WO2019006699A1

WO2019006699A1 - Imide-functional unit, copolymer thereof and their preparation methods, as well as their uses

Info

Publication number: WO2019006699A1
Application number: PCT/CN2017/091845
Authority: WO
Inventors: Xugang GUO; Jianhua Chen; Xianhe ZHANG
Original assignee: South University Of Science And Technology Of China
Priority date: 2017-07-05
Filing date: 2017-07-05
Publication date: 2019-01-10

Abstract

An imide-functional unit of Formula I, a copolymer thereof and their preparation methods, as well as their uses in a thin-film transistor. The imide-functional unit is an effective electron-deficient building block for constructing high-performance polymer semiconductors due to its solubilizing ability, extended π conjugation length, close π-π stacking, and appropriate electron-withdrawing ability and lower-lying FMOs versus the previously reported imide-functional units.

Description

[Title established by the ISA under Rule 37.2] IMIDE-FUNCTIONAL UNIT, COPOLYMER THEREOF AND THEIR PREPARATION METHODS, AS WELL AS THEIR USES

FIELD

The present invention belongs to the field of semiconductor material, in particular relates to an imide-functional unit, a copolymer thereof and their preparation methods, as well as their uses.

BACKGROUND

π-conjugated organic and polymeric semiconductors have attracted substantial attentions owing to their great potentials for applications in low-cost, flexible, light-weight electronic devices such as organic n-channel thin-film transistors (OTFTs) , organic photovoltaics (OPVs) , and organic light-emitting diodes (OLEDs) . ^[1-6] Among them, most of high-performance polymer semiconductors for OPV and OTFT applications are constructed based on the donor-acceptor (D-A) strategy, ^[7-8] which results in good intramolecular charge transport and close intermolecular stacking for efficient charge carrier hoping between neighboring molecules. The physical properties and device performance of D-A polymers can be readily modulated by modifying the donor and acceptor units. For achieving improved n-channel OTFT performance, the design and synthesis of novel electron-deficient building blocks with optimized geometry and opto-electrical property plays a critical role. ^[9, 10] The acceptor units leading to promising n-channel OTFT performance typically have two key features, i.e. high degree of backbone planarity and large electron affinity. ^[1, 11] Therefore, tremendous efforts have been made to develop novel acceptor units with such features for improving the performance of n-channel OTFTs. ^[12] However, most of high-mobility n-type polymer semiconductors constructed by D-A strategy typically resulting in narrow bandgaps with high-lying HOMOs, which lead to facile hole injection and pronounced p-channel performance, hence suffering from low I_on/I_off ratios of 10³-10⁴. ^[13, 14] Hence, the HOMOs also should be taken into consideration in order to achieve ideal n-channel OTFT performance.

Imide-functionalized building blocks are considered as the most promising electron-deficient units for constructing high-performance n-channel polymer semiconductors in OTFTs, which are mainly contributed to their intrinsic advantages including: (a) the strong electron-withdrawing capability of imide groups, which suppresses the lowest unoccupied molecular orbitals (LUMOs) of organic semiconductors, resulting in facile electron injection and stabilizing the generated organic anions； (b) the good π-conjugation with the neighboring arenes due to the enforced compact geometry and/or intramolecular non-covalent interactions, such as hydrogen bond and sulfur-oxygen interaction； (c) the solubilizing N-alkyl chains, which are distal from conjugated backbones, offering good solubility without generating significant steric hindrance, therefore fine-tuned film morphology, close intermolecular π-stacking, and long range ordering can be obtained. ^[1] To date many imide-functionalized electron-deficient units have been reported, such as perylenediimide (PDI) , ^[15-24] naphthalenediimide (NDI) , ^[25-30] phthalimide (PhI) , ^[31-37] , thienopyrroledione (TPD) ^[38-43] , and bithiopheneimide (BTI) . ^[10, 44-48] The polymer semiconductors derived from these building blocks show the most promising OTFT performance.

In our previous work, we have reported several families of high-performance polymer semiconductors by incorporating NDI, ^[49, 50] PhI, ^[51-54] TPD, ^[55, 56] and BTI^[57-60] and systematically investigated their device performance in OTFTs and OPVs and elucidated their structure-property correlations. In 2008, a novel electron deficient unit, the highly planar BTI, was synthesized and the derived BTI homopolymer exhibits an excellent electron mobility > 0.01 cm² V^-1 s^-1 with a current on/off ratio (I_on/I_off) of 10⁷. ^[10] Molecular weight optimization in combination with device engineering leads to enhanced film crystallinity and an improved electron mobility of ～0.2 cm² V^-1 s^-1 was obtained in top-gated OTFTs. ^[59] Recently, Osaka and co-workers designed and synthesized a doubly BTI-fused building block dithienylthienothiophenebisimide (TBI, Formula 1) . The corresponding polymers exhibit versatile performance in OPVs as donor materials or acceptor materials as well as in OTFTs as n-type and p-type semiconductors with charge carrier mobility of 10^-3 ～10^-2 cm² V^-1 s^-1. ^[61]

