Controlling small molecule crystallization through molecular engineering of precursor phases

Both experimental and theoretical studies of crystallization traditionally focus on nucleation and growth from isotropic media such as melts and solutions. We discuss here crystallization from orientationally anisotropic precursor phases, a phenomenon that has been studied for macromolecules such as conjugated polymers but has remained largely unexplored for small molecules. Specifically, we discuss crystallization from the columnar hexagonal phase of organic semiconductor, HAT6. Through a combination of polarized optical microscopy (POM) and grazing incidence wide angle X-ray scattering (GIWAXS) we demonstrate that controlling molecular packing in the precursor liquid crystalline phase provides a route to grow textured crystals. We conclude by providing examples from other material systems, such as vapor-deposited glasses, where engineering orientational anisotropy in precursor phases has been used to determine crystallization outcomes. These results taken together establish that controlling molecular packing of the crystallization medium is a power route to control the final structure of small-molecule crystals.