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 center dot center dot center dot 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 center dot center dot center dot H, C center dot center dot center dot H, and C center dot center dot center dot 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.