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 pillar5arenes as host structures using two atomic force microscopy (AFM) techniques at the single molecular level. Frequency modulation AFM observations revealed that densely assembled pillar5arenes 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.
Asakawa et al. (Wed,) studied this question.