Construction Of Dynamic Molecular Crystals And Their Stimuli-responsive Properties

Dynamic molecular crystals are the smart materials that can response to external stimuli(such as heat,force,light and solvent vapor,etc.)and output feedback signals by mechanical behaviors(such as rotation,jumping,bending,folding and crimping).They highly ordered molecular stacking structure endows them fast response speed,efficient energy transfer and accurate motion behavior,which makes them have a wide application prospect in the next generation of bionic robots,smart actuators,high-performance opto/electronic devices and so on.However,most crystals are often broken and damaged in the process of mechanical response due to the hard and fragile characteristics of crystals,which has become a technical bottleneck that restricts its rapid development.The emergence of flexible crystals is expected to solve this problem.However,the examples of the report are limited,which brings difficulties to the mechanism research.Moreover,the uncontrollablly mechanical response behavior further limits their functional applications.In order to construct a dynamic response crystal with flexibility and realize the controllable response behavior,this paper mainly does the following researches:Based on this background,a series of bissalicylaldehyde molecular crystals were studied.They exhibited elastic/plastic bending behavior under external forces.In this process,the layered structure was proved to play a key role in flexible response of crystals.Layered crystals can respond to external forces by changing the molecular distance or by slipping slightly between layers,thus avoiding the fragmentation caused by the failure of stress release in time.This result is of theoretical significance to design flexible crystal from bottom to top.In addition,according to the optical properties and the layered structures of crystal,it has been successfully used in flexible optical waveguide devices and dual output optical waveguide devices.According to the research of flexible crystals,cyanostilbene that can undergo[2+2] cycloaddition reaction,was introduced into the similar layered structure to construct the light-stimulus crystals with flexible dynamic responses.The two adjacent molecules in the crystal that can undergo [2+2] photocycoaddition are called?-dimer.They are the basic photoactive units.In the crystal,the intra-?-dimer and inter-?-dimer hierarchical architectures both affect the dynamic responsive behaviors,which provides abundant opportunities for the regualtions to this system.Firstly,it has been proved that the orientation of ?-dimers in crystals has a great influence on the deformation of crystals.In this work,the ?-dimer,as the photoactive units,will generate expansion force in the [2+2] cycloaddition reaction.In this single crystal system,the direction of tension is controlled by the orientation of ?-dimer in crystal.When the direction of tension is consistent with the direction of crystal axis,2D bending occurs.When the tension direction and the crystal axis direction present a certain Angle,the 3D spiral is realized.Micro-crystals arraies with unitary orientation were fricated by using a capillary bridge to control their dewetting.These micro-crystals materials can be bended at the designated location by controlling the irradiation position.Secondly,the effect of intra-?-dimer and inter-?-dimer hierarchical architectures on the dynamic response behavior of molecular crystals has been investigated.(Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(naphthalen-2-yl)acrylo-nitrile(BNA)can be crystallized to two polycrystals by self-assembly strategy.The two crystals have totally different intra-?-dimer and inter-?-dimer hierarchical architectures,which endows them with distinct reactivity,functions and photoresponsive properties.Here,two different types of solid-state [2+2] photocycloaddition reaction:(i)a typical olefin-olefin cycloaddition within the symmetric ?-dimers of BNA-? and(ii)a rare olefin-aromatic ring cycloaddition within the offset ?-dimers of BNA-? have been achieved.Secondly,the layered packing of ?-dimer in BNA-? exhibits outstanding flexibility upon UV irradiation.While,the reversible photocycloaddition/thermal cleavage process(below 100 °C)accompanies with unique fluorescence changes can be realized in crossed packing of ?-dimers in BNA-?.According to the different light response behavior of molecular crystal,micro-scale photoactuators and the light-writable anti-counterfeiting materials are prepared.Finally,the molecular arrangement within ?-dimer was determined to affect on the dynamic response behavior of crystals.It is found that the dynamic response behavior of crystals is dependent on the rate of [2+2] cycloaddition reaction.The crystals with rapid reaction rate exhibit obvious deformation,while the crystal with slow reaction rate has no obvious mechanical response.Interestingly,we found that the ?-dimer of cyanostilbene containing pyridine groups is hard to undergo [2+2]cycloaddition,although the molecular arrangement within ?-dimer satisfy the topochemistry rules.We speculated that the head-to-head arrangement would inhibit the activity of the cycloaddition reaction due to the polarity of the double bond in cyanostilbene molecules.In the co-crystals system,the molecules in ?-dimer were transformed to head-to-tail arrangement,resulting in the rapid cycloaddition reaction rate.Thus,the mechanical response behaviors of the crystal were obtained.In conclusion,this paper reveals that the layered structure is the crucial factor to construct flexible crystals.Based on this research,the cyanostilbene molecules that can undergo [2+2] cycloaddition have been used to construct similar layered crystals,which produced flexible deformation under light stimulus.The dynamic response behaviors of molecular crystals were successfully regulated by adjusting the intra-?-dimer or inter-?-dimer molecules packing.The functional applications of molecular crystals were explored based on their different dynamic response behaviors.