Regioselective complexation of metal ion in chromogenic calix[4]biscrowns

A series of 1,3-alternate chromogenic azo-coupled calix[4]biscrowns in which the crown size varied with crown-5 and crown-6 have been synthesized. From the results of UV/vis band shift upon metal ion complexation, metal ions were entrapped only by the upper crown loop, causing the hypsochromic shift on the UV/vis spectra. Calix-[4]bis(crown-5)(crown-6) revealed K+ ion selectivity while calix[4]bis(crown-6)(crown-6) showed Cs+ ion selectivity caused by a size complementarity between hosts and guest ions. From the UV band shift of 4 in which the NO2 group is replaced by the NH2 group, we observed bathochromic shift upon the metal ion addition, indicating that the metal ion is encapsulated in the lower crown ring because of strengthened pi-cation interaction by introducing the electron-donating NH2 unit regardless of the steric hindrance between two azo-phenyl groups adjacent to the crown ring.