A Combined Study of Mesomorphism, Optical and Electronic Properties of Novel Columnar Liquid Crystals
Columnar mesophases of discotic liquid crystals (DLCs) are a separate class of organic semiconductors. Anisotropic charge transport, high charge carrier mobility, and self-organizing properties have been regarded as superior properties of DLCs but their performance in devices, such as organic light-emitting diodes, has been inferior to the best performing conventional organic semiconductors. Our group recently demonstrated that a DLC of donor-acceptor structure can bias electron over hole conduction if the occupied and unoccupied frontier orbitals are localized on different parts of the molecule and only the unoccupied frontier orbitals electronically interact in columnar stacks. This thesis describes the first systematic approach to different types of donor-acceptor DLCs and the investigation of their mesomorphism and optoelectronic properties. In spite of their large aspect ratios, four board-shaped donor-acceptor-donor quinoxalinophenanthrophenazine (QPP) dyes (Chapters 2 and 3) induce complex columnar mesomorphism over wide temperature ranges. These QPP derivatives also show strong fluorescence in solution and as thin films. This strong fluorescence in columnar mesophases is explained with a nonparallel orientation of the elongated donor-acceptor cores in the columnar stacks that minimizes interactions between transition dipole moments of co-facially stacked molecules. Opto-electronic properties suggest that the QPP derivatives are organic semiconductors. Inspired by the donor-acceptor concept, we generated novel ionic DLCs based on imidazole and imidazolium structures (Chapter 5). Unexpectedly, only the ionic triphenylenoimidazolium derivatives display columnar mesomorphism. Strong ionic interactions between the cores are apparently required for the formation of columnar mesophases. Overall, these compounds are rather unusual because few ionic DLCs have the charge localized on the aromatic core. Most often the charged group is attached via spacer chains. A first review (Chapter 4) dedicated to thermotropic ionic DLCs was published in the Isr. J. Chem. Finally, two star-shaped donor-acceptor DLCs based on a hexaazatriphenylene core (Chapter 6) are reported that display columnar mesomorphism over wide ranges of temperature. The initial microwave conductivity data indicate that both compounds are probably electron conductors and ToF mobility measurements are presently conducted to verify this conclusion.
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