Synthesis,Characterization And Dielectric Properties Of The Molecule-based Phase Transition Compounds Based On Triethylene Diamine Derivatives

Phase transition materials are always accompanied by the change of the heat capacity,molecules or ions in motion,the change of the structure and physical properties.It is crucial to prepare new phase transition materials,not only for the exploration of novel physical properties,but also for the theoretical study of structure–property relationships.In order to find new phase transition and dielectric materials,based on reading a lot of phase transition and dielectric materials on the related literature,modified high symmetry heterocyclic ligand 1,4-diazabicyclo [2.2.2]octane?Dabco?with chemical method,we obtained three kinds of quaternary ammonium salt based on Dabco.Eight kinds of novel metal organic or molecular ion phase transition compounds prepared by the synthesis of three ligands,the phase transition point,molecular structures of in two phases and dielectric properties of the compounds were further studied.1.Three metal-organic coordination phase transition compounds with high dielectric constant,low dielectric loss properties were prepared by the self-assembly of [CH₃CH₂-Dabco]⁺·Br^-and transition metal ions M(M = Zn²⁺,Co²⁺)respectively:[CH₃CH₂-DabcoH][ZnCl₄]?1?;[CH₃CH₂-Dabco H][CoCl₄]?2?;[CH₃CH₂-Dabco H][ZnBr₄]?3?;The crystal structures and phase transition temperature of three compounds were determined by variable temperature single crystal structure analysis and differential scanning calorimetry measurements?DSC?.The structural phase transitions were further conducted by ariable-temperature PXRD experiments,as also confirmed by dielectric measurements.Dielectric constant undergoes abrupt changes in the vicinity of phase transition,while the magnitude of the variations is related to the char-acteristics of such transition.DSC result of three conpounds show that the compound occurs phase transition when the temperature cooling to 241 K,257 K and215 K.The three conpounds belong to homologous series,variable-temperaturesingle-crystal X-ray diffraction suggested that the three compounds underwent phase transitions from room-temperature phase with a space group of Pnma to lower-temperature phase with a space group of P21/c.The ordering of twisting motions of Dabco rings and the position changes of molecular may drive the phase transition.2.Two organic molecular-ionic compounds were prepared by the self-assembly of [?CH₃?₂CH-Dabco]⁺·Br^- with sodium tetrafluoroborate and perchloric,respectively,[?CH₃?₂CH-DabcoH][Na?BF₄?₃]?4?;[?CH₃?₂CH-DabcoH][2ClO₄]?5?;The crystal structures of two compounds were determined by variable temperature single crystal structure analysis and the differential scanning calorimetry measurements?DSC?display that only conpound 4 was phase transition material with one-dimensional chain structure based on sodium tetrafluoroborate.DSC result of conpound 4 show that the compound occurs phase transition when the temperature cooling to 176 K,the structural phase transition was determined by the simulated XRD comparison at two temperatures.Variable-temperature single-crystal X-ray diffraction suggested that the compound 4 underwent phase transitions from room-temperature phase with a space group of P63 to lower-temperature phase with a space group of P21/c,as also studied by dielectric measurements,unexpectedly,there is no obvious dielectric anomaly in the measured frequency range.The ordering of twisting motions of Dabco rings and the formation of hydrogen bonds may drive the phase transition.3.Four metal-organic coordination compounds with phase transition properties were prepared by the self-assembly of [CH₃?CH₂?₃-Dabco]⁺·Br^- and transition metal ions M(M = Zn²⁺,Co²⁺,Ni²⁺)respectively:Zn[CH₃?CH₂?₃-Dabco]Cl_2.5Br_0.5?6?;Co[CH₃?CH₂?₃-Dabco]Cl_2.6Br_0.4?7?;Ni[CH₃?CH₂?₃-Dabco]Cl_2.5Br_0.5?8?;Zn[CH₃?CH₂?₃-Dabco]Br₃?9?;The crystal structures and phase transition temperature of four compounds weredetermined by variable temperature single crystal structure analysis and differential scanning calorimetry measurements?DSC?.The structural phase transition was determined by the simulated XRD comparison at two temperatures.DSC result of four conpounds show that the compounds occur phase transition when the temperature cooling to 186 K,199 K,192 K and 194 K.The four conpounds belong to homologous series,variable-temperature single–crystal X-ray diffraction suggested that the four compounds underwent phase transitions from room-temperature phase with a space group of Pca21 to lower-temperature phase with a space group of P21.The disorder–order transformation of the terminal two C atoms of the[CH₃?CH₂?₃-Dabco] cation triggered by the decrease in temperature may drive the phase transition.At the same time,the dielectric properties of the four compounds were studied,only compound 7 observed a reversible dielectric response near the transformation point.