Azophenol appended (thia)calix[4]arenes for colorimetric sensing of anions: a complexation induced extended conjugation.

Iminoazophenol appended calix[4]arene/thiacalix[4]arene derivatives 1 and 2 of cone and 3 of 1,3-alternate conformation have been synthesized and examined for their chromogenic anion recognition abilities towards different anions like fluoride, chloride, bromide, iodide, acetate, dihydrogenphosphate, nitrate and hydrogensulphate by UV-vis spectroscopy. Receptors 1-3 show an unusual red shift in UV-vis spectra upon binding with these anions, with a colour change visible to naked eye in case of fluoride, acetate and dihydrogenphosphate ions at lower concentration of anions. This red shift in the absorption spectra is accounted by anion complexation induced sigma-extended conjugation.