Chiral Phase Amplification of 1,1'-Binaphthyl Thin Films by Thermal Treatment ​

1,1'-Binaphthyl is a model molecule for axial chirality with the possibility to interchange between the two enantiomers by changing its molecular conformation. The molecule crystallizes in three distinct crystalline phases: the racemic phase, the chiral phase and the transient chiral phase. This work studies phase transition mechanisms from the racemic to the chiral phase in thin films. Thin films are prepared by solution processing using drop casting and spin coating, with additional post-deposition treatment by solvent vapour annealing. The structure of thin films is investigated in situ as a function of temperature using hot stage optical microscopy to study thin film morphological changes and X-ray diffraction to determine the corresponding crystallographic changes during the phase transition. Depending on the initial thin film structure three distinct transition pathways to chiral phase pure thin films are observed: i) a solid-solid transition in drop casted thin films around 120 °C driven by chiral seed crystals, ii) a liquid-solid transition in solvent vapor-annealed films near 146 °C due to the melting of the racemic phase followed by the recrystallization into the chiral phase and iii) a sublimation assisted purification in spin-coated thin films around 120 °C, where the racemic crystallites sublime and the chiral phase remains.  While the first two transition mechanisms also occur in bulk material, the sublimation-driven pathway is observed exclusively in thin films. Generally, the initial thin film morphology and the crystallographic properties determine which phase transition pathways occur.