Synthesis Of Diarylethylene Derivatives And Study On Their Crystal Photomechanical Properties

As a member of photo-responsive materials,the photo-mechanical-responsive materials are able to produce macroscopic continuous motion or deformation under the stimulus of light,which has been a hot topic in the field of materials research.Common photomechanical response materials include organic polymers,liquid crystalline,and crystals.Among them,the photomechanical molecular crystals have attracted increasing attention.On one hand,they have great potential for applications in light harvesting,mechanical actuators,molecular machines,optical sensors,and smart switches.On the other hand,they have highly ordered structures exhibiting higher Young's modulus and shorter light response time,and the reaction process can be controlled in advance.In order to obtain highly efficient photomechanical molecular crystals,it is important to understand the correlations between molecular structure,stacking modes,and photomechanical properties.In this thesis,the diarylethylene compounds with novel structures were designed,the effects of different functional groups on molecular packing were investigated,and the photomechanical properties of different shapes of crystals were studied.Some innovative results have been obtained and outlined below:1)The naphthyl acrylic acid and naphthyl methyl acrylate derivatives FNAA,ClNAA,BrNAA,FNAMe,ClNAMe and BrNAMe were synthesized.Under UV irradiation,the needle-like FNAA and BrNAMe crystals bent away from the UV light.The strip-like crystals of ClNAA bent toward the irradiated light firstly,then bent away from the light.The flake-like crystal of BrNAA exhibited twisting and finally had a right-handed helix structure.After the UV irradiation,the appearance of the ~1H NMR signals in the high-field region corresponded to the protons in cyclobutane,illustrating that the topo-photodimerization was the driving force for the photomechanic effects of the molecular crystals.We explained the causes of the bending toward and away from the light as follows:the strain could be yielded and accumulated in the phototropic surface of the crystal when the movements of the molecular on the photodimerization process.The release of the strain induced the bending of FNAA and BrNAMe crystals.For ClNAA crystals,in the initial seconds of UV irradiation,the photodimerization caused the phototropic surface to generate contraction force from the ends to the center,causing the crystal to bend toward the light firstly.On prolonging the UV irradiation time,the cycloaddition reaction gradually penetrated into the crystal,causing more strain accumulation,and the slowly release of the cumulative stress caused the crystal to bend backwards the light source.Meantime,the photo-dimerization occurred in the surface of the flake BrNAA crystal,which caused the contraction of the phototropic surface taking place in the direction of top-left to bottom-right,resulting in the right-handed helix of the crystal.In addition,the Hirshfeld surface analysis of FNAA crystal revealed that the proportion of H…F,H…O were 12.6%and 19.4%,respectively,which caused the molecular stacking in a head-to-head mode,so that only one photodimerization products was yielded.Therefore,the introduction of halogens and carboxylic acids in the molecular were beneficial to increase the intermolecular hydrogen bonds,which were good for[2+2]cycloaddition reaction.2)(E)-1-(2-(perfluorophenyl)vinyl)naphthalene(PFBV1N),(E)-1-fluoro-4-(2-(perfluorophenyl)vinyl)naphthalene(PFBV1NF),(E)-1-methoxy-4-(2-(perfluorophenyl)vinyl)naphthalene(PFBV1NM),(E)-2-(2-(perfluorophenyl)vinyl)naphthalene(PFBV2N)and(E)-4-(2-(perfluorophenyl)vinyl)quinoline(PFBV4Q)were synthesized,and their photomechanical behaviors were studied.The rod-like crystals of PFBV1N,PFBV1NF and PFBV4Q as well as the block-like crystal of PFBV2N showed the photosalient behaviors,and light-induced bending was observed for the needle-like crystals of PFBV1NF and the lamellar-like crystal of PFBV1NM.The appearance of the ~1H NMR signals for the protons in cyclobutane illustrated that light-induced[2+2]cycloaddition reaction happened in the crystals of perfluorophenylvinylnaphthalene derivatives and perfluorophenylvinylquinoline.Notably,after irradiation the microcrystals of PFBV1N,PFBV1NF,PFBV1NM,PFBV2N and PFBV4Q for 5 min by 365 nm light(3 W),the dimerization conversions efficiency were 90%,85%,93%and 100%,82%,respectively,meaning that the highly efficient photochemical reactions were responsible for the mechanical motions of the crystals.During the efficient light-induced[2+2]cycloaddition reactions,the strain was generated and accumulated in the crystals.On the one hand,the accumulated strain could not be released continuously in the crystals with low aspect ratios(such as rod-like and block-like crystals),so that photosalient behaviors were observed on account of the rapid release of the strain.In the case of the needle-like and long lamellar-like crystals with high aspect ratios,they showed light-induced bending because the accumulated strain was released slowly.In