Mechanistic study on the structure-property relationship of flexible organic crystals

Flexible crystals have attracted extensive attention for their potential as smart materials, such as flexible field-effect transistors, wearable devices and flexible displayers. Developing flexible materials with specific properties still remains challenging because the design standards for flexible organic crystals are still not well established. Herein, we reported two Schiff-based molecules, together with their polymorphs, which exhibit different photophysical behaviors and mechanical properties. It was found that, due to the distinguished molecular stacking arrangements, they exhibit diverse fluorescence properties. MN-I exhibits orange emission at 590 nm, while MN-II shows yellow emission at 579 nm. MO-I shows bright yellow-green fluorescence at 538 nm, while the emission spectrum of MO-II cannot be measured due to the presence of photochromism caused by its distorted molecular conformation. Furthermore, MN-I exhibits better elasticity than MN-II, while MO-I and MO-II are plastic and brittle, respectively. Detailed crystallographic analyses and energy framework calculations were performed to reveal the mechanisms, which suggest that the differences in the strength of intermolecular interactions and the diverse types of intermolecular interactions caused by the differences in molecular conformation lead to flexible properties.