Solvent Polarity Influenced Polymorph Selection of Polar Aromatic Molecules

The crystallization and polymorph control of rigid polar molecules by solvent polarity was investigated for the case of 1,2,9,10,11-pentafluorote-tracene (F5TET), an extended pi-conjugated molecule with large in-plane dipole moment. Liquid assisted crystallization from polar solvents results in a dipole-parallel molecular arrangement with a crisscross packing. By contrast, nonpolar solvents lead to a more stable polymorph, which exhibits a dipole-antiparallel arrangement with a slip-stacked packing. Since for both polymorphs no solvents are incorporated in the crystal lattice, we attribute the different growth modes to a screening of the electrostatic forces by the solvent during nucleation. The results emphasize that solvent polarity must be considered when exploring the polymorphic landscape of molecular materials, which is particularly important for organic semiconductors that typically consist of pi-conjugated molecules with rather low solubility, thus hampering normal solution crystallization.