Doctor of Philosophy, University of Akron, 2010, Polymer Science

Organic materials are part of the 3rd generation photovoltaics which focus on providing cost-effective energy production and ease of use. Moreover, organic materials are easy to process, are chemically flexible, and are easy to handle. One method to overcome an intrinsic disadvantage in organics, small exciton diffusion lengths, is to utilize π-π stacking in aromatic materials. One example is discotic liquid crystals which can self-assemble to form continuous charge conduction pathways. A new series of porphyrin based discotic liquid crystals had been previously synthesized and had shown photovoltaic efficiencies of 0.7%. This was very high for a potentially unaligned discotic phase. To understand the reason for this, the structure and morphology of these materials was characterized using DSC, WAXD, SAED and PLM amongst other techniques. The alignment of discotic liquid crystals has been problematic in the past due to their high viscosities. Only thermal gradient alignment and magnetic field alignment proved successful in large scale orientation of these materials. This series showed a stable liquid crystalline phase at room temperature and also a metastable solvent induced crystal phase. The peripheral n-alkyl chain length was varied from C8 to C12. The liquid crystalline phase for all samples showed a ribbon-like morphology. WAXD showed a hexagonally packed columnar structure with significant helical ordering within the columns. A coiled coil structure has been proposed as the only possible structure. This stable Col*hh phase was seen in all the samples. This work is the first observed evidence of a coiled coil structure for an achiral discotic liquid crystal. This would indicate that discotic columnar systems mimic biological systems where coiled coils are found commonly. Sample QE12C also showed a unique undulating morphology. To understand the morphology the n-C14 alkyl chain length molecule was investigated. Characterization showed that the undulating phase (open full item for complete abstract)

Committee: Stephen Cheng Dr. (Advisor); Mark Foster PhD (Committee Chair); Dhinojwala Ali PhD (Committee Member); Carri Gustavo PhD (Committee Member); Pang Yi PhD (Committee Member) Subjects: Materials Science; Physics; Polymers

PHD, Kent State University, 2023, College of Arts and Sciences / Department of Chemistry and Biochemistry

Liquid crystals are known as the fourth state of matter, and it is found between two condensed phases solids and liquids. Liquid crystals were first discovered in 1888 by an Austrian botanist and the term “Liquid crystals” was termed in 1904 by a German physicist. In the long run wide varieties of liquid crystals are known. In this thesis the classes of discotic and calamitic liquid crystals are covered. They have potential applications as supercapacitors and organic semiconductors. The chapters of this thesis are divided into following topics: 1. Tetrafluoro-tetraalkoxy triphenylene discotic compounds. There is a misbelief amongst researchers that long flexible tails on the periphery of a rigid aromatic core are a prerequisite for formation of discotic liquid crystals (DLCs). In this study we have synthesized triphenylene based liquid crystals which have significantly shorter tails on the periphery of the core and still the mesophase is retained. These molecules defy the classical understanding about discotic liquid crystals. 2. Influence of fluorination on the mesogenicity of 6,7,10,11-tetramethoxytriphenylene compounds. Amongst the short tailed DLCs, 1,2,3,4-tetrafluoro-6,7,10,11-tetramethoxy triphenylene has the shortest tails that can retain the mesophase. In this study, we try to learn about the influence of the extent and position of fluorination on the compounds mesogenicity. 3. Chloro-fluoro triphenylene discotic compounds. In this study we have synthesized triphenylene molecules with only fluorine and chlorine substituents, yet we were able to find mesophases in many of these compounds. These compounds belong to the class of strictly tail free compounds with columnar mesophases. 4. No-tail but larger aromatic core discotic compounds. Since we have a few different sets of short tail or no-tail DLCs based on the triphenylene core, we aim to find this behavior in compounds with a larger aromatic core. In this study, we have been able to synthesize s (open full item for complete abstract)

Committee: Robert Twieg (Advisor) Subjects: Chemistry; Organic Chemistry

PHD, Kent State University, 2022, College of Arts and Sciences / Department of Physics

Discotic liquid crystals are typically described as consisting of a rigid molecular core surrounded by long flexible tails. This work explores a group of tail-free discotic liquid crystals, which possess a liquid crystalline mesophase despite completely lacking tails. These compounds are based on 1,2,3,4-tetrafluorotriphenylene with additional electronegative substituents, and form hexagonal columnar mesophases. We describe their behavior using a combination of thermal analysis, X-ray scattering, charge mobility measurements and molecular dynamics simulations.

