Thermal expansion is investigated for halogen-bonded co-crystals containing molecules that exhibit dynamic motion, lack motion ability, or experience static disorder.

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.

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.

The impact of intermolecular interactions and molecular motion on solid-state properties is an active field of interest for chemists and materials scientists. For example, cocrystallization has been shown to modify and/or enhance the solid-state behaviors of a molecule when compared to the single-component solid. Here, we describe a series of cocrystals containing bis(pyridin-4-ylmethylene)benzene-1,4-diamine (BPDI) and ditopic or tritopic hydrogen-bond-donor molecules that are conformationally flexible. The components in all the cocrystals self-assemble through hydroxyl-pyridine heterosynthons to afford one-dimensional chains due to the conformations of the donor molecules. BPDI is torsionally flexible, and in cocrystals with ditopic hydrogen-bond donors, the molecule is almost planar, whereas in cocrystals with tritopic hydrogen-bond donors, BPDI is significantly twisted. This twisting in BPDI affects crystal packing and affords higher thermal expansion coefficients. Cocrystallization of BPDI with resveratrol, another torsionally flexible molecule, induces molecular pedal motion in BPDI and results in larger expansion behavior in the cocrystal.

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...