Conformational control of electron-rich calix[6]arene skeleton by paraquat recognition

Calix[6]arene derivatives having electron-donating groups were designed and synthesized for the recognition of electron-deficient organic guests. These compounds exhibited relatively well-resolved 1H NMR spectra while most calix[6]arene derivatives reported previously showed broad spectra. The 1H NMR spectra clearly indicated that the studied calix[6]arene derivatives adopted a 1,2,3-alternate conformation as the dominant conformation. The hydroxy derivative formed a host–guest complex with paraquat molecule, in which the calix[6]arene framework exclusively adopted a cone conformation. Thus, an efficient conformational conversion from the 1,2,3-alternate to the cone conformation took place upon the host–guest complexation.