Negatively charged crown ethers for binding paraquat in water

A water soluble negatively charged fluorescent 1,4-benzo-1,5-naphtho-36-crown-10-based host has been devised and synthesized. As shown by proton NMR, ESI mass spectrometry and UV-vis spectroscopy, it binds paraquat with a 1:1 stoichiometry and an association constant of 4.50 (± 0.02) × 10 3 M −1 in water. Its complexation with paraquat in water was further investigated by fluorescence emission spectroscopy. The results revealed that when paraquat was added to the water solution of the host, the fluorescence emission of the host was quenched by the charge transfer between the crown ether host and paraquat guest. A similar bis- p -phenylene-34-crown-10-based host was synthesized as a contrasting host. Lacking the naphthalene unit, the second host binds paraquat with a 1:1 stoichiometry and a lower association constant, 1.04 (± 0.04) ×10 3 M −1 , in water. This demonstrated that the naphthalene unit could not only act as a strong fluorescence group but also improve the π-π stacking interactions between the host and guest.