Giant (Mar 2023)
Molecular design principles of helical pyramidal chirality self-organized from achiral hexakis(alkyloxy)triphenylene
Abstract
2,3,6,7,10,11-Hexakis(alkyloxy)triphenylene (HATn) containing n = 4 to 12 carbons in its alkyl groups, are a classic group of discotic molecules self-organizing columnar liquid crystals. The structure of the crystalline assemblies of these discotic molecules were neglected since they were invented 45 years ago. Recently, we discovered that the crystal state of HAT4 consists of a highly ordered 8/1 helical chiral pyramidal 3D self-organization. In this publication we report the structural analysis of all 3D self-organizations of HATn containing n = 4 to 12 carbons in their achiral alkyls. Unexpectedly, the highly ordered 8/1 helical chiral pyramidal crystalline columns assembled from the crown-conformation of HAT4 is encountered also in the crystalline periodic array of HAT5. HAT6 self-organizes a 5/1 helical chiral pyramidal 3D column. HAT7 to HAT10 maintain their crown conformation in their 3D supramolecular pyramidal columns. However, their 3D pyramidal assemblies exhibit a nonhelical criss-cross arrangement. HAT11 and HAT12 self-organize 3D nonhelical columns from criss-cross arrangements of disc-like conformers. The structures of all these 3D helical and nonhelical pyramidal as well as of nonhelical discotic columns were resolved for the first time at the molecular level by reconstruction of their oriented fiber X-ray diffractograms with the help of molecular models. These results demonstrate that a single self-assembling building block can self-organize either helical chiral pyramidal or nonhelical pyramidal or even nonhelical discotic assemblies via two of its constitutional isomers. This outcome prompts essential questions related to the current standing of helical self-organizations. The most important question is, why over the past many years the scientific community explored helical self-organizations derived from long alkyl groups when the ideal alkyls for this process seem to be short? A hypothetic explanation of the findings reported here and of their impact on the field of helical self-organizations is presented.
