Frozen Interphase Domain and Mechanism of the Snakelike Macroscopic Motion in a Dynamic Crystal Solvate

Dynamic molecular crystals capable of structural transformations have significant potential in functional materials. In this study, we investigate the phase transitions and mechanical behavior of 4-hydroxy-N′-(4-methylbenzylidene)benzohydrazide N-methylpyrrolidin-2-one solvate (HMBB·NMP). Thermal analysis and single-crystal X-ray diffraction reveal the existence of three solvate polymorphs, along with a single-crystal-to-single-crystal transition between 106 and 124 °C. This structural transformation induces macroscopic motion, characterized by a distinct snakelike deformation, as observed through polarized optical and hot-stage microscopy. The snakelike motion can be explained based on the cell/supercell relation between the two crystal phases and the molecule-to-molecule mapping of the transition. Nanoindentation studies demonstrate notable variations in mechanical properties between the two phases, with significant differences in hardness and elastic modulus. Partially transformed single-crystal samples, in which the two phases coexist along with an interphase region, can be obtained by quenching the transition at room temperature. This has allowed a remarkable characterization of the interphase region by space-resolved Raman spectroscopy. The results are consistent with a model in which the interphase region, about 7 μm long, is formed by domains filled with one or the other of the two structures. In moving across the interphase, it is the concentration of daughter phase domains and of parent phase domains that regularly increases and decreases, respectively.