Progress of Research on Organic/Organometallic Mechanoluminescent Materials

Functional materials with unique properties or specific functions, have been developed greatly in the areas of information, aerospace, energy, biology and so forth. Recently, organic/organometallic mechanoluminescence (ML) has attracted considerable attention owing to its unique optical properties induced by external stimulus, which demonstrates great potential as a candidate for sensing of impact, stress, tension or pressure, display and lighting, as well as imaging. In this review, the recent progress on organic/organometallic ML materials, the relationship between molecular structures and properties, their luminescent mechanisms, as well as the applications are summarized. Currently, the organic/organometallic ML systems mainly contain small molecules, including organometallic complexes and pure organic compounds, and polymers. In comparison to the design and preparation of materials, the progress of underlying mechanisms still remains ambiguous without a universal acknowledgement. Up to now, it is generally accepted that organic/organometallic ML materials should have non-centrosymmetric molecular structures, dipolar structures and piezoelectric properties. Because when the crystals are stimulated under grinding, rubbing, cutting, cleaving, shaking, scratching, compressing, or crushing, the center asymmetric molecular structures of organic/organometallic materials are broken, resulting in disruption of the crystal and electronic discharge at the crack surface, then emitting obvious light from the surface of solids state. This organic ML mechanism is mainly derived from the mechanism of inorganic ML. Mechanisms of organic/organometallic ML materials need to be verified by further experiments and theoretical study. As to the mechanism of mechanically activated luminescence of pure organic polymers, it was reported that when the material was stimulated by mechanical forces, the excited state went back to the ground state and emitted light. This review will focus on the recent progress of organic/organometallic mechanoluminescent materials including rare-earth organometallic complex, transition organometallic complex, pure organic small molecule materials and pure organic polymer materials, and their mechanisms during the past decades. Finally, the challenges and the outlook of the organic/organometallic ML have been discussed.