Syntheses and molecular recognition abilities of functionalized calixarenes.

Calixarenes are cyclic oligomers synthesized from phenols and formaldehyde. We designed and synthesized a variety of new host molecules from calixarenes. They acted as cyclic ionophores with crown-like functions by introducing ionophoric groups. In particular, calix [5] arene and calix [6] arene with carboxylate groups acted as “super-uranophiles” exhibiting the record-breaking uranyl affinity (K=1018.4-19.2 M-1) and uranyl selectivity (1010.6-17). Inclusion of organic guest molecules in solution was effected for the first time by using water-soluble calixarenes which had either anionic, cationic, or neutral hydrophilic groups. Chiral, water-soluble calixarenes changed their conformation upon inclusion of guest molecules, which was conveniently detected by the CD spectra and provided several lines of important information for the host-guest-type complex formation. Through these studies, it was demonstrated that the moderate rigidity and the remaining conformational freedom are important characteristics of the complex formation with calixarene-based host-molecules.