Synthesis and Characterization of Regioregular, Amphiphilic Semifluoroalkyl-Substituted Polythiophenes and Cofacial Bis(oligothienyl)naphthalenes
Watt, Shannon L.
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Conjugated polymers and oligomers have been widely studied based on their wide range of useful properties and applications. Given the importance of self-assembly and charge transfer in the development of conjugated materials for use in electronic applications, it is crucial to: (i) prepare functional materials by molecular design, (ii) evaluate the structure-property relationships of new materials, and (iii) develop fundamental understanding of electronic structure and charge transport behavior. The use of conjugated polymeric materials in electronic applications relies on control of the assembly and orientation of the polymer chains in the solid state. Conjugated polymers with liquid crystalline behavior could be used to implement an additional level of control over orientation and resultant properties. Substitution of the conjugated polythiophene backbone with semifluoroalkyl side chains (i.e., the diblock -(CH2)m(CF2)nF) has afforded materials with unusual properties. The mutual immiscibility of the aromatic backbone, the alkyl side-chain segments, and the fluoroalkyl side-chain termini provides control over supramolecular packing. A series of eight polymers has been synthesized, in which the lengths of the alkyl (m) and fluoroalkyl (n) segments are varied. One regiorandom analogue and two poly(3-alkylthiophene)s were also synthesized for comparative purposes. The structure, molecular weight, and regioregularity of the polymers were evaluated using a variety of techniques. The semifluoroalkyl-substituted polymers have been systematically studied to determine the effect of side chain length and m:n block ratios on their solution state, liquid crystalline, and solid state properties. The effect of side chains on conjugation was determined, where solubility allowed, by solution-state UV-visible and fluorescence spectroscopy. The thermal and liquid crystalline properties of the homopolymers were evaluated by DSC, variable-temperature X-ray diffraction, and polarized optical microscopy. Several semifluoroalkyl-substituted polythiophene homologues show liquid crystalline behavior. Molecular packing and charge transport are key factors governing the use of conjugated materials in electronic applications. A wide variety of oligomers have been studied as models for charge migration in conjugated polymers. One-dimensional models do not adequately represent two-dimensional charge transport; thus, a variety of two-dimensional, covalently-linked models have been developed. Previous work by our group, and others, led to the proposal of bis(oligothienyl) compounds as models to study the interaction of the ð-conjugated chains. Previous reports by other researchers described the synthesis and characterization of hydrogen-terminated analogues of 1,8-bis(oligothienyl)naphthalenes. However, these materials proved to be unsuitable for use as charge transport models, as they were subject to irreversible polymerization upon oxidation. Installation of methyl groups at the terminal a-positions of 1,8-bis(oligothienyl)naphthalenes allowed us to create a series of models in which conjugated chains are held in close proximity. This provides access to multiple redox states, and future systems based on these molecules may be used as models for charge transport or as functional materials for incorporation into devices.