Effects of Geometry and Electronic Structure on the Molecular Self-Assembly of Naphthyl-Based Dimers

Three new series of symmetric dimers containing a naphthoyloxybenzyl (NB), benzoyloxynaphthyl (BN), and naphthoyloxysalicyl (NS) mesogenic core linked to an alkylene spacer via an imino group were synthesized. The effects of the variant spacer parity as well as the variant core structure on the mesomorphic properties have been studied. The dimers having NB and BN mesogenic units display intercalated smectic structures regardless of the spacer parity. In contrast, bilayer smectic and Col_rec structures are observed for the NS core compounds with even and odd spacers, respectively. The influence of geometric and electronic factors on the mesomorphic behavior, in particular on the molecular packing within the smectic phase, is discussed based on conformational and dipolar considerations following DFT calculations using model molecules. The difference in self-organization of symmetric naphthyl-based dimers appears to be governed by the competition between geometric factors and dipole–dipole interactions between identical mesogenic units

Sensitivity of the N_TB phase formation to the molecular structure of imino-linked dimers

<p>Here we report on the synthesis and mesomorphic properties of a series of imino-linked dimeric molecules. In order to improve our understanding of the structure–N_TB phase correlations, we have studied the impact of geometric and electronic factors arising from varying mesogenic units, different spacer lengths and from the ratio (<i>n/m</i>) between the lengths of terminal chains (<i>n</i>) and spacer (<i>m</i>). From the perspective of the molecular geometry, the results show that the stability of the N_TB phase results from increasing effective molecular bending and with the broadening of the mesogenic unit, in particular near the spacer, and that the <i>n</i>/<i>m</i> ratio plays a substantial role in conjunction with the specific mesogenic unit. A computational study of the electronic properties shows that a broadening of the mesogenic core in the vicinity of the spacer is associated with an increased anisotropy of the electrostatic potential distribution. Within a given series of materials our study suggests that the incidence of the N_TB phase and its thermal stability are governed by the synergy of specific geometrical factors and the anisotropy of the electrostatic potential distribution of the mesogenic core.</p

A Liquid-Crystalline Phenylene-Based Shape-Persistent Molecular Spoked Wheel

Molecular spoked wheels with an all-phenylene backbone and different alkoxy side chain substitution patterns were synthesized using a cobalt-catalyzed [2 + 2 + 2] cycloaddition and subsequent template-directed cyclization via Yamamoto coupling. The two-dimensional organization of the molecules at the solid/liquid interface was investigated by means of scanning tunneling microscopy, allowing imaging of the molecular structure with submolecular resolution. With the right proportion of the flexible alkyl corona to the rigid core, mesomorphic behavior of one compound could be observed over a wide temperature range

Geometric aspects influencing N-N_TB transition - implication of intramolecular torsion

<p>Herein we report a comprehensive study on novel carbonyl- and ethenyl-linked symmetric dimers that combine synthesis, mesomorphic properties and molecular modelling. The study has been focused on the impact of geometry imposed by the linkage group on the incidence of the twist-bend nematic (N_TB) phase. Comparison of the mesomorphic properties of these two series complemented with computational studies of conformational space around the linkage group points molecular curvature and intramolecular torsion plays important role in the appearance of the N_TB phase and can be regarded as the basic structural requirements for design of new twist-bend nematogen materials.</p

Crystallization of Poly(ethylene oxide) with a Well-Defined Point Defect in the Middle of the Polymer Chain

Poly­(ethylene oxide) (PEO) is a polymer of great interest due to its prevalence in biomedical, pharmaceutical, and ion conductive systems. In this study, the crystallization behaviors of a PEO with 22 monomer units (PEO₂₂) and a PEO having the same degree of polymerization but with an additional 1,4-disubstituted 1,2,3-triazole ring in central position of the chain (PEO₁₁-TR-PEO₁₁) are investigated. PEO₁₁-TR-PEO₁₁ shows one type of lamella crystal after cooling to <i>T</i> = 0 °C, but structural changes during heating below their final melting are detected by WAXS, DSC, POM, and solid-state NMR spectroscopy. The lamella thickness increases, but simultaneously the helix–helix distance decreases and an additional Bragg reflection appears at 2θ = 22.1°. A model is proposed which explains these structural changes by incorporation of the TR ring into the crystals which are additionally stabilized by attractive C–H···π interactions of the TR rings. Additionally, two different types of extended chain lamella crystals are found in PEO₂₂ by SAXS which are discussed in the context of fractionation caused by the molar mass distribution obtained from MALDI-ToF data

Crystallization of Poly(ethylene oxide) with a Well-Defined Point Defect in the Middle of the Polymer Chain

Poly­(ethylene oxide) (PEO) is a polymer of great interest due to its prevalence in biomedical, pharmaceutical, and ion conductive systems. In this study, the crystallization behaviors of a PEO with 22 monomer units (PEO₂₂) and a PEO having the same degree of polymerization but with an additional 1,4-disubstituted 1,2,3-triazole ring in central position of the chain (PEO₁₁-TR-PEO₁₁) are investigated. PEO₁₁-TR-PEO₁₁ shows one type of lamella crystal after cooling to <i>T</i> = 0 °C, but structural changes during heating below their final melting are detected by WAXS, DSC, POM, and solid-state NMR spectroscopy. The lamella thickness increases, but simultaneously the helix–helix distance decreases and an additional Bragg reflection appears at 2θ = 22.1°. A model is proposed which explains these structural changes by incorporation of the TR ring into the crystals which are additionally stabilized by attractive C–H···π interactions of the TR rings. Additionally, two different types of extended chain lamella crystals are found in PEO₂₂ by SAXS which are discussed in the context of fractionation caused by the molar mass distribution obtained from MALDI-ToF data

An Azo-Bridged Ferroelectric Liquid Crystal with Highly Enhanced Second and Third Harmonic Generation

A laterally azo-bridged trimer ferroelectric liquid crystal (FLC) incorporating a strong chromophore along its polar axis was synthesized and characterized by polarized-light optical microscopy , differential scanning calorimetry , two-dimensional X-ray diffraction analysis, electro-optical measurements, and nonlinear optical (NLO) investigations. This mesogen exhibits a thermodynamically stable enantiotropic SmC* phase and a bistable ferroelectric switching in a surface stabilized cell with bookshelf geometry. It gives the resonance-enhanced <i>d</i>₂₂ coefficient of 28 pm V^–1 (λ = 1.369 μm) for second harmonic generation (SHG), the largest NLO susceptibility reported to date for all FLCs. At the same wavelength, a new type of helicoidal phase matching assisted by the helical SmC* structure was identified. When the second harmonic wavelength of 780 nm is far away from the resonance wavelength (λ_max = 572 nm), the <i>d</i>₂₂ coefficient is reduced to 6.8 pm V^–1 (λ = 1.56 μm). In addition to a strong SHG activity, the trimer also shows a strong third harmonic generation (THG) with an estimated third-order nonlinear susceptibility of χ⁽³⁾ = ∼3 × 10^–11 esu (λ = 1.56 μm), among the largest χ⁽³⁾ value reported from THG measurements for liquid crystals. This work enables viable applications of FLCs in nonlinear optics and offers an innovative approach to develop new FLCs with larger NLO strength