Polymorphism,Solid-state Phase Transition And Properties Of Crystalline Molecular Complexes

Polymorphism and solid-state phase transition are two classical and important issues in crystallography,especially for molecular crystals.In this field,two major questions have attracted the most attentions by physicists,chemists and crystallographers:1.what are the key factors that determine a particular crystalline form will be generated from a compound?2.What is the fundamental molecular mechanism of solid-state phase transitions?To answer these two questions,researchers should fully study many types of polymorphism and solid-state phase transition in molecular crystals.The better way is to design new crystals which may have polymorphism and related solid-state phase transition.There are two pathways to achieve this goal:1.design crystals by CSP(crystal structure prediction)technology which is the calculation of the crystal structures and crystalline forms from the first principles;2.design crystals by crystal engineering which is the empirical methodology based on the existing experimental data.Crystal engineering is the undersranding of intermolecular interactions in the context of crystal packing and the utilization of such understanding in design of new solids with desired physical and chemical properties.The concept of synthon and how it may be used as module in retrosynthetic analyis is now deeply embedded into the theory and paractice of crystal engineering.Utilization of crystal engineering to design crystalline molecular complexes,especially for organic cocrystals and organic-inorganic hybrid crystals,has unique advantages.Based on the above considerations and our early work,this thesis is focused on designing molecular complexes,crystal growth,polymorphism,solid-state phase transitions and relationships between structures and properties.The main content is as follows:1.To design and study a series of 8-hydroxyquinoline-based cocrystals:Two crystalline forms of bis-(8-hydroxyquinolinato)palladium(Ⅱ)-tetracyanoquinodimethane(PdQ2-TCNQ)were successfully discovered.The mechanical force inducing single-crystal-to-single-crystal phase transition(SCSC)in PdQ2-TCNQ system were investigated.And the differences of SCSC phase transitions between the PdQ2-TCNQ and CUQ2-TCNQ were explained at the molecular-level.Two different stoichiometric ratio 1,2,4,5-tetracyanobenzene-p-bis(8-hydroxyquinolinato)copper(II)CUQ2-TCNB and CuQ2-(TCNB)2 were successfully obtained.Moreover,other polymorph of CuQ2-(TCNB)2,named CuQ2-(TCNB)2-II,is also obtained.In this work,some crystalline molecular complexes with the mechano-induced phase transitions were discovered.The polymorphism and the related phase-relationship of these 8-hydroxyquinoline-based cocrystals are compared in detail at molecular-level.These results give us deeper insights into understanding mechano-induced phase transitions in crystalline molecular complexes.2 A thermally-induced reversible SCSC(single-crystal-to-single-crystal)phase transition in coronene-TCNB cocrystals was demonstrated.More important,the self-healing behavior was discovered in this system and a resonable mechanism for the self-healing behavior was suggested at molecular-level on the basis of the structural,microscopic and thermal analysis data.This contribution extends the self-healing concept into cocrystal realm and enriches the fundamental understanding of phase transitions in crystalline molecular complexes.3 The discovery of new structures or phases using surfactants as additives has been illustrated for organic-inorganic hybrid bromoplumbates.By employing this method,the first 3D hybrid bromoplumbates and a pair of polymorphs of MV2+PbBr3 are obtained.The polymorphism and the mother liquor-induced SCSC phase transition was investaged.We believe that this surfactant-mediated crystallization strategy could be employed as a promising strategy to prepare new crystalline organic-inorganic hybrids with distinctive physical and chemical properties.Our work demonstrate well that it is possible to design new crystalline molecular complexes with polymorphism and solid-state phase transitions based on crystal engineering.Our studies enrich the fundamental understanding of the molecular mechanism of solid-state phase transitions in molecular crystals.