The Role Of "Halogenation Effect" In Organic-inorganic Hybrid Phase Change Materials

In recent years,organic-inorganic hybrid phase change materials have been widely used in nonlinear optical switches,wearable electronic devices and memory.Due to their excellent performance,high mechanical strength,easy machining,easy film forming and flexible properties,they have become one of the hot topics of researchers.In the exploration process of multifunctional organic-inorganic hybrid phase change materials,molecular design strategies such as"symmetry breaking","quasi-spherical theory"and"fluorosis effect"are introduced to regulate the phase change temperature,so as to conduct further research.Therefore,in this paper,a series of high temperature phase transition organic-inorganic hybrid materials were successfully synthesized by using halogenation effect on the basis of fluorination effect.Using phenylethylamine as organic parent,the molecular design strategy of"halogenation effect"was used,that is,halogen atoms（F、Cl、Br）were used to replace hydrogen atoms at any site of the benzene ring,and nine halogenated phenylethylamine compounds were obtained:2-fluorophenylamine（2-FPEA）、2-chlorophenylamine（2-CPEA）、2-bromophenylamine（2-BPEA）、3-fluorophenylamine（3-FPEA）、3-chlorophenylamine（3-CPEA）、4-fluorophenylamine（4-FPEA）、4-chlorophenylamine（4-CPEA）and 4-bromophenylethylamine（4-BPEA）reacted with Pb Cl₂、Pb Br₂、Sb Cl₃、Bi I₃ and Cd Cl₂ in a ratio of 2:1 to obtain compounds 1～16.Among them,compounds 1～7 are lead-based organic-inorganic hybrid perovskites:[（2-FPEA）₂Pb Cl₄]（1）、[（2-CPEA）₂Pb Cl₄]（2）、[（2-BPEA）₂Pb Cl₄]（3）、[（3-FPEA）₂Pb Cl₄]（4）、[（4-FPEA）₂Pb Br₄]（5）、[（4-CPEA）₂Pb Cl₄]（6）、[（4-BPEA）₂Pb Cl₄]（7）,and confirmed its purity by infrared and powder XRD,and then through a series of characteristics such as single crystal diffraction,DSC and UV-visible absorption spectrum.The crystal microstructure of compounds 2～7 was found to be typical two-dimensional A₂BX₄ structure.The phase transition points of compounds 1、2、3 and 6 were determined to be 440 K、517 K、510 K and 522K,respectively.Compound 5 had two phase transition points of 451 K and 515 K,respectively,while compound 7 also had two phase transition points of 509 K and 531K,respectively,but the phase transition point at 509 K is irreversible.Such high phase transition temperatures of compounds 2～7 are rare in existing lead-based organic-inorganic hybrid perovskites,indicating that the introduction of"halogenation effect"can increase the phase transition temperatures of compounds.The UV band gap values of compounds 1～7 at room temperature were 3.52 e V、3.50 e V、3.52 e V、3.37e V、2.84 e V、3.43 e V and 3.41 e V,respectively.It was found that the change of organic amines did not change the optical properties of the compounds.Compounds 8～11 are antimon-based organic-inorganic hybrid perovskites:[（2-FPEA）₂Sb Cl₅]（8）、[（3-FPEA）₃Sb₂Cl₉]（9）、[（3-CPEA）₃Sb₂Cl₉]（10）、[（3-BPEA）₃Sb₂Cl₉]（11）,its purity was determined by infrared and powder XRD tests.The crystal structure of compound8 is one-dimensional A₃BX₅,and the crystal structure of compounds 9～11 is zero-dimensional A₃B₂X₉,which indicates that the change of organic amines affects the dimension of compounds.It was determined that compound 8 did not undergo reversible phase transition,while compound 9 and 10 had phase transition points of350 K and 369 K,respectively.Compound 11 was a phase change substance with three phase transition points of 331 K、348 K and 366 K,respectively,indicating that the halogen substitution site on the benzene ring had an effect on its properties.The optical bandgap values of compounds 8～11 were 3.21 e V、3.53 e V、3.53 e V and 3.49 e V,respectively.Compounds 12～14 are bismuth-based organic-inorganic hybrid perovskites:[（3-FPEA）₃Bi I₆]（12）、[（3-CPEA）₃Bi I₆]（13）、[（3-BPEA）₃Bi I₆]（14）.Compound 12 was found to be a zero-dimensional A₃BX₆ structure by single crystal diffraction.The phase transition points of compounds 12～14 were determined by DSC to be 335 K、411 K and 401 K,respectively.The UV band gap values of compounds12～14 at room temperature are 2.14 e V、2.09 e V and 2.09 e V,respectively,indicating that the semiconductor properties of compounds are related to the selection of inorganic ligands.Compounds 15 and 16 are cadmium-based organic-inorganic hybrid perovskites:[（3-FPEA）₂Cd Cl₄]（15）and[（4-FPEA）₂Cd Cl₄]（16）.Single crystal diffraction shows that compounds 15 and 16 are typical two-dimensional A₂BX₄structures.The phase transition points were determined by DSC to be 383 K and 384K respectively,and there was little difference in the phase transition temperature between the two.It preliminatively indicated that the influence of halogen substitution sites on the benzene ring on the phase transition temperature is not very obvious in the cadmium-based organic-inorganic hybrid perovskite,but is very obvious in the lead-based organic-inorganic hybrid perovskite.To sum up,in this paper,based on the basic theory of halogenation effect,sixteen organic-inorganic hybrid perovskites based on lead,antimony,bismuth and cadmium in different dimensions were designed and synthesized.Except for compound 8,which did not undergo phase transition,all the other compounds underwent reversible phase transition.It was also found that substituting hydrogen atoms at different sites on benzene ring by the same halogen atom could increase the phase transition temperature of the compound,and substituting hydrogen atoms at the same site on benzene ring by different halogen atoms could also increase the phase transition temperature of the compound,which opened up the research field of organic-inorganic hybrid phase change materials.