Abstract
Thermally responsive molecular crystals exhibiting programmable mechanical motions hold significant promise for applications in smart actuators, sensors, and drug delivery systems. However, achieving precise control over their phase transition thermodynamics remains a fundamental challenge. A series of isomorphic 5-fluorocytosine/fatty acid cocrystals is reported where the phase transition temperatures vary across an interval of 100 K with increasing alkyl chain. Two distinct transition pathways are unveiled: i) a cooperative single-crystal-to-single-crystal transition (II-III) accompanied by explosive mechanical motions, and ii) a reconstructive transition (I-III) following classical nucleation-growth mechanisms. The cooperative phase transition (II-III) induces remarkable expansion, with a striking +64.4% expansion along the layer stacking direction and a -16.9% contraction perpendicular to the (001) plane, leading to dynamic phenomena such as jumping, rotating, and splitting. Notably, the transition temperatures (T(t, II-III)) exhibit linear dependence on coformer chain length (from C10 to C18), a correlation attributed to interlayer hydrophobic interactions. This work provides a versatile approach for designing molecular crystals with tunable thermo-mechanical properties, offering new opportunities for advanced applications in dynamic functional materials.