Cooperative Host-Guest Complexation in Densely Assembled Host Structures on Surfaces Revealed at the Single-Molecular Level.

Reversible association and dissociation are fundamental processes in the behavior of molecular systems. Host-guest chemistry based on supramolecular formation through noncovalent intermolecular interactions is a typical model of a molecular system based on association-dissociation. High-density assembly of host structures on surfaces enables the unique host-guest complexation phenomena, such as cooperative effects that differ significantly from those in homogeneous solution systems. However, analytical methods applicable to the phenomena at solid-liquid interfaces are limited and often provide only average or statistical information on the host-guest complexation event. Therefore, such molecular-scale phenomena that are lost in averaging are difficult to understand. Here, we show direct observation of association-dissociation with densely assembled pillar[5]arenes as host structures using two atomic force microscopy (AFM) techniques at the single molecular level. Frequency modulation AFM observations revealed that densely assembled pillar[5]arenes exhibit positive cooperativity, whereby inclusion of a guest at one host site enhances guest binding at neighboring sites separated by one neighboring host molecule, likely due to lateral host-host interactions. In addition, the dynamics of reversible association-dissociation processes of host-guest complexation were also visualized by high-speed AFM.