Development of Cross-Conjugated Polymers for Sensing Applications

Abstract

Conjugated polymers are an important class of materials for electronic applications. Compared to conventional inorganic semiconductors, they offer mechanical flexibility, solution processability, and tunable electronic properties. Recently, the research interest in polymer-based sensing technology has grown considerably due to the increasing demand from emerging fields such as the Internet of Things (IoT), smart packaging, and healthcare electronics. Sensors based on conjugated polymers have demonstrated promising performance towards various stimuli such as liquid chemicals, gaseous compounds, and temperature. However, they still suffer from several limitations, including insufficient stability, reversibility, and manufacturing challenges. This thesis aims to address these issues through the exploration of novel material designs based on cross-conjugated building blocks, which have received less research interest compared to linear conjugated structures due to their inherent lower carrier mobility. For sensing applications, their unique ability to transform into a linear conjugated structure under specific stimuli could be beneficial for enhanced sensor sensitivity and selectivity. Additional design strategies are employed for enhanced sensing performance, including the incorporation of hydrogen-bonding sites for reversible sensing and the development of intrinsically conductive polymers to eliminate the need for external dopants, potentially improving device stability. In the first part of the study, dihydropiperazine (DHP) is chosen as the target cross-conjugated building block. A novel building block bisindolin-dihydropiperazine (IDHP) is developed and further copolymerized with a thienothiophene (TT) unit to constitute the cross-conjugated polymer, PIDHPTT. IDHP monomer exists as a cross-conjugated lactam but converts to a conjugated lactim form within the polymer. Neighboring DHP units in the lactim form facilitate this process through π-bridges, demonstrating a vinylogous effect, which has previously only been observed in small molecules. The OH groups in the lactim DHP interact more strongly with fluoride ions (F-) than other halides (Cl-, Br-). A water-gated organic field effect transistor (WGOFET) sensor based on PIDHPTT shows excellent sensitivity (LOD = 0.28 μM) and selectivity for fluoride ions over other halide ions, in addition to excellent reversibility and high stability in ambient and aqueous environments, demonstrating the potential of this polymer design for aqueous chemical sensing applications. Next, two thiophene-flanked DHP-based polymers, PTDbT-ET and PTDbT-T, are developed and synthesized with the eco-friendly DArP method. Incorporation of tri-ethylene glycol (TEG) side chain significantly raises their HOMO energy level higher than the ambient oxygen oxidative potential, enabling spontaneous doping by oxygen gas in the presence of moisture. Due to the higher abundance of TEG groups in PTDbT-T, it possesses a larger energy trap between its HOMO and oxygen oxidative potential, forming a more stable charge transfer complex (CTC) and can maintain its conductivity by storing in a moisture-free environment. When tested toward volatile organic compound (VOC) gaseous analytes, PTDbT-T-based chemiresistive sensors demonstrate excellent repeatability and stability, in addition to high sensitivity and selectivity to ethanol (LOD = 3.07 ppm) over other alcohol species, demonstrating the potential of this alternative strategy to develop dopant-free conductive polymers for chemiresistive gas sensor applications. In the second part of the study, a novel cross-conjugated polymer and the first polymeric analogue of a quinhydrone-like charge-transfer complex with intrinsic conductivity, poly(3,4-dihydroxythiophene-alt-thiophene-3,4-dione) (P(HOT-DOT)), is designed and synthesized. The ammonia-coordinated polymer P3 generates a perfectly balanced 1:1 donor-acceptor architecture that promotes self-doping and stabilizes polarons with spontaneous air oxidation. The polymer exhibits a narrow bandgap, broad near-infrared absorption, and high intrinsic conductivity (∼0.29 S cm-1), enabled by an ultrasmall π-π stacking distance (3.25 Å) despite its cross-conjugated backbone. Flexible temperature sensors fabricated from P3 show high stability, rare positive temperature coefficient (PTC) behavior, and reproducible and linear thermal responses over multiple cycles (TCR = 0.113 ± 0.00045%/°C). Ongoing and future studies of this material should focus on expanding other basic coordination groups for higher material stability and targeting unique electronic properties for high-performance organic electronics applications.