Uniaxially Oriented, Highly Ordered, Large Area Columnar Superstructures of Discotic Supramolecules using Magnetic Field and Surface Interactions

Abstract

The unique anisotropic physical properties of columnar discotic liquid crystals (DLCs) have attracted considerable interest for their potential applications as electronic devices, such as field effect transistors (FETs), [1,2] photovoltaics, [3] and light emitting diodes (LEDs). [4,5] The typical DLC molecule consists of a relatively rigid aromatic core with several flexible aliphatic side chains. Depending on their molecular structure and thermodynamic state, DLCs exist in various phases such as nematic, hexagonal columnar and rectangular columnar. [6] In the columnar phase, the intermolecular p-p orbital overlap between neighboring aromatic cores is significant, providing extremely anisotropic and order-dependent, quasi-one-dimensional, charge carrier mobility along the stacked cores. [7,8] For many practical applications, however, it is crucial to obtain a thin film of uniaxially oriented and highly ordered columnar superstructures of DLC molecules covering the macroscopic area of a substrate. [9] For example, the unidirectional 'edge-on' alignment, with the DLC columnar directors parallel to the substrate, is required for field effect transistors. On the other hand, the 'face-on' arrangement, with DLC columnar directors perpendicular to the substrate, is generally best for photovoltaic devices and light emitting diodes. [10] Recently, various alignment techniques utilizing temperature gradient, [11] concentration gradient, [12] pre-aligned layer, [13] electric field [14] and magnetic field [15] have been reported. However, more facile approaches to achieve uniaxially oriented long-range ordering of DLC columnar superstructures with clear identification of the ordered structures are still being actively explored.Here, we report a simple method to obtain a uniaxially oriented and highly ordered columnar superstructure of a DLC, using the alkyl-substituted metalloporphyrinoid, cobalt octa(n-decylthio)porphyrazine (CoS10), on substrates over a centimeter length scale. This method simultaneously utilizes the interactions of DLCs with both an easily accessible external magnetic field (ca. 1.0 T) and a functionalized surface of substrate. The substrate was functionalized with octadecyltrichlorosilane (OTS) to increase the affinity of the surface with the alkyl side chains of the discotics, resulting in an 'edge-on' arrangement of DLC columns. The detailed structures of the ordered and oriented columnar DLCs on substrates were investigated using the grazing incidence small angle X-ray scattering (GISAXS) technique, which is a very powerful tool to investigate the internal nanoscale structures in thin films. [16,17] The GISAXS measurements show that when a spin-coated CoS10 thin film on an OTS-functionalized silicon wafer is cooled from its isotropic phase to the liquid crystal phase in the presence of an external magnetic field of 1.07 T, it forms highly oriented 'edge-on' columnar superstructures with the columnar directors perpendicular to the applied magnetic field. The oriented superstructure shows a long range o...

Nanostructured Organic and Hybrid Solar Cells

“…When heated to an isotropic state, external stimuli such as electric [ 29 ] or magnetic [ 30 ] fi elds can induce long range orientation of the molecules which may be retained in the LC state. Furthermore, local defects in the supra-molecular organization can be overcome by a self-healing procedure which is highly desirable for increased device lifetimes.…”

Effect of Film Thickness on the Columnar Packing Structures of Discotic Supramolecules in Thin Films

“…Our previous study indicates that the rectangular columnar phase of CoS10 has p2 gg planar symmetry. [13] As the film thickness is increased, the peaks in the GISAXS patterns become broader along the azimuth angle and eventually split into multiple peaks. Considering the anisotropy of the GISAXS patterns depending on the relative orientation of the incident X-ray beam with respect to the applied magnetic field, it is clear that all the magnetically aligned CoS10 films with different film thicknesses (from 49 to 845 nm) form uniaxially oriented 'edge-on' columnar superstructures with their columnar directors perpendicular to the applied magnetic field.…”

“…Recently, we developed a new method to fabricate uniaxially aligned and highly ordered columnar DLCs on substrates over a macroscopic length scale, simultaneously utilizing an applied magnetic field and the interaction of DLC molecules with a functionalized substrate. [13] When a spin-coated cobalt octa(ndecylthio)porphyrazine (CoS10, Figure 1 a) thin film on an octadecyltrichlorosilane (OTS)-functionalized substrate is cooled from its isotropic phase to liquid crystalline phase in the presence of an external magnetic field (1.07 T), it forms an uniaxially oriented and highly ordered rectangular columnar structure, with the columnar directors perpendicular to the applied magnetic field and parallel to the substrate surface. This uniaxially oriented and highly ordered columnar packing structure of CoS10 in a thin supported film provides an excellent test bed to study the effects of film thickness on the orientation and ordering of columnar DLCs on substrates.…”

Self-assembled 1D Semiconductors: Liquid Crystalline Columnar Phase

“…Choi and co-workers designed discotic metallomesogens, i.e. cobalt octa(n-alkylthio) porphyrazine derivatives that can interact with an external magnetic field to uniaxially align columnar phase [139,140]. Planar alignment was achieved, by simultaneously utilizing an applied magnetic field (1.0 T) and the interaction of the discotic metallomesogen with an octadecyltrichlorosilane (OTS) functionalized substrate.…”