Solid-State Photochemical Reactions Of 1-Cyano-1,2-Diarylethene Derivatives And The Photoactuation Of Crystals

Photomechanical crystal materials are formed by self-assembly of photosensitive molecules,which typically contain nitrogen-nitrogen double bonds,carbon-nitrogen double bonds or carbon-carbon double bonds.Photosensitive molecules containing carbon-carbon double bonds can undergo cis-trans isomerizations,electrocyclic reactions or cycloaddition reactions upon UV light.Due to the tightly ordered stacking of molecules in crystal materials,the stress generated by atomic displacement during molecular photochemical reactions was amplified through intermolecular interactions,resulting in macroscopic photomechanical behavior of crystals.Therefore,photomechanical crystal materials can convert light energy into mechanical energy and are widely used in actuators,molecular machines,smart switches and artificial muscles.To date,although a large number of photomechanical crystal materials have been reported,the mechanism of photomechanical effect of crystal materials is still under debate,and there are still shortcomings in the control of photochemical reactions in crystals.In addition,internal factors such as molecular packing and intermolecular interactions,as well as external factors such as the direction and intensity of light illumination and crystal size,have different effects on the photomechanical performance.In this thesis,a series of cyanostilbene derivatives were synthesized to explore their photomechanical properties and mechanism in molecular crystals.The photoreaction types of the cyanostilbene with different aromatic ring substituents in crystals and stimulus-responsive properties of crystals were investigated.Additionally,it is also studied that the effects of different molecular stacking on the luminescence and photomechanical properties of molecular crystals.Some innovative results have been obtained and outlined below:（1）A series of（Z）-3-（naphthalen-1-yl）-2-phenylacrylonitrile derivatives Z-HPhNaH,Z-FPhNaH,Z-ClPhNaH,Z-BrPhNaH,Z-HPhNaF,Z-FPhNaF,Z-ClPhNaF,Z-BrPhNaF and Z-FPhNaOMe have been synthesized via Knoevenagel condensation reaction.The photoisomerization took place in the molecular crystals,which triggered the crystals to bend backward from the light source rapidly.In particular,the significant photomechanical motions were observed in the initial several seconds of UV irradiation,during which the conversion ratio from Z-to E-isomer was less than 0.3%.Such fast photoactuation induced by subtle photoisomerization might be originated from the relatively large change in molecular width after photochemical reaction and the strong intermolecular interactions to amplify the deformation at molecular level into macroscopic mechanical motion of the crystals.Therefore,the crystals performed reversible photo-induced bending and unbending by changing the irradiation directions for more than 100 times on account of the retained crystal integrity during photoactuation and the good elasticity of the crystals.Besides,the needle-like crystal also exhibited photomechanical behavior in water.（2）A series of phenyl-2-naphthylacrylonitrile derivatives,t-Bu NA,Me NA,DMe NA,DBNA and DOMe NA,were synthesized.The photomechanical behavior of molecular crystals and the influence of molecular dihedral angles on molecular stacking and the types of solid-state photochemical reaction were investigated.The crystals of t-Bu NA and Me NA showed photomechanical bending under UV irradiation,while the crystals of DMe NA exhibited photoinduced cracking and jumping.The dihedral angle between the benzene ring and the naphthalene ring in Me NA was 29.07°,and adjacent molecules stacked in a completely parallel manner.The distance between the double bonds is 3.952?,which is less than 4.2?.Therefore,Me NA can undergo[2+2]cycloaddition in crystals.When t-butyl group was introduced at the end of the phenyl-2-naphthylacrylonitrile molecular skeleton,the dihedral angle between the benzene ring and the naphthalene ring in t-Bu NA becomes 