We studied the orientation of the hexagonal columnar mesophase formed by self-organization of a perfluorinated supramolecular dendrimer containing a carboxyl (-COOH) headgroup and three perfluorinated (-CF 3) tails at surfaces modified with self-assembled monolayers (SAMs). The SAMmodified surfaces studied were composed of an Au(111) substrate modified with one of five types of SAM. The SAM molecules used all had an -SH headgroup, but different terminal groups (-CF 3, -CH3, and -OH) and different spacer chain lengths. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and contact angle microscopy results revealed that the lattice parameters and structure of the perfluorinated supramolecular dendrimer are retained, but the orientation of the columns is strongly affected by the characteristics of the SAM surface. The supramolecular columns took on a planar alignment on the -CF 3 and -OH terminated SAM surfaces, but exhibited a perpendicular orientation on the -CH3 terminated SAM surface. These variations in column alignment can be attributed to the types of molecular interactions between the terminal groups of the SAM molecules and the perfluorinated core/tails of the supramolecular columns. However, the surface morphology and orientation was not affected by changing the space chain length of the SAM molecules used.
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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.