Mechanofluorochromic materials, which are dependent on changes in physical molecular packing modes, have attracted considerable interest over the past ten years. In this review, recent progress in the area of pure organic mechanofluorochromism is summarized, and majority of the reported organic mechanofluorochromic systems are discussed, along with their derived structure-property relationships. The existence of a structural relationship between aggregation-induced emission compounds and mechanofluorochromism is recognized based on our recent results, which considered aggregation-induced emission compounds as a well of mechanofluorochromic materials. The established structure-property relationship will guide researchers in identifying and synthesizing more mechanofluorochromic materials.

A fluorescent compound, 9,10-bis(2-(10-hexyl-10H-phenothiazin-3-yl)vinyl) anthracene, has been synthesized and studied. The results show that the compound possesses piezofluorochromic properties as well as aggregation-induced emission enhancement effect. The spectroscopic properties and morphological structures are reversibly exhibited upon pressing (or grinding) or annealing (or fuming). The piezofluorochromic nature is generated through phase transformation under the stimulus of external pressure. The reason for the phase transformation caused by external pressure is ascribed to the twisted conformation of the molecule which leads to poor solid molecular packing and weak interactions in the interfaces of lamellar layers confirmed by its single-crystal X-ray diffraction analysis.

Persistent luminescence from purely organic materials is basically triggered by light and electricity, which largely confines its practical applications. A purely organic AIEgen exhibits not only persistent photoluminescence, but also transient and persistent room-temperature mechanoluminescence. By simply turning on and off a UV lamp, tricolor emission switching between blue, white, and yellow was achieved. The data from single-crystal structure analysis and theoretical calculation suggest that mechanism of the observed persistent mechanoluminescence (pML) is correlated with the strong spin-orbit coupling of the bromine atom, as well as the formation of H-aggregates and restriction of intramolecular motions in noncentrosymmetric crystal structure. These results outline a fundamental principle for the development of new pML materials, providing an important step forward in expanding the application scope of persistent luminescence.

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Figure 4. PL spectra of TPE-An in water/THF mixtures. The insets depict: the changes in PL peak intensity (upper right) and its emission images in pure THF and 90 % water fraction of water/THF mixture under 365 nm UV illumination.