Photoreactions of p-benzo-, p-naphtho- and p-anthraquinones with ascorbic acid.
The photoreduction of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives, e.g. dimethylBQ, trimethylBQ, duroquinone, bromoNQ, methoxyNQ, methylAQ and dimethylAQ in acetonitrile-water by ascorbate was studied by time-resolved UV-vis spectroscopy using 20 ns laser pulses at 308 nm and continuous 254 nm irradiation. The semiquinone radical (*QH/Q*(-)) is formed after H-atom transfer from ascorbate to the quinone triplet state. The rate constant for quenching is k(q)=(2-9) x 10(9) M(-1) s(-1). Termination of the radicals takes place in the micros-ms range. The results are compared with those initiated by electron transfer from DABCO under similar conditions, where the k(q) values are similar, but the termination of Q*(-) takes place by electron back transfer not yielding hydroquinones. Specific properties of the quinone triplet state, e.g. self-quenching, nucleophilic water addition and the effects of structure are discussed.
Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an der Ruhr, Germany.
Helmut
H
Görner
D000880:Anthraquinones / Q000737:chemistry* / Q000528:radiation effects; D001205:Ascorbic Acid / Q000737:chemistry* / Q000528:radiation effects; D016227:Benzoquinones / Q000737:chemistry* / Q000528:radiation effects; D005609:Free Radicals / Q000737:chemistry / Q000528:radiation effects; D006859:Hydrogen / Q000737:chemistry; D009285:Naphthoquinones / Q000737:chemistry* / Q000528:radiation effects; D010084:Oxidation-Reduction / Q000528:radiation effects; D010777:Photochemistry / Q000379:methods; D014466:Ultraviolet Rays
D000880:Anthraquinones; D016227:Benzoquinones; D005609:Free Radicals; D009285:Naphthoquinones; D006859:Hydrogen; D001205:Ascorbic Acid