Taking Supramolecular Assembly to the Next Level: Assembly of Flexible Molecules with Complex Internal Structure and Chemistry

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Karl Freed of the University of Chicago is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division to investigate equilibrium self-assembly of molecules into non-covalently bonded ordered supramolecular structures. This phenomenon is pervasive in chemistry, materials science, and biology, and its control has numerous applications in industry and medicine. Earlier studies elucidated general underlying principles of this spontaneous and reversible organization based on models that ignored details of the internal structure of the molecules. The current work is extending those theoretical advances to account for the strong and often subtle influence of the chemical structure of the assembling units, their geometrical shapes, and their mutual (often directional) interactions. The Lattice Cluster Theory formalism developed earlier is being generalized to include (i) strong and/or attractive interactions and (ii) flexible/deformable molecules. The goal is to describe five qualitatively different types of self-assembly of structured units, including assembly into chains and network structures. The intramolecular and intermolecular structure and thermodynamics contributions are nonadditive, adding computational challenge but also relevance to realistic chemical association. This new class of self-assembly theories enables examination of the ubiquitous enthalpy-entropy compensation phenomenon, where modification of a system by what is anticipated to produces large changes in the enthalpy (of a reaction) is largely compensated by a corresponding change in the entropy (and vice versa).The bottom-up manufacture of materials for applications in industry and medicine is made possible when molecules in solution come together and organize spontaneously. The circumstances under which this occurs have become clearer from the work of Freed and his group, who are developing computational tools describing the chemical association of neighboring molecules in this process of equilibrium self-assembly. Different structures may result, for example, chains and network structures, and the outcomes can be influenced by the shapes and vibrating and rotating behavior of the individual molecules. The goals of the work are to understand how different outcomes occur for different molecules and to use this knowledge to control self-assembly so that it can be fully exploited in bottom-up material design.