Studies on the Growth of Metastable Metal Phthalocyanine Crystals by Physical Vapor Transport Process

Abstract

Metal phthalocyanine (MPc) molecules are old organic dyes named and identified in their structure since the 1930s. They have attracted great attention due to their properties that can be caused by having a molecular structure with 18 pi-electrons and various central metals. In particular, MPc's high charge carrier mobility, strong absorption bands in the infrared region, and photosensitizer properties are still widely applied to electronic devices, optical devices, and cancer therapy. These properties largely arise from two characteristics: the intrinsic characteristics of the molecules themselves of MPc and the characteristics caused by the molecular packing from the crystal structure. Various and complex chemical processes that modify the molecule's functional groups are required to improve the intrinsic properties of the molecule itself. This process may not only increase the price of the product, but also adversely affect its characteristics as the content of impurities increases. On the other hand, in order to improve the properties of the crystal structure, this can be implemented by controlling weak intermolecular interactions without inducing a chemical reaction. However, due to weak interaction between molecules, molecular packing varies depending on crystal growth conditions, making it difficult to control. In this study, to improve the properties of MPc, we aimed to implement it as a physical vapor transport (PVT) method, aiming to study crystal growth that controls the crystal structure. The PVT method is a method of making crystals based on the difference in vapor pressure according to temperature, and is suitable for the formation of MPc crystals due to low solubility and sublimation. In addition, because the PVT method has a high purity through the sublimation process, it produces single crystals with fewer impurities and defects. This has the advantage of making it possible to get high performance of the application as well as correlations between crystal structure and properties. However, the PVT method mainly produces thermodynamically stable forms, so obtaining metastable forms in high yields or discovering new metastable forms is still a challenging task. In this study, novel synthesis methods for obtaining metastable MPc crystals by introducing new variables in the PVT process were presented, which resulted in securing improved properties. Part 1 explains the basic contents needed to understand Part 2 - 4 and the overall overview of this thesis. Part 1.1 introduces the general features of molecular crystals. Unlike inorganic crystals, molecular crystals are bound by weak intermolecular interactions, resulting in several characteristics. Therefore, we introduce weak intermolecular interactions, and describe what kind of molecular packing leads to, changes in crystal properties and crystal characteristics. Part 1.2 describes MPcs, one of the molecular crystals. MPcs show various characteristics and properties depending on their composition and crystal structure, and describe how they are applied in electronics, photonics, and biomedics. In addition, the PVT method, one of the methods of forming MPc crystals, was described, and the principles and characteristics of MPcs were introduced. Part 1.3 introduces polymorphisms from a thermodynamics and kinetics perspective and examines the current status and limitations through research cases that effectively control them. Part 2 reports a method of acquiring metastable a-form zinc phthalocyanine (ZnPc) nanowires in high yield using PVT methods. a-form ZnPc nanowire is a photosensitive agent for cancer therapy and not only shows excellent photodynamic properties, but can also be used in the body due to its high water dispersibility. However, due to the mixture of stable b-form, there were restrictions on applying them. Through this study, the size of the crystal was limited by adjusting the flow rate of the carrier gas in the PVT system, which made the a-form stable and obtainable at high yield. As the flow rate increased from 50 sccm to 2000 sccm, the width of the nanowire decreased significantly from about 500 nm to 50 nm, and accordingly, the content of a-form increased significantly from about 25% to 98%. Furthermore, we demonstrated that it is applicable to copper phthalocyanine, securing versatility for these strategies. Part 3 makes a new C-phase titanyl phthalocyanine (TiOPc) tube crystal and reports high photoconductivity properties. The conventional C-phase was not obtained by crystal, but was reported only in powder form, so the exact crystal structure and its electrical characteristics could not be confirmed. Through this study, we proposed a method in which the C-phase can be stably formed into a crystal by introducing surface modifications of the substrate in the PVT method. As a result, it was possible to create a new tube-type TiOPc crystal that had not been previously reported, and showed 74 times photoconductivity, much higher than previously known a- and b-phases, showing its applicability as optoelectronic devices. In addition, the correlation between crystal structure and properties was discovered by identifying the cause of the high photoconductivity of the C-phase through single crystal structure, photoluminescence, and absorption spectrum analysis. Part 4 presents a method for strategically making chiral crystals from achiral molecules by introducing a screw dislocation in the C-phase TiOPc tube introduced in Part 3. There are cases of forming chiral crystals from achiral molecules, but synthetic strategies for this are still insufficient. Through this study, helical tube crystals with chiral properties were successfully synthesized by introducing screw dislocation through the substrate temperature control during the crystal growth process in the PVT method. It was confirmed through Burgers vector analysis that the introduction of screw dislocation creates instability between interfaces and causes distortion to twist them, resulting in helical crystals. This is the first report to create helical crystals by introducing screw dislocations to organic molecular crystals, suggesting a new strategy for creating an unstable form of helical crystal from achiral molecules.