Dendritic Ligands for Magnetic Suspensions in Liquid Crystals

Martin Hähsler,Silke Behrens

doi:10.1002/ejoc.201901450

Martin Hähsler, Silke Behrens

Open Access

https://doi.org/10.1002/ejoc.201901450

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Abstract

The synthesis of long‐chain, aliphatic and space filling dendritic ligands containing (pro)mesogenic, aliphatic or nitrile biphenyl moieties for the stabilization of magnetic nanoparticles in liquid crystal hosts is described. A Negishi or Sonogashira cross‐coupling is exploited as a key step in the synthetic sequence. These synthetic procedures enable the synthesis of various ligands which can be easily adapted to different types of liquid crystals [e.g. 4‐pentyl‐4′‐cyanobiphenyl (5CB)]. For instance, a three‐step sequence (i.e. etherfication, Sonogashira–Hagihara cross‐coupling and Steglich‐esterfication) yields a dendritic ligand in 77 % overall yield starting from literature known compounds. The length of the ligand is important to stabilize the magnetic nanoparticles, and, therefore, the length of the ligand may be easily modified by this approach. Indeed, the established synthesis can readily tackle this issue.

Highlights

Suspensions of magnetic nanoparticles (MNPs) in liquid crystals (LCs) combine physical properties of both materials
We have shown the synthesis of variousmesogenic ligands with linear and dendritic structures using a Sonogashira cross-coupling reaction as a key step
Our approach represents a convenient and practical route which delivers themesogenic ligands in good overall yields, minimizes the apparative effort and allows different end and anchoring groups to be introduced, respectively. This is an important issue with respect to the functionalization of MNPs in LC hosts and the future application of the resulting hybrid materials

Summary

Introduction

Suspensions of magnetic nanoparticles (MNPs) in liquid crystals (LCs) combine physical properties of both materials. These properties of the hybrid materials include electro-optical, magneto-optical (static and dynamic) and magneto-rheological properties not observed for the individual components.[1] In 1970, Brochard and de Gennes suggested that doping of LCs with shape-anisotropic MNPs leads to an increase in magnetic susceptibility .[2] For the first time, the macroscopic collective behavior of ferromagnetic γ-Fe2O3 nanorods (500 × 70 nm) in N-(4-methoxy-benzylidene)-4-butylaniline (MBBA, Figure 1) was demonstrated experimentally by Amer et al in 1983.[3] In 2013, a ferromagnetic nematic phase with spontaneous magnetization was realized by Mertelj et al embedding ferromagnetic BaFe11.5Sc0.5O19 nanodiscs (70 × 5 nm) in 4-pentyl-4′-cyanobiphenyl (5CB, Figure 1).[4]. Lower particle concentrations minimize potential interactions and the number and size of aggregates.[3,4] In order to prevent particle aggregation and phase separation, specific (pro)mesogenic ligands have been introduced to functionalize the particle surface

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