Photoswitchable Flexible-Brittle Transition with Reversible Cracking and Healing in a Photochromic Crystal

Soft materials with ordered structures have attracted much attention due to their potential applications, such as flexible electronics, optical devices, and sensor materials. In particular, organic crystals that exhibit elastic and plastic deformation, as well as self-healing capability, are attractive owing to their shape adaptability and low cost. Although reversible photoinduced modulation of mechanical properties has been reported, organic crystals capable of repeated fracture and subsequent crack healing remain rare. Here, we report a photoresponsive organic crystal that permits reversible photoswitching between flexible and brittle states, as well as photoinduced cracking and healing. The developed crystals exhibit one-directional (1D) elastic and plastic deformation, along with photoinduced cracking and healing upon UV and visible light irradiation. The mechanisms of the flexibility and reversible crack recovery phenomena were made clear by X-ray crystallographic analysis and various microscope techniques. Furthermore, the durability and mechanical properties of the flexible crystals during alternating irradiation of UV and visible light are demonstrated. This research provides new insights into flexible organic crystals and opens up possibilities for using photoreactive crystalline materials for future applications in self-healing materials and flexible electronics.