Syntheses,Structural Phase Transitions And Properties Of Molecular-Based Cocrystals Based On N,N-Dimethylisopropyl Ammonium

The exploration of solid-state molecular motion can provide insight into the relationship of dynamic structures with various physical and chemical properties,which has attracted considerable attention of researchers.In the conventional concept,molecules within crystals are confined in a highly dense close-packing environment and thus only show constrained and small-amplitude vibrations.To enable large-amplitude atomic/molecular motions in solid-state,two main strategies that aim at generating motion-free confined space,non-porous crystals with loose and low-density packing or engineering crystals with permanent porosity,have been adopted in the past few decades.By utilizing the former strategy,the realization of large-amplitude molecular motions（such as swinging,flipping,hopping or rotating）for a series of small-size quasi-spherical molecules with rigid tetrahedral skeletons,such as（Me₃NOH）⁺,（Me₃NNH₂）⁺,（Me₃NCH₂X）⁺（X=F,Cl,or Br）,have been intensively studied.However,by comparison with the solid-state motions of small-sized rigid molecules,those of moderate-sized flexible molecules is in infancy but are anticipated to be more complex,where the motions of such molecules as a whole may be synergistically accompanied by a local intra-molecular deformation.According to the above ideas,a flexible（i-Pr NHMe₂）⁺cation of moderate-sized and quasi-spherical was selected as the research object.It can be viewed as a pseudo-symmetric ammonium,in consideration of the similar geometry of its two half-parts（-CMe₂H and-NMe₂H）,the similar-size C/N atoms and similar bond lengths.In this thesis,three crystalline molecular-based compounds with different structures were synthesized i.e.,zero-dimensional supramolecular compound,one-dimensional chain-like coordination polymer and three-dimensional perovskite-like coordination polymer.The molecular motion and structural phase transition of（i-Pr NHMe₂）⁺cations in solid-state confined space were studied by differential scanning calorimeter,variable-temperature single crystal X-ray diffraction,variable-temperature/frequency dielectric spectrum,and molecular dynamics（MD）simulation.The main research contents are as follows:（1）The zero-dimensional supramolecular compound（i-Pr NHMe₂）₂[Sn Cl₆]（1）was synthesized.Compound 1 undergoes a structural phase transition at 359 K,the room temperature phase crystallizes in the P2₁/n space group and the high temperature phase crystallizes in the F4 3m space group.During heating and cooling cycle,variable-temperature/frequency dielectric spectrum of compound 1 presents a dielectric bistability,thermal hysteresis is 16 K,the variable-temperature crystal structure analysis and MD simulations indicated that the structural phase transition was mainly due to the change of the（i-Pr NHMe₂）⁺cation from statically ordered at room temperature to dynamically disordered at high temperature.（2）The one-dimensional chain coordination polymer（i-Pr NHMe₂）[Cd Br₃]（2）was synthesized.Dielectric relaxation was observed in the variable-temperature/frequency dielectric spectrum of compound 2.Through single crystal X-ray diffraction shows that（i-Pr NHMe₂）⁺cations have an order-disorder transition,and the dielectric relaxation was attributed to an orientation polarisation caused by the dynamic change of the disordered fraction of（i-Pr NHMe₂）⁺cations.（3）The three-dimensional cage hybrid rare-earth double perovskite（i-Pr NHMe₂）₂[Rb Ce（NO₃）₆]（3）was synthesized.Compound 3 undergoes structural phase transition at 258 K and 283 K.Single crystal structure,MD simulations and variable-temperature/frequency dielectric spectrum revealed that this is due to a static to dynamic disorder change of the（i-Pr NHMe₂）⁺cation.Based on the above studies,it is found that the moderate-sized quasi-spherical（i-Pr NHMe₂）⁺cations have the mechanism of molecular"order-disorder"transformation in different confined space accompanied by the change of intra-molecular torsion angle,which leads to the dielectric relaxation or structural phase transition of crystalline molecular-based compounds 1,2 and 3.This study provides a meaningful insight into the solid-state MD of moderate-sized quasi-spherical molecules that feature a flexible inner core.