such as optically low bandgaps and electrically semiconducting properties. [1] These systems have been extensively investigated for promising applications in organic photovoltaics, photodetectors, thermoelectrics, and organic field-effect transistors (OFETs). [2] Most π-conjugated platforms, quinoidal compounds, have attracted attention from researchers because of their structural specificity and unique and noteworthy characteristics such as high electrical charge transport, non-linear optical and magnetic properties, which are uncommon in organic materials. [3] The quinoid structure is well known for its resonance form of a general aromatic structure, which has a planar structure linked by double bonds between each cyclic ring. [4] In terms of electronic state, the quinoid structure has a closed-shell form. When the closed-shell quinoid structure is extended, it can be converted into an openshell diradical structure due to the tendency to recover aromaticity. [5] These open-shell diradical characteristics of quinoidal compounds could promote interesting spin-induced magnetic behaviors and singlet fission processes. [6] Isatin-terminated quinoids have been used to incorporate quinoidal platforms in the conjugated polymer backbone. [7] Isatin may contain halogen atoms at the 6-position, which allows Pdcatalyzed carbon-carbon cross coupling polymerization (Stille and Suzuki coupling) resulting in para-connected conjugation pathway in the entire polymer backbone. Moreover, isatin-terminated quinoids have crucial advantages because they are easily synthesized via indophenine reactions under ambient conditions and allow easy structural modification. To date, a few isatinbased quinoidal conjugated polymers have been developed by researchers, including our group. [8] Whereas most quinoidal small molecules have demonstrated n-type field-effect transistor behaviors with strong electron-withdrawing terminal units such as dicyanomethylene groups, conjugated polymers composed of isatin-terminated quinoids rarely exhibit n-type behaviors because of the relatively weak electron-withdrawing nature of the isatin unit. [9] A conjugated polymer that contained S,S-dioxidized thienoquinoids demonstrated unipolar n-type semiconducting behaviors because of its strong electron withdrawing SO groups. [8a,10] To achieve n-type semiconducting behaviors, azaaromatic compounds may be incorporated into the conjugated building An open-shell quinoidal conjugated polymer exhibiting n-type semiconducting behavior is successfully synthesized and characterized. An electron-deficient azaaromatic unit is proven to reduce the energy levels of frontier orbitals via the electronegative nitrogen atom and steric hindrance within the polymer backbone. A synthesized azaquinoidal bithiophene (azaQuBT) is a quinoidal bithiophene that is end-functionalized with a pyridine ring. The open-shell quinodial conjugated polymer, poly(azaquinoidal bithiophene-thiophene), PazaQuBT-T, is synthesized using azaQuBT and thiophene. The extended quinoidal building ...

Quinoid derivatives can be applied to various applications as organic semiconducting materials owing to their unique intrinsic optoelectronic properties compared to those of existing aromatic compounds. Structural modification of the...

Quinoid platforms have garnered considerable attention as promising candidates for conjugated building blocks because of their superior charge transport properties and high electrical conductivity. Because the quinoid structure can easily adjust the optoelectronic properties by controlling the length of the quinoid core, it has the advantage of choosing the desired purpose. However, double-bond linkages within the quinoid structure could generate geometric isomers, which hinder the purification process and understanding of a well-defined structure–property relationship. Herein, the conformational locking concept was exploited to fix the molecular configuration via intramolecular nonbonding S···O interactions using the 3,4-ethylene­dioxythiophene (EDOT) unit as a quinoid core. Two EDOT-based quinoidal building blocks with varying core lengths were simultaneously and easily obtained through the indophenine reaction and validated to have only one isomer-free configurational structure via nuclear magnetic resonance analysis. A vinylene unit was used as an aromatic counterpart to establish isomer-free quinoidal polymers with high coplanarity, leading to efficient charge transport. Two polymers are termed PQmEDOTV and PQbEDOTV. Both polymers exhibited changes in the optical and electrochemical properties in accordance with the core length of the quinoidal building blocks. Although the two polymers are composed of EDOT as the core, PQbEDOTV is significantly affected by the electron-donating nature of EDOT because it contains two EDOT units in the quinoidal backbone, resulting in more high-lying frontier molecular orbital levels compared to PQmEDOTV. Consequently, the PQmEDOTV- and PQbEDOTV-based organic field-effect transistors demonstrated ambipolar and unipolar p-channel operations with hole mobility up to 0.18 cm2 V–1 s–1, respectively.

In this work, we synthesized and characterized two quinoidal small molecules based on benzothiophene modified and original isatin terminal units, benzothiophene quinoidal thiophene (BzTQuT) and quinoidal thiophene (QuT), respectively, to investigate the effect of introducing a fused ring into the termini of quinoidal molecules. Extending the terminal unit of the quinoidal molecule affected the extension of π-electron delocalization and decreased the bond length alternation, which led to the downshifting of the collective Raman band and dramatically lowering the band gap. Organic field-effect transistor (OFET) devices in neat BzTQuT films showed p-type transport behavior with low hole mobility, which was ascribed to the unsuitable film morphology for charge transport. By blending with an amorphous insulating polymer, polystyrene, and poly­(2-vinylnaphthalene), an OFET based on a BzTQuT film annealed at 150 °C exhibited improved mobility up to 0.09 cm2 V–1 s–1. This work successfully demonstrated that the extension of terminal groups into the quinoidal structure should be an effective strategy for constructing narrow band gap and high charge transporting organic semiconductors.

Conjugated polymer nanoparticles (CP NPs) that could absorb the first near-infrared (NIR-I) window have emerged as highly desirable therapeutic nanomaterials. Here, a quinoidalconjugated polymer (QCP), termed 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 band gap, NIR-I absorption, and diradical features. The obtained PQ was coated with a poly(ethylene glycol) (PEG) moiety, affording nanosized 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 PTTs under an 808 nm laser irradiation. This study demonstrates that QCPs are promising active agents for noninvasive anticancer therapy using NIR-I light.