Ding and co-workers reported the synthesis and photovoltaic application of a pentacyclic amide-based lactam acceptor unit, thieno [2', 3': 5, 6] pyrido [3, 4-g] thieno [3, 2-c] isoquinoline-5, 11 (4H, 10H) -dione (TPTI, Formula 2) . The TPTI-based polymers exhibit promising photovoltaic performance as p-type materials in OPVs. ^[62]

Thereafter Pei, ^[63] Takimiya, ^[64] and Kim ^[65] reported the TPTI isomer thieno [20, 30: 4, 5] -pyrido [2, 3-g] thieno [3, 2-c] quinoline-4, 10 (5H, 11H) -dione (TPT, Formula 3) and its derived polymers for p-channel OTFTs. Attributed to the fused structure of TPTI and TPT unit, the corresponding polymers possess high-degree of backbone planarity and substantial crystallinity, resulting in large hole mobilities up to 0.58 cm² V^-1 s^-1. ^[63]

However, limited by the weaker electron-withdrawing ability of amide group in TPTI and TPT, the HOMO and LUMO energy levels of the resulting polymers are relative high and hence inappropriate for n-channel OTFTs owing to the large electron injection barrier and pronounced p-channel performance.

SUMMARY

In order to promote n-channel performance in OTFTs, less electron rich group or more electron-deficient group is need to lower the FMOs without sacrificing good solubility, close packing and so on. The present invention provides the design and synthesis of the novel imide-functional unit dithienylbenzodiimide (TBDI, Formula I) and that its incorporation into copolymers affords semiconductors (Formula II) with good solubilizing ability, extended π conjugation length, close π-π stacking, and appropriate electron withdrawing ability and lower-lying FMOs.

In order to achieve this purpose, the present invention employs the following technical solutions:

An imide-functional unit of the Formula I,

wherein

R is a straight or branched alkyl, preferably having 5-15 carbon atoms, and more preferably having 7-12 carbon atoms,

X is H or F atom.

Compared to the TBI unit, the imide-functional unit dithienylbenzodiimide (TBDI) or dithienylfluorobenzodiimide (FTBDI) replaces the central thienothiophene of TBI with a less electron rich benzene or fluorobenzene moiety, ^{[1, 66, 67]} which should lower the HOMO level and suppress p-channel performance in OTFTs for the resulting polymers. Combining the advantages of TPTI and TPT by attaching additional carbonyl group, the new building block TBDI and FTBDI should lead to the derived polymer semiconductors with lower-lying FMOs versus the polymer analogues based on the amide-functionalized TPTI and TPT. Single crystal structure of TBDI indicates that the imide-functional unit features a non-planar backbone but with a remarkable π-π stacking distance of

Such close intermolecular π-π stacking is beneficial to intermolecular charge transport. ^[60]

In another aspect, the present invention provides a copolymer of the imide-functional unit of the invention having the Formula II,

wherein

is an aromatic unit can be the same as or different from the building block, n is 5-100.

Preferably, on the basis of the technical solution provided by the present invention,

is selected from the following group:

wherein R is a straight or branched alkyl, preferably having 5-15 carbon atoms, and more preferably having 7-12 carbon atoms.

To verify the above materials design strategy, thiophene, 3, 4-difluorothiophene, bithiophene, 4, 4'-didodecyl-2, 2'-bithiophene, 3-dodecyl-3'- (undecyloxy) -2, 2'-bithio-phene, difluorobithiophene, 2, 2'-bithiazole, 4, 4'-bis (octyloxy) -5, 5'-bithiazole, 2, 5-bis (2-ethylhexyl) -3, 6-di (thiophen-2-yl) pyrrolo [3, 4-c] pyrrole-1, 4 (2H, 5H) -dione, and 5, 6-difluoro-4, 7-bis (4-hexylthiophen-2-yl) benzo [c] [1, 2, 5] thiadiazole were chosen as the in-chain ∏ aromatic unit for constructing a copolymer semiconductor series. It will be seen that the resulting TBDI-based polymers exhibit promising device performance in n-channel organic thin-film transistors, with electron mobility (μ_e) up to 0.4 cm² V^-1 s^-1 and current modulation ratio (I_on/I_off) of 10⁶-10⁷. These results demonstrate that TBDI is an efficient building block for enabling polymer semiconductors with unipolar n-channel OTFT performance. Materials structure-property-device performance correlations are established here, and offer useful insights into organic electronics materials design. We believe that more promising performance can be realized by optimizing the chemical structures of TBDI-based materials.

