Photoreactions of 2-methyl-5-isopropyl-1,4-benzoquinone

The photochemistry of the title compound (thymoquinone: TQ) in solution was studied by time-resolved UV-Vis spectroscopy and 20 ns laser pulses at 248 nm. The results are compared with those of 2,5-dimethyl-1,4-benzoquinone (Me2BQ). The triplet state of Me2BQ reacts with amines, such as triethylamine (TEA) or 1,4-diazabicyclo[2.2.2]octane (DABCO) via electron transfer and with 2-propanol via H-atom transfer. The semiquinone radical of Me2BQ is formed as the main intermediate and the second-order decay leads to the hydroquinone. The quantum yield of photodecomposition (Φd, λirr=254 nm) of Me2BQ is low in acetonitrile, but substantial in the presence of donors, where also the semiquinone radical is large. In contrast, Φd is substantial for TQ in neat acetonitrile even in the presence of oxygen. No triplet state was detected for TQ in any solvent at room temperature and the first transient (Tz) is assigned to a zwitterion, formed after an intramolecular H-atom transfer from the side chain. The lifetime of Tz in acetonitrile or tert-butanol is ca. 2 μs and the decay is enhanced by water, acetic acid or 2,2,2-trifluoroethanol. Conversion of the zwitterion into hydroquinones, not involving free radicals, is proposed. In the presence of other alcohols or amines, however, conversion of Tz into radicals and their termination takes place. The effects of solvent polarity and other specific properties of intramolecular H-atom transfer with TQ are discussed.