Investigation of the Complexation between 4-Aminoazobenzene and Cucurbit[7]uril through a Combined Spectroscopic, Nuclear Magnetic Resonance, and Molecular Simulation Studies.

Cucurbiturils are well known for their ability to form supramolecular systems with ultrahigh affinities binding. Inclusion complex between 4-aminoazobenzene and cucurbit[7]uril has been investigated in aqueous solution by ultraviolet (UV)-spectroscopy, 1H NMR, and molecular simulations. 4-aminoazobenzene shows high affinity in acidic solutions while no association was detected in neutral solutions. The thermodynamic properties of complex formation are investigated using both UV spectroscopy and nuclear magnetic resonance (NMR) measurements. Our results highlight that the high binding constant between CB7 and 4AA (log K = 4.9) is the result of a large negative change in Δr H° (-19 kJ/mol) and a small positive change in TΔr S° (9 kJ/mol). The analysis of the experimental data lead to hypothesis on the structure of the complex. We have used molecular dynamics simulation to interpret experiments. Interestingly, the cis-trans isomerization of aminoazobenzene is considered. All the results are discussed and compared with those previously obtained with other host molecules.

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, ß-cyclodextrins (ß-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 ß-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 ß-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 ß-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 ß-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.

Associations of Water-Soluble Macrocyclic Hosts with 4-Aminoazobenzene: Impact of pH.

An investigation of the pH effect on the inclusion complexes of ß-cyclodextrins and calixarenesulfonates with 4-aminoazobenzene was conducted both by experiments and molecular simulations. The whole thermodynamic characterizations of the association between hosts and 4-aminoazobenzene ( K, Δr G0, Δr H0, and TΔr S0) were determined by UV-visible spectroscopy. ß-Cyclodextrin inclusion complexes are not affected by pH change unlike those obtained with calixarenes. All the studied systems were enthalpically favored. Nevertheless, the entropic behavior is different depending on the host. In order to interpret these experimental results, molecular simulations were used to calculate the number of atoms inserted into the cage-like host compounds and the number of water molecules expelled from the cavity.

Why is the association of supramolecular assemblies different under homogeneous and heterogeneous conditions?

The origin of the change of the thermodynamic properties of association between homogeneous and heterogeneous systems is highlighted by numerical experiments.

Free energy calculations in electroactive self-assembled monolayers (SAMs): impact of the chain length on the redox reaction.

The free energy approach is used to study the effect of the relative chain length of the two constituents of electroactive self-assembled monolayers (SAMs) on gold. In this study, the ferrocene groups are exposed to the electrolyte solution. This situation is achieved by using shorter diluent alkanethiol chains. To this end, the mixed monolayers formed by the self-assembly of 11-ferrocenylundecanethiol and butanethiol FcC(11)S/C(4)S and of 6-ferrocenylhexanethiol and butanethiol FcC(6)S/C(4)S onto a gold surface are studied. Calculation of enthalpy and entropy differences are also performed using molecular simulations. Additionally, the electrochemical signatures of these systems are determined to allow a direct comparison with our calculations. The thermodynamic properties are discussed in terms of enthalpy and entropy changes. Two effects account for the thermodynamic behavior. The first one involves the ion pairing between the ferrocenium group and the perchlorate anion. The second one concerns the desolvation of the first hydration shell of the anions. Finally, this work is also completed with a microscopic description associated with an energy characterization of these SAMs as a function of the surface coverage under conditions close to experiments.

Toward a prediction of the redox properties of electroactive SAMs: a free energy calculation by molecular simulation.

We report free energy calculations of FcC(6)S-/C(4)S-Au and FcC(6)S-/C(12)S-Au binary self-assembled monolayers (SAMs) formed by one ferrocenylhexanethiolate chain and alkylthiolate chains. We demonstrate that the free energy perturbation methods are able to reproduce the positive shift of the redox potential when the coadsorbed butanethiolate C(4)S chains are replaced by dodecanethiolate C(12)S chains. The different contributions to the Ewald summation involved in the perturbation process are thoroughly described. We complete the study by a microscopic description of the binary SAMs before and after oxidation. The molecular dynamics (MD) simulations evidence that the formation of the ion-pair between the ferricinium and a single perchlorate anion of the supporting electrolyte is more favored in FcC(6)S-/C(12)S-Au SAM.