Molecular Conformation of Bent-Core Molecules Affected by Chiral Side Chains Dictates Polymorphism and Chirality in Organic Nano- and Microfilaments

Abstract

The coupling between molecular conformation and chirality is a cornerstone in the construction of supramolecular helical structures of small molecules across various length scales. Inspired by biological systems, conformational preselection and control in artificial helical molecules, polymers, and aggregates has guided various applications in optics, photonics, and chiral sorting among others, which are frequently based on an inherent chirality amplification through processes such as templating and self-assembly. The so-called B4 nano- or microfilament phase formed by some bent-shaped molecules is an exemplary case for such chirality amplification across length scales, best illustrated by the formation of distinct nano- or microscopic chiral morphologies controlled by molecular conformation. Introduction of one or more chiral centers in the aliphatic side chains led to the discovery of homochiral helical nanofilament, helical microfilament, and heliconical-layered nanocylinder morphologies. Herein, we demonstrate how a priori calculations of the molecular conformation affected by chiral side chains are used to design bent-shaped molecules that self-assemble into chiral nano- and microfilament as well as nanocylinder conglomerates despite the homochiral nature of the molecules. Furthermore, relocation of the chiral center leads to formation of helical as well as flat nanoribbons. Self-consistent data sets from polarized optical as well as scanning and transmission electron microscopy, thin-film and solution circular dichroism spectropolarimetry, and synchrotron-based X-ray diffraction experiments support the progressive and predictable change in morphology controlled by structural changes in the chiral side chains. The formation of these morphologies is discussed in light of the diminishing effects of molecular chirality as the chain length increases or as the chiral center is moved away from the core-chain juncture. The type of phase (B1-columnar or B4) and morphology of the nano- or microfilaments generated can further be controlled by sample treatment conditions such as by the cooling rate from the isotropic melt or by the presence of an organic solvent in the ensuing colloidal dispersions. We show that these nanoscale morphologies can then organize into a wealth of two- and three-dimensional shapes and structures ranging from flower blossoms to fiber mats formed by intersecting flat nanoribbons.

Stereochemistry and Twisted Crystals

“…In a remarkable family of compounds, flat nanoribbons, twisted nano-and micro-filaments, and nanocylinder conglomerates form depending on the substitution pattern (Figure 10). [89] The compound lacking stereogenic centers forms a conglomerate of nanofibers as before, and introduction of chiral side chains leads to stereoselective assembly. The lower symmetry compounds, with a chiral "tail" on one side of the molecule and achiral on the other, lead to microfilaments or nanocylinders as a consequence in the change of the interlayer interactions arising from the position of the stereogenic centers.…”

Highly Tunable Circularly Polarized Emission of an Aggregation-Induced Emission Dye Using Helical Nano- and Microfilaments as Supramolecular Chiral Templates

“…However, apart from CPL amplification, the tunability of such systems has yet not been explored. Aside from N*-LCs generating or templating CPL, bent-core LC systems (BCLC) forming now temperature-dependent as well as cooling rate tunable helical nano- or microfilaments − are yet underutilized.…”

“…What has been reported for such helical nanoassemblies is that BCLC-based nanofilaments themselves show fairly low g lum values (on the order of | g lum |≈ 10 –3 ). ,, However, if used as templates, such morphologically chiral nanofilaments can serve as active as well as tunable templates with respect to overall dimensions, helical pitch, surface fraction of exposed aggregation sites, and polymorphism, with all these parameters allowing control over the overall CPL output. To demonstrate such active templating, we here investigate four distinct types of B4 chiral nanotemplates (Figure ) whose structures and morphologies were previously thoroughly studied and reported. − ,, These nanofilament or nanocylinder B4 phase morphologies were realized by strategically incorporating chiral centers in aliphatic side chains at the core-chain junctures in each side chain with identical (BC1 and BC2) or opposite configuration (BC3) or in only one of the side chains, either in the shorter meta -side of the molecule (BC4) or in the longer para -side (BC5) . With chiral centers located in each side of the aliphatic chains, a dual-modulated helical nanofilament (HNF mod2 ) phase forms (both upon slow as well as rapid cooling) due to an intralayer mismatch between the top and bottom molecular halves of these bent-core molecules that can only be relieved by local saddle splay with negative Gaussian curvature .…”

Luminescent assemblies of pyrene-containing bent-core mesogens: liquid crystals, π-gels and nanotubes

“…[39][40][41][42][43][44][45][46][47][48] Concerning mesophase organizations, bent-core liquid crystals (BCLCs), which were discovered around 1996, 49 and their nonconventional mesophases have received significant attention in the soft matter area in the last decades. [50][51][52][53][54][55][56][57] Their compact packing has allowed the development of novel liquid crystal phases with polar and in certain cases chiral arrangements, providing materials with very attractive functional properties. 54,56,[58][59][60][61] Ferro-and antiferroelectric switching, piezoelectric or nonlinear optical responses from their polar molecular non-centrosymmetric order, have been reported even by using achiral molecules.…”