Thermal expansion behavior is tuned by incorporating motion-capable or -incapable molecules into organic solids.

Hydrogen bonding is one of the most widely used noncovalent interactions for assembling multicomponent materials. Specifically, hydrogen bonds involving carboxylic acids and pyridines are exceptionally reliable, and although frequently utilized, the influence of COO−H•••N and C−H•••O hydrogen bonds on thermal expansion (TE) behavior is underexplored. Here, we describe a series of isostructural cocrystals wherein the components self-assemble into two-dimensional (2D) hydrogen-bonded sheets through a combination of COO−H•••N and C−H•••O hydrogen bonds. We describe the contribution of these two classic interactions to the TE behavior of the cocrystals. Specifically, two cocrystals exhibit 2D zero TE, and two cocrystals exhibit one-dimensional zero TE. This rare behavior results from the interactions sustaining the 2D hydrogen-bonded sheets, which work in tandem to control TE within the sheets.

A series of aromatic organic molecules functionalized with different halogen atoms (I/ Br), motion-capable groups (olefin, azo or imine) and molecular length were designed and synthesized. The molecules self-assemble in the solid state through halogen bonding and exhibit molecular packing sustained by either herringbone or face-to-face π-stacking, two common motifs in organic semiconductor molecules. Interestingly, dynamic pedal motion is only achieved in solids with herringbone packing. On average, solids with herringbone packing exhibit larger thermal expansion within the halogen-bonded sheets due to motion occurrence and molecular twisting, whereas molecules with face-to-face π-stacking do not undergo motion or twisting. Thermal expansion along the π-stacked direction is surprisingly similar, but slightly larger for the face-to-face π-stacked solids due to larger changes in π-stacking distances with temperature changes. The results speak to the importance of crystal packing and intermolecular interaction strength when designing aromatic-based solids for organic electronics applications.