Renaissance of Organic Triboluminescent Materials.

Solid-state luminescence of organic dyes is an elusive frontier, and understanding and designing solid-state stimuli-responsive materials is not trivial. "Mechanoluminescence" (ML) or "triboluminescence" (TL), which is associated with fracture or force-initiated luminescence from a material, is currently attracting new interest. Fracturing the surfaces of organic crystals ordered in noncentrosymmetric space groups can electronically excite the surface and neighboring molecules through piezo- or pyroelectric effects, and this can result in luminescence when the molecules relax back to their ground states. The combined duration of these two consecutive phenomena leads to force-generated luminescence or TL. Although TL has been known for a very long time, examples of TL-active materials are scarce, but are increasing as synthetic and characterization procedures develop. The question is now whether the relatively rare phenomenon of TL needs to be reevaluated to obtain a broader understanding of the subject.