Unveiling Discotic Liquid Crystalline Phase Changes through Supramolecular Disc Polymorphism at Their Disc-Driven Gel State

Abstract

Supramolecular interactions can create a stable spatial architecture that resists reorientation in response to an external electromagnetic field. In contrast, liquid crystal molecules with weak intermolecular interactions can easily align with an external field but are unable to form robust supramolecular architectures. As a result, it has been challenging to develop supramolecular gelators that exhibit liquid crystalline properties. Nonetheless, if we can confine the supramolecular interaction within a short 2D area and the resulting supramolecular plate can organize itself according to the external field, we can integrate both the opposing behaviors in the same materials. A 2D self-assembly based gelator material is employed here to address this issue (Sahoo et al. Langmuir 2009, 25, 8742). Therein, a gradual increase in an alkyl chain length at the primary amine part in primary ammonium dicarboxylate (PAD) salts transforms 2D hydrogen-bonded networks (HBNs) into 1D with a consequent change in the crystalline phase. Cumulative weak van der Waals interactions with increasing alkyl chain length in the primary amine part metamorphose a 2D network into a kinetically stable 1D network at higher temperatures and exhibit gelation ability. Heating a 2D plate partially melts it to a 2D disc, which on further heating generates a 1D fiber, arranged radially to retain the disc shape after metamorphosis. Can this phase-changing ability with supramolecular plate stacking efficiency impart discotic behavior in the gel state? From this hypothesis, the experiments show that once the salt in DMSO is added above minimum gelator concentration and heated gently, the supramolecular disk is arranged in a discotic fashion, exhibiting a liquid crystalline property even in a gel state. The fibrous disc may have the potential to allow photon passage for the construction of a new kind of display device.

Introducing Dihedral Angle Torsion in a Flexible Dicarboxylic Acid: Evolution from a Sequential Symmetry Condensing Liquid Crystalline Gel to a Step-Halting Heat-Set Gel

“…However, the crystalline phase changing abilities through a sequentially symmetry condensing ability was never explored until last year. 16 A PAD salt, efficient in exhibiting temperature dependent polymorphism, 19 was made flexible by providing a flexible acid backbone. However, as the intermolecular molecular interactions self-assemble and lock the molecules from external electromagnetic fields in the supramolecular (gel) state, the supramolecular system can never align stepwise to exhibit complete liquid crystalline properties.…”

“…Therein, we can introduce the supramolecular gelling properties in the liquid crystal system. 16 The 2D disc supramolecular structure with temperaturedependent crystalline phase changes introduces mesogenic characteristics to the gel, with the notable feature that crystalline phase changes do not disintegrate the supramolecular shape, thus retaining the mesogenic properties. A recent work shows that a PAD salt, namely, pentadecyl ammonium adipate (Figure 1a) gel (Figure 1b) exhibited mesogenic behavior in both the solvated state and solid condition.…”

“…Without adding any external liquid crystal or gelling systems, just the PAD salts can impart both the gelation and the liquid crystalline properties for proving the opposing natures of supramolecular systems and liquid crystal systems. 16 The conformational flexibility in adipic acid sustains the 1D HBN even after disintegrating the van der Waals interactions while heating. 13,16 From differential scanning calorimetry (DSC), crystalline phase changes were observed at 69.8 °C (solid-smectic), 79.2 °C (smectic-nematic), and 120.9 °C nematic−isotropic in a DMSO gel (at MGC, 2.60 wt %) (Figure 1c).…”

“…16 The conformational flexibility in adipic acid sustains the 1D HBN even after disintegrating the van der Waals interactions while heating. 13,16 From differential scanning calorimetry (DSC), crystalline phase changes were observed at 69.8 °C (solid-smectic), 79.2 °C (smectic-nematic), and 120.9 °C nematic−isotropic in a DMSO gel (at MGC, 2.60 wt %) (Figure 1c). 16 The 2.6 and 3 wt % gels were melted (T gel ) at 77 and 85 °C, showing that the gel with a proper gelator concentration can survive in smectic or nematic phases.…”

“…Heating such 2D crystal stepwise reduces the symmetry by unzipping the van der Waals interactions to generate a heat-driven 1D hydrogenbonded network (step 1 and step 2 in Scheme 1). 16 Heating the plate partially melts it to form a disc and exhibits a smectic phase (step 2 in Scheme 1). As heat is a form of electromagnetic (em) energy, it stacks the liquid crystal discs in step 3.…”