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The crystal structure of a commercially available anthracene derivative, anthracene‐9‐thiocarboxamide, is reported here for the first time. The compound undergoes a [4+4] cycloaddition in the solid state to afford facile synthesis of the cycloadduct (CA). The cycloaddition is also reversible in the solid state using heat or mechanical force. Due to the presence of unpaired, strong hydrogen‐bond donor atoms on the CA, significant solvatomorphism is achieved, and components of the solvatomorphs self‐assemble into four different classes of supramolecular structures. The CA readily crystallizes with a variety of structurally‐diverse solvents including those containing oxygen‐, nitrogen‐, or pi‐acceptors. Some of the solvents the CA crystallized with include thiophene, benzene, and the three xylene isomers; thus, the CA was employed in industrially‐relevant solvent separation. However, in competition studies, the CA did not exhibit selectivity. Lastly, it is demonstrated that the CA crystallizes with vinyl‐containing monomers and is currently the only compound that crystallizes with both widely used monomers 4‐vinylpyridine and styrene. Solid‐state complexation of the CA with the monomers affords over a 50 °C increase in the monomer's thermal stabilities. The strategy of designing molecules with unused donors can be applied to achieve separations or volatile liquid stabilization.

 

Achieving substantial anisotropic thermal expansion (TE) in solid‐state materials is challenging as most materials undergo volumetric expansion upon heating. Here, we describe colossal, anisotropic TE in crystals of an organic compound functionalized with two azo groups. Interestingly, the material exhibits distinct and switchable TE behaviors within different temperature regions. At high temperature, two‐dimensional, area zero TE and colossal, positive linear TE (α=211 MK⁻¹) are attained due to dynamic motion, while at low temperature, moderate positive TE occurs in all directions. Investigation of the solid‐state motion showed the change in enthalpy and entropy are quite different in the two temperature regions and solid‐state NMR experiments support motion in the solid. Cycling experiments demonstrate that the solid‐state motions and TE behaviors are completely reversible. These results reveal strategies for designing significant anisotropic and switchable behaviors in solid‐state materials.

 

Control over thermal expansion (TE) behaviors in solid materials is often accomplished by modifying the molecules or intermolecular interactions within the solid. Here, we use a mixed cocrystal approach and incorporate molecules with similar chemical structures, but distinct functionalities. Development of mixed cocrystals is at a nascent stage, and here we describe the first mixed cocrystals sustained by one‐dimensional halogen bonds. Within each mixed cocrystal, the halogen‐bond donor is fixed, while the halogen‐bond acceptor site contains two molecules in a variable ratio. X‐ray diffraction demonstrates isostructurality across the series, and SEM‐EDS shows equal distribution of heavy atoms and similar atomic compositions across all mixed cocrystals. The acceptor molecules differ in their ability to undergo dynamic motion in the solid state. The synthetic equivalents of motion capable and incapable molecules were systematically varied to yield direct tunabililty in TE behavior.

 

The solution and mechanochemical synthesis of two cocrystals that differ in the stoichiometric ratio of the components (stoichiometric cocrystals) is reported. The components in the stoichiometric cocrystals interact through hydrogen or hydrogen/halogen bonds and differ in π‐stacking arrangements. The difference in structure and noncovalent interactions affords dramatically different thermal expansion behaviors in the two cocrystals. At certain molar ratios, the cocrystals are obtained concomitantly; however, by varying the ratios, a single stoichiometric cocrystal is achieved using mechanochemistry.

 

Thermal expansion (TE) behavior in solid-state materials is influenced by both molecular and supramolecular structure. For solid-state materials assembled through covalent bonds, such as carbon allotropes, solids with higher dimensionality (i.e., diamond) exhibit less TE than solids with lower dimensionality (e.g., fullerene, graphite). Thus, as the dimensionality of the solid increases, the TE decreases. However, an analogous and systematic variation of the dimensionality in solid-state materials assembled through noncovalent bonds with a correlation to TE has not been studied. Here, we designed a series of solids based on dimensional hierarchy to afford materials with zero-dimensional (0D), 1D, and 2D hydrogen-bonded structures. The 2D materials are structural analogues of graphite and covalent-organic frameworks, and we demonstrate that these 2D solids exhibit unique biaxial zero TE with anisotropic and colossal TE along the π-stacked direction (α ∼ 200 MK–1). The overall behavior in the 2D hydrogen-bonded solids is similar to 2D covalent-bonded solids; however, the coefficient of TE along the π-stacked direction for these hydrogen-bonded solids is an order of magnitude higher than in 2D graphite or phosphorus allotropes. The hierarchal materials design strategy and correlation to TE properties described herein can be broadly applied to the design and synthesis of new solid-state materials sustained by covalent or noncovalent bonds with control over solid-state behaviors. 

A strategy for modifying thermal expansion properties in dichroic, charge-transfer cocrystals is described. A solid-state Diels–Alder reaction is used to covalently connect adjacent molecules in the cocrystal, and thermal expansion along the direction of these bonds is reduced when compared to the unreacted cocrystals. 

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. 

Pedal motion or static disorder in single-component solids containing imine groups is demonstrated. Unique solid-state behaviors including colossal biaxial positive thermal expansion in one solid and a temperature-dependent phase transition in another are discussed. Imines exhibit torsional flexibility, which differs from the isoelectronic azo and olefin groups and influences solid-state behaviors.