Mechanochromism and mechanical deformation of organic crystals: tunable packing structure for achieving luminescence reversibility and elasticity

Organic luminescent crystals with mechanochromic and mechanical deformation properties are fascinating owing to their vast application prospects. In this study, two types of cyanostyrene derivatives, FPPA and FPLA, and their polymorphs were prepared to systematically examine the relation between mechano-responsive properties and crystal packing structures. Crystallographic and spectroscopic analyses and theoretical simulations revealed that the luminescence switching triggered by mechanical grinding originates from the crystal-to-crystal transition between the two polymorphs. Anisotropic shearing induces the molecules in the crystal with loose packing to reorganize into a more compact packing structure to counteract mechanical grinding disturbances. A thermo-induced phase transition could be used to recover the original luminescence. In addition, the needlelike crystals of Pr-I and Pentyl-I with interlocked and corrugated structures display outstanding elasticity when subjected to localized external forces. Reversible and fast luminescence switching between the two polymorphs of FPLA with green and cyan emissions was successfully applied in an information storage and encryption system.