Study On The [2+2] Cycloadditon Of Phenyl Conjugated Diene Nitrile Derivatives And Photomechanical Properties Of The Molecular Crystals

Photomechanical crystal materials are intelligent materials capable of efficiently converting light energy into mechanical energy.These materials possess advantages such as rapid response,remote control capability,and easy characterization and modification,making them a recent research hotspot.Photomechanical effects,including cracking,bending,curling,jumping,twisting and so on,have been widely reported.Recent studies have indicated that photochemical reactions,such as trans-cis isomerization,electrocyclic reactions,tautomerism,and cycloaddition reactions,are the driving forces of photomechanical behaviors of molecular crystals.Among them,solid-state[2+2]cycloaddition reaction not only allows adjustments of molecular packing in crystals through crystal engineering or molecular engineering,but also enables the stereospecific synthesis of cyclobutane derivatives,which is an important structure in many natural products and drugs.Therefore,photo-induced[2+2]cycloaddition reactions have become a common approach for driving photomechanical motion of molecular crystals.Currently,reports on photomechanical motion of organic crystal driven by photo-induced[2+2]cycloaddition reactions primarily based on diarylethenes,chalcones,and cyanostilbene derivatives,while researches on the conjugated diene derivatives are relatively few.In this study,a series of novel conjugated dienenitrile derivatives were designed to investigate the effects of different functional groups on molecular packing and reactivities of[2+2]cycloaddition in crystals,as well as the photomechanical effects of molecular crystals.Innovative research results were obtained and outlined as follows:(1)Different substituted phenyl conjugated diene malononitrile derivatives were synthesized:(E)-2-(3-(4-bromophenyl)allylidene)malononitrile(Br AM),(E)-2-(3-(4-chlorophenyl)allylidene)malononitrile(Cl AM),(E)-2-(3-(4-fluorophenyl)allylidene)malononitrile(FAM),(E)-2-(3-phenylallylidene)malononitrile(HAM),and(E)-2-(3-(4-methoxyphenyl)allylidene)malononitrile(Me OAM).Crystals of Br AM,FAM,HAM and Me OAM were obtained and the photomechanical effects of their crystals were studied.Upon irradiation of UV light,only the HAM and FAM crystals exhibited photomechanical behaviors,of which the HAM plate-like crystals displayed photoinduced cracking and the FAM strip-like crystals showed backlight bending and rolling.The irradiation time dependent ~1H NMR spectra of microcrystals indicated that only HAM,FAM and Cl AM could undergo[2+2]cycloaddition reactions upon exposure to UV light.Through the investigation of single crystal X-ray diffraction(XRD)of HAM and FAM as well as the obtained photodimerization products,the mechanism for photomechanical motion was proposed.After UV light irradiation,photodimerization firstly occurred on the irradiated surface of the crystal,leading to structural changes and internal atomic movements within the crystal,resulting in the accumulation of stress.The release of stress led to the cracking of HAM crystals and the bending and rolling of FAM crystals.Additionally,although the crystals of Br AM and Me OAM were gained,only the single crystal structure of Me OAM was obtained,and it was found that the molecular packing in the crystal did not meet the Schmidt’s criteria,preventing[2+2]cycloaddition reactions.This work has developed a fast-responsive photomechanical conjugated diene organic crystal material,laying a solid foundation for its further applications in areas such as artificial muscles,actuators,photonic switches,and sensors.(2)Different substituted phenyl conjugated dienenitrile derivatives were synthesized:(2Z,4E)-5-(4-bromophenyl)-2-(2,4-difluorophenyl)penta-2,4-dienenitrile(Br DE),(2Z,4E)-5-(4-chlorophenyl)-2-(2,4-difluorophenyl)penta-2,4-dienenitrile(Cl DE),(2Z,4E)-2-(2,4-difluorophenyl)-5-phenylpenta-2,4-dienenitrile(HDE),(2Z,4E)-2-(2,4-difluorophenyl)-5-(p-tolyl)penta-2,4-dienenitrile(Me DE),(2Z,4E)-2-(2,4-difluorophenyl)-5-(4-methoxyphenyl)penta-2,4-dienenitrile(Me ODE),and(2Z,4E)-2-(2,4-difluorophenyl)-5-(4-(dimethylamino)phenyl)penta-2,4-dienenitrile(Me NDE).The crystals were obtained,and the topophotochemistry reactions of the molecules and the photomechanical effects of the crystals were studied.After irradiation of UV light,needle-like crystals of Br DE and Cl DE exhibited photo-induced backlight bending,rod-like crystals of HDE displayed photo-induced cracking,and plate-like crystals of Me DE showed photo-induced fragmentation.However,crystals of Me ODE and Me NDE did not exhibit photomechanical motion.The ~1H NMR spectra of microcrystals of Cl DE,Br DE,HDE,Me DE,Me ODE and Me NDE after UV irradiation revealed that only microcrystals of Cl DE,Br DE,HDE and Me DE underwent[2+2]cycloaddition reactions.Through analysis of the single crystal structures of Cl DE,Br DE,HDE and Me DE,as well as simulation of crystal morphologies using Mercury 4.1.0 software,it was found that the unit cells expanded on the irradiated surface of the crystals with the occurrence of[2+2]cycloaddition,resulting in photomechanical motion.Additionally,the irradiation time dependent ~1H NMR spectra of HDE and Me DE indicated that only one type of dimer was formed during the photoreaction,and it was confirmed to beα-type dimers through the single crystal structures.In the single crystal structures of HDE and Me DE,it was observed that the adjacent reaction molecules had a small degree of overlap in theπorbitals,and only the vinyl pair at theα,β-position satisfied Schmidt’s rules,allowing for the specific occurrence of[2+2]cycloaddition reactions.On the other hand,the ~1H NMR spectra of microcrystals of Cl DE and Br DE exhibited multiple signals corresponding to H atoms in different kinds of cyclobutane rings,indicating that the[2+2]cycloaddition reactions in these compounds were not selective.The crystal structures of Cl DE and Br DE showed a high degree of overlap between adjacent molecules,and both the vinyl pairs at theα,β-position and theγ,δ-position satisfied Schmidt’s rules,allowing both the vinyl pairs to undergo[2+2]cycloaddition reactions,resulting in a lower selectivity of their photochemical reactions.This work not only developed a new series of crystal materials with rapid response to light,but also achieved the specificity of photodimerization reactions in conjugated diene nitrile derivatives,which can be applied to the selective synthesis of cyclobutane derivatives.