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

Chiral Brønsted Acid Catalyzed Enantioselective aza-Friedel–Crafts Reaction of Cyclic α‑Diaryl <i>N</i>‑Acyl Imines with Indoles

Asymmetric addition of indoles to cyclic α-diaryl-substituted <i>N</i>-acyl imines, which are generated <i>in situ</i> from 3-aryl 3-hydroxy­isoindolinones, is described. The transformation proceeds smoothly with a broad range of indoles and isoindolinone alcohols using a SPINOL-derived chiral Brønsted acid catalyst to afford α-tetrasubstituted (3-indolyl)­(diaryl)­methanamines in excellent yields and enantioselectivities (up to 98% yield, up to >99:1 e.r.). The origin of stereochemical induction is supported by DFT calculations and experimental data

Theoretical model for the Frank elastic moduli in the intercalated SmA_b phase of bent-shaped dimers

In our previous works we have shown that the elastic properties of the intercalated SmAb phase formed by bent-shaped dimers are governed by the nematic-like behaviour of the secondary director m that is associated with the projection of the molecular axes of the monomers on the plane of the smectic layer. From the experiment, the corresponding three Frank-like moduli K11m, K22m and K33m related to the secondary director demonstrate the usual behaviour of the Frank moduli of the nematics formed by rod-like molecules: monotonously increase with decreasing temperature. This is contrary to the temperature dependence of the elastic moduli for the primary director of N and NTB phases formed by bent-shaped dimers (for which the bend elastic constant decreases with temperature to zero). However, the values of the Frank-like moduli for SmAb were found to be smaller than their nematic-phase equivalents, and demonstrate a strong and unusual anisotropy, with K11m : K22m : K33m ratio being approximately 30 : 1 : 10. Here we present a theoretical model based on the assumption of the nematic-like order within the smectic layers that provides a qualitative explanation of the experimental results.</p

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