Preparation And Study Of Photoresponsive Elastic Crystals Of Azopyridine

In recent years,the photomechanical response properties of organic crystals have become a hotspot in the field of functional materials research due to its tremendous application potential in molecular machinery,drug delivery,mechanical conduction and so on.Understanding the molecular mechanism and general laws of crystal photomechanical motion has important theoretical guiding significance for the development of new organic functional materials.Nowadays,the research is mainly focus on the effect of molecules properties and external factors on photoresponsive mechanism.However,the dependence of photomechanical motion on crystal structure has not been reported yet,especially for the intermolecular interaction.In this context,3,3’-azopyridine(3AP)and 4,4’-azopyridine(4AP)molecular crystals were prepared and systematically studied for their photomechanical deformation.Firstly,the preparation and structure analysis of 3AP and 4AP molecule crystals were studied.The crystals were prepared by cooling crystallization and sublimation crystallization method respectively.The crystal structures were characterized by powder X-ray diffraction(PXRD)and single crystal X-ray diffraction(SCXRD).The results suggested that 4AP-form 1,4AP-form 2 and 3AP crystals were obtained,of which 4AP-form 2 was a new crystal.Secondly,the photoisomerization of azopyridine molecules and process of photomechanical response of crystals were studied.UV-vis absorption spectroscopy and 1H Nuclear Magnetic Resonance Spectra(1H NMR)were used to analyze the ability of azopyridine molecules to undergo photoisomerization in the liquid,and polarizing microscope(PLM)was used to observe the deformation of the three crystals under the UV light.It was found that different crystals had various performance of photomechanical motion.On this basis,PXRD,solid-state UV spectroscopy and atomic force microscopy(AFM)were used to investigate the mechanism of crystal movements.It was found that the structural transformation and its degree were the reasons for the different macroscopic mechanical deformation in the crystalsThen,the molecular mechanism of photomechanical deformation was analyzed systematically.Hirshfeld surface of the molecules in the 3AP and 4AP crystals and energy framework of 3AP and 4AP crystals were calculated via CrystalExplore software,and the geometric configuration of cis-/trans-isomers were simulated by Gaussian software.The results showed that 4AP-form 2 had more π-π stacking interaction and less energy binding,and the molecular arrangement in the diagonal direction of the crystal surface caused the crystal to produce obvious bending-twisting motion.The energy binding in the c-axis of the 3AP crystal prevented motions of molecules,so that the 3 AP crystal only exhibited bending motion.Finally,cocrystal of 4AP and 5-chlorosalicylic acid(5ClSA)was prepared by solution evaporation based on the above research on the weak interaction of crystals.The crystal structure was confirmed by SCXRD,and the photoresponsive behavior and elastic bending deformation of the crystal were observed under PLM.The crystal structure was analyzed by Hirshfeld surface calculation and energy framework.The results showed that the weak photomechanical deformation in the cocrystal was related to the excessive hydrogen bond;the staggered arrangement of molecules and the three-dimensional isotropic energy distribution facilitated the elastic bending of 4AP-5ClSA cocrystal.