Chromogenic indoaniline armed-calix[4]azacrowns

Two novel chromogenic 1,3-alternate calix[4]azacrown (1) and calix[4]-bis-azacrown (2) in which an indoaniline chromophore was attached on the nitrogen atom of the azacrown unit with one methylene spacer were synthesized. The ¹H NMR spectrum of the ligand 1 and Ca²⁺ proved that the metal ion is entrapped by the calix[4]azacrown unit and by the conjugated indoaniline system. From the UV/vis band shifts upon metal ion complexation, Zn²⁺ ion was found to give the largest band shifts compared to other metal cations, indicating that Zn²⁺ ion (K_a = 18 760 M^-1 for 1 and K_a = 19 930 M^-1 for 2) was selectively encapsulated by the calix[4]azacrown cavity with assistance of the pendent indoaniline sidearm.