Study On The [2+2] Cycloaddition Of Styrylindazole Derivatives And The Photomechanical Effects Of The Molecular Crystals

Photomechanical crystal materials that efficiently transfer light energy into mechanical energy,are widely applied due to their advantages of fast response speed and non-contact control.It has been proven that photothermal effect and photochemical reactions are the driving force of photomechanical effect.Among various photochemical reactions that drive the photomechanical motion of crystal materials,the solid-state[2+2]cycloaddition reaction can not only provide driving force for the photomechanical crystal materials exhibiting photoinduced mechanical motions,such as bending,curling and twisting.It can also be used to synthesize cyclobutane derivatives with stereospecificity.Thus,the exploration of photodynamic molecular crystal materials induced by[2+2]cycloaddition has received extensive attention.Besides,indazole derivatives are widely found in natural products and pharmaceutical molecules,which could be used for the inhabitation of cancer cell growth and development of multi-targeted chemotherapy drugs.Therefore,the study in[2+2]cycloaddition reactions of indazole derivatives and their photomechanical properties of their molecular crystals are helpful for the development of photodynamic materials with valuable applications.Herein,a new series of styrylindazole derivatives and indole-substituted chalcone derivatives were synthesized,whose crystals with photomechanical properties were also prepared.Some innovative research achievements are outlined as follows:(1)Styrylindazole derivatives with different fluorine substitution were synthesized:(E)-3-(4-fluorostyrene)-1-methyl-1H-indazole(FPIZ),(E)-1-methyl-3-(3,4,5-trifluorostyrene)-1H-indazole(TFPIZ),(E)-1-methyl-3-(2-(pentafluorophenyl)vinyl)-1H-indazole(PFPIZ),(E)-3-(2,6-difluorostyrene)-5-fluoro-1-methyl-1H-indazole(FDFPIZ),and(E)-5-fluoro-1-methyl-3-(2-(pentafluorophenyl)vinyl)-1H-indazole(FPFPIZ).Their photomechanical,photochromic,and acidochromic properties were investigated.When irradiated with 365 nm UV light,the needle-like crystals of PFPIZ exhibited the backlight bending followed by phototropic bending,while needle-like crystals of FDFPIZ and FPFPIZ only showed simple backlight bending.However,no photomechanical motions occurred on FPIZ and TFPIZ crystals.By comparing the crystal structures of PFPIZ,FDFPIZ,FPFPIZ and their photodimer,it was found that the[2+2]cycloaddition reaction would result in the expansion of the phototropic surface on crystals,which would further lead to the backlight bending of the crystal.Different from other crystals,crystal of PFPIZ further bent towards light upon continuous irradiation of UV light,which was a consequence of change of crystalline phase of the phototropic crystal caused by the photothermal effect.The molecular packing of phototropic surface became disordered and the contraction of the phototropic surface resulted in the bending towards light.However,the stacking parameters of FPIZ and TFPIZ crystal went beyond the Schmidt’s criteria,so that they could not undergo[2+2]cycloaddition reactions.In addition to photoinduced bending behaviors,the crystals of PFPIZ,FDFPIZ and FPFPIZ also exhibited photochromic behavior under UV irradiation.The fluorescence color of PFPIZ changed from blue to white,and the fluorescence color of FDFPIZ and FPFPIZ changed from dark blue to light blue.The formation of photodimer during the process of photodimerization destroyed the intermolecular interaction,which resulted in the reduced fluorescence intensity of PFPIZ,FDFPIZ and FPFPIZ and relevant photochromic phenomenon.Since indazole in FPIZ,TFPIZ,PFPIZ,FDFPIZ and FPFPIZ is alkaline,the fluorescence intensity of these compounds decreased significantly with the increasing concentration of trifluoroacetic acid(TFA)in solutions and film.The TFA concentration-dependent ~1H NMR indicated that strong intramolecular charge transfer from the indazole ring to phenyl ring occurred in the protonated Styrylindazoles,which resulted in the acidochromism and fluorescence quenching.This work developed a crystal material capable of rapid response to light and TFA,which can be applied to the development of photoactuators and fluorescent sensing materials.(2)The(E)-3-(1-methyl-1H-indazole-3-yl)-3-phenylprop-2-en-1-one derivatives(series of Me IP-3:FMe IP-3,Cl Me IP-3,3DFMe IP-3,4DFMe IP-3,5DFMe IP-3,TFMe IP-3)and(E)-1-(1-methyl-1H-indazole-3-yl)-3-phenylprop-2-en-1-one derivatives(series of Me IP-1:FMe IP-1,3DFMe IP-1,5DFMe IP-1,TFMe IP-1)were synthesized.It has been found that[2+2]cycloaddition reaction can occur in Me IP-3 series molecules in crystal under UV irradiation.After UV irradiation for 30min,the FMe IP-3,Cl Me IP-3,3DFMe IP-3,4DFMe IP-3,5DFMe IP-3,TFMe IP-3could undergo[2+2]cycloaddition reactions in yields of 24%,12%,85%,70%,32%,and 50%,while the Me IP-1 compounds are photo-inert.By comparing the single-crystal structures,good molecular planarity was found in the series of Me IP-3crystals.Adjacent molecules stacked in parallel with the distances between the"olefin pairs"smaller than 4.2(?),indicating[2+2]cycloaddition reactivity.In the Me IP-1crystals,large torsion angles between the indazole and phenyl rings were observed.The distances between the"olefin pairs"in crystal stacking were larger than 4.2(?),which did not meet the Schmidt’s rules.Thus,[2+2]cycloaddition reactions could not occur.These results provide a basis for designing chalcones with[2+2]cycloaddition reactivity.Except for Cl Me IP-3 in series of Me IP-3 with a low[2+2]cycloaddition conversion rate,other molecular crystals all exhibited photomechanical behaviors under UV irradiation.Such photomechanical crystal provide a substantial basis for the potential applications in remote controlled intelligent robots,artificial muscles and other fields.