Classification of crystal structures of extended aromatic hydrocarbons and the systematic relations

Abstract

Crystal structures of extended aromatic hydrocarbons are reviewed by applying the tools developed in my previous paper. Many large-area compounds have stacking (γ) structures, in which usually the longest edge with the maximum number of parallel peripheral hydrogen atoms forms the non-parallel (T-type) intercolumnar contact because the intermolecular energy is maximized due to the electrostatic attraction. The herringbone (HB) structure is regarded as the extreme limit. When the long edge is curved or blocked by substituents, a pitched π-stack structure is realized in which the molecular terminal forms the non-parallel contact. Since compounds with five-membered rings have tilted edges, unsymmetrical wedge-like molecules realize modified HB structures, but symmetrical molecules form a kind of stacking (θ) structure with a large dihedral angle of 130°. The stacking (γ) structure of large-area aromatic hydrocarbons is situated at the connecting point of the HB, pitched π-stack, and θ-structures because these structures have the same lattice symmetry.