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 π-cation interaction by introducing the electron-donating NH₂ unit regardless of the steric hindrance between two azo-phenyl groups adjacent to the crown ring.