New chromogenic azocalix[4]arene podands incorporating 2,2′-bipyridyl subunitsElectronic supplementary information (ESI) available: mole ratio plot for mixtures of 4a and Zn(CF3SO3)2; UV-Vis spectra for 4a (5 × 10–6 mol l–1) when Zn2+ (5 × 10–5 mol l–1) is added to the CH2Cl2 host solution. See http://www.rsc.org/suppdata/nj/b2/b209528a/

Six new calix[4]arenes 2, 3 and 4 functionalized with 2,2′-bipyridyl and azophenyl groups at the lower rim and the upper rim respectively have been synthesized and their complexing properties towards Zn2+ have been determined by UV-Vis titrations in organic media. The conformational properties of these ligands were obtained by 1H NMR spectroscopy. The stoichiometries of zinc complexes were determined by both the mole ratio method and Job plots. A stoichiometry of 1∶1 was found for the complexes Zn-2 and Zn-3 and 1∶2 for the complex Zn-4 as a function of the number of grafted bipyridyl groups. The association constants for 1∶1 complexes were determined by using the Benesi–Hildebrand plot and also by linear regression analysis.