Molecular Description of Grafted Supramolecular Assemblies on Gold Surfaces: Effect of Grafting Points and Chain Lengths

The association of 4 aminoazobenzene (4AA) with two different water-soluble hosts, β-cyclodextrins (β-CD) and calixarenesulfonates (CnS), was studied in heterogeneous conditions using molecular simulations. This situation is achieved by immobilization of macrocycles onto a gold Au(111) surface. Several factors that can influence the binding properties are investigated here through the chain length of alkylthiols spacer of the surface-immobilized host and the number of attachment points to the surface. A conformational change of β-CD as a function of the chain length is evidenced upon grafting on the gold surface, whereas CnS does not show any changes. It is then possible to tune the thermodynamic properties of β-CD by changing the grafted chain length and forming a larger hydrophobic region. The mechanisms of insertion of guests into the cavities are similar to those obtained in a homogeneous system. 4AA is included longitudinally in the β-CD cavity, and it interacts rather with the sulfonate groups of the CnS located at the outer edge of the cavity.

Thermodynamics of Supramolecular Associations with Macrocyclic Water-Soluble Hosts

The thermodynamic study of the complexation of the β-cyclodextrins and p-sulfonatocalix[n]arenes (CnS) with the 4-aminoazobenzene was reported and was carried out by molecular dynamics simulations. We determined the whole thermodynamic properties (K, Δr G°, Δr H°, and TΔr S°) using the potential of mean force (PMF) technique and more precisely the adaptive biasing force method. Depending on both the nature of the host molecule and the pH of the solution, the PMF profiles present different shapes and energy minima. Considering the complexity of these PMF profiles, we are also interested in the structural properties of these associations. Hence, we calculated hydrogen bonds, Lennard-Jones and electrostatic energies, the number of atoms of the guest molecule inserted inside the cagelike host molecule, and the number of water molecules expelled from the cavity.

How does the dehydration change the host-guest association under homogeneous and heterogeneous conditions?

In this study, the thermodynamic properties of association of some inorganic ions (ClO4(-) and SO4(2-)) with β-cyclodextrins (β-CD) in aqueous solution are determined under both free β-CD and surface confined β-CD conditions using atomistic simulations. The potential of mean force (PMF) is calculated as a function of the environment and the thermodynamic properties of association are deduced by integrating the free energy profiles. No inclusion complex between SO4(2-) and β-CD is detected. Nevertheless, the PMF curve obtained for gold-confined CD seems to evidence a small minimum at a larger separation distance that shows specific interactions such as hydrogen bonding outside the cavity. As concerns ClO4(-), our simulations reveal the formation of an inclusion complex with free β-CD in perfect agreement with the available experimental results. Nevertheless, we do not detect any formation of the host-guest inclusion complex under heterogeneous conditions. Finally, the differences observed as a function of the anions are interpreted through an atomistic description. The general trend of weaker complex stabilities with the increasing free energy of hydration of the anions is found in homogeneous systems.