Research On Molecular Crystal Functional Materials Of Azobenzene Derivatives And Their Structure-function Relationship

Stimuli-responsive crystal materials are able to change their physical or chemical properties in response to external stimuli,such as light,mechanical stress,temperature,electricity,p H and so on,which have aroused great interests in science and engineering for their applications as actuators,sensors,switches and biomimetics,etc.However,achieving one multi-stimuli responsive crystal that could perform multiple functionalities simultaneously is still challenging.In order to solve the problem of single stimulus response mode of the currently reported crystal materials in practical applications,this disseration focuses on investitation of the influence of isomer phase state,molecular conformational and packing polymorphs on tuning stimuli-responsive behaviors of photoactive azobenzene derivatives.Firstly,two azobenzene derivatives,(4-cyano-4’-ethoxy)azobenzene(Az C2)and(4-cyano-4’-pentoxy)azobenzene(Az C5),were synthesized,and their single crystals and microcrystals with polymorphism were prepared.UV-vis absorption spectra showed the photoresponsive properties of the above compounds in solution.Single crystal X-ray diffraction indicated that the two crystal forms of Az C2 were stacking polymorphs,while the two crystal forms of Az C5 were conformational polymorphs.The DSC and variable-temperature PXRD results showed that the four kinds of crystals have thermostability,which improves the durability of the materials in practical applications.Secondly,three completely distinct photoinduced deformation motions under the same irradiation conditions but at different temperature were achieved with mono-component molecular crystals by tuning the isomer phase states.Bending away from the light source,bending towards the light source and fully bidirectional motions were obtained in AZC5 Form I microcrystals respectively.The molecular mechanisms underlying the photomechanics were systematically investigated from the viewpoints of molecular packing,phase states,and limitation of crystal lattice size by PXRD,AFM and crystallographic data.It was found that photoisomerization,photoinduced phase transition and the competition between them were the reasons for distinct photoinduced deformation motions in the same microcrystal.Thirdly,the effect of molecular conformations on the photomechanical behaviors of the molecular crystals of azobenzene derivatives was systematically investigated.The photomechanical behaviors of two conformational polymorphs of Az C5 microcrystals were observed by microscope upon the same UV irradiation.The ability of photoisomerization of the two polymorphs were compared by in-situ Raman spectra.The theoretical calculation of intermolecular interaction energies indicated that the isomerization ability of Form I crystal was relatively weak and its energy distributions were poorly anisotropic,leading to bending motion away from the UV light.In contrast,the isomerization ability of Form II was relatively strong and its energy distributions were highly anisotropic,resulting in photosalient behavior.Finally,the multi-stimuli responsive functional crystal materials of azobenzene derivatives were constructed by simply tuning crystal polymorphs.These two crystal polymorphs of Az C2 showed remarkable difference in photomechanical bending motion,mechanical property and photoluminescence under the same stimulus.Multiple experimental techniques including PXRD,AFM,SEM,Raman microscopy,nano-indentation,DMA,solid-state UV diffuse reflection spectroscopy and fluorescence spectroscopy were used to compare and analyze the properties of the two polymorphs.Combined with the crystallographic data,it was found that these distinct mechanical and optical behaviors were attributed to different molecular packing.Form I crystal with a herringbone structure tends to bending away from the UV light,break under mechanical stress and emit yellow-green fluorescence,while Form II crystals with a cross structure tend to bending toward to the light source,elastic deformation under mechanical stress and emit red fluorescence.