| Crystal phase transition materials exhibit unique optical,thermodynamic,electrical,mechanical and magnetic properties under external stimuli.Structural phase transition is accompanied by the changes of physical and chemical properties,showing a series of bistable functional switching properties such as dielectric,ferroelectric,nonlinear optics,ferroelastic,etc.These multifunctional molecular phase transition crystals have a wide range of applications in data storage,intelligent switching,energy conversion and transfer,laser detection,ferroelectric storage and other fields.Molecular phase transition materials mainly include organic compounds and organic-inorganic hybrid materials.Due to its advantages of simple preparation method,low cost,high crystallinity,light weight,easy film formation,good mechanical flexibility,high biological compatibility,adjustable structure,etc.,it is applied to flexible electronic devices,biomimetic materials and wearable devices.The excellent properties were determined by crystal structures.By the strategies of molecular design modification and crystal regulation engineering,the composition and structure were changed to improve the functional molecular-based phase transition crystals.The mechanism of phase transition is mainly caused by the order-disorder motion of organic amine cations under thermal stimulation.Moreover,based on the study of crystal structures and phase transition mechanism,the exploration of multifunctional dielectric,ferroelastic,fluorescence,thermochromism and other aspects are helpful to promote the development and application of molecular phase transition materials.Based on this,several different organic amines and metal ions or inorganic acid ions respectively were assembled to construct a series of new phase transition compounds.A series of photo-electric switchable materials with multiple functional responses have been successfully designed and prepared by using the transition crystals or thin films.And the synthesis method,phase transition mechanism and multifunctional properties of the compounds were investigated systematically.The specific research contents are as follows:（Ⅰ）A simple two-dimensional layered perovskite compound 1[C5H14N]2[Cd Cl4](C5H14N:Amylamine)was obtained.It exhibits obvious thermal anomalies,rapid dielectric responces and symmetry breaking under thermal stimulation.Moreover,the hybrid materials exhibit temperature-dependent red luminescence through Mn doping strategy.The dielectric/fluorescent dual response switching materials triggered by thermal exhibit quickly response and high fatigue resistance.It will be another step towards the practical application of two-dimensional perovskite stimuli response materials in smart electronic devices.（Ⅱ）A novel two-dimensional organic-inorganic hybrid perovskite compound 2[2,5-DCA]2[Cu Cl4]（2,5-DCA:2,5-dichloroamylamine）was obtained by combining with chloro-substituted 2,5-dichloroamylamine and thermochromic Cu2+.The compound has both dielectric and thermochromism response properties under thermal stimulation.The appearance color of the crystal changes reversibly between yellow-green and red-brown,and it has high fatigue resistance and band gap adjustability.This study demonstrates a reliable strategy for preparing thermochromic materials with multiple stimuli responses and provides ideas for the study of two-dimensional lead-free perovskite in smart windows.（Ⅲ）A series of organic amine salts including compound 3 nicotinic acid chlorine N-Cl（C6H7N2O2Cl）,compound 4 nicotinic acid bromide N-Br（C6H7N2O2Br）and compound 5nicotinic acid iodine N-I（C6H7N2O2I）were designed and synthesized by halogen substitution strategy.Among them,compound 4 shows excellent ferroelasticity,and the Curie temperature is as high as 402 K,which is higher than that of other reported organic salt ferroelastics so far.Under polarized light microscopy,reversible ferroelastic domain changes were observed,and the compound exhibited regular fluorescence characteristics by halogen substitution.Combined with the ferroelastic phase transition at high temperature and the optical properties of organic amine salts,the potential applications of photoelectric hybrid ferroelastic switching materials are prospected.This will enrich the multifunctional synthesis strategies and the applications of organic salt ferroelastic family.（Ⅳ）By halogen substitution strategy,the Cl atom was replaced by larger Br atom at the end of organic group,which increases the phase transition temperatures and realizes the regulation from single to dual phase transitions in molecular crystal structures.The phase transition temperature increased from 368 K（compound 6[CTA]2[Mn Br4],CTA is（3-chloro-2-hydroxypropyl）trimethylammonium）to 383 K and 398 K（compound 7[BTA]2[Mn Br4],BTA is（3-bromo-2-hydroxypropyl）trimethylammonium）,showing bistable dielectric double switching.Domain structures of two ferroelastic compounds varies with the change of temperature,and both have good reversibility and fatigue resistance.In addition,both compounds exhibit charming bright green and yellow-green fluorescence properties,respectively,as bistable fluorescent switches.Therefore,this work provides guidance for the synthesis and application of multifunctional stimuli-response optical-electrical ferroelastic,and has an important significance for further exploration of multifunctional molecular switching materials.In conclusion,in this paper,molecular modification and crystal engineering strategies are used to optimize and enrich the research of molecular dielectric and ferroelastic phase transition compounds with multiple optical-electrical responses,and chemical analysis and interpretation,so as to have a deeper understanding and research on the relationship between crystal structures and physical properties. |
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