•Two new polymers based on BTPBF unit were synthesized and characterized.•Two polymers showed ultra broad absorption spectra (covering 400–1600 nm) and remarkably low bandgaps (ca. 0.8 eV).•The planar structure and π-stacking orientation can be optimized by the positions of side-chains.•Polymer exhibited excellent ambipolar mobilities of as high as over 0.8/0.19 cm2 V−1s−1.Two new D−A polymers (PBTPBF-HH and PBTPBF-TT) based on (3E, 7E)-3,7-bis(4-(2-decyltetradecyl)-4H-thieno[3,2-b]pyrrole-5,6-dione)benzo[1,2-b:4,5-b']difuran-2,6(3H,7H)-dione and (E)-2-(2-(thiophen-2-yl)vinyl)thiophene units with different side-chain positions (head-to-head and tail-to-tail) were synthesized, and the side-chain positions were optimized with respect to their planarity, microstructure and performance as organic thin-film transistors. Both the polymers showed broad absorption spectra (covering 400–1600 nm) and remarkably low bandgaps (ca. 0.8 eV). PBTPBF-HH containing head-to-head linkages had a dual texture, in which face-on and edge-on crystallites coexisted. In contrast, PBTPBF-TT containing tail-to-tail linkages mainly exhibited an edge-on texture. Consequently, PBTPBF-TT showed a much higher transport performance than PBTPBF-HH when evaluated using bottom-gate/top-contact organic thin-film transistors. The best mobilities of above 0.80 cm2 V−1s−1 and 0.19 cm2 V−1s−1 were obtained for hole and electron, respectively, at the optimized thermal annealing and in the presence of a high boiling point additive. Overall, this study showed that a minimal change in their side-chain positions dramatically optimized the planarity, microstructure, π-stacking orientation, and charge transport performance.

•A new thieno-isoindigo derivative dye and its polymer were designed and synthesized.•The polymer showed ultra broad absorption spectra that spanned across the near-infrared region and remarkably low bandgap.•The new polymer showed highly balanced hole and electron transport characteristics.•Polymer nanoparticles showed excellent photothermal conversion efficiency under 980 nm irradiation.A new thieno-isoindigo derivative, (3E,7E)-3,7-bis(4-(2-decyltetradecyl)-4H-thieno[3,2-b]pyrrole-5,6-dione)-5,7-dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione (BTPDI), was designed and synthesized. A donor−acceptor conjugated polymer (PBTPDI-TT) was also synthesized with this new unit as the acceptor and thieno[3,2-b]thiophene as the donor. The microstructure, photophysical, electrochemical, field-effect properties and photothermal performances were investigated. The polymer showed a broad absorption spectrum that spanned across the near-infrared (NIR) region (780–1300 nm), with a very low bandgap (∼0.95 eV), deep lowest unoccupied molecular orbital, and suitable highest occupied molecular orbital levels. As a result, the polymer-based organic field-effect transistors showed highly balanced hole and electron transport characteristics with a hole mobility of 0.027 cm2 V−1s−1 and an electron mobility of 0.022 cm2 V−1s−1. The polymer nanoparticles showed good reusability under NIR (980 nm) irradiation and could also effectively convert NIR light to heat at an excellent efficiency of 20.3%.Download high-res image (189KB)Download full-size image

The aldol reaction is a facile green synthetic method and has been widely used in the synthesis of small molecules. In this study, we attempted to prepare conjugated polymers using aldol polymerization for the first time. Two isoindigo-based conjugated polymers (PIID-DT and PBIBDF-DT) were synthesized successfully in excellent yields and with high molecular weights (MGPCn up to 53.9 kDa) using toluene as the solvent and an acid as the catalyst. The entire synthesis route is economical and environmentally friendly, and the traditional drawbacks such as the use of organometallic reagents, toxic tin monomers, and other environmentally harmful compounds often encountered in Stille and Suzuki cross-coupling reactions could be avoided. Moreover, the isoindigo-based polymers prepared by the newly established aldol polymerization were also evaluated as organic field-effect transistors with bottom-gate/top-contact devices and exhibited excellent mobilities as high as 0.16 and 0.26 cm2 V−1 s−1 with a high Ion/Ioff ratio of 105 for PIID-DT and PBIBDF-DT, respectively. Therefore, aldol polycondensation has environmentally friendly characteristics that can be applied in the green synthesis of isoindigo-based conjugated polymers.