Committee: Brett Ellman (Advisor); Robert Twieg (Committee Member); Arkaprabha Konar (Committee Member); Samuel Sprunt (Committee Member); John Portman (Committee Member) Subjects: Physics

PHD, Kent State University, 2016, College of Arts and Sciences / Department of Physics

Organic electronics offer the possibility of producing low cost, flexible, and large area electronics. Organic semiconductors (OSCs) (organic polymers and crystals), used in organic electronics, are promising materials for novel optical and electronic devices such as organic light emitting diodes, organic field effect transistors, organic sensors, and organic photovoltaics (OPVs). OSCs are composed of molecules weakly held together via van der Walls forces rather than covalent bonds as in the case of inorganic semiconductors such as Si. The combined effect of small wave function overlap, spatial and energetic disorder in organic semiconducting materials lead to localization of charge carriers and, in many cases, hopping conduction. OSCs also differ from conventional semiconductors in that charges photogeneration (e.g., in OPVs) proceeds via the production, diffusion, and dissociation of excitons. Liquid crystalline OSCs (LCOSCs) are semiconductors with phases intermediate between the highly ordered crystalline and completely disordered liquid phases. These materials offer many advantages including facile alignment and the opportunity to study the effects of differing intermolecular geometries on transfer integrals, disorder-induced trapping, charge mobilities, and photogeneration efficiency. In this dissertation work, we explored the photogeneration and charge transport mechanisms in a few model smectic and discotic LCs to better understand the governing principles of photogeneration and charge transport using conventional and novel methods based on the pulsed laser time-of-flight charge carrier transport technique. Four major interrelated topics were considered in this research. First, a sample of smectic LC was aligned in order to compare the resulting hole mobility to that of an unaligned sample, with the aim of understanding how the intermolecular alignment over large length scales affects the hopping probability. The role of the polarization of the photogene (open full item for complete abstract)

Committee: Brett Ellman Dr. (Committee Chair); Samuel Sprunt Dr. (Committee Member); John Portman Dr. (Committee Member); Björn Lüssem Dr. (Committee Member); Robert Twieg Dr. (Committee Member); Michael Tubergen Dr. (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Physics; Solid State Physics

PHD, Kent State University, 2010, College of Arts and Sciences / Department of Physics

Organic semiconductors(OSs) have stirred huge commercial interest due to their potential applications in electronic and optoelectronic devices such as field effect transistors, photovoltaic cells, and organic light-emitting diodes. Major benefits of OSs over conventional semiconductors include mechanical flexibility, low temperature processing, very low cost, and ease of fabrication in large area electronic devices on plastic and paper substrates. Liquid crystals (LCs) are particularly interesting classes of OSs, both from the standpoints of fundamental physics and practical applications. Systems we studied include a thiophene-benzene-thiophene-based smectic (1,4-di-(5-n-tridecylthien-2-yl)-benzene). This material exhibited polaron band behavior with very impressive hole transport (> 0.1 cm2/Vs with the smectic-F phase templating large domains of more ordered phases with very large mobilities. The mobilities are high enough to be of practical interest. Another project involved calamitic LCs with pyridine-thiophene-thiophene-pyridine cores (5, 5'-di-(alkyl-pyridin-yl)-2, 2' bithiophenes). We found both electron and hole mobilities to be strongly electric field dependent but very weakly dependent on temperature. Pyridine-based LCs often exhibit very high order smectic phases and are therefore of interest as OSs. However, the mobilities of these materials were found quite low, even in high-order phases. We were able to describe some part of our data using Basseler's theory of hopping conduction in disordered systems. We also studied charge transport in a triphenylene-based discotic LC (1-nitro-2, 3, 6, 7, 10, 11-hexakis (pentyloxy) triphenylene). This material showed strong temperature and field dependent hole mobilities described by disorder dominated one-dimensional hopping. Since the columnar phase exists over a wide range of temperatures, such photo-conducting materials may be very useful for applications in electronics. Finally, we developed a technique to m (open full item for complete abstract)

Committee: Brett Ellman (Committee Chair); Antal Jakli (Committee Member); Almut Schroeder (Committee Member); Samuel Sprunt (Committee Member); Robert Twieg (Committee Member) Subjects: Physics

PHD, Kent State University, 2006, College of Arts and Sciences / Chemical Physics

A series of new 3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) fluorescent dyes has been synthesized and their fluorescent properties studied. Some of the prepared DPP compounds have been utilized in single-molecule fluorescent spectroscopy experiments. The dyes have been shown to posses high quantum yields and useful (up to 84 nm) Stokefs shifts. Maleimide, hydroxy, halogen and some other functional groups have been incorporated into the DPP bicyclic structure. N,NŒ-Diarylated DPPs have been prepared by two new methods. Cysteine-specific fluorescent probes based on Nile Red and DCDHF fluorophore cores have been synthesized and applied to testing of the local polarity and conformational changes in a single-cysteine mutant of GroEL chaperonin of E. coli. A series of iodinated aromatic compounds has been prepared and purified to study charge transport in organic molecular crystals. Several polyalkyl- and polyalkoxy- acenes (anthracenes, tetracenes) and some important key intermediates en route to polyalkoxypentacenes have been prepared. None of the synthesized compounds exhibited mesogenic properties. A reliable, highly effective purification protocol has been elaborated for purification of hexapentyloxytriphenylene and its analogs. New trinitro-HAT-5 has been synthesized and characterized as a discotic liquid crystal in the temperature range 34c140 ‹C.

Committee: Robert Twieg (Advisor) Subjects: Chemistry, Organic