Synthesis of Highly Ordered Polymer Nanostructures Based on Polyphenylene Vinylenes (PPVs)

Author

Xinju Zhu, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2015

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Plunkett, Kyle

Abstract

Poly(phenylene vinylene)s (PPVs) are a prototypical class of CPs that has been well studied in terms of optical, electronic, and structural properties. To build off of this knowledge, a series of well-defined polychromophores with a backbone composed of alternating PPV oligomers (trimer, pentamer, or septamer) with flexible (n-decyl or tetraethylene glycol) or rigid linkers (adamantane, diamantane, or azobenzene) were created. Depending on the structure of the PPV chromophore, variations in the polymer chain morphology was found as evident by thin film and single molecule morphology modulation. Thin film fluorescence showed broad, red-shifted emission for septamer PPV copolymers with tetraethylene glycol linkers, which suggests stronger inter-chromophore interactions in the solid state. Such low energy complexes were not observed in smaller PPV chromophore polymers. Single-molecule excitation polarization spectroscopy confirmed these bulk measurements by showing the longer chromophore containing polymers (septamer) adopted more anisotropic nanostructures than the shorter chromophore polychromophores (trimer and pentamer) and indicates a new synthetic approach to aligned polymer nanostructures. To probe the relevance of regioregularity in these new polychromophore PPV systems, three pentamer derivatives of 2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylenes (MEH-PPVs) with varying degrees of side chain regioregularity were prepared. Before polymerization into the polymeric structures, the physical properties of the individual oligomers was probed. As expected, the solution phase cyclic voltammetry (CV) and absorption spectra of each oligomer were found to be essential identical. In contrast, the thin film spectra reveal that the more regioregular pentameric oligomers possessed a new, red-shifted shoulder structure. These results suggest that the packing order is highly influenced by side chain regioregularity even for pentameric PPV oligomers. In addition to the polychromophore work, this thesis also details our work in creating new tunable electron acceptors based on cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAH). Small molecule and polymeric donor-acceptor copolymers that utilize a cyclopenta[hi]aceanthrylene (CPAA) core were prepared. A series of substituted CPAA derivatives with electron donating (NH2, OCH3), neutral (H), and electron withdrawing (COOH, CF3, CN, NO2) functionalities were investigated and showed the optoelectronic properties of the resulting materials could be systematically varied. In addition, we demonstrate a one-step post-polymerization modification that converts three high-band gap poly(arylene ethynylene)s into low band gap donor-acceptor copolymers containing CPAAs. The methodology relies on a palladium-catalyzed cyclopentannulation between an aryl halide and the ethynylene groups in the polymer backbone. The new strategy provides an opportunity to build up high molecular weight materials via efficient Sonogashira cross-coupling polymerizations and then convert those benign materials into more custom materials via new aryl-aryl bonding after cyclopentannulation.