Three donor–acceptor polymers based on (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo-[1,2-b:4,5-b]-difuran-2,6(3H,7H)-dione (BIBDF) and three kinds of dialkylated bithiophenes with head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT) connectivity were synthesized by the Stille coupling reaction. Their photophysical and electrochemical properties, electronic device performance, and microstructure were investigated. We found that the alkyl chains substituted near the thiophene–thiophene connection cause less steric hindrance than those near the BIBDF–thiophene connection. Therefore PBIBDF-HH exhibited the preferred planarity, crystallinity, and molecular orientation, yielding the highest field-effect mobility. A maximum electron mobility of 1.23 cm2 V−1 s−1 and a maximum hole mobility of 0.37 cm2 V−1 s−1 were obtained for PBIBDF-HH-based devices. These results show that the substitution sites on the bithiophene units play an important role in the determination of molecular organization and the resulting device performance.

Single-component near-infrared phototransistors based on ambipolar organic semiconductor nanowires have been investigated and compared with their corresponding thin-film counterparts. The nanowire organic phototransistors (NW-OPTs) showed photocurrent/dark-current ratios and photoresponsivities as high as 1.3 × 104 and 440 mA W−1 for the p-type channel, and 3.3 × 104 and 70 mA W−1 for the n-type channel, respectively, upon near-infrared illumination with an intensity of 47.1 mW cm−2. These were much higher values compared to their thin-film counterparts. The enhancement of the near-infrared photoresponse could be attributed to the larger trap density originating from the semiconductor/insulator interface and the semiconductor/air interface. The performance of NW-OPTs was demonstrated to open up new possibilities to improve the near-infrared photoresponse of single-component devices.

Conjugated polymers containing alternating donor/acceptor units have strong and sharp absorbance peaks in near-infrared (NIR) region, which could be suitable for photothermal therapy. However, these polymers as photothermal transducers are rarely reported because of their water insolubility, which limits their applications for cancer therapy. Herein, we report the donor–acceptor conjugated polymer PBIBDF-BT with alternating isoindigo derivative (BIBDF) and bithiophene (BT) units as a novel photothermal transducer, which exhibited strong near-infrared (NIR) absorbance due to its low band gap (1.52 eV). To stabilize the conjugated polymer physiological environments, we utilized an amphiphilic copolymer, poly(ethylene glycol)-block-poly(hexyl ethylene phosphate) (mPEG-b-PHEP), to stabilize PBIBDF-BT-based nanoparticles (PBIBDF-BT@NPPPE) through a single emulsion method. The obtained nanoparticles PBIBDF-BT@NPPPE showed great stability in physiological environments and excellent photostability. Moreover, the PBIBDF-BT@NPPPE exhibited high photothermal conversion efficiency, reaching 46.7%, which is relatively high compared with those of commonly used materials for photothermal therapy. Accordingly, in vivo and in vitro experiments demonstrated that PBIBDF-BT@NPPPE exhibits efficient photothermal anticancer efficacy. More importantly, PBIBDF-BT@NPPPE could simultaneously encapsulate other types of therapeutic agents though hydrophobic interactions with the PHEP core and achieve NIR-triggered intracellular drug release and a synergistic combination therapy of thermo-chemotherapy for the treatment of cancer.Keywords: conjugated polymer; donor−acceptor polymer; photothermal conversion; photothermal stability; synergistic combination therapy

•Three polymers based on BDD unit were synthesized and characterization.•This is the first investigation on the OTFTs of BDD-based polymers.•The OTFT performances were optimized by tailoring structure and annealing.•The optimized P3 exhibited balanced hole and electron transport.•The phototransistors showed a high photocurrent/dark-current ratio of 6.4 × 103.Three copolymers (P1, P2, and P3) based on benzodithiophenedione and diketopyrrolopyrrole units were synthesized by Suzuki reaction. The thermal, optical, electrochemical, microstructure, charge transport and phototransistor properties of the copolymers were also investigated. The field-effect performances of the copolymers were optimized by tailoring their structure and using thermal annealing. The three copolymers exhibited ambipolar transport under vacuum, but hole characteristics in air. The optimized copolymer P3 exhibited a balanced hole and electron mobility of 0.022 and 0.028 cm2 V−1 s−1, respectively, under vacuum conditions, but hole transport in air with a hole mobility of as high as 0.034 cm2 V−1 s−1. Phototransistors based on P3 showed a photocurrent/dark current ratio of 6.4 × 103 under near-infrared light illumination (808 nm). This is the first investigation on the field-effect and phototransistor performances of benzodithiophenedione-based copolymers.

