In the preceding chapters, the mechanofluorochromism of polymers, dye-doped polymers, metal complexes and some organic compounds has been discussed. It is already known that the fluorescent properties of molecules in the solid state depend mainly on the molecular arrangement, conformational flexibility and intermolecular interactions, such as monomer-excimer transformation, dipole-dipole interaction or conformational planarization, in which mutual inter-conversions, e.g., crystal-crystal, crystal-amorphous or crystal-liquid crystal may be involved. Any modification in the molecular packing and conformation of the fluorophore would affect the HOMO-LUMO energy levels and alter the fluorescent properties. Thus, controlling the mode of molecular packing (i.e., the aggregation states) to achieve the dynamic control of highly efficient and reversible solid-state fluorescence is more attractive for both fundamental research and practical applications because of its advantages of low pressure demand and good reversibility. However, as described in the previous chapters, mechanochromic fluorescent materials that are dependent on changes in physical molecular packing modes are still rare. This rarity may be attributed to two major issues.¹  First, predicting and designing materials that exhibit polymorphism is difficult. Each identified compound reported seems to be an isolated event, which makes the identification of a general characteristic difficult. Secondly, the fluorescence efficiency of organic fluorescent materials often becomes very weak when they are in the solid state because of the aggregation-caused quenching (ACQ) effect. Consequently, observing the mechanochromic fluorescent phenomenon becomes quite difficult.