Rationalizing Guest-Driven Cooperative Effect in a Hierarchical Host-Guest Aqueous Assembly.

Molecular recognition is a key step in the cooperative interactions that govern the functions of hemoglobin and microtubules. However, in artificial systems based on molecular recognition, understanding the various facets of cooperative interactions during hierarchical assembly in aqueous media is often overlooked. Here, we report cooperative effects arising from proximity-induced π-interactions between guest molecules with an anthracene core (An≡Py), which facilitate enhanced binding to cucurbit[8]uril in water, leading to higher-order (2:2) complexation and hierarchical organization into 2D flake-like structures. A less favored 2:2 binding mode, owing to the size disparity between the anthracene core and the host cavity, is formed through concealed guest-interactive species as rationalized by spectroscopic, microscopic, and computational analyses. Furthermore, a multistate pathway is evident in an enthalpy-driven binding process obtained by changing the titration protocol in isothermal titration calorimetry, replacing the guest in the sample cell with the host. Contrary properties of a reference analog (Bz≡Py) with changes in the hydrophobic π-core encompass the delicate nature of guest-guest interactions in determining the resultant binding stoichiometry. Our study on CB[8]-guest systems qualitatively mines out a previously elusive molecular interaction, simulated from a subtle tuning of the π-scaffold, that drives a cooperative hierarchical assembly process in water.