In another aspect, the present invention provides a preparation method of the imide-functional unit described herein wherein R is branched alkyls comprising:

(1) adding dimethyl 2, 5-dibromoterephthalate, methyl 2- (trimethylstannyl) thiophene -3-carboxylate， an organic solvent into a reaction vessel, and purging the mixture with an inert gas；

(2) adding Pd (PPh₃) ₄ and then purging an inert gas, heating the mixture to 80-140 ℃℃；

(3) adding alkali carbonate, an organic solvent or water to the mixture and refluxing the mixture at 40-90 ℃；

(4) adding SOCl₂ into a reaction vessel；

(5) adding alkyl amine and heating the mixture to 140-160 ℃；

(6) adding Br₂ and FeCl₃ and heating the mixture to 40-90 ℃；

(7) extracting the reaction mixture and washing；

(8) concentrating the organic layer and purifying to give the imide-functional unit.

Preferably, on the basis of the technical solution provided by the present invention, in step (1) , the mole ratio of the dimethyl 2, 5-dibromoterephthalate to methyl 2- (trimethylstannyl) thiophene-3-carboxylate is 1: 2-4, preferably 1: 2.2-3, more preferably 1: 2.5；

preferably, the ratio of the organic solvent to the 2, 5-dibromoterephthalate is 2-10 mL/mmol, preferably 3-7 mL/mmol；

preferably, the organic solvent is selected from THF, EtOH, dioxane, DMF, toluene or a mixture thereof；

preferably, time of the purging is more than 10 minutes, preferably more than 20 minutes, more preferably 30 minutes；

preferably, the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture thereof；

preferably, in step (2) , the mole ratio of the Pd (PPh₃) ₄ to 2, 5-dibromoterephthalate is 1: 5-20, preferably 1: 8-15, more preferably 1: 10；

preferably, time of the purging is more than 5 minutes, preferably more than 10 minutes, more preferably 20 minutes；

preferably, the mixture is heated to 80-140 ℃； preferably to 111 ℃；

preferably, the heating is conducted under microwave irradiation；

preferably, in step (3) , preferably, the alkali carbonate is selected from K₂CO₃, Na₂CO₃, Li₂CO₃, LiOH, NaOH, KOH or a mixture thereof；

preferably, the organic solvent is selected from THF, EtOH, dioxane, DMF or a mixture thereof；

preferably, the mixture is refluxed at 70 ℃；

preferably, in step (4) , the ratio of SOCl₂ is 1-5 mL/mmol, preferably 2-4 mL/mmol；

preferably, the ratio of alkyl amine to 2, 5-bis (3- (chlorocarbonyl) thiophen-2-yl) terephthaloyl dichloride is is 1: 2-4, preferably 1: 2.2-3, more preferably 1: 2.5；

preferably, the mixture is heated to 140-160 ℃； preferably to 150 ℃；

preferably, in step (6) , the mixture is heated to 40-90 ℃； preferably to 60 ℃；

preferably, in step (7) , the reaction mixture is extracted with an organic solvent, preferably with DCM；

preferably, the washing is conducted with water and brine；

preferably, in step (8) , the concentrating is conducted under reduced pressure；

preferably, the purifying is conducted by column chromatography using petroleum ether as an eluent.

In another aspect, the present invention provides a preparation method of the copolymer of the invention, comprising:

(1) adding the imide-functional unit of claim 1, an aromatic unit material, tris (di-benzylideneacetone) dipalladium (0) (Pd₂ (dba) ₃) , and tris (o-tolyl) phosphine (P (o-tolyl) ₃) into a reaction vessel, and subjecting the reaction vessel and the mixture to an inert gas；

(2) adding an organic solvent； sealing the reaction vessel under an inert gas flow and then stirring while heating；

(3) adding 2- (tributylstanny) thiophene and stirring the reaction mixture while heating, then adding 2-bromothiophene and stirring the reaction mixture while heating；

(4) after cooling to room temperature, dripping the reaction mixture into methanol containing hydrochloric acid；

(5) drying the solid precipitate obtained in step (4) to give the crude product, and then extracting the crude product；

(6) after the extracting, concentrating the polymer solution, and then being dripped into methanol, collecting the solid and drying to obtain the copolymer.

Preferably, the mole ratio of the electron-donating unit of the invention to the aromatic unit material is 1: 0.5-2, for example, 1: 0.8, 1: 1.2, 1: 1.8 and so on, preferably 1: 0.8-1.5, more preferably 1: 1.

