Renaissance of Organic Triboluminescent Materials.

Solid-state luminescence of organic dyes is an elusive frontier. Thus, understanding and designing solid-state stimuli responsive materials is not trivial, and is being continually pursued in recent times. 'Mechanoluminescence' (ML) or 'triboluminescence' (TL), which is associated with fracture or force initiated luminescence from a material, is finding new interests in very recent times. Fracturing the surfaces of organic crystals ordered in non-centrosymmetric space groups, can electronically excite the surface and neighboring molecules via piezo- or pyro- electric effects. Such electronic excitation of organic dye molecules can result in observation of luminescence when the molecules relax back to their ground-states. Thus, in the combined duration of these two consecutive phenomena, we observe force generated luminescence or triboluminescence (TL). Although TL is known for a very long time, examples of TL active materials are scarce. Progress in modern synthetic tools, characterization and understanding new corners of solid-state luminescence are increasing the frequency of our encounter with TL active materials. At this moment, we need to ask the question if we can re-evaluate the relatively rare phenomenon of TL and reach a broader understanding of this subject. The silent renaissance of organic triboluminescent materials might have significant impact in the frontiers of solid-state emissive organic materials.