Issue 1, 2004

Photoprocesses of p-naphthoquinones and vitamin K1: effects of alcohols and amines on the reactivity in solution

Abstract

The photochemistry of 1,4-naphthoquinone (NQ), the 2-methyl, 2,3-dichloro and 2-bromo derivatives, and vitamin K1 was studied in non-aqueous solvents by time-resolved UV-vis spectroscopy after ns laser pulses at 248 and 308 nm. The triplet state of the NQs reacts with alcohols and amines, e.g. triethylamine (TEA) and DABCO, yielding semiquinone radicals (HQ˙/Q˙). They are the major intermediates and their second-order decay kinetics depend on the properties of the additives and the medium. Transient conductivity measurements suggest the occurrence of photoinduced electron transfer from amines to the triplet state of NQs in acetonitrile. The photoconversion (λirr = 254 nm) of NQs to the 1,4-dihydroxynaphthalenes (H2Q) was measured in the absence and presence of varying concentrations of electron and H-atom donors, and the quantum yield was found to increase with increasing electron- or proton-donor concentration. The mechanisms of photoreduction of NQs by propan-2-ol and TEA in acetonitrile exhibit a number of similarities. Oxygen quenches the triplet state, thereby forming singlet molecular oxygen. Oxygen also reacts with the semiquinone radical, thereby forming HO2˙/O2˙ radicals, and reacts with H2Q, thereby re-forming the quinone. A different pattern, involving intramolecular H-atom transfer, holds for vitamin K1, where 1,3-quinone methide (1,3-QM) diradicals were observed in acetonitrile prior to formation of two 1,2-QM tautomers, but a triplet was not. The decay of the 1,3-QM intermediates becomes faster in the presence of alcohols and amines due to proton-transfer reactions.

Graphical abstract: Photoprocesses of p-naphthoquinones and vitamin K1: effects of alcohols and amines on the reactivity in solution

Article information

Article type
Paper
Submitted
11 Jun 2003
Accepted
08 Sep 2003
First published
30 Sep 2003

Photochem. Photobiol. Sci., 2004,3, 71-78

Photoprocesses of p-naphthoquinones and vitamin K1: effects of alcohols and amines on the reactivity in solution

H. Görner, Photochem. Photobiol. Sci., 2004, 3, 71 DOI: 10.1039/B306670N

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