Structural Features in Crystals of Derivatives of Benzene with Multiple Contiguous Phenyl Substituents

Hexaphenylbenzene and related derivatives of benzene with multiple contiguous phenyl substituents have well-defined nonplanar topologies. Their structures limit conjugation and disfavor intermolecular pi-pi and C-H center dot center dot center dot pi interactions. Such compounds therefore show higher HOMO-LUMO gaps, lower degrees of self-association, less efficient packing, and higher solubilities than planar analogues. These characteristic properties underlie the growing utility of nonplanar phenyl-substituted benzenes in science and technology. Surprisingly, no systematic structural analysis of compounds of this type has been reported. We have now examined Structural features in crystals of hexaphenylbenzene, 1,2,4,5-tetraphenylbenzene, 1,4-dimethyl-2,3,5,6-tetraphenylbenzene, pentaphenylbenzene, 1-methyl-2,3,4,5,6-pentaphenylbenzene, and an octaphenyl-p-quinquephenyl. These compounds tend to crystallize in sheets held together by van der Waals contacts and small numbers of C-H center dot center dot center dot pi interactions. Although the nonplanar topologies or these compounds do not eliminate C center dot center dot center dot H contacts entirely, the ratios of H center dot center dot center dot H contacts to C center dot center dot center dot H contacts greatly exceed those found in analogous aromatic compounds that can assume planar structures. Our structural analyses promise to help explain the characteristic behavior of nonplanar aryl-substituted arenes and to accelerate the discovery of new applications.