Preferably, the mole ratio of the tris (dibenzylideneacetone) dipalladium (0) (Pd₂ (dba) ₃) to tris (o-tolyl) phosphine (P (o-tolyl) ₃) is 1: 4-15, for example, 1: 6, 1: 9, 1: 13 and so on, preferably 1: 6-10, more preferably 1: 8； the Pd loading is 0.005-0.1 equiv, preferably 0.01-0.06.

Preferably, the reaction vessel and the mixture are subjected to 1-5 pump/purge cycles with Ar.

Preferably, the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture thereof.

Preferably, in step (2) , the inert gas is selected from any one of Ar, N₂, He, Ne, or a mixture thereof.

Preferably, the organic solvent is selected from any one of anhydrous toluene, benzene, chlorobenzene, DMF, or a mixture thereof.

Preferably, the ratio of the organic solvent to the electron-donating unit is 10-75 mL/mmol, preferably 5-50 mL/mmol.

Preferably, the heating is conducted at 50-170 ℃ for 1-72h, preferably at 80-150 ℃ for 3-50h.

Preferably, the heating is conducted under microwave irradiation.

Preferably, the heating is conducted by 80 ℃ for 10 minutes, 100 ℃ for 10 minutes, and 140 ℃ for 3 h under microwave irradiation.

Preferably, in step (3) , the heating is conducted at 80-170 ℃ for more than 0.2 h, preferably at 100-160 ℃ for more than 0.4 h.

Preferably, the heating is conducted under microwave irradiation.

Preferably, the heating is conducted under microwave irradiation at 140 ℃ for 0.5 h, then adding 2-bromothiophene and stirring the reaction mixture at 140 ℃ for another 0.5 h.

Preferably, the mole ratio of the 2- (tributylstanny) thiophene to the electron-donating unit is 0.1-0.5: 1, for example, 0.2: 1, 0.4: 1 and so on, preferably 0.2: 0.4-1.

Preferably, the mole ratio of the 2-bromothiophene to the electron-donating unit is 0.2-1.5: 1, for example, 0.4: 1, 0.8: 1, 1.3: 1 and so on, preferably 0.4: 0.8-1； preferably, in step (4) , the methanol contains 0.5-10mL hydrochloric acid， preferably 0.5-10mLof 5-20 mol/L hydrochloric acid.

Preferably, the dripping is conducted under vigorous stirring, preferably is conducted for at least 0.5 h, preferably at least 1 h.

Preferably, in step (6) , the dripping is conducted under vigorous stirring.

Preferably, the collecting is conducted by filtration.

Preferably, the drying is conducted under reduced pressure.

In still another aspect, the present invention provides a use of the copolymer according to the present invention in n-channel thin-film transistor.

The raw material used in above preparation method can be prepared by known method in the art or by the following method described below or buying on the market.

We report herein the design and synthesis of a novel imide-functionalized building block, dithienylbenzodiimide (TBDI) , which features a non-planar backbone but a close π-stacking of

as revealed by single crystal structure study. The twisted backbone and intrinsic electrical property of TBDI leads to the TBDI-based polymer semiconductors with low-lying FMO level. Low-lying FMOs should promote n-channel but suppress p-channel performance in OTFTs, which greatly differs from most high-performance n-type polymer semiconductors reported to date, showing narrow bandgap, high-lying HOMOs, and substantial p-type characteristics. In spite of its twisted backbone, all TBDI-based polymers show high-degree of film crystallinity with close π-stackings of

Attributed to the strong electron withdrawing imide group, all TBDI-based polymers show low-lying HOMOs and LUMOs, which facilitate electron injection but suppress hole injection. OTFT characterizations show that TBDI-bithiophene based OTFTs exhibit ambipolar characteristics with electron and hole mobility of 0.15 and 0.015 cm²V^-1s^-1, respectively. TBDI-thiophene and TBDI-difluorobithiophene OTFTs can achieve unipolar n-channel OTFT performance with electron mobility of 0.11 and 0.40 cm²V^-1s^-1, respectively. Our study reveals that dithienylbenzodiimide is a highly promising building block for enabling high-performance n-channel organic thin-film transistors and non-planar building block also can be used for constructing high mobility polymer semiconductors.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. The chemical structure of TBDI-based polymers.

Figure 2. Top view (a) , side view (b) , and intermolecular arrangements (c) of the TBDI-C8 single crystal.

Figure 3. Optimized geometries of TBDI-based polymers with 2.5 repeat units. The DFT calculation was performed at B3LYP/6-31G*level.

Figure 4. Calculated molecular orbitals of TBDI-based polymers.

Figure 5. The TGA curves of TBDI-based polymers.

Figure 6. The DSC curves of TBDI-based polymers.

