Synthesis of new mesogens and investigation of their liquid crystalline properties

Abstract

Liquid crystals have been a subject of curiosity since the day they were discovered, and as time passed and their use developed, they gained an indispensable place in the rapidly expanding technology of the modern world. As a result of Friedrich Reinitzer's inability to explain this strange phenomenon, which he realized at the end of his work with turnips and which initially suggested that he could not purify cholesteryl benzoate, he sent Otto Lehnman samples. Otto Lehnman, who was the first person to try to define the Liquid Crystal Phase phenomenon as we define it today, who was a very competent name in the field of crystallography, attempted this definition with the expression "Fluid Crystals" in his time. Although he faced serious opposition because he introduced this definition in his period, the reality of this notion is obvious as it is brought by studies and modern science. Today, these intermediate phases, which are exhibited by liquid crystalline substances, are called mesophase, while chemical molecules displaying this feature are called mesogenes. Thanks to the studies on them, more than one mesophase of Liquid Crystals has been defined today. More than one mesophase, nematic, smectic, columnar, cholesteric (chiral nematic) and lyotropic, enables different usage areas and new materials are being developed in this context day by day. In this context, the main focus of this thesis has been to design, synthesize, characterize and examine the usage areas of different types of new mesogens, ranging from high molecular weight to low molecular weight or covalent to non-covalent bonded structures. For this reason, the studies in this thesis have been revealed in the form of presenting different articles under five main headings. In the first two studies, different complexes of known liquid crystal mesogens with different polymer chains, which are designed to carry hydrogen bond acceptor side-substituted groups, were prepared to form hydrogen bonds, and the change in their mesomorphic properties was investigated. They touch on a very important point, as the use of high molecular weight liquid crystal materials can spread to areas as wide as the usage areas of the polymers for which they are designed. In the third study, an original material from the chalcone compound class, which has very important biological and optical properties, was prepared in order to form a hydrogen bonded liquid crystal complex material and its properties were investigated. The focus of the fourth study was the establishment of a hydrogen bond between a new carbamate molecule, which has both hydrogen bond donor and acceptor properties, and LC11, a mesogen with previously studied boundaries. After investigations on that, drastic changes in the mesomorphic properties of the LC11 mesogen were observed. These changes were studied computationally and experimentally then the reasons for the change which were experimentally observed, were explained by computational methods. In the fifth study, two new mesogens were designed, synthesized and characterized. These new cholesterol-based materials are completely covalently bonded structures and do not require any intermolecular bonds to display mesomorphic properties. The dielectric properties of these materials, which have both biological and optical properties, have also been studied. In first study, a new polystyrene-based side chain liquid crystalline polymer (EP-PS-LC11)) has been prepared from polystyrene having ethylpiperazine moiety as a hydrogen bond acceptor polymer and 11-(4-cyanobiphenyl-4(-oxy) undekan-1-ol (LC11) as a bond donor bymolecular self-assembly processes via hydrogen-bond formation between nitrogen of ethylpiperazine on polymer chain and hydroxyl group of the LC11. The formation of hydrogen bond has been confirmed by using FTIR spectroscopy. The liquid crystalline behavior of the EP-PS-LC11 has been investigated by a differential scanning calorimeter (DSC) and polarized opticalmicroscopy. The thermal behaviors of the H-bonded liquid crystalline polymer have been investigated by DSC measurements. The dielectric properties and ac conductivity mechanism of EP-PS-LC11 have also been investigated by impedance spectroscopy within the frequency interval of 5 Hz–15 MHz. The dielectric relaxation type of EP-PS-LC11 has been analyzed by fitting dispersion curves of ε′-ω. The system obeys nearly Debye and non-Debye type relaxations for the low and high frequency regions. In addition, it has been revealed that since the dielectric strength decreases by increasing frequency, the LC compositemolecules can align more easily at high frequency. Moreover, the variation of imaginary component of dielectric constant with frequency shows two relaxation peaks. While the low frequency relaxation peak corresponds to molecular vibration, the high frequency relaxation peak is attributed to molecular orientation. The frequency dependence of ac conductivity has also been analyzed by means of frequency exponent, s. Depending on the frequency deal with, the system exhibit nearly dc, correlated barrier hopping (CBH) and super linear power law (SLPL) behaviors. In the second paper, a new PEG-containing liquid crystalline side chain block copolymer HBC-PEG550 was prepared from poly (ethylene glycol)-b-poly(2-(diethylamino) ethyl methacrylate copolymer (BC-PEG550) and 