5,11,17,23-Tetrakis[(p-carboxyphenyl)azo]-25,26,27,28-tetrahydroxy Calix[4]arene: Crystal Structure and pH Sensing Properties

Treatment of 5,11,17,23-tetrakis[(p-carboxyphenyl)azo]-25,26,27,28-tetrahydroxy calix[4]arene (2) with HCl in DMF or NaOH in MeOH produced 5,11,17,23-tetrakis[(p-carboxyphenyl)azo]-25,26,27,28-tetrahydroxycalix[4]arene center dot 4DMF (2 center dot 4DMF) and 5,11,17,23-tetrakis[(p-carboxyphenylsodium)azo]-25,26,27,28-tetrahydroxycalix[4]arene (3), respectively, which were characterized by elemental analysis, IR, UV-vis, H-1 NMR and C-13 NMR. An X-ray analysis of 2 center dot 4DMF revealed that its calix[4]arene core adopts a flattened cone conformation in which opposed phenyl groups take parallel or sharply inclined positions. The intra- and intermolecular hydrogen-bonding interactions and the pi center dot center dot center dot pi interactions form a 2D hydrogen-bonded wavelike network. Compound 2 had a unique reversible color change in a wide pH range from 1 to 13.5 and showed interesting pH sensing properties.