Figure 7. The UV-vis absorption spectra of TBDI-based polymers in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 8. The UV absorption spectra of TBDI-T in o-DCB solution (1x 10^-5 M) in different temperature.

Figure 9. The UV absorption spectra of TBDI-DT in o-DCB solution (1x 10^-5 M) in different temperature.

Figure 10. The UV absorption spectra of TBDI-DFDT in o-DCB solution (1x 10^-5 M) in different temperature.

Figure 11. The cyclic voltammograms of TBDI-based polymer films in 0.1 M (n-Bu) ₄N^. PF₆ acetonitrile solution with the Fc/Fc⁺ as the internal standard.

Figure 12. Transfer (up) and output (down) characteristics of TBDI-T (a, e) , TBDI-DT (b, f) , and TBDI-DFDT (c, g) OTFTs in n-channel regime at the optimized annealing temperature (250, 220, and 250 ℃, respectively) with Au source/drain electrodes. The transfer (d) and output (h) characteristics of TBDI-DFDT OTFTs at the optimized annealing temperature (250 ℃) with Al source/drain electrodes, the OTFTs show unipolar n-channel performance.

Figure 13. AFM height and phase images (5 × 5 μm) of TBDI-T (a, d) , TBDI-DT (b, e) and TBDI-DFDT (c, f) as cast films.

Figure 14. AFM height and phase images (5 × 5 μm) of TBDI-T (a, d) , TBDI-DT (b, e) and TBDI-DFDT (c, f) films after annealing at the optimized temperature (250, 220, and 250 ℃, respectively) .

Figure 15. GIXD images of TBDI-T (a) TBDI-DT (b) TBDI-DFDT (c) and the corresponding linecuts of the TBDI-based polymer films in out-of-plane and (d) in-plane (e) . The as cast films were casted on silicon substrate for thin-film transistor fabrication.

Figure 16. GIXD images of TBDI-T (a) TBDI-DT (b) TBDI-DFDT (c) and the corresponding linecuts of the TBDI-based polymer films in out-of-plane and (d) in-plane (e) . The films were casted on silicon substrate under the optimal conditions for thin-film transistor fabrication.

Figure 17. The ¹H NMR of compound TBDI monomer.

Figure 18. The ¹³C NMR of compound TBDI monomer.

Figure 19. The HRMS of compound TBDI monomer.

Figure 20. The UV-vis absorption spectra of TBDI-DTR in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 21. The UV-vis absorption spectra of TBDI-TRTOR in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 22. The UV-vis absorption spectra of TBDI-DFT in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 23. The UV-vis absorption spectra of TBDI-DTz in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 24. The UV-vis absorption spectra of TBDI-TzOR in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 25. The UV-vis absorption spectra of TBDI-BT in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 26. The UV-vis absorption spectra of TBDI-DPP in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 27. The UV-vis absorption spectra of Homo-TBDI in chloroform solution (1 × 10^-5 M) and in film states (spin coated from 3 mg mL^-1 chloroform solution) .

Figure 28. A BGTC device comprising TBDI-based polymers of the invention.

DETAILED DESCRIPTION

To facilitate understanding of the present invention, the embodiment of the present invention is exemplified as follows. A person skilled in the art should be appreciated that the embodiments are merely used to help understand the present invention and should not be regarded as specific limits on the invention.

All reagents and chemicals were commercially available and were used without further purification unless otherwise stated. Tetrahydrofuran and toluene were distilled from Na/benzophenone, and anhydrous dichloromethane and acetonitrile were distilled from CaH₂. Dimethyl 2, 5-dibromo-terephthalate (1) and methyl 2-(trimethylstannyl) thiophene-3-carboxylate (2) were prepared following published procedures. ^[61, 69] All other reagents were used as received except where noted. Unless otherwise stated, all manipulations and reactions were carried out under argon using standard Schlenk line techniques. Polymerizations were carried out on Initiator+ Microwave Synthesizer (Biotage, Sweden) .

¹H and ¹³C spectra were recorded on a Bruker Ascend 400 MHz spectrometer. HR-MS measurements were performed on a Bruker APEX II FT-ICRMS spectrometer. Elemental analyses (EA) of monomer and polymers were performed on Vario EL Cube at Shenzhen University (Shenzhen, Guangdong) . Polymer molecular weights were measured on Polymer Laboratories GPC-PL220 high temperature GPC/SEC system at 130 ℃ vs polystyrene standards using trichlorobenzene as eluent. Differential scanning calorimetry (DSC) curves were recorded on a differential scanning calorimetry with heating rate of 10 ℃/min in nitrogen atmosphere. Thermogravimetric analysis (TGA) curves were recorded on a Mettler, STARe TA Instrument. UV-Vis absorption spectra were recorded on a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer. Cyclic voltammetry (CV) measurements were carried out using a CHI760E voltammetric analyzer with 0.1 M tetra-n-butyl-ammonium hexafluorophosphate (Bu₄NPF₆) in acetonitrile as supporting electrolyte. A platinum disk, platinum wire and silver wire were employed as working electrode, counter electrode and reference electrode, respectively. Polymer films were drop-coated from chloroform solutions on a Pt working electrode. The scan rate was 100 mV/s. And Fc/Fc⁺ was used as internal reference for all measurements. AFM measurements of pure polymer films were performed by using a Dimension Icon Scanning Probe Microscope (Asylum Research, MFP-3D-Stand Alone) in tapping mode.

