Electric‐Field‐Assisted Alignment of Supramolecular Fibers

A molecular wire consisting of a metal/molecule/metal junction can be regarded as the basic building block for future nanoelectronics applications. Alongside the great effort expended in the last ten years on the use of single molecules as electroactive components, there is also a growing interest centered on the use of supramolecular architectures as electroactive species to bridge metallic electrodes. The supramolecular approach can enhance the mechanical and electronic properties of the wire, which should improve the performance of electronic devices. One major challenge in the study of charge transfer across organic molecules is achieving reproducible attachment between metallic electrodes. Although different electrode pairs have been employed, including break junctions, lithographically tailored nanoelectrodes, and a solid substrate and a conductive tip of an atomic force microscope or a mercury drop, new, scalable routes to the controlled incorporation of nanometer-scale objects in the gap between nanoelectrodes are required. The manipulation and alignment of an anisotropic object using dielectrophoretic forces in an electric field has been successfully accomplished with a variety of different structures including metal and semiconducting nanoparticles and nanowires, DNA molecules, carbon nanotubes, block copolymers, ZnO–organic complexes, and dendron rod–coil ribbons. This has recently led, for example, to improved emission properties of single conjugated polymer molecules. The possibility of applying this technique to supramolecularly engineered nanostructures is thus of major interest in view of their reversible self-assembling properties under external stimuli such as temperature and chemical environment. We provide here the first direct quantitative determination of the electric-field-assisted alignment of single organic supramolecular fibers self-assembled at a surface. We have chosen a gel-forming functionalized 1,3,5-triamide cis,cis-cyclohexane derivative (cyclohexane trisamide gelator (CTG), Fig. 1a) that is known to self-assemble into supramolecular fibers in aqueous solution through the formation of hydrogen bonds. Due to its wider applicability for electronic applications, we present here attempts to form similar fibers in an organic solvent. Fibers were deposited from solution onto two gold electrodes arranged in a source–drain geometry with micrometer-scale separation. During deposition, a DC voltage was applied between the two electrodes and the system was cooled below its sol–gel transition temperature (Tsol–gel). In the gel, the three intermolecular hydrogen bonds among the amide moieties of CTGs are both parallel to one another and perpendicular to the plane of the cyclohexane ring (see Fig. 1a inset), endowing strong, self-complementary, and uniaxial intermolecular interactions that are necessary to enforce quasi-1D self-assembly. Since each hydrogen bond has a dipolar nature, the identical orientation of at least two of the three H-bonded networks provides the columnar architecture with a large dipole moment, making the CTG fibers excellent candidates to study the effect of applied electric fields. Micrometer-length CTG fibers were observed preferentially on the top of the negatively charged electrode and tended to align with the local electric field, which has been calculated for our system geometry using finite element analysis. Given C O M M U N IC A TI O N S