Restricted Rotation in Hexaarylbenzenes

Hexaarylbenzenes exist in a conformation in which the peripheral rings are perpendicular to the plane of the central ring on the nmr time scale. Hexaarylbenzenes substituted in ortho or meta positions of the peripheral rings display restricted rotation about the single bonds joining the central and peripheral rings, and complex stereoisomerism and stereoisomerization behavior results. The rotational barriers observed are relatively high (up to ~38 kcal/mol), and even rings bearing only a m-methyl substituent lead to barriers of ~16 kcal/mol.