maleimide, which has a lifetime of several milli-seconds at similar concentrations. The effects of concentration, solvent and pH are discussed in relation to a proposed general mechanism for the photoreduction.