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) by molecular self-assembly processes via hydrogen bond formation between amine group of HBC-PEG550 and hydroxyl group of the LC8. The formation of H bond was confirmed by using FTIR spectroscopy. The liquid crystalline behavior of the HBC-PEG550 was investigated by differential scanning calorimeter (DSC) and polarized optical microscopy. Dielectric properties of BC-PEG550 and HBC-PEG550 has been studied by impedance spectroscopy. Real and imaginary parts of complex dielectric constant, impedance, and energy loss tangent factor for BC-PEG550 and HBC-PEG550 have been characterized in the frequency range of 100 Hz–15 MHz. To investigate a low molecular weigth system, a new chalcone based nematic schlieren texture liquid crystalline material was prepared from 3-(4-(dimethylamino) phenyl)-1-(4-hydroxyphenyl) prop-2-en-1-one (DMAC) and 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) by molecular self-assembly processes via hydrogen bond formation between tertiary amine group of DMAC and hydroxyl group of the LC8. Hydrogen bond formation was observed by FTIR spectroscopy. Dielectric properties of DMAC has been studied by impedance spectroscopy. The Cole-Cole analysiswas used to determine the Resistance-Capacitance (RC) components equivalent circuit situation of doped LC8 on theDMAC. Dependent angular frequency variation of dielectric parameters and ac conductivity properties are non-Debye type relaxation mechanism of interfacial dopant in the liquid crystal molecular structure of the reorientation polarization effects. To understand the effect of hydrogen bond, it is presented a combined experimental and theoretical study on the novel hydrogen-bonded liquid crystalline complex (UR-LC11) exhibiting both nematic and smectic phases upon cooling. The complex was prepared by mixing 2-(2-methoxyethoxy) ethylbutyl carbamate (UR) as H-bond acceptor with calamitic mesogen 4′-((11- hydroxyundecyl) oxy)-[1,1′-biphenyl]-4‑carbonitrile (LC11) as H-bond donor. The complex was characterized by FTIR technique and its liquid crystalline properties were studied by differential scanning calorimetry (DSC) and polarized optical microscope (POM). The experimental IR spectra were compared with theoretically obtained IR spectra by Density Functional Theory (DFT) to suggest the structure of hydrogen-bonded liquid crystal (LC). The molecular dynamics (MD) simulationswere performed to understand the impact of hydrogen bonding on the mesomorphic behaviour of the complex and the temperature dependency of the transitions between the mesophases. We determined that UR-LC11 is a stable H-bond acceptor/donor type complex and a single H-bond forms between the carbonyl oxygen atom of the amide moiety of UR and the hydrogen atom of the terminal hydroxyl group of the LC11. Although LC11 is present only in nematic liquid crystalline form, the new complex displayed both nematic and smectic phases during cooling. The reason for the two distinctive LC phases was explained by the presence of hydrogen bond interactions, which provides structural flexibility. Besides, H-bond maintains uniaxial rod shape of the molecule to promote self-assembly behaviour and induces positional ordering in the smectic phase. The enhancement in the self-assembly of the H-bonded chains in the complex is reflected in the increased ΔHfusion values. Due to the intermolecular π-π interactions of the phenyl rings and the formation of strong dipoles on the backbone, especially at the cyanobiphenyl end of the chains, the longrange directional order of the dipoles along their long axes are preserved at elevated temperatures and nematic to isotropic phase transition is observed at around 370 K both experimentally and theoretically. After working on hydrogen-bonded complexes, for the purpose of investigating newly designed biologically effective and chiral mesogens two materials prepared. Cholesteryl chloroformate which is known liquid crystal material was modified to give the new molecules a long-lasting LC phases with the stability of the carbamate and aromatic functions. Two new mesogens cholesteryl 1H-imidazole-1-carboxylate (Cho-Imi) and Cholesteryl (4-((E)-phenyldiazenyl) phenyl) carbamate (Cho-Diazo) were synthesized starting from Cholesteryl chloroformate and (E)-4-(phenyldiazenyl) aniline in the presence of triethylamine (TEA) as an acid scavenger at room temperature. Structural characterization of the obtained LC compounds was performed by FTIR and H-NMR. These new compounds, which show more than one LC phase different from the LC phases of the starting material, showed that they are promising as a result of the analyses. The mesomorphic properties were examined by polarized optical microscope (POM) and thermal properties of theLCcompounds were determined by DSC and TGA. The dielectric properties of synthesized liquid crystal samples were investigated using dielectric spectroscopy technique. It was seen that both new syntheses mesogen cholesteryl carbamates-based liquid crystal samples exhibit non-Debye type relaxation properties. The cole–cole analysis by dielectric spectroscopy was used to determine the equivalent devices of this new Cholesteryl carbamate-based mesogens. The samples have different dielectric properties in different frequency regions will provide significant flexibility as a working frequency region in engineering, biological and genetic application areas.