particular,the topo-[2+2]cycloaddition reaction led to the emission enhancement of the microcrystals of PFBV1N,PFBV1NF,PFBV1NM,PFBV2N and PFBV4Q in the initial seconds of UV irradiation.Taking PFBV1NF as an example,the solid fluorescence quantum yield increased from1.12%to 15.79%after UV irradiation for 40 s.The possible reasons for emission enhancement of the microcrystals could be explained as follow,the formation of the dimers in the crystals could weaken the intermolecularp-pinteractions,which might lighten the fluorescence of the monomer molecule.In addition,the fluorescence resonance energy transfer from the dimers to olefins might also be another favorable factor for the enhancement of the emission.3)New diarylethene derivatives containing benzoxazole(NBO)and benzothiazole(NBT)had been synthesized.Light-induced trans-cis isomerization of NBO and NBT took place in crystals,and only induced the needle-like crystals of NBO to bend backwards away from the UV light source.The ratio of the integral values for the signals at?=8.62 and 6.93 ppm was about 1/0.05,so about 5%of E-NBO molecules were transformed into Z-NBO when the crystals were irradiated with UV light for 5 s.Therefore,strain would be produced and accumulated rapidly on the surface of the crystals facing the light,which would trigger motions on molecular level.Therefore,the bending of the needle-like crystal backwards away from the light source became the pathway for the uniform release of strain.Unlike the NBO,after the crystals of NBT were irradiated by UV light for 5 s,only ca.1%E-NBT molecules were isomerized into Z-NBT.Therefore,the low isomerization efficiency of crystals of NBT would not produce enough strain in a short time,and thus,no photomechanical behavior could be observed.4)The photo-reactive 5-chloro-2-(naphthalenylvinyl)benzo[d]oxazols(BOV1N,BOV1NF,BOV1NM,BOV2N,and BOV2NM)had been prepared.The needle-like crystals of BOV1N,BOV2N and BOV2NM bent away from the UV light upon irradiation.The gel fibers of BOV1N swung quickly backwards to the light,and the fiber-like crystals of BOV1NF showed rapid and large bending deformation under UV light.Unlike other crystals,the needle-like crystals of BOV1NM exhibited slipping and swinging induced by the UV light.The ~1H NMR spectral changes for the crystals before and after UV irradiation suggested that photodimerization was the driving force for the light-induced macroscopic mechanic movements.Since the photo-induced[2+2]cycloaddition shortened the distance of the olefin pairs and made the terminal units stretch outside the molecular long axle,the strain in the phototropic surface of the crystal could be yielded and accumulated.The release of the accumulated strain led to the bending,slipping and swinging of the needle-like and fiber-like crystals backwards to the UV light.In addition,the Hirshfeld surface analysis of BOV1N revealed that the proportion of H…Cl,H…O were 12.8%and 4.9%,which indicated that the introduction of Cl atom could increase the ratio of intermolecular hydrogen bonding interactions in the entire crystal,and promote the accumulation of molecules in the crystal to facilitate the photodimerization reaction.5)4-fluorosubstituted naphthylvinylbenzoxazole(BO1N4F),benzothiazole(BT1N4F)and 4-methoxy substituted naphthylvinylbenzoxazole(BO1N4M),benzothiazole(BT1N4M)were synthesized.The rod-like and needle-like crystals of BO1N4F showed light-induced photosalient and bending behaviors,respectively.Cracks were observed in the slice-like crystal BT1N4F under UV light.BO1N4M and BT1N4M crystals did not show any change upon UV light irradiation.The ~1H NMR spectral changes for the microcrystals before and after UV irradiation suggested that[2+2]cycloaddition was the driving force for the light-induced macroscopic mechanic movements.After UV irradiation of the microcrystals of BO1N4F for 5 min,ca.54%of the BO1N4F molecules were converted into the main product of the dimer(D-BO1N4F),and ca.7%were converted into the byproduct of the dimer.However,the photochemical conversion rate of BT1N4F was only 15%and 2%,lower than that of BO1N4F.Therefore,slice-like crystal BT1N4F only exhibited weak photomechanical phenomena under UV irradiation.After the microcrystals of BO1N4M and BT1N4M were irradiated for 10 min and 30 min,respectively,1%of the monomer molecules were converted into dimers,and the low photodimerization reaction efficiency could not induce the photomechanical movement of the crystal,which proved that the substituents had important effects on the photodimerization reaction.The above phenomenon showed that the photodimer reaction efficiency and crystal shape had significant effects on the photomechanical effect.In addition,the fluorescence emission of BO1N4F and D-BO1N4F in the film could be quenched by strong acid vapors(such as TFA and HCl),and the detection limit of TFA vapors in film were 3.2 ppm and 2.6 ppm,respectively.Therefore,the light-induced[2+2]cycloaddition reaction of diarylethene not only enhanced the performance in sensory property of the film,but also became the driving force for the photomechanical effects.