Exploring flexibility in molecular crystals: bending responses to light and mechanical stress

Crystalline materials are traditionally known for their brittleness when subjected to external stress. However, recent advancements have enabled the creation of flexible crystals through specific molecular architectures. These innovative materials can deform in response to light or mechanical stress, making them promising candidates for smart materials, sensors, actuators, and optoelectronic devices. While considerable research has been conducted on the effects of individual external stresses, the investigation of flexible crystals that respond to multiple stimuli and their bending mechanisms remains to be improved. This review aims to elucidate the mechanisms that govern the flexibility of crystals responsive to both light and mechanical stimuli. We will analyze the mechanical properties related to 1D or 2D assembly, [2 + 2] or [4 + 4] photodimerization, and molecular isomerization and provide insights on designing new molecular structures to enhance the flexibility of crystals. Additionally, we will address current challenges in the field and propose future research directions to advance the development of flexible molecular crystals.