Date of Award

3-24-2019

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

First Advisor

Stephan Eichhorn

Keywords

Dyes, Liquid Crystals, Mesophase, Organic Semiconductors, Self-assembly, Triazines

Rights

CC-BY-NC-ND

Abstract

The development of electronic devices based on organic materials has been an extensive effort over the last two decades, in the search for more efficient, cost effective, and bio-compatible alternatives to inorganic semiconductors. Interest in discotic liquid crystals arises from their capacity to combine the high charge carrier mobilities found in crystalline compounds with the solubility, processability and flexibility encountered in amorphous materials. This dissertation focuses on the development of mesomorphic dyes based on 1,3,5-triazines. The thesis begins with structures possessing a more conventional liquid crystal design (e.g. symmetric disk-shaped molecules with six or more side- chains) and progresses towards the synthesis of liquid crystals with little to no side-chains. The removal of side-chains may help promote 3-D charge transport and alleviate the detrimental effects associated with defects and charge trapping sites that are commonly encountered in discotic liquid crystals. The objective of this thesis is to develop structure property relations for these types of triazine derivatives. It starts with the molecular design, and is followed by the synthesis and characterization of mesomorphism and crystal structures, and finally, important optical and electronic properties that are also relevant to organic electronics. Chapter 2 presents the synthesis and characterization of board-shaped liquid crystals based on diketopyrrolopyrrole and isoindigo dyes. The triazines serve as branching points to decrease the aspect-ratio of these typically rod-shaped molecules to generate columnar oblique and columnar rectangular phases, respectively. Chapter 3 explores the propensity for symmetric tristhiophenyl triazine derivatives with three to six alkyl side-chains with methylene linking groups to generate liquid crystalline structures. Despite the addition of polarizable/polar groups to the molecules periphery, no liquid crystalline behaviour was observed. This is mainly attributed to the molecules conformational flexibility; and the non-coplanar side-chains in triazines bearing 4,5-substitued thiophenes disrupt the disc-shaped structure that is important for mesomorphism of discotic molecules. The use of only three side-chains and an ester connecting group (Chapter 4) between the thiophene and side-chains afforded the less common discotic nematic phase, although it was metastable. A single crystal structure showed that the carboxylate linking group allowed for coplanar conformations of the side-chains and introduced strong dipole-dipole interactions with neighboring molecules that help stabilize the liquid crystal phase. Much more stable liquid crystal phases were obtained in Chapter 5 when the ester groups were exchanged with cyano acrylate groups that extended the π-system and are more electron withdrawing. Owing to the molecules’ conformational variability, these molecules possess numerous conformers with similar energies and different shapes (e.g. disk-like, rod-like, t-shaped structures). As a result, depending on the length of the side-chains used, different liquid crystalline phases were observed including the discotic nematic (triazines with ethyl, butyl, and octyl chains), smectic A and columnar lamellar phases (triazines with octyl and dodecyl chains). This demonstrates that shorter chains prefer nematic phases due to poor microphase segregation, while longer chains gave higher ordered phases because they are able to interdigitate and nanosegregate more effectively. Chapters 6 and 7 explore the use low-symmetry and conformational flexibility to drive mesomorphism of discotic structures without side-chains. In Chapter 6, low-symmetry triazines possessing three different heterocycles (pyrazole, thiophene, furan, benzothiophene, carbazole) attached exhibited high solubilities (>250 mg/mL) in organic solvents despite their lack of solubilizing chains. While mesomorphism was not observed in these structures, the crystalline phase was heavily destabilized due to the low-symmetry and 2-fold positional disorder of all attached heterocycles but carbazole. In Chapter 7, these low-symmetry triazines were converted to meta-substituted donor-acceptor structures to reduce the HOMO-LUMO gap of these molecules and potentially induce mesomorphism through strong π-stacking interactions. Triazines substituted with thiophenes bearing pyrrolidine (donor) and vinyl malononitrile (acceptor) exhibited a soft crystalline mesophase with short π-stacking distances of 3.39 Å. In addition, strong meta-coupling between donor and acceptor arms was observed leading to a broad absorption spectrum up to 650 nm.

Available for download on Thursday, April 02, 2020

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