Abstract
Although molecular crystals have long been considered to be intrinsically brittle, a study of molecular crystals that are capable of plastic or elastic bending upon applying mechanical stress recently attracted significant attention. Malleable molecular crystals often need to meet specific criteria regarding the intermolecular interaction patterns within the crystal structure. Accordingly, examples have been reported where one polymorph shows bending, while other polymorphs of the same compound exhibit fracturing upon exposure to mechanical force. Here, we have succeeded in preparing bent crystals of an intrinsically fragile polymorph. Methylated flufenamic acid (1) can form three different polymorphs, i.e., 1α, 1β, and 1γ, of which 1β is difficult to isolate. Under mechanical force, the crystals of 1α exhibit remarkable plastic deformation, while those of 1γ are readily broken. Similar to the mechanical properties, the emission properties of 1 differ depending on the polymorph, i.e., 1γ exhibits a shorter-wavelength emission maximum and much higher emission quantum yield than 1α. Remarkably, both the unbent and bent forms of the 1α crystals can undergo a phase transition to the 1γ phase upon exposure to ethyl acetate. In this manner, phase transitions of the mechanically bent crystals of polymorph 1α afforded bent crystals of the intrinsically fragile polymorph 1γ. These findings may lead to a potential post-modification method for the preparation of functional flexible materials with enhanced emission properties in order to expand their applications.