The synthesis of the key TBDI building block is straightforward, and Scheme 1 depicts the Synthesis of TBDI monomer and TBDI-based polymers.

More detailed TBDI monomer and polymer synthetic and characterization information is reported as follows. Dimethyl 2, 5-dibromo-terephthalate (1) and methyl 2- (trimethylstannyl) thiophene-3-carboxylate (2) were prepared following published procedures. ^[61, 69] Still coupling between 1 and 2 affords compound 3 with a yield of 44％under microwave condition at 150 ℃, which is then followed by hydrolysis to provide compound 4. The compound 4 is reacted with SOCl₂ overnight to give compound 5, which can be used for the next step without further purification. After the removal of SOCl₂, the alkyl amine is added and the reaction mixture is heated to 150 ℃ to afford compound 6 with a yield of 40％. The final monomer is obtained with a high yield > 90％by brominating compound 6 using Br₂/Fe. ^[58] .

The TBDI monomer was copolymerized with tin product or borate product of ∏aromatic unit under a standard Stille or Suzuki polymerization condition using microwave as the heating source to give the TBDI-based polymers. Figure 1 depicts the molecular structure of TBDI-based polymers. The product polymers were purified by Soxhlet extractions using methanol, acetone, n-hexane, dichloromethane, and chloroform as the solvents. The chloroform fraction was collected and dried under vacuum.

Monomer and Polymer Synthesis-wherein R is a branched alkane

(3) : dimethyl 2, 5-bis (3- (methoxycarbonyl) thiophen-2-yl) terephthalate synthesized by following Scheme 2

Compound 1 (351 g, 1 mmol) , 2 (915 mg, 3 mmol) , Pd (PPh₃) ₄ (100 mg) and DMF (15 mL) were added to a reaction tube under nitrogen atmosphere. The reaction tube was heated to 150 ℃ for 4 hours with a microwave reactor. After cooling to room temperature, the DMF was evaporated under a reduced pressure. 100 mL methanol was added to the round bottom flask and then filtrated. The solid was washed with water (30 mL) and methanol (30 mL) to give compound 3 as gray solid (210 mg, yield 44％) .

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 8.03 (s, 2H) , 7.56 (d, J ＝ 5.3 Hz, 2H) , 7.37 (d, J ＝ 5.3 Hz, 2H) , 3.73 (s, 6H) , 3.70 (s, 6H) . ¹³C NMR (100 MHz, CDCl₃) δ(ppm) : 165.92, 163.19, 147.94, 134.72, 133.40, 133.31, 129.44, 128.99, 124.70, 52.35, 51.59.

(4) : 2, 5-bis (3-carboxythiophen-2-yl) terephthalic acid was synthesized by following Scheme 3

Compound 3 (351 g, 1 mmol) , KOH (915 mg, 3 mmol) , H₂O (15 mL) and THF (15 mL) were added to a round bottom flask. The reaction mixture was heated to 70 ℃ overnight. After cooling to room temperature, the THF was evaporated under a reduced pressure. The aqueous solution was cooled to 0 ℃, and then 2 mL hydro-chloric acid was added. The product 4 was obtained by filtration and then washed by water (30 mL) as white solid (210 mg, yield 44％) . The product was pure without any further purification.

¹H NMR (500 MHz， DMSO) δ (ppm) : 12.61 (s, 4H) , 7.80 (s, 2H) , 7.65 (d, J ＝ 5.3 Hz, 2H) , 7.44 (d, J ＝ 5.3 Hz, 2H) . ¹³C NMR (100 MHz， DMSO) δ (ppm) : 167.04, 163.96, 147.63, 134.64, 134.35, 133.09, 130.83, 129.65, 125.98.

(5) : 2, 5-bis (3- (chlorocarbonyl) thiophen-2-yl) terephthaloyl dichloride synthesized by following Scheme 4

Compound 4 (351 g, 1 mmol) and SOCl₂ (15 mL) were added to a round bottom flask in nitrogen atmosphere and then heated to 80 ℃ overnight. The excess SOCl₂ was removed by evaporation. The crude product of 5 was directly used in the next step without any further purification.

