Thermotropic MIDA Boronates as a Case Study for the Role of Dipolar Interactions in Liquid Crystalline Self‐Assembly

Abstract

A series of MIDA (N-methylimino diacetic acid) boronates carrying 4-alkoxy, 3,4-bisalkoxy, or 3,4,5-trisalkoxyphenyl substituents were synthesized and their mesomorphic properties characterized by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques such as small- and wide-angle X-ray scattering (SAXS and WAXS, respectively). Most derivatives were liquid crystalline. In the case of mono- and bisalkoxy-substituted derivatives, C chains already induced smectic A (SmA) mesophases despite the bulky MIDA head group. With increasing chain length, columnar hexagonal (Col ) phases replaced SmA phases in the disubstituted series. Quantum chemical calculations on a series of MIDA boronates show that the B-N bond is a dative bond with a positive charge on the boron atom and negative charges on the nitrogen and oxygen atoms. In addition, no π-interaction between the aryl moiety and B-N bond was found, thus the mesogenic unit is electronically decoupled from the MIDA head group. These theoretical findings were supported by IR and Raman spectra as well as by asingle crystal structure analysis of 4-ethoxyphenyl MIDA boronate. Calculations of the electrostatic potential of the MIDA boronate reveal a special polarity pattern that can support the formation of a two-dimensional network and is likely to explain the liquid crystalline self-assembly. The absence of any electronic cross-talk between the MIDA head group and B-aryl or B-alkyl substituents allows the efficient tailoring of the mesophase type through variation of the substituents.

Columnar Propeller‐Like 1,3,5‐Triphenylbenzenes: Probing the Effect of Chlorine on the Suzuki Cross‐Coupling and Liquid Crystalline Properties

“…During examination of precursors 6 by DSC and POM we noticed liquid crystalline behaviour for MIDA boronates 6a , c , d which is in line with previous studies …”

“…The DSC results for MIDA boronate 6c exhibit that one chloro substituent in the meta ‐position led to a higher melting temperature of 97 °C, but lowered the mesophase‐to‐isotropic liquid transition to 141 °C in the heating scan relative to the transition temperatures of [3,4‐bis(dodecyloxy)phenyl]‐MIDA boronate 13 (Figure , Table ). Upon cooling the clearing temperature is maintained while supercooling for the mesophase–Cr transition to 78 °C and two further Cr–Cr transitions at 66 °C and 60 °C were observed.…”

“…Williamson etherification of 5‐bromo‐3‐hydroxyphenol 11a with bromododecane in the presence of K 2 CO 3 gave aryl bromide 8a in 63 % yield (Scheme ) . Subsequent treatment with n BuLi in THF at –78 °C, triisopropyl borate and finally N ‐methyliminodiacetic acid (MIDA) according to literature procedures, yielded boronate 6a in 63 %. Dakin oxidation of 5‐bromo‐3‐chloro‐2‐hydroxybenzaldehyde 12 gave bromide 11c quantitatively, which was etherified with bromododecane to yield bromide 8c .…”

“…Mesophase widths of liquid crystalline MIDA‐boronates 6 and star‐shaped triphenylbenzenes 5 obtained by DSC upon second heating (heating rate 5 K min –1 ). Data for [3,4‐bis(dodecyloxy)phenyl]‐MIDA boronate 13 and 4a were taken from the literature , …”

Large Electro‐Optic Kerr Effect in Ionic Liquid Crystals: Connecting Features of Liquid Crystals and Polyelectrolytes

“…12OCB was obtained from Synthon Chemicals (99 %). 3,5‐DMB was synthesized analogue to a previously described method . The details of the synthesis as well as the physical and spectral properties of 3,5‐DMB can be found in the supporting information.…”