Origin of Solubility Behavior of Polar π-Conjugated Crystals in Mixed Solvent Systems

We investigate the origins of solubility behavior of polar p-conjugated crystals in mixed solvent systems and the influence of nonpolar aromatic solvents on solubility and crystal growth characteristics. In solvent mixtures of polar nonaromatic methanol and nonpolar aromatic toluene (or o-xylene), solubility of three analogous polar crystals, consisting of a mu-conjugated phenyltriene bridge between electron donor and acceptor groups, shows a maximum at the midpoint of composition. The maximal solubility mole fraction in such mixed solvent systems can be as high as 67 times as that in a single solvent. For comparison, a mixed solvent system consisting of only polar nonaromatic solvents (methanol and acetonitrile) is also examined. Main intermolecular interactions between the solute and solvent molecules in mixed solvent systems, related to the solubility behavior, are experimentally provided by the UVvis absorption spectroscopy and H-1 NMR measurements. In the presence of aromatic solvents, additional strong interactions between the solvent and pi-conjugated solute molecules, such as face-to-face and edge-to-face pi-pi interactions including p center dot center dot center dot HOAr hydrogen bonds are clearly observed. The aromatic characteristics of solvents also importantly influence the morphology and polymorphism of single crystals grown by solution crystal-growth methods.