Intramolecular tension-driven self-recovering mechanochromism in organic microcrystals

Self-recovering mechanochromic fluorescent (MCF) crystalline materials, requiring no extra treatment for cyclic use, are highly significant for practical applications. However, exploring design principles for these materials remains challenging due to the fracture and disorder of microcrystals. In this study, we present a twisted molecule, Np-H, which exhibits varying rates of self-recovering MCF behavior in its polycrystalline forms, Np-H-1 and Np-H-2, thereby revealing for the first time the crucial effect of intramolecular tension. Single-crystal X-ray diffraction analysis and theoretical calculations reveal that Np-H-1, with smaller torsional angles, exhibits a greater increase in intramolecular tension and enhanced intrinsic vibrations in response to mechanical stimuli, resulting in an accelerated self-recovery rate as compared to Np-H-2. Moreover, the counterparts composed of peripheral methyl or interposed phenyl provide further evidence to the pivotal relationship between intramolecular tension and self-recovering MCF behavior. Notably, Np-H-1 microcrystals exhibit bright emission and can self-recover within 20 minutes after grinding with easy repeatability, enabling the application of chameleon painting inks and edge-ball detection in table tennis.