용액상 풀러렌 분자들의 결정화 조절

Abstract

This thesis introduces factors that affect self-assembly of fullerenes, and presents studies of controlled fullerene crystallization. Part I introduces fundamental information that is necessary to understand parts II–IV. Section I.1 summarizes the structures, solubility, and spectroscopic properties of fullerenes. Molecule shape, distance between carbon atoms, and angles in pentagons/hexagons are introduced. Solubility of fullerenes in organic solvents that are used for fullerene crystallization are shown. Spectroscopic information (absorption/emission/vibration) which is very important to understanding of fullerenes and their interactions with other molecules, is summarized. Section I.2 describes crystallization methods, particularly direction/location-controlled crystallization, and heterostructure crystallization using fullerenes. To aid understanding, gas-phase crystallization and solution-phase crystallization methods are described separately. Especially, solution-phase crystallization, drop-drying, solvent vapor annealing, and reprecipitation are used in parts II–IV. Direction/location-controlled crystallizations to obtain fullerene crystals for specific purposes are introduced; the methods include vertical growth of fullerene crystals by using solution-phase crystallization with vertically aligned template, and location-controlled growth of fullerene crystals by using gas-phase crystallization. Heterostructure crystallization is described; it involves crystallizations using polymer-fullerene or organic molecule-fullerene. Part II presents a method to achieve vertical crystallization of C60 nanowires on a substrate. This alignment is required to apply crystals to specific fields, but the method is quite limited, especially methods that use conjugated molecules. Most methods have economic limitations because they demand high temperature, specific template/substrate, or physical vapor transport environment. We use solvent vapor annealing (SVA) to simply grow fullerene nanowires vertically on an arbitrary substrate without a template. Inspired by the observation that the direction of fullerene crystallization is affected by the direction of solvent movement during drop-drying process, SVA was used because it can maximize vertical movement of solvents (i.e., solvent evaporation). The number of solvent molecules in action during evaporation is proportional to vapor pressure of solvent. Thus, the degree of vertical growth (ratio of vertical crystals to laterally grown crystals) of fullerenes follows the order of vapor pressure of solvents. By applying the same method, vertically grown 2D fullerene crystals and anisotropic pentacenetetrone can be obtained. Considering its applicability to other crystallization system, the SVA process with an appropriate selection of molecule and solvent has possible applications in vertical growth of various organic/conjugated molecules. Part III presents different fullerene crystallizations using the same solvent combination. One limitation to crystallization of fullerene to alter their morphologies is that specific solvent molecules should be replaced. However, we report a simple method to control the morphology of C70 crystals to be cubic or tubular by controlling the volume ratio between two solvents. These crystals have different crystal structures (cubic vs hexagonal) as a result of different ratios of mesitylene in the crystal. Cubes form when mesitylene is dominant; tubes form when it is deficient. The same tendency was confirmed when different alcohols (antisolvents) were used. To describe this dependence on concentration, we proposed a new mechanism that starts from the homogeneous solution because of good miscibility of mesitylene and isopropyl alcohol. The absolute amount of mesitylene near C70 molecules is determined by the ratio between mesitylene and IPA, and determines the crystallization pathways to form the C70·2(mesitylene) or C70·0.7(mesitylene) phase, which result in C70 cubes and C70 tubes, respectively. Part IV presents a method to obtain novel fullerene crystals (fullerene flowers). Solution-phase crystallization is an effective method to change the local environment around solute molecules by modifying the kinds of solvents and the ratio between two solvents. Because solvent-environment-dependent crystallization of fullerene is already well known, fullerene is a candidate for use in fabricating new crystals with it. We used C70-mesitylene system that can crystallize as cubic, tubular, and bumpy rod-shaped crystals, and used C60 as the other shape-controlling factor. As a result, flower-shaped fullerene crystals having six petals with 6-fold symmetry formed. They were composed of C60, C70, and mesitylene. We suggest their growth mechanism as follows; lower solubility of C70 than C60 causes dominant participation of C70 in the initial crystallization and it forms center, then C60 participation (with C70) affects petal formation. The antisolvent, ethanol, makes a significant contribution to this mechanism; it determines the order in which each fullerene participates in the growth. By exploiting this mechanism, other various fullerene crystals can be grown. One, two, and multi-deck (of petals) fullerene flowers were obtained by controlling the start point of petal formation. Fullerene flowers selectively grown at center-to-center to both tips of C60 tubes and edge-grown fullerene flowers can be obtained using different C60 tubes and fullerene flowers as seed crystals, respectively.