Water soluble tetra-anionic calix[4]arene fluorophore for selective sensing of Fe2+ and Fe3+

Abstract

Water soluble 1,3-alternate calix[4]arene containing four ethynylbenzoic acid is synthesized through a Sonogashira copper-palladium catalyzed coupling reaction between 1,3-alternate tetraiodocalix[4]arene and 4-ethynylbenzoate followed by base catalyzed hydrolysis of the four ester groups. In dimethylsulfoxide solution, the synthesized compound exhibit maximum absorption and emission at 317 and 418 nm, respectively, with high quantum efficiency of 0.36 comparable to the tetraester precursor ( = 0.40). The tetracarboxylic compound however gives low quantum efficiency of 0.6% in phosphate buffer pH 8 solution due to hydration stabilizing internal charge-transfer (ICT) state. The quantum efficiency in aqueous media can be improved by nonionic surfactant Brij 58 due to reducing hydration around the fluorophores. The fluorescence signal of the phosphate buffer solution of the compound in the presence of Brij 58 is selectively quenched by Fe2+ and Fe3+ with different Stern-Volmer constant (Ksv) of 3.5 × 104 M-1 and 8.6 × 103 M-1, respectively.