Design,Fabrications And Properties Of Cyanostilbene-Based Dynamic Molecular Crystals

Organic molecular crystals are a class of macroscopic solid materials formed by electroneutral organic molecules through weak noncovalent supramolecular interactions.Depending on the molecular shape,strength,and direction of intermolecular interactions amongst other factors,organic molecules can form crystalline materials with diverse morphologies and physical and chemical properties.Therefore,organic molecular crystals have gained significant attention from scholars.However,as research on molecular crystals has deepened,the disadvantages of these crystals have become more apparent.For instance,the morphology of molecular crystals is usually uncomplicated,comprising mainly of 1D fibers or 2D sheets.Additionally,crystalline materials are often unresponsive to external stimuli and brittle when subjected to external forces.The morphological manipulation of crystals is also typically based on direct contact,which can cause irreparable damage to the crystals.Consequently,the emergence of flexible,dynamically responsive crystals is anticipated to solve the problems.Dynamic molecular crystals are a new class of intelligent materials that can sense external stimuli and output mechanical responses as feedback signals.The highly ordered molecular stacking structure inherent in the crystal can give it high efficiency in energy conversion.Dynamic molecular crystals show excellent potential for use in new bionic intelligent machines and high-performance optoelectronic devices.Nonetheless,there are very limited reports on dynamic molecular crystals,and there is a complete lack of effective theoretical systems to guide their design and synthesis.This dissertation outlines a series of studies based on cyanostilbene molecules aimed at constructing a more diverse range of flexible dynamic response crystals and achieving controlled regulation of their mechanical response behavior.Chapter 2 presents the successful construction of molecular crystals with layered structures,CS-H and CS-O,by the rational design of molecules using orthogonal relationships between intermolecular complementary hydrogen bonds andπ-πinteractions.Helices spontaneously formed in solution crystallization or distorted to form crystal helices under UV light illumination.Crystal structure studies clarified the key role of weak interlayer interactions in layered crystals for adaptive crystal deformations.Weaker interlayer interactions induced deformation by slight interlayer slippage or dislocation upon superlattice formation or photoreaction,achieving helical crystals with complex morphology.Additionally,the interlayer interaction effectively affected the molecular assembly pattern within the layers,which significantly influenced the crystal’s responsiveness to UV light.Under UV irradiation,theπ-πdimer of closely arranged CS-H molecules in the crystal underwent[2+2]cyclization photoreaction,inducing lattice changes and providing the driving force for crystal distortion.The findings provide insights into the spontaneous formation of crystal helices and the dynamic design and synthesis of photomechanical response crystals.In Chapter 3,we addressed the crystal brittleness issue by constructing an example of CS-A molecule with amide hydrogen bonding.The aim was to enhance the mechanical elasticity of the crystal through intermolecular hydrogen bonding and break the inherent concept of hydrogen bonding in the design of elastic crystals.Flexible molecular crystals with 2D elasticity were successfully obtained.Structural studies revealed that amide-amide hydrogen bonding and multiple intermolecular interactions induced symmetry-breaking stacking of cyanostilbene molecules,forming supramolecular hydrogen-bonded columnar structures that exhibited chiral 2₁ helices.This chiral helical column structure played a key role in the elastic bending of the crystal.Moreover,brick-like stacking was formed between the 2₁-helix columns,and a honeycomb-like interaction network was generated,ensuring no irreversible plastic slippages and the integrity of the crystal during bending.Our approach inspires the design and construction of elastic molecular crystal materials in the future.In Chapter 4,we aimed to construct a molecular crystal with both mechanical elasticity and photomechanical bending to target the problems of brittleness and poor photoresponsiveness of molecular crystals.Inspired by the work in Chapter 3,we innovatively introduced the homochiral effect of molecular assembly.We designed a V-shaped CS molecule with chirality and used its homochiral assembly effect to achieve the objective of this chapter.First,for crystal elasticity,the homochirality promoted the formation of hydrogen-bonded molecular chains that possessed sufficient freedom of molecular motion.Second,for photo-mechanical bending,the homochiral assembly formed a tile-window structure with a large free volume,thereby effectively promoting the Z-E isomerization of cyanostilbenes.Additionally,the hydrogen bonds oriented along the long axis of the crystal played a"gearing"role in force transmission during crystal deformation.We investigated Rac-CS crystals to further support homochiral effects.Our findings provide a promising strategy for the design of organic crystals with multiple stimuli-responses.In summary,this thesis first reveals the critical role of the interlayer structure of molecular assemblies for the crystal’s adaptive denaturation ability and photoresponsiveness.Hydrogen-bonded molecular crystals with excellent elasticity were successfully constructed by modulating hydrogen-bond assembly.Finally,by chiral expansion of the molecular structure,the homochiral effect was innovatively used to construct hydrogen-bonded molecular crystals with both elasticity and photoresponsivenes.