Structure–Property Relationships in Hydrogen-Bonded Liquid Crystals

Abstract

The structural impact of hydrogen-donating moieties on the liquid crystalline behavior of hydrogen-bonded assemblies (HBAs) is comprehensively investigated. Therefore, a series of phenol derivatives such as phenol, catechol (CA), resorcinol (RE), hydroquinone (HQ), pyrogallol, hydroxyhydroquinone, and phloroglucinol (PHG) were combined with alkoxyazopyridines (Ap-N) yielding 49 new HBAs, which were studied with respect to their mesomorphic properties. The present study revealed significant differences in the liquid crystalline behavior of the structurally diverse assemblies, ranging from the absence of a mesophase to smectic or nematic phases. In contrast to previous studies a comprehensive crystallographic analysis provides insight into the structure–property relationships of the assemblies and proves a correlation between the supramolecular architecture and the macroscopic properties (=liquid crystallinity). More specifically, comparison of the single crystal data with the 2D X-ray diffraction patterns indicates that linear assemblies tend to form crystalline or smectic phases (for the HQ and RE, respectively), while a bent-shaped assembly yields nematic phases (for CA and PHG). Furthermore, our results suggest that segregation of aliphatic and aromatic segments, as observed in the solid state structures, supports the formation of stable mesophases.

Polymorphism of hydrogen-bonded star mesogens – a combinatorial DFT-D and FT-IR spectroscopy study

“…[56][57][58][59][60][61][62][63][64] In contrast to these binding motifs, the interaction between phenol (PH) and pyridine (pyr) is signicantly weaker leading to a higher exibility and enhances structural diversity. 46,[65][66][67] In order to nd a suitable starting geometry for our study, we initially investigated the PH/pyr interaction by conformational analysis (details see ESI, Chapter 3 †). Hereby, we obtained 121 optimized conformer geometries, which differ in their energies (zero point energy ZPE and Gibbs free energy G; ESI Fig.…”

“…Since subtle changes in their architecture have a tremendous impact on the geometries and physical properties of the hydrogen-bonded liquid crystals, a precise prediction of their structure-property relationships is challenging. 19,46 Lee et al demonstrated that hydrogen-bonded star mesogens occur in two different morphology depending on the number and substitution pattern of the peripheral alkoxyl groups. They found smectic or columnar arrangements of the star mesogens.…”

“…Based on a modular approach combining hydrogen bond donating core units (phloroglucinol, PHG) with hydrogen bond accepting side chains (azopyridine, Ap), we reported a variety of liquid crystalline materials with photo-switchable properties, which exhibited fast and reversible phase transitions (nematic N / isotropic I) upon irradiation with a commercially available laser pointer (405 nm, 5 mW). 22,46 Applying this concept, we investigated the impact of the core 46 and side chain on the liquid crystalline behaviour of these assemblies. For instance, the degree and pattern of uorination control the nature of the mesophase (nematic or smectic) and enhance the temperature range of liquid crystalline phase.…”

“…2). 22,24,46 However, so far we did not investigate the shape of the hydrogen-bonded assemblies in their mesophase.…”

Alkyl‐Alkyl Interactions in the Periphery of Supramolecular Entities: From the Evaluation of Weak Forces to Applications

“…Based on our findings, that exclusively the phloroglucinol assemblies showed mesomorphic behavior, a comprehensive study of the structural impact of hydrogen‐bond donor moieties on the liquid crystalline behavior was performed . Therefore, the substitution pattern of the hydrogen‐bond donor was systematically varied (phenol ( PH ), hydroquinone ( HQ ), resorcinol ( RE ), catechol ( CA ), pyrogallol ( PYG ) and hydroxyhydroquinone ( HHQ ).…”