Strategies For Manipulating Solid State [2+2] Cycloaddition And Photoactuation Properties Of Molecular Crystals

In recent years,light-stimulis responsive mechanical molecular crystals have received much attention.As the materials are irradiated by UV light,they typically exhibit macroscopic mechanical movements,such as bending,twisting,cracking,and salient behaviors,having potential applications in actuators,artifial muscles,flexible electronics,and other fields.The macroscopic mechanical motions are mainly from the accumulation of strain caused by photo-induced molecular or atomic displacement in the crystal lattice.Photothermal/photochemical reactions are the driving forces to induced photomechanical motions.Generally,photochemical reactions in photoactuation materials mainly involve E-Z isomerization,electrocyclization,[2+2]cycloaddition,and[4+4]cycloaddition.Among them,the molecular crystals materials driven by photo-induced[2+2]cycloaddition have fast response,and their ordered and clear structure can be characterized by the single crystal XRD technique,which is helpful to understand intrinsic reacton mechanism.However,the study on molecular crystals driven by[2+2]cycloaddition and the photomechanical behaviors is still in its infancy,and the factors affecting the macroscopic mechanical properties remain to be further explored.（1）The three group of chalcone derivatives and their analogues involving halogen atoms（X=F,Cl,Br）have been synthesized.Firstly,the nearly planar acyclic chalcone derivatives were inclined to undergo photo-induced stereospecific[2+2]cycloaddition,which triggered the crystals to exhibit macroscopic motions of bending or cracking.In particular,the single-crystal-to-single-crystal transformation happened upon UV irradiation of their crystals,which was helpful for the understanding photomechanical effects.The group of cyclic 3,4-dihydronaphthalen-based chalcone analogues possess a more twisted conformation,and they tend to take place trans-cis isomerization.No photomechanical effect was observed for the crystals of the cyclic chalcone analogues due to the lower isomerization rate.The twist degree of chroman-based chalcone analogues molecules was in between of the first two,[2+2]cycloaddition and trans-cis isomerization simultaneously underwent in their crystals.Photo-induced bending and twisting were observed for the crystals of chroman-based chalcone analogues.Therefore,the differences in molecular dihedral angles in?α,β-unsaturated ketones were responsible for their photochemical characters and in turn to tune the photomechanical effects.In this work,a bridge between the molecular structures and solid-state photochemical reactions triggered photomechanical crystals is built.（2）The cyclic chalcone analogues of（E）-2-（2,4-dichlorobenzylidene）-2,3-dihydro-1H-inden-1-one（BIO）,（E）-2-（naphthalen-2-ylmethylene）-2,3-dihydro-1H-inden-1-one（NIO）,（Z）-2-（2,4-dichlorobenzylidene）benzofuran-3（2H）-one（BFO）,and（Z）-2-（2,4-dichlorobenzylidene）benzo[b]thiophen-3（2H）-one（BTO）have been synthesized.The single crystal structures of the four compounds revealed that the geometric parameters of“olefin pairs”satisfy Schmidt’s criteria.However,only compounds BFO and NIO undergo photo-induced[2+2]cycloaddition,while BIO and BTO are photo-inert.Combined with the single crystal structures and Hirshfeld surfaces analysis,we found that the interactions of C=O···H（CH₂）in the crystal of BIO and Cl···S/C=O···H（C₆H₄）in the crystal of BTO restrict the movement of the carbon-carbon double bond and prevent[2+2]cycloaddition.However,in the crystals of BFO and NIO,the O atom or CH₂ group in BFO or NIO directly linked to the C=C double bond is free,allowing for easy movement of double bonds and photoinduced[2+2]cycloaddition.Moreover,the needle-like crystals of BFO and NIO exhibited photo-induced bending behaviors driven by[2+2]cycloaddition.This work highlights the importance of intermolecular interactions in the design of photodimerization triggered photomechanical molecular crystalline materials with potential applications in smart materials.（3）The（E）-2-（2,4-difluorostyryl）quinoline（24FQL）and（E）-2-（3,5-difluorostyryl）quinoline（35FQL）are synthesized.Besides,the compounds of 24FQL or35FQL with the conformers of tetrafluoroterephthalic acid（TFPTA）or tetrafluorobenzoic acid（TFBA）to form corresponding cocrystals（24FQL/TFBA,24FQL/TFPTA,35FQL/TFBA and 35FQL/TFPTA）have been prepared.It has been found that 24FQL and 35FQL in the pristine crystals could take place[2+2]cycloaddition,and the crystals showed cracking behaviors under UV light irradiation.In addition,the introduction of the conformers could affect the photochemical reactivity and the photomechanical behaviors of the cocrystals.When the conformers were introduced in the lattice of 24FQL,the[2+2]cycloaddition conversion of 24FQL decreased.In contrast,the photochemical reactivity of35FQL was significantly improved in the cocrystals.It was ascribed to changes of the packing structures of 24FQL or 35FQL molecules in crystalline lattices.Moreover,the cocrystals containing 24FQL or 35FQL exhibited more remarkable photomechanical behaviors,for example,the block-like crystal of 24FQL/TFBA exhibited rapid photo-induced cracking,elongating,and bending behaviors.Combining the crystal structures and Hirshfeld surface analysis,the intermolecular interactions in cocrystals of 24FQL/TFBA might be stronger than those in crystals of 24FQL.During the[2+2]cycloaddition of24FQL in the cocrystals more intermolecular interactions would be broken,giving low conversion to the dimerization product but more significant photomechanical performance.Hence,the cocrystal strategy can be a useful tool to harmonize the photochemical reactivity and photomechanical behaviors of the potentially photoactive molecular crystals.It provides a facile strategy to design photoresponsive materials with enhanced performance in energy conversion systems.（4）The（Z）-2-methyl-4-（naphthalenylmethylene）oxazol-5（4H）-one derivatives（B1NMO and B2NMO）have been synthesized.The plate-like B1NMO displays fast cracking behaviors driven by[2+2]cycloaddition upon UV light irradiation.Meanwhile,its crystal exhibits photoluminescence enhancement.Based on DFT,we proposed the mechanism of emission enhancement.At the intial period of UV irradiation,molecules tend to form molecular pairs,then photodimerization occurs.Prolonging the irradiation time,the number of isolated molecules increase,the excited state molecules transfer to their ground states with the form of fluorescence,and the fluorescence intensity of the crystal gradually increases.On the other hand,the crystal of B2NMO behaves photo-stable behavior under UV irradiation,which could be abscribed to its low photochemical reactivity.However,because its energy level of S₁ state in isolated molecule is closed to one of S₁ state inπ-dimer,the reactivity of[2+2]cycloaddition significantly increases after heating its crystal.Furthermore,the composite film prepared through incorporating PVDF with B1NMO,the film can display distinct photo-induced bending away the light source motion upon irradiation,which extends the application prospect of the material.