Crystalline solid retains memory of anisotropy in precursor liquid crystalline phase

Abstract

From chemical reactivity to mechanical strength, crystal texture plays a central role in determining the key properties of solids. In organic electronics, texture determines the charge carrier mobility of the active layer in devices such as organic field effect transistors (OFETs). Current methods of controlling the texture of thin films of organic crystals through solution processing are restricted by the stochastic nucleation that accompanies rapid solvent evaporation. We report here a facile method to create biaxially textured crystals of thin films of discotic organic semiconductor, HAT6 (2,3,6,7,10,11-Hexakis(hexyloxy)triphenylene). We first direct the self-assembly of the precursor discotic liquid crystalline (DLC) state; the columnar hexagonal (Col_H) phase of HAT6 is biaxially aligned in lithographically fabricated microchannels. Synchrotron X-ray scattering measurements reveal the direction of π-stacking is perpendicular to the channel walls. The biaxial alignment and the directionality of π-stacking in the liquid crystalline phase are transferred to the crystalline state through rapid cooling. Contrary to previous reports, we observe preservation of anisotropy upon crystallization of DLCs. As a significant fraction of organic semiconductors exhibit a liquid crystalline phase, we expect our method of creating textured crystals to be of broad interest.