Chromogens based on non-benzenoid aromatic systems. Part II. Arylazo-derivatives of 1,2,3,4-tetrachlorocyclopentadiene

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Abstract
Several arenediazonium salts have been coupled to 1,2,3,4-tetrachlorocyclopentadiene and the products have been shown by i.r. spectroscopy to exist exclusively as hydrazones in the solid state. The visible absorption spectra show that the same situation exists in solution, and the effect of substituents on the spectra can be interpreted most readily in terms of the hydrazone structures. In the presence of bases the hydrazones readily form anions which absorb at longer wavelengths than the parent compounds. The long wavelength bands of the anions exhibit an opposite substituent effect to that observed for the hydrazones, and the shifts of the hydrazone bands due to ionisation are approximately proportional to the Hammett σ constants of substituents in the aryl ring.

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