Study of entropy-driven self-assembly of rigid macromolecules

Entropy Drives Interpolymer Association in Water: Insights into Molecular Mechanisms

Interpolymer association in aqueous solutions is essential for many industrial processes, new materials design, and the biochemistry of life. However, our understanding of the association mechanism is limited. Classical theories do not provide molecular details, creating a need for detailed mechanistic insights. This work consolidates previous literature with complementary isothermal titration calorimetry (ITC) measurements and molecular dynamics (MD) simulations to investigate molecular mechanisms to provide such insights. The large body of ITC data shows that intermolecular bonds, such as ionic or hydrogen bonds, cannot drive association. Instead, polymer association is entropy-driven due to the reorganization of water and ions. We propose a unifying entropy-driven association mechanism by generalizing previously suggested polyion association principles to include nonionic polymers, here termed polydipoles. In this mechanism, complementary charge densities of the polymers are the common denominators of association, for both polyions and polydipoles. The association of the polymers results mainly from two processes: charge exchange and amphiphilic association. MD simulations indicate that the amphiphilic assembly alone is enough for the initial association. Our proposed mechanism is a step toward a molecular understanding of the formation of complexes between synthetic and biological polymers under ambient or biological conditions.

Molecular Motion and Conformational Interconversion of Ir(I)·COD Included in Rebek's Self-Folding Octaamide Cavitand

We report experimental and theoretical evidence of restrained axial rotation for heteroleptic L2·Ir(I)·1,5-cyclooctadiene (COD) complexes included in the aromatic cavity of Rebek's self-folding octaamide cavitand. At 298 K, the axial spinning motion of the included organometallic guests was slow on the (1)H NMR time scale and produced a proton spectrum for the bound host indicative of C2 symmetry. Signals corresponding to aromatic protons of the bound host coalesced at 323 K, indicating that the spinning process of the included guest became fast on the (1)H NMR time scale and that the complex approached C4 symmetry. Surprisingly, lowering the temperature of the solution to 193 K induced an additional splitting of the proton signals observed at room temperature for both the bound host and the included guest. We propose the emergence of a new element of chirality in the complexes, which was associated with a slow interconversion, on the (1)H NMR time scale between the two chiral twisted-boat conformers of the chelated COD included in the already chiral cavity of the container. This leads to the inclusion complexes existing in solution as pairs of two racemic diastereomers. We estimated that the racemization barrier for the two cyclochiral conformers of the Ir(I) chelated COD was 5 kcal mol(-1) higher as an included organometallic complex than as free in solution. Furthermore, we performed a van't Hoff plot and determined that the inclusion of the organometallic complex in the cavitand was endothermic and exclusively driven by entropy (ΔH = 5.9 kcal mol(-1) and ΔS = 33.9 cal mol(-1) K(-1)).

Molecular motions in functional self-assembled nanostructures

The construction of "smart" materials able to perform specific functions at the molecular scale through the application of various stimuli is highly attractive but still challenging. The most recent applications indicate that the outstanding flexibility of self-assembled architectures can be employed as a powerful tool for the development of innovative molecular devices, functional surfaces and smart nanomaterials. Structural flexibility of these materials is known to be conferred by weak intermolecular forces involved in self-assembly strategies. However, some fundamental mechanisms responsible for conformational lability remain unexplored. Furthermore, the role played by stronger bonds, such as coordination, ionic and covalent bonding, is sometimes neglected while they can be employed readily to produce mechanically robust but also chemically reversible structures. In this review, recent applications of structural flexibility and molecular motions in self-assembled nanostructures are discussed. Special focus is given to advanced materials exhibiting significant performance changes after an external stimulus is applied, such as light exposure, pH variation, heat treatment or electromagnetic field. The crucial role played by strong intra- and weak intermolecular interactions on structural lability and responsiveness is highlighted.

Entropy-controlled 2D supramolecular structures of N,N'-bis(n-alkyl)naphthalenediimides on a HOPG surface

The two-dimensional supramolecular structures of a series of N,N'-bis(n-alkyl)naphthalenediimides (NDIs), whose chain lengths span from C3 to C18, at a liquid-HOPG surface interface, studied by STM and FM-AFM, are assigned with the help of molecular dynamics/molecular mechanics calculations to demonstrate that the C3- and C4-NDIs show lamellar structures, the C4- to C12-NDIs show honeycomb (KAGOME) structures, and the C14- to C18-NDIs show lamellar structures again. The change in supramolecular structure depending on chain length can be explained semiquantitatively by the balance of entropy and enthalpy terms to show the importance of "self-avoiding walk" of the alkyl chain in entropy terms.