Crystal Growth,Solid-State Phase Transition And Properties Of Molecular Complexes:Studied In Crystal Engineering

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 crystal engineering is a natural outcome of the interplay between molecules and crystals. How to predict the crystal structure from specified molecules? How do molecules organize themselves in crystals? These are two core questions that are being addressed by the crystal engineering. Crytsal engineering is an interdiscipline that lies at the crossroads of materials science, organic chemistry, structural chemistry and X-ray crystallography.With developing of modern science and technology, crystal engineering has come a long way since the 1980s. A number of terms (solid-state reaction, polymorph, cocrystal, coordination polymer, metal-organic framework, hydrogen bond, halogen bond) have been lively discussions. Crystal engineering has played significant roles in many fields, such as solid-state organic chemisty, pharmaceutics and materials science.However, in order to answer the two core questions of crystal engineering, researchers need to be more in-depth understanding of the relationships between the intermolecular interactions and the packing patterns in crystalline state, even to understand crystallization processes of molecular materials. In this context, the future of crystal engineering will require researchers to observe the dynamic process how molecules assemble into crystals in real-time. So far, the crystallization of molecular materials has been studied mainly by means of diffraction, spectroscopy and computational method, which do not really visualize the real microscopic kinetic processes. The solid process of molecular materials (such as single-crystal-to-single-crystal phase transition and solid crystal growth, etc.) may able to image the dynamic changes by optical, electronic and scanning probe microscope in real-time. Combining these results with X-ray crystallography will help to fully answer the two core questions of crystal engineering.Based on the above considerations, this thesis is focused on designing molecular complexes, crystal growth, imaging microscopic kinetic processes in solid-state and understanding the relationships between structures and properties. According to crystal engineering, we designed and discovered new crystalline phases of 7,7,8,8-tetracyanoquinodimethane-p-bis(8-hydroxyquinolinato)copper(II) (CuQ2-TCNQ, Form Ⅱ) and 1,2,4,5-te-tracyanobenzene-p-bis (8-hydroxyquinolinato) copper(II) (CuQ2-TCNB). We investigated the mechanical force induced single-crystal-to-single-crystal phase transition (SCSC) in CuQ2-TCNQ. We imaged the solution-free on-surface crystallization of a metal-organic coordination compound [Ni(quinolone-8-thiolate)2] ([Ni(qt)2]) in situ. The experimental results revealed nanoparticle-mediated migration and oriented attachment pathways in solid state growth process in the molecular crystal for the first time. Large sized organic-inorganic hybrid nonlinear optical crystals tri-diethylammonium hexachlorobismuthate (TDCB) and tri-diethylammonium hexabromobismuthate (TDBB) were successfully grown by the slow-cooling method. Optical and other physical investigations such as transmittance spectra, SHG effect, thermal expansion, and laser-induced damage threshold are researched. We also preliminary investigated the relationships between structures and SHG properties in TDCB and TDBB. The main contents are as follows:1. By analyzing the crystal structures of molecular complexes MQ2-TCNQ (M for metal atom) in the Cambridge structural database (CCDC), we identified synthons in the system and utilized such synthons to design new crystalline phases in CUQ2-TCNQ and CUQ2-TCNB. The single crystals of Form IIcuQ2-TCNQ and CUQ2-TCNB were successfully grown. Their crystal structures have been determined.2. We discovered the new polymorph of Form IICuQ2-TCNQ that shows a mechanical force induced single-crystal-to-single-crystal (SCSC) phase transition. The crystallographic studies, microscopic kinetic and thermal analysis of the SCSC phase transition have been investigated. The main conclusions show as follows:(1).The changes in the packing motifs of the molecular layers-the dihedral angle decreasing from 55.6°in Form Ⅱ to 27.7° in Form Ⅰ (decrease≈49.8%) and the repetition period increasing from 8.035 A in Form Ⅱ to 14.098 A in Form Ⅰ (increase ≈75%)-result in the crystal being approximately half its original thickness and double its original length. (2). According to the results of high speed photography and mechanical experiments, we observed the deformation of crystals caused by the phase transiton always along the [100] direction of Form Ⅱ. We explained the phenomenon by analyzing the anisotropy of crystal structure, thermal expansion of crystal axes and attachment energy of crystal planes. That is, the (001) plane has the smallest attachment energy and the a-axis has the biggest thermal expansion coefficient, which help the deformation to occur along the [100] direction.(3). Based on crystal structural calculation and spectroscopy results, we demonstrated there is basically noelectronic state change between the two phases. The differential scanning calorimetry (DSC) measurements indicated that the phase transition is monotropic and temperatures of the phase transition are depended to the degree of crystal size and perfection. (4). Based on the duration of the phase transition and microscopic kinetic observations, we proposed a reasonable mechanism to describe the phase transition as follows:at first, mechanical stimulation on the surface of Form Ⅱcrystals produces a large number of nuclei, which results in molecular rearrangement triggering the phase transition; then, reconstruction of the 2D layers takes place layer-by-layer; finally, the complete transition induces dramatic changes in the dimensions of the crystals.3. By using "retrosynthesis" method in crystal engineering, we found the Ni(qt)2 is a suitable material for observing the thin film to single-crystal transition. Using in-situ high-temperature atomic force microscopy (AFM) during the solvent-free crystallization of an organic compound [Ni(quinolone-8-thiolate)2] ([Ni(qt)2]), we have observed long-range migration of nanoparticles on a silica substrate, and their incorporation into larger crystals, suggesting a non-classical pathway in the growth process of the molecular crystal.4. Organic-inorganic hybrid nonlinear optical crystals TDCB and TDBB were designed under the guidance of the double radical structure model theory. Large sized single crystals were successfully grown by the slow-cooling method. The morphologies, crystal structures, optical and other physical investigations such as transmittance spectra, SHG effect, thermal expansion, and laser-induced damage threshold are researched. The main conclusions show as follows:(1) TDCB and TDBB crystallize in the trigonal system, the R3c space group. Its morphology has been indexed to reveal the major facets of the crystal to be{1120} and {0112}. (2) Transmittance spectra of TDCB show an optical transmission in the entire visible region with the cutoff wavelength at 365 nm. The powder second harmonic generation (SHG) measured by using the Kurtz and Perry technique indicates that TDCB is a phase-matchable NLO material with a SHG efficiency of 1.8 times that of KH2PO4 (KDP). (3) Its specific heat (Cp293K=1.05 J g-1 K-1; Cp380K=1.42 J g-1 K-1) and thermal expansion (α11=2.308 × 10-5 K-1; α33=1.653 × 10-4K-1) were investigated as a function of temperature, and the relationship between the structure and the thermal properties has been discussed. Laser-induced damage threshold measurements show a threshold up to 2.32 GW cm-2. (4) Thermodynamic measurements and temperature-dependent complex dielectric of TDCB reveals that there exists a phase transition in 373 K. (5) The melting point of TDBB is about 191℃. Which means the thermal stability of TDBB is better than TDCB. The SHG efficiency of TDBB is 3.3 times that of TDCB.5. Based on the synthon theory in crystal engineering, we have developed a preliminary analysis of the relationships between the crystal structures and SHG properties in TDCB and TDBB. We found the significant changes of intermolecular interactions between organic and inorganic components. In order to confirm that which synthon is more favorable to SHG property, we conducted some preliminary experiments for TDXB (where X means one atom position is occupied by Cl and Br atoms in different proportion) crystal growth.