Organic crystals, possessing both flexibility and stimuli-responsive properties, are attractive candidates as smart materials, namely sensors, actuators, optoelectronic devices, information storage, medical applications, etc. They are emerging as a new research direction in materials science, namely, crystal adaptronics. For a long time, it has been generally considered that crystals are inclined to crack upon external force, which limits their application as elements. Recently, however, more and more crystals that display excellent deformation have been reported, which sparks interest in their potential applications as stimuli-responsive flexible smart materials. This comprehensive review summarizes these remarkably flexible crystals with stimuli-responsive (light, thermal, electrical and multiple) properties reported to date, delves into their response mechanisms and structure–property relationships, and discusses the design strategies for endowing flexible organic crystals with stimuli-responsive properties. Finally, the challenges and future outlook in this field are discussed.

Growing high-quality crystals with ideal properties is of great importance. The morphology of crystal is one key factor reflecting product quality, as it can affect the performance of products and downstream operations. In this work, the current state of crystal morphology modification is reviewed from different perspectives. First, the most widely used crystal growth models are discussed. Then, a variety of crystal morphology control methods, which include adjustment of crystallization operation parameters, addition of foreign molecules, change of different solvents, membrane assistance, the addition of external physical fields and the use of ball milling are summarized. As for applications, the control of crystal morphology has application potential in pharmaceutical and material fields, for example, energetic materials and semiconductor materials. Finally, the future development direction of crystal morphology regulation is discussed.

Nucleation of crystals from solution is fundamental to many natural and industrial processes. In this work, the molecular mechanism of conformational polymorphism nucleation and the links between the molecular conformation in solutions and in crystals were investigated in detail by using 5-nitrofurazone as the model compound. Different polymorphs were prepared, and the conformations in solutions obtained by dissolving different polymorphs were analysed and compared. The solutions of 5-nitrofurazone were proven to contain multiple conformers through quantum chemical computation, Raman spectra analysis, 2D nuclear Overhauser effect spectroscopy spectra analysis and molecular dynamics simulation. The conformational evolution and desolvation path was illustrated according to the ¹H NMR spectra of solutions with different concentrations. Finally, based on all the above analysis, the molecular conformational evolution path during nucleation of 5-nitrofurazone was illustrated. The results presented in this work shed a new light on the molecular mechanism of conformational polymorphism nucleation in solution.

The crystal habit is one of the most important properties for active pharmaceutical ingredients (APIs), which can significantly affect their downstream processing. In this study, to better understand the role of solvent in crystal habit evolution, the crystal interface–solvent interactions and their effect on the crystal habit were investigated using cefradine as a model compound. Molecular dynamics simulation was applied to calculate the interactions between the solute (cefradine) and solvent (organic/water mixture) molecules occurring at the crystal/solution interface. It turned out that the interactions between the solvent and (101) face are diminished in the mixed solvent of methanol, ethanol and acetonitrile, thereby lowering the solvation barrier to crystal growth. In contrast, for the solvent of DMA, DMF and DMSO, the solute–solvent interaction is stronger at the (011) and (01-1) faces, which lowers the adsorption rate of solute molecules onto these faces and consequently inhibits the crystal growth along the a-axis. The developed knowledge based on the crystal interface–solvent interactions can form a basis for further optimization of the crystal habit of cefradine.