Structural Complexity, Network Formation and Spontaneous Chirality in Soft-Matter Self-Assembly

ConspectusIn modern science, chemists excel at programming interactions and functionalities on the molecular scale to design and create novel materials with diverse and tailored functionalities. Self-assembly, a major mechanism that amplifies these interactions, enables the emergence of well-defined structures at higher levels and larger length scales. The intricate interplay among molecular-scale properties (polarity, chirality, etc.), specific intermolecular interactions (hydrogen bonding, dispersion forces, π-stacking, etc.), nanoscale segregation, and global order gives rise to complex self-assembly behaviors that hold great promise for generating materials with novel properties and functions. While self-assembly has been extensively studied in solutions, on surfaces, and within solid-state materials, the understanding of complex soft self-assembly in highly dynamic but ordered fluids still remains in its infancy due to the interplay between entropic and enthalpic contributions. In this Account, we elucidate complex soft self-assembly in 3D networks formed by simple organic molecules involving π-conjugated rod-like building blocks. Two different types of compounds derived from linear π-conjugated polyaromatic rods have been designed, and their soft self-assembly was studied by different methods including synchrotron X-ray scattering and resonant soft X-ray scattering. One type of compound, called polycatenars, has alkyl chains attached to both ends, and the other one, the bolapolyphiles, have linear or branched alkyl chains side-on attached and polar glycerols at each end. Both types of compounds form network phases, differing in the orientation of the rods, with respect to the struts forming the network. In the first group, the rods are organized perpendicular (transversal) to the network direction which allows them to develop an intermolecular helical twist. These supramolecular helices propagate chirality through space and induce mirror-symmetry breaking in soft and fluid systems, which is highly relevant to the spontaneous emergence of uniform chirality, especially biological chirality. In the bolapolyphiles, the polar glycerols organize into supramolecular spheres, which are interlinked by bundles of parallel arranged rods forming the struts interconnecting the spheres into networks, in this case with the rods aligned parallel (longitudinal) to the network. These bundles of parallel rods can be considered as bonds, having defined lengths, linking the supramolecular spheres at the junctions with coordinate numbers ranging from 3 to 14. In total, 8 different networks, divided into single-, double-, and triple-networks, have been found, among them those formed by cubic, octahedral, and tetrahedral frames, including the I-WP network and the A15 type Frank Kasper network. Here, structural complexity arises from the delicate balance between optimizing sphere-packing and minimizing infinite periodic minimal surfaces. In summary, by leveraging specifically designed low molecular weight amphiphilic or polyphilic rod-like molecules equipped with multiple interactions, we have successfully extended the frontiers of programmable self-assembly from the domain of solid-state materials into the realm of soft matter, liquid crystals, and isotropic liquids. The insights into structural complexity and symmetry breaking have profound implications for understanding the emergence of chirality, as well as for the rational design of advanced soft materials. Potential applications include soft addressable and multifunctional structural, optical, chiroptical, and electronic materials.