Thienoisoindigo-Based Conjugated Polymers Synthesized by Direct Arylation Polycondensation.

A series of thienoisoindigo (TIG)-based conjugated polymers (CPs) with high molecular weights are synthesized by direct arylation polycondensation (DArP) by using TIG derivatives as CBr monomer and multi-halogenated thiophene derivatives, i.e., (E)-1,2-bis(3,4-difluorothien-2-yl)ethene (4FTVT), (E)-1,2-bis(3,4-dichlorothien-2-yl)ethene (4ClTVT), 3,3',4,4'-tetrafluoro-2,2'-bithiophene (4FBT), and 3,3',4,4'-tetrachloro-2,2'-bithiophene (4ClBT), as CH monomers. Density functional theory (DFT) calculations reveal the high selectivity between α-CH bonds in 4FTVT, 4ClTVT, 4FBT, and 4ClBT and ß-CH bonds in TIG CBr monomer. All four resulting CPs exhibit low optical bandgaps of ca. 1.20 eV and ambipolar transport characteristics with both electron and hole mobility above 0.1 cm2  V-1  s-1 as elaborated with organic thin-film transistors (OTFTs). The polymer TIG-4FTVT delivers the best device performance. With this polymer, n-channel OTFTs with electron mobility up to 1.67 cm2  V-1  s-1 and p-channel OTFTs with hole mobility up to 0.62 cm2  V-1  s-1 are fabricated by modifying source/drain electrodes with polyethylenimine ethoxylated (PEIE) and MoO3 , respectively, to selectively inject electrons and holes.

Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor-Acceptor-Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization.

Molecular design and precise control of thin-film morphology and crystallinity of solution-processed small molecules are important for enhancing charge transport mobility of organic field-effect transistors and gaining more insight into the structure-property relationship. Here, two donor-acceptor-donor (D-A-D) architecture small molecules TRA-IID-TRA and TRA-TIID-TRA comprising an electron-donating triarylamine (TRA) and two different electron-withdrawing cores, isoindigo (IID) and thienoisoindigo (TIID), respectively, were synthesized and characterized. Replacing the phenylene rings of central IID A with thiophene gives a TIID core, which reduces the optical band gap and upshifts the energy levels of frontier molecular orbitals. The single-crystal structures and grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that TRA-TIID-TRA exhibits the relatively tighter π-π stacking packing with preferential edge-on orientation, larger coherence length, and higher crystallinity due to the noncovalent S···O/S···π intermolecular interactions. The distinctly oriented and connected ribbon-like TRA-TIID-TRA crystalline film by the solution-shearing process achieved a superior hole mobility of 0.89 cm2 V-1 s-1 in the organic field-effect transistor (OFET) device, which is at least five times higher than that (0.17 cm2 V-1 s-1) of TRA-IID-TRA with clear cracks. Eventually, rational modulation of fused core in the π-conjugated D-A-D small molecule provides a new understanding of structural design for enhancing the performance of solution-processed organic semiconductors.

Molecular Design and Photothermal Application of Thienoisoindigo Dyes with Aggregation-Induced Emission.

Organic fluorescent dyes with aggregation-induced emission (AIE) property have an extensive application range, especially in the fields of imaging, labeling, and adjusting microprocesses in aggregated environments. In particular, the thienoisoindigo skeleton, which exhibits an outstanding electron-withdrawing capacity in optoelectronic materials, has been defined as a promising AIE candidate. For instance, by installing AIE blocks or other rotatable groups at two terminal sites, such as various arylamine groups, thienoisoindigo derivatives can be efficiently turned to be functional AIE structures. In this work, a thienoisoindigo derivative with AIE characteristics, namely, TII-TPE, was developed. This AIE system was expanded by linking typical AIE fragments, namely, tetraphenylethene, with the proposed thienoisoindigo derivative, which exhibited typical AIE fluorescence in the 600-850 nm range and maintained high photostability. Then, employing the reported derivative TII-TPA coating thienoisoindigo and triphenylamine as a contrast, aggregated TII-TPE and TII-TPA nanoparticles were prepared and demonstrated photothermal conversion efficiencies of 36.2 and 35.6%, respectively. Moreover, both nanoparticles were evaluated as photothermal therapeutic (PTT) agents in a tumor mouse model, which showed to significantly inhibit tumor growth after four treatment cycles in vivo. This work not only presents an enriched thienoisoindigo system but also provides a pattern for subsequent construction of functional AIE molecules.

Facile Preparation of a Thienoisoindigo-Based Nanoscale Covalent Organic Framework with Robust Photothermal Activity for Cancer Therapy.

The covalent organic frameworks (COFs) so far are usually built with small aromatic subunits, which makes their absorption spectra mainly located in the high-energy part of the visible region. In this work, we have developed a COF with a low band gap by integrating electron-deficient thienoisoindigo and electron-rich triphenylamine. The intramolecular charge-transfer effect combining the extended length of the π-conjugated backbone of COF endow it with broad absorption even to the second near-infrared region. After optimizing the solvent, a uniform size and colloidal stable COF is obtained. Benefiting from the coplanar structure of the monomer, this COF achieves a considerable photothermal conversion efficiency (PCE) of 48.2%. With these advantages, it displays convincing cancer cell killing effect upon laser irradiation in vitro or in vivo. This work provides a simple and practical method to acquire promising a COF-based phototherapy reagent that is applied in biomedicine field.

