Stepwise and Dramatic Enhancement of Anion Recognition with a Triple-Site Receptor Based on the Calix[4]arene Framework Using Two Different Cationic Effectors

Synthesis and binding behavior of a novel multi-responsive host 1, in which two esters, two
polyether moieties, two urea sites, and two bipyridine units as ion binding sites are arranged on the calix[4]arene skeleton, is reported. 1 recognizes Na+ and Ag+ simultaneously and quantitatively and captures
an anionic guest. The ability of 1 to recognize anions, including CF3SO3- and BF4-, remarkably increases
in a stepwise manner using Na+ and Ag+ as effectors. The enhancement of the Ka eventually reaches
factors of 1500 and 2000 for NO3- and CF3SO3-, respectively, in the presence of both Na+ and Ag+,
compared to the free 1. The regulation of binding of multiple ligands may be applicable to multistep cascade
systems for the amplification of molecular events, and further studies in this field could provide insight
applicable to more advanced molecular devices.