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

Treatment of 5,11,17,23‐tetrakis(p‐carboxyphenyl)azo]‐25,26,27,28‐tetrahydroxy calix4]arene ( 2 ) with HCl in DMF or NaOH in MeOH produced 5,11,17,23‐tetrakis(p‐carboxyphenyl)azo]‐25,26,27,28‐tetrahydroxycalix4]‐arene·4DMF (2·4DMF) and 5,11,17,23‐tetrakis(p‐carboxyphenylsodium)azo]‐25,26,27,28‐tetrahydroxycalix4]‐ arene ( 3 ), respectively, which were characterized by elemental analysis, IR, UV‐vis, ¹H NMR and ¹³C NMR. An X‐ray analysis of 2·4DMF revealed that its calix4]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 π···π 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.