Date of Award
2-5-2025
Publication Type
Dissertation
Degree Name
Ph.D.
Department
Chemistry and Biochemistry
Supervisor
S. Holger Eichhorn
Rights
info:eu-repo/semantics/embargoedAccess
Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-No Derivative Works 4.0 International License.
Abstract
Flexible side-chains are an integral part of the molecular design of thermotropic liquid crystals as their structure, position, and number controls the type of mesomorphism and phase transition temperatures. However, these aliphatic side-chains generate electronically insulating regions that limit charge transport and often complicate their purification and processing. Hence, removal of these side-chains while retaining liquid crystalline character is desirable for producing mesomorphic organic semiconductors with enhanced charge transport properties. Few reports on calamitic and discotic liquid crystals without side-chains exist, though their structure-property relationships are not well understood, and the reported compounds are generally not suitable for use in organic electronic applications, as they tend to be insufficiently conjugated or exhibit impractically high mesophase transition temperatures. This dissertation establishes a greater fundamental understanding for how mesomorphism can be induced without the use of side-chains and provides molecular structural design criteria that can be followed to give predictive structure-mesomorphism relationships for generating specific mesophase types within specific temperature ranges. This ultimately has generated core-only mesomorphic organic semiconductors with superior charge transport properties to liquid crystals bearing side-chains. Presented in each chapter is the molecular design, synthesis and characterization of libraries of compounds for comparison of their optical and mesomorphic properties to determine structure-property relationships. Supporting evidence was also provided by crystal structure analysis and computational calculations. Chapters 2 and 3 focus on generating core-only discotic liquid crystals using a 2,4,6-(hetero)aromatic-1,3,5-triazine core. Chapter 2 explores the use of donor and acceptor groups on thiophene rings attached to an unsymmetrically substituted triazine core. A few compounds display soft crystalline mesophases and the first nematic discotic core-only mesophase is reported. Also unprecedented is the strong electronic communication between meta-conjugated subchromophores viii that was experimentally observed as a broad absorption tail, which is reasoned due to low symmetry and rotational isomerism. Chapter 3 incorporates fluorine and chlorine groups on symmetrical and unsymmetrical 2,4,6-trithienyl-1,3,5-triazine derivatives as these halogens have been shown to induce mesomorphism. Unfortunately, none was observed in these compounds, but extension of the conjugated core by adding two additional thiophenes to each triazine arm afforded a columnar hexagonal plastic mesophase. Chapters 4, 5, 6 and 7 focus on generating core-only calamitic liquid crystals, with Chapter 4 serving as the foundational study where aspect ratio (molecular length divided by width), polarizability, dipole moment and rotational energy barriers were molecular and electronic properties investigated to help explain observed mesomorphism. The minimum aspect ratio to induce mesomorphism was determined to be 2.7, while functionalization of the 5-position of the terminal thiophene ring was also required for inducing mesomorphism. Variations to this terminal functional group affected mesophase structure (nematic and/or smectic A) and phase transition temperatures. Exchange of heteroatoms within these conjugated rings was also tested, including the incorporation of pyridine and diazine rings instead of benzene in Chapter 5. Chapter 6 investigates the effects of exchanging benzoxazole for benzimidazole, where methylation of one benzimidazole nitrogen was essential for retaining nematic mesomorphism, and an attempt at the first ionic core-only calamitic liquid crystal by methylation of the second nitrogen was performed. Finally, Chapter 7 explored the effect that using fluorine as lateral substituents has on this molecular template, where substitution of fluorine group(s) onto specific rings could enhance, hinder or have minimal impact on resulting mesomorphism. Key results were fluorination of the inner benzene ring reducing phase transition temperatures and fluorination of benzoxazole’s benzene ring increasing nematic phase stability at the loss of the smectic A phase. Surprisingly, these all-aromatic calamitic structures displayed strong fluorescence in solution and solid-state, while their quantum yields appear to have an excitation wavelength dependence.
Recommended Citation
Rothera, Jacob Gerard, "Generation of Core-Only Calamitic and Discotic Liquid Crystals for Use in Organic Electronics" (2025). Electronic Theses and Dissertations. 9645.
https://scholar.uwindsor.ca/etd/9645