Partially Controlling Molecular Packing to Achieve Off-On Mechanochromism through Ingenious Molecular Design

The control of molecular packing at the molecular design stage is significant but challenging. Herein, a strategy is provided to regulate the molecular packing of pyrene derivatives based on steric and electronic effects. Two tertiary butyl groups are introduced as isolation groups to enlarge the intermolecular distance to accommodate the tendency of monomer-like packing. By increasing the electron-withdrawing ability of substituents, tunable and tighter packing motifs can be realized owing to the decreasing electron density of the pyrene ring. Accordingly, coupled with quenched emissions in monomer-like packing and bright emissions in dimer-like packing, compound 4-(2,7-di-tert-butylpyren-4-yl)benzaldehyde (PPCHO) shows an off-on mechanochromism with high performance (contrast ratio reaching 400 compared with the normal figure of <100), demonstrating the power of molecular design. Encryption based on the unique off-on mechanochromism characteristic of PPCHO demonstrates good sensitivity and reversibility.