Large‐Area, Highly Aligned Cylindrical Perfluorinated Supramolecular Dendrimers Using Magnetic Fields

Abstract

The self-assembly of supramolecular dendrimers allows the rapid construction of nanometer-sized structures with regularly ordered features that depend on the shape of the molecules and the relative strength of the intra-and intermolecular interactions. [1][2][3][4][5][6] In particular, the cylindrical phases of supramolecular dendrimers have recently attracted considerable attention because of their potential applications as optoelectronic materials, [7][8][9] selective membranes, [10][11][12][13] and in the creation of novel nanopatterning templates. [14,15] It is commonly accepted that most flat, tapered dendrimers self-assemble into cylindrical mesophases with nanometer-scale building blocks.[2] As in block-copolymer self-assemblies, [16][17][18][19] controlling the orientation of cylindrical supramolecular dendrimers over a large area is essential to realizing their applications. However, the lack of long-range order and the orientation of the cylinders represents a critical limitation to realizing an ordered nanomaterial, although cylinders from supramolecular self-assemblies have many advantages, including the ability to achieve very small feature sizes (less than 10 nm), flexibility in chemical functionality, and fast stabilization of the molecular ordering and nanostructure due to the reversible and non-covalent interactions of supramolecular molecules.[20]Previously, there have been reports that the orientation of the cylinders can be controlled by surface anchoring, and a high degree of orientation was indicated by transmission electron microscopy (TEM). [2][3][4]13,21] Here we report on a dramatic improvement in the degree of control and selectivity in the orientation of fan-shaped supramolecular cylinders over large areas; this has been achieved by applied magnetic fields. We show that these cylindrical domains become aligned perpendicular to the magnetic field direction, with the aromatic parts of the cylinders orienting themselves parallel to the magnetic-field vector. This method can be used to create a grain size in the plane of the film that is much larger than that which can be achieved using surface alignment techniques. [2][3][4] Our strategy relies on the magnetic-field-induced alignment of aromatic molecular materials due to the diamagnetic properties of aromatic groups.[22] Calamitic liquid crystals containing aromatic groups have a positive diamagnetic anisotropy, whereas discotic liquid crystals have a negative diamagnetic anisotropy. This is due to the component of the diamagnetic susceptibility perpendicular to a benzene ring being greater than the in-plane component.[23] Thus, it is expected that fanshaped dendrimers containing an aromatic system will affect the cylindrical alignment in the presence of magnetic fields (Scheme 1). Supramolecular dendrimers can interact with a magnetic field through the diamagnetic anisotropy of its repeating units. The energy that a repeating unit gains through the interaction with a magnetic field is dependent on the orientation of the unit relative to...

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