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This thesis is concerned with the design, synthesis and properties of thermotropic liquid crystals based on the fluorene and 9,10-dihydrophenanthrene core units. Derivatives of 9,10-dihydrophenathrene were found to be both expensive to prepare and difficult to purify, and attention was concentrated on fluorene derivatives. Simple 2,7-disubstituted fluorenes are either monotropic or exhibit enantiotropic smectic phases. Accordingly a third benzene ring was directly coupled to the fluorene core to give a 2-phenylfluorene. The homologous series of 2-(4-cyanophenyl)-7-alkylfluorenes from ethyl to nonyl was prepared via a five step convergent synthesis, and the compounds were isolated as white crystalline solids. All the compounds exhibited enantiotropic nematic phases, and the heptyl, octyl and nonyl homologues also exhibited a smectic A phase. The parent compound 2-(4-cyanophenyl)fluorene with no alkyl chain was prepared by a similar route, but did not exhibit an enantiotropic mesophase. Some physical properties of 2-(4-cyanophenyl)-7-pentylfluorene were measured, and many values {e.g. dielectric anisotropy and birefringence) were similar to those for the commercially successful 4-pentyl-4"-cyano-p-terphenyl. However 2-phenylfluorene material had a higher viscosity. These properties could be related to the crescent shape of the 2-phenylfluorene core, which is dictated by the bridging methylene group. Lateral substitution of the fluorene core was investigated in an attempt to prepare low melting point derivatives. Four C9 alkylated compounds were prepared, and these were found to be oils. However no mesophases were detected. Some 2-(4-alkoxyphenyl)-7-alkylfluorenes were prepared as low birefringence analogues, but these were found to exhibit smectic phases at high temperatures. The analogous alkoxy compound 2-pentoxy-7-(4-cyanophenyl)fluorene was prepared but could not be satisfactorily purified. Preliminary investigations into partial reduction of the fluorene core were carried out, but these did not progress beyond an early stage.

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Single-walled carbon nanotubes (SWNTs) are envisioned as one of the most promising materials for next-generation electronic devices such as field-effect transistors, photovoltaics, new power sources and bio/chemical sensors. In particular, solution processable SWNT networks are of great interest for flexible and stretchable electronics. However, most of these applications specifically require pure semiconducting (sc-) or metallic (m-) SWNTs. However, large scale synthetic methods for SWNTs always produce a mixture of semiconducting and metallic carbon nanotubes. In recent years, several biochemical separation techniques such as DNA assisted separation, density gradient ultracentrifugation, and gel chromatography techniques have been utilized to separate semiconducting and metallic SWNTs. Although these methods can be used for sorting SWNTs according to their chiralities, they are either time-consuming or not easily scalable. In addition, the supramolecular functionalization of SWNTs with conjugated polymers has received a great deal of attention due to its capability to extract sc-SWNTs via simple sonication and centrifugation steps within a few hours. Furthermore, π-conjugated polymers can be modified by suitably changing monomers and/or comonomers, and it is also easy to control molecular weight and solubility of resulting polymers in organic solvents and aqueous media. There is also the possibility for selectively extracting specific chirality (n, m) nanotubes using specifically designed macromolecular structures. Except for its application to the separation of SWNTs, the supramolecular complexes of π-conjugated polymer and SWNTs have potential applications in many research areas such as new composite materials. After a brief overview of the current work related to the investigation of the supramolecular interaction between various conjugated polymers and SWNTs (chapter 1), synthesis of a series of different types of fluorene-based conjugated copolymers and their supramolecular complex formation properties with SWNTs are described (chapter 2, 3, 4, 5 and 6). In order to understand the effect of conjugated polymer backbone and side-chain structure on formation of supramolecular complexes with SWNTs, several crucial factors were investigated by: (1) altering the polymer backbone composition; (2) introducing different solubilizing (functional) groups while the polymer backbone remains the same; (3) changing the side-chain functional groups, and (4) introducing different polymer repeat units with varying physical and chemical properties. The experimental results indicated that all of the resulting polymer-SWNTs complexes possess excellent (or moderate) solution stability in organic solvents such as tetrahydrofuran (THF), toluene, and xylene. It was also demonstrated that the interaction between the conjugated polymers and SWNTs is strongly influenced by polymer structure; even minor changes on side-chains have a significant effect on the selectivity of the polymers in dispersing specific SWNT structures. This investigation highlights the potential importance of incorporating different types of heterocyclic aromatic rings (electron rich or electron poor), and introducing side chains with varying electronic and geometric structure on selective solubilization of SWNTs. Polymer molecular weight and solvent properties also strongly influence the π-conjugated polymer assisted dispersion of specific chirality SWNTs. Although some progress has been made, the search for a conjugated polymer that selectively solubilizes specific SWNT chiralities on large scale remains a challenge. / Thesis / Doctor of Philosophy (PhD)

This thesis presents a study of the photophysical properties of a number of fluorene molecules used for organic semiconductor lasers. These results are then combined with lasing results to assess what the important properties in an organic semiconductor laser material are. Photophysical measurements were performed on a family of oligofluorenes; results show a redshift in the peak absorption and emission wavelengths with increasing length. There is also an increase in the molar extinction coefficient and photoluminescence quantum yields of the molecules. Transition dipole moments also increase with length, but fluorescence scales slower than absorption due to self-trapping occurring at longer molecular lengths. This study was then expanded to two families of star-shaped molecules with fluorene arms and differing cores. These molecules have three arms connected to either a central benzene unit or a larger truxene core. These molecules show an increase in PLQY and roughly three times higher molar extinction coefficients than comparable linear oligofluorenes. The star-shaped molecules PLQY and transition dipole moments are both greater than their linear oligofluorene counterparts. Energy transfer was then studied in the truxene-cored molecules, which showed that the symmetry of the molecule was broken due to interactions with the solvent. Energy transfer was observed on two timescales; a fast 500 fs process which is attributed to a localisation onto a single arm to emit, and a 3-10 ps second decay component, and was assigned to resonant energy transfer between the arms. Both decays were found to be wavelength dependent. Lasing results were then obtained for the benzene cored molecules. It was found that star-shaped molecules present improved lasing characteristics with lower ASE and lasing thresholds. These results were compared with those obtained for truxene-cored molecules whose rigid core provides them with better lasing and ASE characteristics.

by Chow Kim-fung. / Bibliography: leaves 112-114 / Thesis (M.Ph.)--Chinese University of Hong Kong, 1985

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 143-162).

Thesis (Ph. D.)--University of Texas at Austin, 2009. / Title from PDF title page (University of Texas Digital Repository, viewed on Aug. 12, 2009). Vita. Includes bibliographical references (leaves 99-105).

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