Small Angle Neutron Scattering Studies of Discotic Liquid Crystals under External Magnetic Fields

The columnar discotic liquid crystals (DLCs), which consist of disc-shaped molecules with an aromatic core and side chains, show one dimensional conductivity along overlapping π orbitals of stacked cores. Therefore, DLCs have been investigated for applications such as FETs, photovoltaics, and LEDs. For practical applications, uniaxial alignment of the columnar DLC in a large scale is crucial. Various alignment techniques for columnar DLCs have investigated during two decades. In general, the techniques utilize interfacial interactions, shear forces, external fields, temperature and concentration gradient, most of which are coupled with substrate effects. Magnetic alignment is one of the most efficient methods to produce ordered molecular materials in a bulk scale. The free energy of magnetic alignment for DLCs can be expressed as <Galign = -1/2 ΔX(H·n)2, where ΔX is the anisotropy of magnetic susceptibility, H is an applied magnetic field, and n is the columnar director. Due to the large diamagnetic anisotropy of the aromatic core (ΔX < 0), the columnar DLCs align perpendicular to the applied magnetic field