•Two polymers based on pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized and characterized.•The LUMO and bandgap were lowered by introducing a more electron deficient unit.•The polymers maintained the deep HOMO level and the devices exhibited a PCE of 3.66% with a high Voc of 0.91 V.Two donor–acceptor polymer semiconductors based on highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized by Stille cross-coupling polymerization, and their thermal property, photophysical property, electrochemical property, microstructure, application as organic thin-film transistors, and photovoltaic property were investigated. Because of the strong electron-accepting characteristic of pyrrolo[3,4-d]pyridazine-5,7-dione, the new polymers (P1 and P2) exhibited much wider absorption, smaller bandgaps (1.70 vs 1.98 eV), and deeper LUMO levels (−3.60 vs −3.37 eV) than those of a phthalimide-based polymer PBDT-PhBT (Figure 1). The fabricated organic thin-film transistor devices exhibited hole-transport behavior, and the highest mobility of 1.14 × 10−3 cm2 V−1 s−1 was obtained. The bulk-heterojunction solar cells based on the two polymers as the electron donors and PC71BM as the electron acceptor showed a high open-circuit voltage and achieved a power conversion efficiency of 2.71% and 3.66% for polymers P1 and P2, respectively.

An acceptor–π–acceptor (A1–π–A2) type polymer based on phthalimide and diketopyrrolopyrrole units has been designed and synthesized by Stille cross-coupling reaction. The photo-physical, electrochemical and transistor properties have been also investigated. The polymer exhibited broad absorption in the near infrared region and low LUMO/HOMO energy levels. Organic thin-film transistors (OTFTs) devices with common architectures were fabricated to evaluate the device properties. Polymer-based OTFTs devices exhibited n-type charge transfer under vacuum and yielded an electron mobility as high as 0.52 cm2 V−1 s−1. This is a record value for n-type polymers based on phthalimide structure. The OTFTs devices were also investigated in air and displayed p-type transporting behavior with a hole mobility of up to 0.13 cm2 V−1 s−1. The corresponding thin film morphology and polymer packing were also investigated in detail by atomic force microscopy (AFM) and X-ray diffraction (XRD) to correlate with the mobility. The results showed that the polymer film possessed highly uniform polymer nanofibers, a long-range-ordered lamellar structure and a small π–π stacking distance.

Conjugated polymers based on a bis(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF) unit displayed promising performances for their application in organic thin-film transistors (OTFTs). Herein, three new BIBDF-based donor–acceptor (D–A) polymers, containing thieno[3,2-b]thiophene (TT), (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) and (2-(thiophene-2-yl)alkynyl)thiophene (TAT) as donors, were synthesized and characterized. The results indicated that the donor unit plays important roles in affecting the absorption bands, HOMO levels, lamellar packing and π–π stacking distances of the BIBDF-based polymers. The OTFT devices based on the three polymers were fabricated, and their field-effect performance and environmental stability were also characterized. All three BIBDF based polymers showed good n-type field-effect characteristics. The PBIBDF-TT showed the highest electron mobility of 0.65 cm2 V−1 s−1 and the best environmental stability, while the PBIBDF-TAT showed the lowest electron mobility of 0.13 cm2 V−1 s−1. The corresponding crystalline structures and morphologies revealed that the PBIBDF-TT and PBIBDF-TVT showed close π–π distances and long-range ordered, lamellar crystalline structures both of which contributed to the high charge carrier mobility. The PBIBDF-TAT with close π–π distances but poor crystalline structures showed miserable performance. Overall, this work showed the correlation of the microstructures and properties of BIBDF-based polymers, and the field-effect performances can be effectively optimized by introducing different donor units.

