Synthesis and characterization of new H-shaped triphenylene discotic room-temperature liquid crystal tetramers by a copper-free click reaction

Library Number:
Kan Zhang, et al
New J. Chem
Content Type:
Journal Citations
2018   Volume:   42   Page(s):   272-280
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Among various soft materials, discotic liquid crystals (DLCs) have received significant attention ever since they have been discovered by Chandrasekhar in 1977.1–5 In the LC phase, the DLC molecules can self-assemble into one-dimensional (1D) columns as a charge or energy migration channel and then further self-organize into two-dimensional (2D) lattice forms in their bulk form.6–11 Although the degree of order in the molecular alignment of DLCs is critical to liquid crystal devices, the discotic molecules possess full fluctuational, translational, and rotational freedom in their molecular alignment, and this significantly affects the mobility within the columns of DLCs.12,13 Therefore, efforts have been made to enhance the degree of order in the columnar mesophase. More generally, control over the degree of order of the molecular alignment in DLCs is usually achieved by modifying the molecular structure and/or relying on various anchoring effects such as π–π interactions,14–16 hydrogen bonding,17–21 metal complexation,22–24 donor–acceptor effects25,26 and microphase separation effects containing fluorophilic–fluorophobic effects,27–31 hydrophilic–hydrophobic effects,32–35 and other intermolecular interactions.36 Evidently, the self-assembly behavior in DLCs depends significantly on the molecular structure variations, and use of different linkages among the rigid cores to construct DLC dimers or oligomers is an efficient method.37–40

In the reported DLC oligomer series, two types of materials have been prepared; one is a linear-shaped DLC in which the cores are connected one by one via a soft or rigid linkage. The other is a star-shaped DLC in which the cores are linked to a central unit. The units can be a single carbon atom,41,42 benzene,43,44 triazine,45–47 siloxane,48 or another aromatic core. The development of novel shaped DLC molecules should open new perspectives for their use as sophisticated functional materials. Therefore, we have designed and synthesised new H-shaped triphenylene DLC tetramers in which two triphenylene cores are initially connected by carbon–carbon triple bonds. Then, two of the triphenylene dimers are connected by a soft alkyl chain or a rigid biphenyl group linkage via a copper-free [3+2] cycloaddition reaction known as the click reaction.

We know that the click chemistry has been widely investigated and applied in almost every field of synthetic chemistry over the past fifteen years,49–51 and the copper(I)-catalyzed azide-terminal alkyne cycloaddition (CuAAC) is an archetypical example of the five common types of click chemistry.52–58 However, residual trace amounts of copper irons originating from the CuAAC are toxic to living cells and can affect the conduction properties of photo-electronic materials as they can form lattice defects.59–61 Our group has developed the click reaction method of strain-promoted azide–alkyne coupling (SPAAC), and the cycloheptyne substrate has been replaced by linear carbon–carbon triple bonds bearing electron-withdrawing groups.62 As part of an ongoing programme aimed at enhancing the degree of order and charge migration rate, new H-shaped DLCs were achieved under these reaction conditions via a double click reaction with moderate yields.

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