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 π−π and C−H···π 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···π interactions. Although the nonplanar topologies of these compounds do not eliminate C···H contacts entirely, the ratios of H···H contacts to C···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.