Organic soft crystals exhibiting spontaneously reversible mechano-responsive luminescence

Mechano-responsive luminescence, or mechanochromic luminescence (MCL), is a type of luminescence that can be reversibly controlled by the addition of mechanical stimuli. Organic materials exhibiting MCL have been an ongoing area of development since the early 2000s, and the number of reports into such materials has been steadily increasing. While the majority of MCL systems rely on the brittle nature of organic crystalline solids, there is a growing interest in "flexible" organic crystals that exhibit mechanical bending or shape deformation owing to their elasticity/plasticity. Such non-destructive deformed crystals may exhibit a new type of MCL that can be controlled by the magnitude of the force stress. In this review, we describe MCL systems capable of the spontaneous recovery of changes in their luminescent properties in response to the loading/unloading of mechanical stress. We particularly focus on the MCL of flexible crystals based on the density gradient of molecular packing (i.e., elastic and plastic crystals) and an emerging system known as "superelastochromism,” which is based on spontaneously reversible crystal polymorphism. This emerging research area has the potential to play an important role in the promotion of next-generation soft crystals.