A new thieno-isoindigo derivative, bis(5-oxothieno[3,2-b]pyrrole-6-ylidene)benzodifurandione (BTPBF) was synthesized by replacing the outer benzene ring of the isoindigo derivative (BIBDF) with thiophene. This was used for the first time as an acceptor to construct a donor–acceptor polymer (PBTPBF-BT) for organic thin-film transistors. The thermal stability, photophysical-, and electrochemical properties, microstructure, and transistor characteristics were also investigated. Compared to the isoindigo derivative-based polymer PBIBDF-BT, the thieno-isoindigo derivative-based polymer PBTPBF-BT had a much smaller bandgap (0.71 eV), a similar deep LUMO level, (−3.94 eV) and a higher HOMO level (−5.18 eV). Therefore, when Au metal was used as the electrode and the devices were tested under similar vacuum conditions, PBIBDF-BT exhibited n-type transport, whereas PBTPBF-BT exhibited ambipolar transport due to the deep LUMO and suitable HOMO levels. Highest mobilities of 0.45 and 0.22 cm2 V−1 s−1 were obtained for hole and electron, respectively. Under the air condition, the hole mobility of PBTPBF-BT significantly increased to 0.61 cm2 V−1 s−1 and electron mobility was maintained at 0.07 cm2 V−1 s−1. Overall, this study demonstrates that replacing the outer benzene with thiophene can effectively vary the polymer properties such as bandgap, energy levels and, as a result, tune the transport behavior.

A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF)-based low band gap polymer (PBIBDF-BT), containing a solubilizing alkyl chain bithiophene unit as a donor, has been synthesized. The polymer with a low-lying LUMO/HOMO energy level (−4.03/−5.55 eV) exhibits efficient ambipolar charge transport. The electron and hole mobilities are as high as 1.08 and 0.30 cm2 V−1 s−1, respectively.

•Benzodithiophene and benzotrithiophene-based conjugated polymers have been synthesized.•The conjugated- and acyl-side chain substituent had strong impact of on polymer-packing order structures.•The highest mobility of 1.70 × 10−3 cm2 V−1 s−1 was obtained for polymer-based devices.A series of benzodithiophene (BDT) and benzotrithiophene (BTT)-based conjugated polymers (P1–P4), with/without conjugated- and acyl-side chain, have been synthesized by Stille cross-coupling reaction. Their thermal, photophysical, electrochemical properties, devices performances, and microstructure have been investigated. Conjugated-side chain can significantly raise the thermal stability and acyl-side chain can lower HOMO/LUMO energy levels. Organic thin-film transistors (OTFTs) based on conjugated polymers were fabricated and the transistor electrical characterization showed the device performance was sensitive to the conjugated- and acyl-side chain substituent of polymers. A maximum hole mobility of 1.70 × 10−3 cm2 V−1 s−1 was obtained for P1-based devices, which is an order of magnitude higher than those of P3 and P4-based devices. The corresponding microstructures were investigated by grazing-incidence X-ray diffraction (GIXD) to correlate with conjugated- and acyl-side chain dependent carrier mobility of P1–P4. The results showed that the conjugated- and acyl-side chain have an impact on the polymer packing models and device performances.Graphical abstract

A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF)-based low band gap polymer (PBIBDF-BT), containing a solubilizing alkyl chain bithiophene unit as a donor, has been synthesized. The polymer with a low-lying LUMO/HOMO energy level (−4.03/−5.55 eV) exhibits efficient ambipolar charge transport. The electron and hole mobilities are as high as 1.08 and 0.30 cm2 V−1 s−1, respectively.

Three donor–acceptor polymers based on (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo-[1,2-b:4,5-b]-difuran-2,6(3H,7H)-dione (BIBDF) and three kinds of dialkylated bithiophenes with head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT) connectivity were synthesized by the Stille coupling reaction. Their photophysical and electrochemical properties, electronic device performance, and microstructure were investigated. We found that the alkyl chains substituted near the thiophene–thiophene connection cause less steric hindrance than those near the BIBDF–thiophene connection. Therefore PBIBDF-HH exhibited the preferred planarity, crystallinity, and molecular orientation, yielding the highest field-effect mobility. A maximum electron mobility of 1.23 cm2 V−1 s−1 and a maximum hole mobility of 0.37 cm2 V−1 s−1 were obtained for PBIBDF-HH-based devices. These results show that the substitution sites on the bithiophene units play an important role in the determination of molecular organization and the resulting device performance.