Orientational isomerisation of guest molecules in equilibrium in a tubular host crystal formed via halogen and hydrogen bonding

Fine-tuning of the ratio of two different states is important in controlling the structures and functions of crystalline materials. Here we report that clathrated guest molecules inside a tubular host...

Differences in thermal expansion and motion ability for herringbone and face-to-face π-stacked solids

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.

Comparison of computationally cheap methods for providing insight into the crystal packing of highly bromomethylated azobenzenes

For five bromomethylated azobenzenes, namely (E)-[4-(bromomethyl)phenyl][4-(dibromomethyl)phenyl]diazene, C₁₄H₁₁Br₃N₂, (E)-1,2-bis[4-(dibromomethyl)phenyl]diazene, C₁₄H₁₀Br₄N₂, (E)-[3-(bromomethyl)phenyl][3-(dibromomethyl)phenyl]diazene, C₁₄H₁₁Br₃N₂, (E)-[3-(dibromomethyl)phenyl][3-(tribromomethyl)phenyl]diazene, C₁₄H₁₀Br₄N₂, and (E)-1,2-bis[3-(dibromomethyl)phenyl]diazene, C₁₄H₉Br₅N₂, the computationally cheap CLP PIXEL approach and CrystalExplorer were used for calculating lattice energies and performing Hirshfeld surface analysis via the enrichment ratios of atomic contacts. The procedures and caveats are discussed in detail. The findings from these tools are contrasted with the results of geometric analysis of the structures. We conclude that an energy-based discussion of the crystal packing provides substantially more insight than one based purely on geometry, as has so long been the custom in crystallography. In addition, we find a surprising shortage of halogen-halogen interactions in these highly bromomethylated compounds.