Characterizing and Directing Hierarchical Assemblies of Nanocylinders in Sequence-defined Mesogenic Dimers

Abstract

Mesogens are molecules that can access anisotropic, liquid crystalline mesophases with useful optical, magnetic, and electronic properties. Mesogens with a bent shape have restricted rotational freedom along their long axis. Because of this restriction, these mesogens can form a range of low symmetry, smectic ‘Banana’ or ‘B-phases’. Also, under certain conditions, oligomers and dimers synthesized with flexible linkers can adopt a bent intermesogen geometry, allowing them to form B-phases as ‘flexible’ bent core mesogens. In rigid bent-core mesogens and dimers synthesized with a flexible linker, the formation of complex microstructures is triggered both by geometric frustrations from the packing arrangement and by a driving force to escape macroscopic polar order. These features give rise to assemblies with substantial curvature and sometimes supramolecular chirality, even in the absence of point chirality within the chemical structure of the mesogen. Because of their chirality, these specialized microstructures have drawn attention for their potential applications in synthesis and nanofabrication efforts in bio- sensors, optomechanics, and as templates for nanoparticle systems. This work examines the self-assembly of an achiral, asymmetric mesogenic heterodimer which is hypothesized to favor a bent intermesogen geometry. The heterodimer self-assembles into nanocylinders 128 ± 21 nm in diameter. Despite the achirality of the mesogenic subunits themselves, their hierarchical nanocylinder arrangement features planar chirality. To reveal the precise structure of the molecular assembly and affirm the potential of this system for further functionalization, copper-catalyzed azide-alkyne cycloaddition (CuAAC) fluorescent tagging procedures are used to visualize the placement of key functional groups within the nanocylinder assembly. We demonstrate that we can achieve a high degree of long-range order and alignment in the nanocylinder microstructure through seeded nucleation followed by slow, high-temperature isothermal crystallization. These findings explore methods to prescribe nucleation sites and direct the subsequent growth of the nanocylinder microstructures, which is an important step towards exploiting these cylindrical assemblies in device fabrication.