Design,synthesis And Dynamic Response Properties Of Low-dimensional Organometallic Halides Phase Transition Materials

Dynamically responsive molecular materials,which usually change their internal structures with external stimuli and exhibit rich switchable physical responses,have become a kind of functional material with the potential application.Among them,organometallic halides with thermally induced phase transitions can exhibit excellent dielectric transitions and nonlinear optical switching,as well as ferroelectricity properties,making them potentially useful in switching,sensing,and others.In this paper,drawing on the chemical systems methodology,we design and synthesize three classes of organometallic halides by using three modification schemes.Then,we study the various switching behaviors of these crystals,such as dielectric,second harmonic generation（SHG）effect,ferroelectricity,etc.,which are allowed by the symmetry produced by the structural phase transition.In addition,we introduce other physical properties related to metal halide frameworks into dynamically responsive molecular materials based on the intrinsic properties of the metal halide framework,and molecular design and crystal engineering,many interesting functional properties have been realized.Such as thermo-induced fluorescence behavior in optical/electrical multifunctional phase transition materials,semiconducting and unusual SHG switching behavior,etc.in dielectric materials.The realization of multifunctionality enriches the research content of dielectric phase change materials and further analyzes the molecular mechanism of dynamic response.Meanwhile,the design and methodology for the synthesis of multifunctional phase transition materials have been significantly supplemented and developed.The specific research contents are as follows:Part I:High-temperature multi-step switching of dielectric has always been a challenge for the design and synthesis of dielectric phase transition materials.In Chapter 3,three one-dimensional organometallic halides 1-3 were constructed by modifying tetramethylammonium cations to trimethyl N-oxide cations with MCl₂（M=Cd and Mn）and BiCl₃,respectively.Because of the formation of O-H···X hydrogen bonds between organic and inorganic parts,their dielectric phase transition temperature is increased,creating a paradigm of multi-step dielectric phase transition and providing ideas for their design.In addition,combined with the unique properties of different metal chlorides,their related luminescence and semiconducting properties have also been studied preliminarily.This section provides references and ideas for the study of multifunctional materials and optimized properties using enhanced intermolecular forces and regulation of metal ions at the B site in organometallic halides.Part II:To improve dielectric phase transition temperature,a lot of work has been done on isotopic and halogenated modification.But the work on chain ammonium modification is very rare.In Chapter 4,based on halogenation modification of parent chain ethylamine with bismuth halide as the structural primitives,two zero-dimensional organometallic halide material compound 4（NH₃CH₂CH₂F）₃BiCl₆（NH₃CH₂CH₂F:2-fluoroethylamine）and 5（NH₃CH₂CH₂Br）₃BiCl₆（NH₃CH₂CH₂Br:2-bromoethylamine）were synthesized and both of which exhibit dielectric phase transition properties.Combined with the previously reported work（NH₃CH₂CH₂Cl）₃BiCl₆（NH₃CH₂CH₂Cl:2-Chloroethylamine）,halogen modification can effectively tune the crystal structure,phase transition behavior,and dielectric properties.In particular,the phase transition temperature of compound 4 is not only improved but also has excellent nonlinear optical properties by the H/F empirical strategy.This is an attempt to systematically control the phase transition temperature and crystal structure by halogenation modification of chain organic amines.Part III:The strategy of mixing halogens has attracted much attention as a way of tuning the optoelectronic properties in organometallic halide crystals.However,tuning performance in the field of dielectric phase transitions has rarely been reported by this strategy.In Chapter 5,we experiment with this strategy in ultra-flexible zero-dimensional structures.A series of zero-dimensional organometallic halides 6-11 were synthesized based on trimethylchloroethylammonium cation（TMCE）and cadmium halide.The initial material compound 6-（TMCE）₂CdI₄ was converted into compounds 7-11 with polar structures,including two ferroelectric phase transition compounds 7-（TMCE）₂CdI₃Br and 8-（TMCE）₂CdI₃Cl,three nonlinear optical switching materials compound 9-（TMCE）₂Cd Br₄,10-（TMCE）₂Cd Br₃Cl and11-（TMCE）₂Cd Br₃I.Among them,compounds 7 and 8 exhibits a unique high temperature and stable two-step unusual SHG-"OFF-ON-OFF"switching performance,which is higher than the switching temperature and stability of most previous SHG switches.Meanwhile,the structural analysis and properties of the synthesized compounds are studied,and the effect of halogen doping in the system is understood by combining experimental phenomena and empirical theory.The successful case of introducing the mixed halogen strategy into zero-dimensional organometallic halides shows its great potential in optimizing dielectric and other special properties in the field of dielectrics.Overview,the research work carried out in this paper is based on several strategies to optimize and enrich the phase transition of dielectrics,and introduces chemical interpretation,thereby having a deeper understanding and knowledge of the relationship between structure and physical properties.