(6) : TBDI synthesized by following Scheme 5

To a round bottom flask with dry compound 5 was added 2 eq RNH₂ in nitrogen atmosphere. The reaction mixture was carefully seal up and then heated to 150 ℃ for 4 hours. After cooling to room temperature, the dark residue was dissolved in CHCl₃ (2 mL) and then purified by silica gel column chromatography with CHCl₃/petroleum ether (1: 4) as eluent. The compound 6 was obtained as pale yellow solid. 6a (yield: 40％)

¹H NMR (500 MHz, CDCl₃) δ (ppm) : 8.35 (s, 2H) , 7.63 (d, J ＝ 5.2 Hz, 2H) , 7.41 (d, J ＝ 5.2 Hz, 2H) , 4.19 (d, J ＝ 7.2 Hz, 4H) , 1.95 (s, 2H) , 1.29-1.24 (m, 80H) , 0.92-0.84 (m, 12H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 168.24 , 161.69, 142.29, 135.24, 135.03, 134.71, 132.16, 128.68, 114.26, 51.12, 36.87, 31.95, 31.53, 30.17, 29.81, 29.49, 29.39, 26.26, 22.72, 14.16. HRMS (MALDI+) Calcd for: C₆₆H₁₀₄N₂O₄S₂ (M⁺) : 1053.7510, found: 1053.7528.

TBDI monomers: synthesized by following Scheme 6

To a solution of compound 6 (1eq) in CH₂Cl₂ was added a catalytic amount of FeCl₃ and Br₂ (2.2 eq) . And then, the reaction mixture was heated to 55 ℃ for 24 hours. After cooling to room temperature, the reaction mixture was poured to water and extracted by CH₂Cl₂. The combined organic layer was dried by anhydrous Na₂SO₄ and then filtrated. The organic solution was evaporated under a reduced pressure. The yellow residue was dissolved in CHCl₃ (2 mL) and then purified by silica gel column chromatography with CHCl₃/petroleum ether (1: 4) as eluent. The compound 6 was obtained as pale yellow solid in high yield over 90％.

¹H NMR (400 MHz, CDCl₃) δ (ppm) : 8.21 (s, 2H) , 7.59 (s, 2H) , 4.16 (d, J ＝ 7.3 Hz, 4H) , 1.91 (s, 2H) , 1.37 -1.26 (m, 80H) , 0.91 -0.88 (m, 12H) . ¹³C NMR (100 MHz, CDCl₃) δ (ppm) : 168.24, 161.69, 142.29, 135.24, 135.03, 134.71, 132.16, 128.68, 114.26, 51.12, 36.87, 31.95, 31.94, 31.53, 30.17, 29.73, 29.68, 29.67, 29.40, 29.39, 26.26, 22.72, 14.16. HRMS (MALDI⁺) Calcd for: C₆₆H₁₀₃O₄N₂Br₂S₂ (M⁺) : 1211.5700, found: 1211.5714. The ¹H NMR, ¹³C NMR and HRMS were shown in Figure17, 18 and 19, respectively.

General Procedure for Polymerizations via Stille Coupling for Synthesis of TBDI-based Polymers.

An glass tube was charged with two monomers (0.2 mmol each) , tris (dibenzylideneacetone) dipalladium (0) (Pd₂ (dba) ₃) _, and tris (o-tolyl) phosphine (P (o-tolyl) ₃) (1: 8, Pd₂ (dba) ₃: P (o-tolyl) ₃ molar ratio； Pd loading: 0.03-0.05 equiv) . The tube and its contents were subjected to 3 pump/purge cycles with argon, followed by the addition of anhydrous toluene (6-8 mL) via syringe. The tube was sealed under argon flow and then stirred at 80 ℃ for 10 minutes, 100 ℃ for 10 minutes, and 140 ℃ for 3 h under microwave irradiation. Then, 0.1 mL of 2- (tributylstanny) thiophene was added and the reaction mixture was stirred under microwave irradiation at 140 ℃ for 0.5 h. Finally, 0.2 mL of 2-bromothiophene was added and the reaction mixture was stirred at 140 ℃ for another 0.5 h. After cooling to room temperature, the reaction mixture was slowly dripped into 100 mL of methanol (containing 5 mL 12 N hydrochloric acid) under vigorous stirring. After stirring for 4h, the solid precipitate was transferred to a Soxhlet thimble. After drying, the crude product was subjected to sequential Soxhlet extraction with the choice of solvents and sequence depending on the solubility of the particular polymer. After final extraction, the polymer solution was concentrated to approximately 20 mL, and then dripped into 100 mL of methanol under vigorous stirring. The polymer was collected by filtration and dried under reduced pressure to afford deep colored solid as the product.

