Intense mechanoluminescence in an organic donor-acceptor crystal: Grinding induced crystal-to-crystal phase transformation

Mechanochromic luminescent (MCL) materials with sensitive and reversible responses have attracted extensive attention in many fields. Herein, we report a donor-acceptor (D-A) phenothiazine derivative (CzS-CN), whose crystals show obvious hypochromatic shifts upon anisotropic grinding. In this work, three different crystals based on the phenothiazine derivative were prepared by solvent evaporation method. Due to the different solvents used during crystallization, these three crystals show different molecular arrangements and stacking methods, and thus they present different shapes, including needle-like (crystal-A), flake-like (crystal-B) and block-like (crystal-C). In addition, these three kinds of crystals showed mechanochromic luminescent property with a remarkable change in emission colors from yellow to yellow-green after grinding. To our excitement, these crystals can be partially recovered by heating at 120 °C for 5min or recrystallized in solvents. Detailed spectroscopic and structural analyses revealed that the hypochromatic shifted emission under grinding and the recovery of fluorescence under recrystallization or heating are attributed to the crystal-to-crystal transformation. On the basis of these crystals, reversible crystal-to-crystal phase transitions in response to external stimuli (such as heating and recrystallization) are achieved, which is ultimately attributed to the stacking mode and molecular structure. This work reports a kind of MCL material, which shows the phase transition from crystal to crystal, and reveals some important information to better understand the relationship between molecular packing and mechanochromic behaviors.