Crowned dendron: ion-responsive flexibility of macromolecules induced by integrated crown ether moieties

Polybenzyl ether type dendrons bearing the crown ether moieties at the periphery, namely, crowned dendrons were synthesized, and the effect of complex formation on their flexibility with metal-ion binding properties was examined. Upon addition of Na⁺, ¹H NMR spectra of the crowned dendrons in CD₃CN were significantly broadened, reflecting the flexibility restriction of the crowned dendrons by the complex formation with Na⁺. Such a significant flexibility restriction was observed only with Na⁺, although ESI-MS studies revealed that the crowned dendrons formed 1:2 complexes (a metal ion:the crown ether moiety) regardless of the kind of metal ions. The flexibility restriction became significant with increasing dendron generation on the basis of ¹H NMR spectra and spin-lattice relaxation time (T₁) measurements. Binding constants of the crowned dendrons with metal ions in CD₃CN decreased with the increase of the dendron generation, reflecting an influence of the charge repulsion as well as a dendrimer effect to cause the steric hindrance. The examination of UV-vis absorption spectra for complexes of the crowned dendron with metal picrates in THF displayed the formation of a loose ion-pair complex with Na⁺, namely, a typical sandwich type complex. However, in CH₃CN, all metal picrates were solvated to be in a loose ion-pair even without complex formation. These results suggested that the control of macromolecular flexibility with metal ions is feasible by the integration of crown ether moieties with a dendritic structure.