Mechanical-Fluorescent Bifunctional Smart Crystals: Hydrogen-Bond Engineering for Ultra-Flexible Response and Programmable Thermochromic Switching

This study successfully synthesized four organic crystalline materials exhibiting dual-functional responses of mechanical behavior and thermochromic fluorescent by introducing halogen atoms and hydrazide structures into CIM crystals via crystal engineering strategies. The LHIH crystals exhibit excellent reversible elastic bending, with a bending strain angle reaching up to 180°. LHIH's mechanical flexibility stems from the N–H···N interactions in the hydrazone structure, which form a "bridge-like" network of molecules. The resulting slip interfaces and interlocking structure effectively dissipate stress and prevent fracture. At the same time, the CIM, LCIM, BCIM, and BHBH crystals exhibit significant unidirectional fluorescence color switching (e.g., from orange-yellow to dark orange) under thermal stimulation, which is attributed to structural changes caused by temperature-induced lattice distortion. In contrast, the LHIH crystals undergo a unique ternary fluorescence color switching (yellow → dark green → grass green) at a thermal phase transition temperature of 122 °C. This study provides a strategy for the development of multistimuli responsive smart crystals and demonstrates their potential applications in flexible optical sensors, optoelectronic devices, and information encryption.