Thermal expansion in organic solids and impact of pedal motion and crystal packing
Crystal engineering is continuingly gaining attention because it is a reliable and useful tool to construct functional materials. By using this strategy, we can synthesize solid-state materials like cocrystals with desired properties. Cocrystallization is the process of incorporating two or more unique molecules held together by intermolecular interactions. Thermal expansion (TE) describes how a material responds to temperature changes. The expansion in molecular crystals is determined by the bonds that hold them together. Organic solids are sustained by noncovalent interactions like halogen bonds, hydrogen bonds, π-π stacking, or van der Waals forces, which are not always predictable. Therefore, designing organic materials with controllable TE properties is still challenging. Molecular motions can also influence TE in solids. Pedal motion resembles the motion of pedaling a bicycle, and the occurrence of pedal motion can lead to large positive TE along the motion direction. TE influences solid-state properties in materials such as phase transitions and mechanical properties. Thus, understanding TE behaviors is helpful to design functional materials used in thermal actuators, shape memory, and energy storage. This dissertation will focus on designing organic materials through crystal engineering methods and studying the influence of pedal motion and crystal packing on their TE properties. For single-component crystals, the di-halogenated aromatic molecules were synthesized by functionalizing with different motion-capable moieties and halogen groups. The amount of pedal motion is influenced by the motion group identity. Moreover, the solids exhibit two different crystal packing styles, resulting in different TE within the halogen-bonded sheets. Specifically, one molecule of them undergoes reversible solid-state behaviors of anisotropic TE, phase transition, and pedal motion. On the other hand, we have synthesized cocrystals including traditional two-component cocrystals and mixed cocrystals. We describe the TE differences in cocrystals which result from different crystal packing and involved noncovalent interactions. Controlling TE behaviors in mixed cocrystals is achieved by tuning the amount the motion-capable or -incapable molecules in the solids.
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