Photoreactions of p-benzo-, p-naphtho- and p-anthraquinones with ascorbic acid

Abstract

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) × 10⁹ M⁻¹ s⁻¹. Termination of the radicals takes place in the µs–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.