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Extending the conjugation of phthalein and xanthene moieties through N heteroatoms and electron donor monomers
Frazier, Daniel J.
Frazier, Daniel J.
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2025
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Organic conjugated polymers are essential materials in organic semiconductor (OSC) applications. Within the past thirty years, the incorporation of chromophores into polymer backbones have led to improvements in the electronic properties of conjugated polymers and an increase in the efficiencies of OSC devices. The halochromic chromophore moieties xanthene and phthalein have not been fully investigated as components of conjugated polymers due to the obvious break in conjugation at the quaternary carbon connecting the phthalide ring and the two flanking aryl groups. However, the conjugation can be extended via a ring-opening mechanism where the C-O bond of the phthalide lactone breaks, which in turn converts the quaternary carbon to a sp2 hybridized carbon, forming zwitterions with quinoidal linkages within the structure.
The work herein focuses on the design and synthesis of xanthene and phthalein-based polymers where the ring-opening mechanism of the lactone ring is more favorable and accessible in order to extend the conjugation of the polymer backbone. Two different designs are investigated in this thesis research. The first design involves linking xanthene or phthalein moieties with amine groups via Buchwald-Hartwig polyamination reactions. The electron-donating capability of the amine linkages is expected to promote phthalide ring-opening to result in quinoidal resonance structures involving the N heteroatoms, which stabilize zwitterion formation. The second design involves the synthesis of several Donor-Acceptor type polymers via direct arylation polymerization or Stille polycondensation reactions where the strong electron donor comonomers are expected to promote and stabilize the conversion of the xanthene and phthalein moieties into a zwitterionic and extended conjugated form. The conjugation of all synthesized polymers was extended upon doping with a protic acid to facilitate the ring-opening mechanism, as evidenced by narrow optical energy gaps estimated from UV-Vis spectroscopy. Further characterization of the polymers indicated excellent thermal stability and ionic and open-shell diradical character that demonstrated the potential of these materials for OSC applications.
As an additional chapter outside of the main scope of this thesis work, an alternative and selective synthesis for the isolation of pure 3,6-dibromophthalic anhydride by oxidizing 1,4-dibromonaphthalene is described.
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