Synthesis and Characterization of Quinoid-type Conjugated Polymers for Optoelectronic and Therapeutic Applications

Abstract

Quinoidal compounds have attractive features as organic semiconducting materials owing to their distinct properties compared to those of aromatic compounds such as optical, electrical and magnetic properties. This thesis focuses on the development of quinoidal compounds with systematic investigation of structure-property relationship and various applications based on their properties. In Chapter 1, it will provide a general introduction of organic semiconducting materials, quinoid-type compounds, isomer control strategies of quinoidal compounds, organic field-effect transistors (OFETs), photothermal therapy (PTT) and organic solar cells (OSCs). In Chapter 2, two quinoidal building blocks mQEDTT-Br and mQEDOT-Br, incorporating 3,4-ethylenedithiothiophene (EDTT) or 3,4-ethylenedioxythiophene (EDOT), respectively, were synthesized for systematic investigation of chalcogen atom substitution effect in quinoid cores. The substitution of O with S atoms in the quinoid core extended the π-conjugation pathway and introduced a π-electron accepting capability in the quinoid. The S-based quinoid exhibited red-shifted light absorption and lower frontier molecular orbital (FMO) energy levels with a narrowed band gap (~0.2 eV). Instead, O-containing quinoid exhibited single isomer form and higher quinoid character which is favorable for intermolecular packing leading to effective charge transport. Two novel quinoidal conjugated polymers, PmQEDTT-T2 and PmQEDOT-T2, were successfully synthesized. Grazing incidence wide-angle X-ray scattering (GIWAXS) results indicated that the PmQEDOT-T2 film showed highly ordered crystallinity compared to the PmQEDTT-T2 film, confirming that mQEDOT has a higher quinoid character than that of mQEDTT, which is beneficial for charge transport. Organic field-effect transistors (OFETs) using PmQEDOT-T2 demonstrated a higher hole mobility of 0.13 cm2 /Vs than that of PmQEDTT-T2 (0.02 cm2 /Vs). Therefore, atomic substitution on the quinoid core is a simple structural modification that can control presence and type of isomers, quinoid characters and consequent device performance as desired. In Chapter 3, conjugated polymer nanoparticles (CP NPs) have emerged as therapeutic nanomaterials that use the first near-infrared (NIR-I) window, which allows deep penetration of biological tissues with minimal cell damage. Here, a quinoidal conjugated polymer (QCP), referred to as PQ, was developed as a novel class of therapeutic agents for photothermal therapy (PTT). Owing to its intrinsic quinoid structure, PQ exhibits molecular planarity and π-electron overlap along the conjugated backbone, endowing it with a narrow bandgap, NIR-I absorption, and diradical features. The obtained PQ was coated with a polyethylene glycol (PEG) moiety, affording nano-sized and water-dispersed PQ nanoparticles (PQ NPs), which consequently show a high photothermal conversion efficiency (PCE) of 63.2 %, good photostability and apparent therapeutic efficacy for both in vitro and in vivo PTT under 808 nm laser irradiation. This study demonstrates that QCPs are promising active agents for non-invasive anticancer therapy using NIR-I light. In Chapter 4, novel quinoidal building blocks, bQuPheDOT-Br and mQuPheDOT-Br, incorporating 3,4-phenylenedioxythiophene (PheDOT), was synthesized. Using the conformation-locking strategy, bQuPheDOT-Br exists as a single isomeric compound with an entire planar molecular structure, leading to effective π-electron delocalization. In addition, two quinoidal conjugated polymers—PbQPheDOT-T2 and PbQPheDOT-2FT2—were synthesized. Owing to the planar geometry and possible electron delocalization range by phenyl ring incorporation of the bQPheDOT unit, they exhibited a low band gap (~1.3 eV) and NIR light absorption up to 1,200 nm wavelength due to mesomeric effect. Grazing incidence wide-angle X-ray scattering revealed that both polymers showed high crystallinity up to the fourth order of (h00) diffraction peaks after thermal annealing because of the rigid and planar quinoidal backbone. Finally, the charge transport properties of PbQPheDOT-T2 and PbQPheDOT-2FT2 were evaluated by fabricating organic field-effect transistors as active layers with hole mobilities of 0.052 and 0.026 cm2/Vs, respectively.