Dissertations / Theses on the topic 'Bent Core Mesogens'

In der vorliegenden Arbeit wird die Synthese und die Charakterisierung von linearen, sternförmig und dendritisch verzweigten Carbosilanen mit Me2SiH-Funktionen, vorgestellt. So konnten oligomere Carbosilane mit CH2CH2CH2SiMe2- als auch CH2CH2SiMe2-Wiederholungseinheiten dargestellt werden. Als Aufbaureaktionen dienten sowohl Alkylierungsreaktionen mit Allylmagnesiumchlorid als auch Hydrosilylierungsschritte mit Chlordimethyl- und Chlordimethylvinylsilan. Die Überführung der SiMe2Cl-Gruppen in SiMe2H-Funktionen gelang durch die Umsetzung mit Lithiumaluminiumhydrid. Besonderes Augenmerk lag auf dem Nachweis und der Abtrennung der bei diesen Reaktionen auftretenden Nebenprodukten. Einen Schwerpunkt dieser Arbeit bildete die NMR-spektroskopische Charakterisierung obiger Verbindungen, wobei insbesondere 2D-NMR-Methoden und die 29Si{1H}-NMR-Spektros-kopie zum Einsatz kamen. Für die Kohlenstoff-13-Signale dieser Verbindungen konnte aus den gewonnenen Daten der Einfluss der peripheren Substituenten in einem Inkrementensystem zusammengefasst und als Zuordnungshilfe herangezogen werden. Bisher fand ein Teil dieser Verbindungen als Bausteine in flüssigkristallinen Materialien (Arbeitskreis von Prof. Tschierske, Halle) Verwendung und erste Ergebnisse dazu werden diskutiert.

博士
淡江大學
化學學系博士班
96
Depending upon the shape of molecule and the polarizability of constituting moiety, a number of peculiar mesophases can be formed with molecules consisting of various bent cores. Three types of bent shape mesogens are synthesized: hockey stick molecule, banana molecule and bent core molecule. The properties of these mesophases are studied with optical polarizing microscopy, differential scanning calorimetry, X-ray diffraction, and electric field effect. Type 1: Hockey stick molecules—bent molecules have two wings differing greatly in length. Enantiotropic anticlinic smectic C (SmCa), smectic C (SmC), smectic A (SmA) and nematic (N) phases of hockey stick molecules are obtained by tuning the length of hydrocarbon chain located at the meta-position to the carbon-carbon double bond of stilbenyl moiety of calamitic mesogenic skeletons. The thermodynamic properties and some aspects of optical appearance of these phases are indiscernible from those of calamitic mesogens. Distinctions are noted such as appearance of 2-brush defects and two types of domains in the tilted smectic phases. X-ray results indicate existence of clusters in the nematic phase and simple layer structure in the smectic phases. These phases exhibit electric field driven color switching, and all the results imply that all three molecular axes are simultaneously aligned in the N and SmC phases—a behavior unprecedented and different from those of conventional calamitic mesogens. Doping with a chiral calamitic mesogen results in formation of corresponding chiral N and ferroelectric SmC phases, but effect for the SmCa phase is not clear. Small and comparable values of spontaneous polarization are obtained for the doped and non-doped phases. Molecular organization of SmC and SmCa phases are proposed and discussed. Type 2: Banana molecules—bent molecules with two wings equivalent or nearly so. Ani series employed 2,4-disubstituted anisole as the bending unit (BU) and consisted of five phenyl rings linked with ester functional groups. The bulky methoxy group at the bending unit weakens intermolecular force and affects the molecular packing so the higher ordered mesophases disappear and thermal stability of mesophase decreases. For the system with distyrylbenzene as the BU, two systems were synthesized—five and seven rings, (XST)2-B and (XBST)2-B, rspectively. Substitutents NH2 and/or F were also attached to the BU. B2 phases were observed for all the (XST)2-B derivatives but B1 for NH2-(ClST)2-B. The B2 phases are identified to be SmCAPA subphase according to the electrooptic study and the values of spontaneous polarization varies with the electron donating and withdrawing property of the substituents. Comparing to (XST)2-B, molecules (XBST)2-B have two more phenyl rings, one at each wing, and therefore higher polarizability, so the higher ordered phase B1 is observed. The lateral chloro substituent at the terminal phenyl ring of the wings increases the steric effect and lowers the packing order, the B1 phase is converted back to B2. Compounds (XST)2-B are fluorescent for distyrylbenzene being a chromophore. The emission wavelength is blue shifted (388nm) and red shifted (473nm) when F and NH2, respectively, is attached at the BU. This behavior is attributed to the resonance stuctures formed when the electron donating and withdrawing groups are covalent bonded to the distyrylbenzene core. Type 3: Bent core molecules—both wings consist of phenyl rings but are highly un- symmetrical. 1,6-N series consists of naphthalene as the BU and with substitutents located at 1 and 6 positions. Banana phases would appear preferrably with equivalent substituents. For the series R’-ST and R’-BST, 3,4’-disubstituted stilbene is the BU. Increasing the polarizability from aliphatic chain to aromatic ring system for the substituents located at the 3-position would convert the hockey stick mesophase to banana phase. This study shows that bent molecule is capable to form mesophases provided that bent conformation, a mesophase destroying factor, and polarizability of moieties, a mesophase constructing factor, are properly balanced.

