Synthesis and characterization of organic semiconducting materials based on quinoid structure with open-shell and closed-shell character for plastic electronics

Abstract

Quinoid platforms have attracted much attention because of their unique and interesting optical, electrical and magnetic properties, and can be applied to various organic electronics, such as organic field-effect transistors (OFETs), organic photovoltaics and organic spintronics. The focus of this thesis is investigation of synthesis and characterization of quinoidal conjugated compounds for plastic electronics. Chapter 1 provide a general introduction of organic semiconducting materials, quinoidal compounds, open-shell diradical character and organic field-effect transistors (OFETs). In Chapter 2, two novel conjugated polymers incorporating quinoidal thiophene are successfully synthesized. By combining 1D nuclear magnetic resonance (NMR) and 2D nuclear Overhauser effect spectroscopy analyses, the isomeric form of the major quinoid monomer is clearly identified as the asymmetric Z, E‐configuration. The quinoidal polymers are synthesized via Stille polymerization with thiophene or bithiophene. Both quinoidal polymers exhibit the low band gap of 1.45 eV and amphoteric redox behavior, indicating extended conjugation owing to the quinoidal backbone. These quinoidal polymers show ambipolar behaviors with high charge carrier mobilities when applied in organic field‐effect transistors. In addition, the radial alignment of polymer chains achieved by off‐center spin‐coating leads to further improvement of device performance, with poly(quinoidal thiophene–bithiophene) exhibiting a high hole mobility of 8.09 cm2 V−1 s−1, which is the highest value among the quinoidal polymers up to now. Microstructural alteration via thermal annealing or off‐center spin‐coating is found to beneficially affect charge transport. The enhancement of crystallinity with strong π–π interactions and the nanofibrillar structure arising from planar well‐delocalized quinoid units is considered to be responsible for the high charge carrier mobility. In Chapter 3, while quinoidal moieties are considered as emerging platforms showing efficient charge transport and interesting open-shell diradical characteristics, whether these properties could be changed by extension to the conjugated polymer structure remains as a fundamental question. Here, we developed and characterized two conjugated polymers incorporating quinoids with different lengths, which have a stable close and open-shell diradical character, respectively, namely, poly(quinoidal thiophene-thienylene vinylene) (PQuT-TV) and poly(quinoidal bithiophene-thienylene vinylene) (PQuBT-TV). A longer length of a quinoidal core led to enhanced diradical characteristics. Therefore, the longer core length of QuBT was favorable for the formation of an open-shell diradical structure in its monomer and in the quinoidal polymer. PQuBT-TV exhibited high spin characteristics observed by the strong ESR signal, a low band gap, and improved electrochemical stability. On the other hand, as QuT maintained a closed-shell quinoid structure, PQuT-TV exhibited high backbone coplanarity and strong intermolecular interaction, which was beneficial for charge transport and led to high hole mobility (up to 2.40 cm2 V–1 s–1) in organic field-effect transistors. This work successfully demonstrated how the control of the closed/open-shell character of quinoidal building blocks changes charge transport and spin properties of quinoidal conjugated polymers via quinoid–aromatic interconversion. Finally, in Chapter 4, an open-shell quinoidal conjugated polymer showing n-type semiconducting behavior is successfully synthesized and characterized. An electron-deficient azaaromatic unit has proven reduced energy levels of frontier orbitals owing to the electronegative nitrogen atom and steric hindrance within the polymer backbone. The azaquinoidal bithiphene (azaQuBT) was end-functionalized with pyridine ring. The openshellquinodial conjugated polymer is synthesized between azaQuBT and thiophene, namely poly(azaquinoidal bithiophene-thiophene) (PazaQuBT-T). The extended quinoidal building block with open-shell diradical character induce low band-gap and redox amphoterism as well as high spin-induced magnetic characteristics of the resulting polymer. PazaQuBT-T achieve ambipolar charge transport behavior in OFET devices. Through a PEIE treatment onto the contact electrode, PazaQuBT-based OFETs exhibited unipolar n-channel operation with electron mobility up to 0.98 cm2 V-1 s-1.