Chromogens based on non-benzenoid aromatic systems. Part I. Electronic absorption spectra of 5-phenylazotropolones

Abstract

Nine 5-phenylazotropolone derivatives, including the parent compound, have been synthesised and their ultraviolet and visible spectra in acid and alkali recorded. No evidence for hydrazone tautomerisation was found, either in the solid state or in solution. The spectra are discussed, and the influence of the tropolone ring is evaluated by comparison with the related 4-phenylazophenols. Substituent effects on the visible absorption bands are adequately accounted for by theory. The frequency difference between the long wavelength bands of the neutral and ionised forms correlates well with the unmodified Hammett σ values of the substituents, and the significance of this is discussed.