碩士
國立臺灣科技大學
材料科學與工程系
99
This research goal is the design new series of bent-core molecules based on a 3,4′-biphenyldiol central core and containing cinnamoyloxy in the side arms has been synthesized. The influence of the central core as well as that of a fluorine and chlorine substituent on the diphenyl rings in the bent-core compound has also been examined. The mesophases under discussion were investigated by means of polarizing microscopy, differential scanning calorimetry, X-ray diffraction as well as electro-optical experiments. The influence of the molecular structure on the occurrence of the SmCP phase was investigated. Finally, the electro-optical properties of the SmCP phase, such as voltage-transmittance effect, tilt angle, dielectric permittivity, and switching behavior were also measured. As a consequence, the correlation between their electro-optical properties and chemical structures of these compounds are investigated.

In der vorliegenden Arbeit wird die Synthese und die Charakterisierung von linearen, sternförmig und dendritisch verzweigten Carbosilanen mit Me2SiH-Funktionen, vorgestellt. So konnten oligomere Carbosilane mit CH2CH2CH2SiMe2- als auch CH2CH2SiMe2-Wiederholungseinheiten dargestellt werden. Als Aufbaureaktionen dienten sowohl Alkylierungsreaktionen mit Allylmagnesiumchlorid als auch Hydrosilylierungsschritte mit Chlordimethyl- und Chlordimethylvinylsilan. Die Überführung der SiMe2Cl-Gruppen in SiMe2H-Funktionen gelang durch die Umsetzung mit Lithiumaluminiumhydrid. Besonderes Augenmerk lag auf dem Nachweis und der Abtrennung der bei diesen Reaktionen auftretenden Nebenprodukten. Einen Schwerpunkt dieser Arbeit bildete die NMR-spektroskopische Charakterisierung obiger Verbindungen, wobei insbesondere 2D-NMR-Methoden und die 29Si{1H}-NMR-Spektros-kopie zum Einsatz kamen. Für die Kohlenstoff-13-Signale dieser Verbindungen konnte aus den gewonnenen Daten der Einfluss der peripheren Substituenten in einem Inkrementensystem zusammengefasst und als Zuordnungshilfe herangezogen werden. Bisher fand ein Teil dieser Verbindungen als Bausteine in flüssigkristallinen Materialien (Arbeitskreis von Prof. Tschierske, Halle) Verwendung und erste Ergebnisse dazu werden diskutiert.

碩士
國立交通大學
材料科學與工程系所
95
Conventional free radical polymerization has been used to synthesize side-chain copolymers with H-bonded/covalent bent-core mesogen. The molar ratio of resulting polymers exhibiting smectic layer arrangement is characterized by NMR spectroscopy. The synthesis of the desired bent-core complexes required two components ( H donor and H acceptor ) to be mixed in precise equimolecular proportions in a common solvent ( THF ) followed by removal of the solvent. Differential scanning calorimetry, thermo-polarized optical microscopy, wide-angle X-ray diffraction are used to characterized the liquid crystal phases of all studied materials involving covalent low molecular weight bent-core mesogens, acrylate monomers and side-chain H-bonded complexes. The electro-optic measurement is carried out by applying a triangular wave field. It is not until the constitutional ratio of H-bonded/covalent bent-core mesogen in copolymer was tuned to 10/1 that the complex exhibit ferroelectric ( FE ) property. Only one current peak can be clearly identified within a modified triangular wave indicating the FE behavior and Ps value is measured to be 50~130 nCcm-2.

碩士
國立臺灣科技大學
材料科學與工程系
100
This paper demonstrates a method to simultaneously enhance the electro-optical properties and decrease the mesophase temperature of MD-H bent-core liquid crystals by introducing organosiloxane dimeric liquid crystals, C11Si2C11 and C11Si3C11. The differential scanning calorimetry, spontaneous polarization, and switching time were systematically studied. It has been shown that the MD-H bent-core liquid crystals doped with 20wt% dimeric liquid crystals have lower mesogen temperature, faster switching time and lower rotational viscosity.