Thienoisoindigo (TII)-Based Quinoidal Small Molecules for High-Performance n-Type Organic Field Effect Transistors.

A novel quinoidal thienoisoindigo (TII)-containing small molecule family with dicyanomethylene end-capping units and various alkyl chains is synthesized as n-type organic small molecules for solution-processable organic field effect transistors (OFETs). The molecular structure of the 2-hexyldecyl substituted derivative, TIIQ-b16, is determined via single-crystal X-ray diffraction and shows that the TIIQ core is planar and exhibits molecular layers stacked in a "face-to-face" arrangement with short core intermolecular distances of 3.28 Å. The very planar core structure, shortest intermolecular N···H distance (2.52 Å), existence of an intramolecular non-bonded contact between sulfur and oxygen atom (S···O) of 2.80 Å, and a very low-lying LUMO energy level of -4.16 eV suggest that TIIQ molecules should be electron transporting semiconductors. The physical, thermal, and electrochemical properties as well as OFET performance and thin film morphologies of these new TIIQs are systematically studied. Thus, air-processed TIIQ-b16 OFETs exhibit an electron mobility up to 2.54 cm2 V-1 s-1 with a current ON/OFF ratio of 105-106, which is the first demonstration of TII-based small molecules exhibiting unipolar electron transport characteristics and enhanced ambient stability. These results indicate that construction of quinoidal molecule from TII moiety is a successful approach to enhance n-type charge transport characteristics.

Influence of Simultaneous Tuning of Molecular Weights and Alkyl Substituents of Poly(thienoisoindigo-alt-naphthalene)s on Morphology and Change Transport Properties.

To simultaneously assess the impact of molecular weight (Mn) and alkyl substituent variations of polymers on the structural and optoelectronic properties, herein, we conduct a systematic study of a series of poly(thienoisoindigo-alt-naphthalene) (PTIIG-Np)-based polymers containing different alkyl substituents (2-hexyldecyl (HD), 2-octyldodecyl (OD), and 2-decyltetradecyl (DT) chains) and Mn's (low (L) and high (H)). All of the polymers produce almost identical energy levels, whereas their optical spectra show a clear dependence on Mn's and the alkyl substituents. Interestingly, increasing the alkyl substituent sizes of the polymers steadily increases the lamellar d-spacings (d100), ultimately leading to a densely packed lamellar structure for PTIIGHD-Np. In addition, both H-PTIIGOD-Np and H-PTIIGDT-Np exhibit larger π-stacking crystallites than the corresponding low-Mn polymers, while for PTIIGHD-Np, their size increases in the low-Mn batch. Ultimately, L-PTIIGHD-Np shows the best hole mobility of 1.87 cm2 V-1 s-1 in top-gate and bottom-contact organic field-effect transistors (OFETs) with a poly(methyl methacrylate), which is nearly 1 order of magnitude higher than other polymers tested in this study. Our results demonstrate that the simultaneous Mn and alkyl substituent engineering of the polymers can optimize their film morphology to produce high-performance OFETs.

Polarity engineering of conjugated polymers by variation of chemical linkages connecting conjugated backbones.

The fine tuning of the dominant polarity in polymer semiconductors is a key issue for high-performance organic complementary circuits. In this paper, we demonstrate a new methodology for addressing this issue in terms of molecular design. In an alternating conjugated donor-acceptor copolymer system, we systematically engineered the chemical linkages that connect the aromatic units in donor moieties. Three donor moieties, thiophene-vinylene-thiophene (TVT), thiophene-acetylene-thiophene (TAT), and thiophene-cyanovinylene-thiophene (TCNT), were combined with an acceptor moiety, thienoisoindigo (TIID), and finally, three novel TIID-based copolymers were synthesized: PTIID-TVT, PTIID-TAT, and PTIID-TCNT. We found that the vinylene, acetylene, and cyanovinylene linkages decisively affect the energy structure, molecular orbital delocalization, microstructure, and, most importantly, the dominant polarity of the polymers. The vinylene-linked PTIID-TVT field-effect transistors (FETs) exhibited intrinsic hole and electron mobilities of 0.12 and 1.5 × 10(-3) cm(2) V(-1 )s(-1), respectively. By contrast, the acetylene-linked PTIID-TAT FETs exhibited significantly improved intrinsic hole and electron mobilities of 0.38 and 0.03 cm(2) V(-1) s(-1), respectively. Interestingly, cyanovinylene-linked PTIID-TCNT FETs exhibited reverse polarity, with hole and electron mobilities of 0.07 and 0.19 cm(2) V(-1) s(-1). As a result, the polarity balance, which is quantified as the electron/hole mobility ratio, was dramatically tuned from 0.01 to 2.7. Our finding demonstrates a new methodology for the molecular design of high-performance organic complementary circuits.