Charge transfer spectra and reaction intermediates. Part II. Stable crystalline complexes from phenols and NN′-dimethyl-4,4′-bipyridylium (paraquat) salts

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Abstract
Simple and complex phenolic materials are shown to interact with NN′-dimethyl-4,4′-bipyridylium (paraquat) ion giving rise to charge transfer spectra. Well defined, coloured, crystalline, complexes between paraquat dichloride and phenols are readily isolated from acidic solution; they apparently contain four hydroxy-groups per molecule of paraquat salt. From corresponding alkaline solutions the complexes isolated contain phenol both as phenolate ion and as neutral phenol. These break down on acidification, and the procedure is convenient for the isolation and purification of polyhydric phenols. Because of the molecular geometry involved, interaction between 4,4′-dihydroxybiphenyl and paraquat is especially favourable. It is suggested that charge transfer transitions in paraquat–phenol complexes result from incidental favourable orbital overlap, as a consequence of dipole–dipole attractive forces, and that such complexes may play a role in the herbicidal action of bipyridylium salts.

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