2.61°.However,the adjacent t-Bu NA stacked obliquely with the double bonds crossing,and the shortest distance between the double bonds was 4.10?.Interestingly,although the stacking of double bonds did not meet Schmidt’s rule,t-Bu NA could undergo[2+2]cycloaddition in the crystal.It is assumed that the magnitude of the dihedral angle within the molecule also affects the reaction activity of[2+2]cycloaddition.The dihedral angles within DMe NA,DBNA,and DOMe NA molecules are between 59°and 75°,and the experimental results showed that these three molecules underwent only cis-trans isomerization in the crystal and could not undergo[2+2]cycloaddition.（3）（E）-2-（1H-benzo[d]imidazol-2-yl）-3-（4-fluorophenyl）acrylonitrile（E-CN-BIFPh）and（Z）-3-（1H-benzo[d]imidazol-2-yl）-2-（4-fluorophenyl）acrylonitrile（Z-BIFPh-CN）were synthesized.The crystals of E-CN-BIFPhhad two phases:G-phase with green emission and OR-phase with orange emission.E-CN-BIFPhcould undergo[2+2]cycloaddition reaction in its OR-phase crystal with a high reaction conversion so that the crystal in OR-phase exhibited a dramatic photosalient behavior.Because of the distinct packing pattern of molecules,the[2+2]cycloaddition reaction was forbidden in G-phase crystal of E-CN-BIFPh.Z-BIFPh-CN also self-assembled into two types of crystals:the crystal in B-phase with bright blue emission and the crystal in G-phase with green emission.Similarly,Z-BIFPh-CN could undergo[2+2]cycloaddition reaction in its B-phase and the reaction was forbidden in its G-phase.Whereas the crystal of Z-BIFPh-CN in G-phase could transform into its B-phase when it was heated at 105℃.Compared with E-CN-BIFPh,Z-BIFPh-CN had a lower[2+2]cycloaddition reaction conversion so that its crystal in B-phase showed a mild photomechanical behavior.The[2+2]cycloaddition reaction was hindered when the benzene ring in E-CN-BIFPhwas replaced by naphthalene ring because the molecular packing in E-CN-BINa did not meet Schmidt’s topochemical rules.Furthermore,due to the tight stacking of E-CN-BIFPhin OR phase crystal,the OR phase crystals exhibited significant mechanofluorochromism behavior with a blue-shift of 115 nm.However,Z-BIFPh-CN had a relatively loose stacking in the G phase,so that the G-phase crystals exhibited mechanofluorochromism with a blue shift of only 12 nm.（4）Three new phenylquinolinylacrylonitrile derivatives（Z）-2-phenyl-3-（quinolin-4-yl）acrylonitrile)（Z-HPh QLA）,（Z）-2-（4-fluorophenyl）-3-（quinolin-4-yl）acrylonitrile)（Z-FPhQLA）and（Z）-3-（quinolin-4-yl）-2-（4-（trifluoromethyl）phenyl）acrylonitrile（Z-CF₃Ph QLA）were synthesized.It was found that Z-HPh QLA and Z-FPhQLA could form organogels in polar solvents,and compound Z-FPhQLA showed good gelation ability in acetone/H₂O（v/v=1/1.3）with a low critical gelation concentration（CGC）of0.11 wt%.Interestingly,the free standing organogel（FSOG）with remarkable mechanical strength could be formed from gelator Z-FPhQLA based on its strong self-assembling capability.The storage modulus of FSOG was measured as up to 10⁴ Pa level at a low concentration（0.74 wt%）,which was much higher than most LMWGs.The FSOG exhibited reversible external force-induced bending without breaking and it could be cut arbitrarily.Immersing the FSOG in water for several hours afforded the free-standing hydrogel（FSHG）.Since a dense film was formed on the surface of FSHG,the mechanical strength of FSHG was further improved compared with FSOG.When exposed to 365 nm light,the freshly prepared organogel would be collapsed into suspension because Z-FPhQLA underwent the tandem reactions of isomerization,electrocyclization and oxidative aromatization to yield 3-fluorobenzo[i]phenanthridine-12-carbonitrile（FBPD-CN）to destroy the intermolecular interactions among the molecules of Z-FPhQLA.In addition,due to the aggregation-induced emission of compounds Z-HPh QLA and Z-FPhQLA,the gained xerogels exhibited a bright fluorescence.For instance,the blue fluorescence of the xerogel film of Z-FPhQLA could be switched off by TFA（trifluoroacetic acid）vapor with response time of 0.38 s and detection limit of 0.43 ppm.