Tampering with molecular cohesion in crystals of hexaphenylbenzenes.

Hexaphenylbenzene (HPB) and analogous compounds have properties of broad utility in science and technology, including conformationally well-defined molecular structures, high thermal stability, high HOMO-LUMO gaps, little self-association, inefficient packing, and high solubilities. Previous structural studies of HPB and its analogues have revealed persistent involvement of the central aromatic ring in strong C-H...pi interactions. These interactions can be blocked by adding simple ortho alkyl substituents to the peripheral phenyl groups. Comparison of the structures of HPB and a series of ortho-substituted derivatives has shown systematic changes in molecular cohesion and packing, as measured by packing indices, densities, solubilities, temperatures of sublimation, melting points, and ratios of H...H, C...H, and C...C contacts. These results illustrate how crystal engineering can guide the search for improved materials by identifying small but telling molecular alterations that thwart established patterns of association.

Hexaphenylbenzenes as potential acetylene sponges.

Acetylene sponges can be created by taking advantage of the nonplanar geometry of hexaphenylbenzenes and the special capacity of the central aromatic ring to engage in C(sp)-H...pi interactions reinforced by secondary C(sp(2))-H...pi interactions, as revealed by X-ray crystallographic studies and DFT calculations.