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Conformational dynamics of linked discotic mesogens

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Thesis type
(Thesis) Ph.D.
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Disc-shaped molecules often assemble into columnar liquid crystal phases, which are a promising class of materials that can be utilized in applications such as organic light emitting devices (OLEDs) and photovoltaics. Discotic dimers, which consist of two tethered discotic mesogens, also form columnar phases, but only when the linking group exceeds a critical length. However, the effects of the linking group on the self-assembly and solution properties remain poorly understood. A strategy was developed in order to design discotic dimers that self-assemble into liquid crystals even when the linking group is short. These studies investigated the effects of varying this linking group and how it effects the observed solution and liquid crystal properties. Disc-shaped dimers were formed through functionalization of compounds prepared from the condensation of 2,3,6,7-tetrakis(alkoxy)phenanthrene-9,10-diones with 3,4-diaminobenzoic acid. The solution properties of these dimers were studied by a variety of NMR techniques, while the materials properties were characterized via polarized optical microscopy, differential scanning calorimetry, and variable temperature X-ray diffraction. It was confirmed that the flexibly-linked dimers adopted intramolecular folded conformations. This folded conformation facilitated the formation of a columnar hexagonal liquid crystal phase in all cases, despite the short linking group for some of the compounds studied. This folded geometry was found to be largely independent of concentration, temperature, and solvent, indicating that the folded geometry is robust. In order to determine if the flexibility of the linking group was essential to the folded geometry observed for the flexibly-linked dimers, a phenyl linker dimer was prepared in order to investigate the solution and phase behaviour of a conformationally-rigid dimer. This compound cannot adopt a folded conformation in solution or the liquid crystal phase, but nonetheless is able to form a columnar oblique liquid crystal phase. A dimer that had properties of both the rigid and flexibly linked dimers was synthesized by utilizing cyclohexane as a linking group in order to further investigate and quantify the folded conformation adopted by the flexibly-linked dimer. This compound was found to adopt predominately a diaxial conformation, despite the large substituents, which facilitated the formation of a columnar hexagonal liquid crystal phase. A flexibly-linked discotic trimer was prepared in order to investigate higher order oligomers and to investigate what effects the addition of a third disc would have to the solution and materials properties that were observed for the dimeric systems. This compound was found to adopt an intramolecular folded conformation in which only two discs are folded at any given time in solution. This dominant conformation resulted in the absence of liquid crystal phase formation.
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Thesis advisor: Williams, Vance
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