Control of Polymorphism and Alignment in Photochromic Salicylideneaniline Crystals Grown by Directional Crystallization

Molecular crystals hold potential applications in soft and flexible devices because of their periodic arrangements, flexibility to design, lightweight, and tunable supramolecular connections. Their rich polymorphism offers the additional advantage that the properties of different crystal forms can be studied for a given molecular structure. This study focuses on the directional crystallization of salicylideneaniline to control its polymorphic α and β forms. We discovered that upon directional crystallization from nonground β crystals, the metastable α form was yielded with well-aligned crystal habits. In contrast, by grinding β crystals before directional crystallization, the stable β form was surprisingly produced. These polymorphs were determined using X-ray diffraction, and their differences in photochromic behaviors have been investigated. This study confirms directional crystallization as a novel approach to polymorphic control. It provides a promising method for fabricating well-aligned, shape-specific, and desired polymorphic crystalline materials suitable for device fabrication.