Most of all‐fused‐ring π‐conjugated molecules have wide or medium bandgap and show photo response in the visible range. In this work, an all‐fused‐ring n‐type molecule, which exhibits an ultrasmall optical bandgap of 1.22 eV and strong near‐infrared (NIR) absorption with an onset absorption wavelength of 1013 nm is reported. The molecule consists of 14 aromatic rings and has electron donor–acceptor characteristics. It exhibits excellent n‐type properties with low‐lying HOMO/LUMO energy levels of −5.48 eV/−3.95 eV and high electron mobility of 7.0 × 10−4 cm2 V−1 s−1. Most importantly, its thin film exhibits a low trap density of 5.55 × 1016 cm−3 because of the fixed molecular conformation and consequently low conformation disorder. As a result, organic photodetector (OPD) based on the compound exhibits a remarkably low dark current density (Jd) of 2.01 × 10−10 A cm−2 at 0 V. The device shows a shot‐noise‐limited specific detectivity (Dsh*) of exceeding 1013 Jones at 400–1000 nm wavelength region with a peak specific detectivity of 4.65 × 1013 Jones at 880 nm. This performance is among the best reported for self‐powered NIR OPDs.

An n‐Type All‐Fused‐Ring Molecule with Photoresponse to 1000 nm for Highly Sensitive Near‐Infrared Photodetector

Conjugated polymers (CPs) with deep LUMO energy levels (ELUMOs) are essential for developing n-type organic electronics. However, few approaches can markedly lower the ELUMOs of CPs. Herein, this issue is addressed by the introduction of a quinoidal building block into the polymer chain. Two CPs, PIQ-A and PIQ-O, are synthesized based on an indandione-terminated quinoidal unit, with alkyl or oligo(ethylene glycol) side chains on the comonomer, respectively. The CPs show ultralow ELUMOs of ca. −4.30 eV, which is lower by 0.75 eV compared to the analogous polymer with aromatic conjugated structure. The deep LUMO energy levels endow the CPs with unipolar n-type transport behavior in organic thin-film transistors and efficient n-type doping in organic thermoelectrics, with doped PIQ-O showing an optimized electrical conductivity of 0.43 S cm–1 and a power factor of 4.24 μW m–1 K–2. Importantly, the electrical conductivity of doped polymer thin films (∼50 nm) barely changes after a 7 day exposure to air, and the doped PIQ-O film shows an electrical conductivity of 0.39 S cm–1 even after a 15 day exposure to air. This work provides a new approach to develop CPs with ultralow LUMO energy levels, toward high-performance and potentially air-stable n-type organic electronics. 

n-Type Conjugated Polymers Based on an Indandione-Terminated Quinoidal Building Block

Selective doping of a single conjugated polymer (CP) to obtain p‐type and n‐type conductive materials would be highly attractive for organic thermoelectric applications, because it will greatly reduce the time and costs of synthesizing different types of CPs. However, this strategy has rarely been investigated. In this study, two CPs are synthesized, designated PTQDPP‐T and PTQDPP‐2FT, based on a newly developed quinoidal unit with thienoisatin as the termini and a thiophene‐flanked diketopyrrolopyrrole (ThDPP) unit as the quinoidal core. The electron‐rich thiophene rings in thienoisatin and the electron delocalization induced by thienoisatin resulted in polymers with high‐lying highest occupied molecular orbital, and the electron‐deficient nature of ThDPP unit and its quinoidal backbone endowed the polymers with low‐lying lowest unoccupied molecular orbitals. As a result, both polymers can be p‐type and n‐type doped. Because of its high mobility, doped PTQDPP‐2FT performed better in organic thermoelectric devices than the doped PTQDPP‐T. After being doped with FeCl3 and N‐DMBI, PTQDPP‐2FT showed p‐type and n‐type power factors of 278.2 and 2.37 µW m−1 K−2, respectively. These are the best for bipolar (p‐type and n‐type) performances that obtained by selective doping of a single polymer.

Realizing p‐Type and n‐Type Doping of a Single Conjugated Polymer via Incorporation of a Thienoisatin‐Terminated Quinoidal Unit

Enhanced n‐type thermoelectric performance of conjugated polymers based on an indandione‐terminated quinoidal unit through comonomer optimization

Quinoidal compounds have great potential utility as high-performance organic semiconducting materials because of their rigid planar structures and extended π-conjugation. However, the existence of E and Z isomers adversely affects the charge-transport properties of quinoidal compounds. In this study, we developed three isomerically pure oxindole-terminated quinoids by introducing chlorine atoms in the quinoidal core. The synthesized quinoids were confirmed to have a Z,Z configuration by means of 1H NMR spectroscopy, density functional theory calculations, and single-crystal X-ray analysis. Importantly, the strategy of chlorination allowed to maintain low-lying frontier molecular orbital energy levels and ensure favorable intermolecular packing. Consequently, all three quinoidal compounds showed n-type transport characteristics in organic thin-film transistors, with electron mobilities up to 0.35 cm2 V-1 s-1, which is the highest value reported to date for oxindole-terminated quinoids. We believe that our study can provide new guidelines for the design of isomerically pure quinoids with high electron mobilities.

Chlorination-Assisted Isomerically Pure Oxindole-Terminated Quinoids for n-Type Organic Thin-Film Transistors.

Low-bandgap organic semiconductors have attracted much attention for their multiple applications in optoelectronics. However, the realization of narrow bandgap is challenging particularly for small molecules. Herein, we have synthesized four quinoidal compounds, i. e., QSN3, QSN4, QSN5 and QSN6, with electron rich S,N-heteroacene as the quinoidal core and indandione as the end-groups. The optical bandgap of the quinoidal compounds is systematically decreased with the extension of quinoidal skeleton, while maintaining stable closed-shell ground state. QSN6 absorbs an intense absorption in the first and second near-infrared region in the solid state, and has extremely low optical bandgap of 0.74 eV. Cyclic voltammetry analyses reveal that the lowest unoccupied molecular orbital (LUMO) energy levels of the four quinoidal compounds all lie below -4.1 eV, resulting in good electron-transporting characteristics in organic thin-film transistors. These results demonstrated that the combination of π-extended quinoidal core and end-groups in quinoidal compounds is an effective strategy for the synthesis of low-bandgap small molecules with good stability.

Indandione-Terminated Quinoidal Compounds for Low-Bandgap Small Molecules with Strong Near-Infrared Absorption: Effect of Conjugation Length on the Properties.

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Indandione-Terminated Quinoidal Compounds for Low-Bandgap Small Molecules with Strong Near-Infrared Absorption: Effect of Conjugation Length on the Properties.