Dimerization of pillar[5]arene: length-adaptive encapsulation of long-chain guests

Abstract

Encapsulation of long-chain molecules is challenging because host molecules must capture entire guest structures through interactions at multiple points. Cavitand and capsule molecules bind long-chain guests within their cavities, where long-chain guests adopt compressed conformations, to maximize interaction with the walls of the receptors. However, pillar[5]arene exhibits selective binding toward linear guest molecules in an extended chain conformation. It interacts more strongly with shorter guest molecules than with longer ones, as the shorter guests better match the length of the pillar-shaped cavity. Consequently, pillar[5]arene binds poorly to long-chain guest molecules that exceed its length. Herein, we synthesized pillar[5]arene dimers linked by phenyl and biphenyl linkers. Due to the cooperative binding provided by its two cavities, the biphenyl-linked dimer demonstrates length-adaptive behaviors for long-chain α,ω-dibromoalkanes that exceed its own length. This behavior is distinct from that of monomeric pillar[5]arene, which exhibits strict length-selectivity. Computational analysis suggests that the formation of curved host–guest complex structures with longer guests is facilitated by conformational adjustment of the biphenyl linker. Thermodynamic analyses reveal that enthalpic gains from the cooperative binding overcome entropic losses during the formation of the curved host–guest complexes, thus enabling length-adaptive binding for longer-chain guests.