TBDI-T was synthesized by following Scheme 7.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a red solid (104 mg, yield: 93％) . ¹H NMR (400 MHz, CDCl₃) : δ 8.17 (br) , 7.63 (br) , 4.62 (br) , 1.99 (br) , 1.36 (br) , 0.96 (br) . Anal. Calcd for C₇₀H₁₀₄N₂O₄S₃ (％) : C, 74.15； H, 9.25； N, 2.47； O, 5.64； S, 8.48. Found (％) : C, 73.96； H, 9.33； N, 2.43； S, 8.56.

TBDI-DT was synthesized by following Scheme 8.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a purple solid (111 mg, yield: 92％) . ¹H NMR (400 MHz, CDCl₃) : δ 8.01 (br) , 7.54 (br) , 4.43 (br) , 1.64-1.38 (br, ) , 0.96 (br) . Anal. Calcd for C₇₄H₁₀₆N₂O₄S₄ (％) : C, 73.10； H, 8.79； N, 2.30； O, 5.26； S, 10.55. Found (％) : C, 71.2； H, 8.72； N, 2.18； S, 10.15.

TBDI-DFDT was synthesized by following Scheme 9.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a red solid (102 mg, yield: 81％) . ¹H NMR (400 MHz, CDCl₃) : δ 8.49 (br) , 7.73 (br) , 4.45 (br) , 1.63-1.35 (br, ) , 0.94 (br) . . Anal. Calcd for C₇₄H₁₀₄F₂N₂O₄S₄ (％) : C, 71.00； H, 8.37； F, 3.04； N, 2.24； O, 5.11； S, 10.25. Found (％) : C, 70.65； H, 8.49； N, 2.19； S, 10.24.

TBDI-DTR was synthesized by following Scheme 10.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a purple solid (120 mg, yield: 90％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 20.

TBDI-TRTPR was synthesized by following Scheme 11.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a blue solid (115 mg, yield: 93％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 21.

TBDI-DFT was synthesized by following Scheme 12.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a red solid (108 mg, yield: 88％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 22

TBDI-DTz was synthesized by following Scheme 13.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a red solid (104 mg, yield: 91％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 23.

TBDI-TzOR was synthesized by following Scheme 14.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a blue solid (120 mg, yield: 86％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 24.

TBDI-BT was synthesized by following Scheme 15.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a blue solid (135 mg, yield: 89％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 25.

TBDI-DPP was synthesized by following Scheme 16.

The solvent sequence for Soxhlet extraction was methanol, acetone, hexane, dichloromethane and chloroform. The polymer was obtained as a dark green solid (140 mg, yield: 87％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 26.

Homo-TBDI was synthesized by following Scheme 17.

The solvent sequence for Soxhlet extraction was methanol, acetone, and hexane. The polymer was obtained as a blue solid (50 mg, yield: 65％) . The corresponding UV-Vis absorption spectra of chloroform solution (10^-5 M) and in film solid state are shown in Figure 27.

After polymerizations, the polymer chains were end-capped with mono-functionalized thiophene⁶⁸. The polymers were collected by precipitation in methanol, which were then subjected to purification via Soxhlet extractions using different solvent sequences, depending on the polymer solubility. The identity and purity of the product polymers were supported by ¹H NMR as well as by elemental analysis. All the polymers exhibit good solubility in common organic solvents for device fabrication. Polymer molecular weights were measured by gel permeation chromatography (GPC) versus polystyrene standards. Number average molecular weight, M_ns are summarized in Table 1.

Table 1 Molecular weight, optical absorption, and electrochemical data of the TBDI-Based polymers.

^a E_g ^opt ＝ 1240/λ_onset (eV) ； ^b Calculated from CV； ^c Calculated from E_HOMO and E_g ^opt : E_LUMO ＝ E_HOMO + E_g ^opt

Single Crystal Structure of Dithienylbenzodiimide

In order to better understand the structure-property correlations of these new TBDI-based polymer semiconductors, the single crystal of dithienylbenzodiimide (TBDI-C₈) with a short and linear n-octyl chain was cultivated by slow diffusing MeOH into the concentrated TBDI-C₈ solution in CHCl₃ as pale yellow acicular crystal. The crystal structure of TBDI-C₈ was determined. The crystal of TBDI-C₈ belongs to the triclinic system with

and

as well as α ＝ 76.86 (5) °, β ＝ 81.81 (3) °, and γ ＝ 88.16 (4) °. As illustrated in Figure 2, TBDI-C₈ shows a symmetrical structure with benzene as the center. However, it exhibits a high degree of backbone torsion with the dihedral angle between benzene (or thiophene) ring with adjacent imide rings up to 31° (or 26°) . In comparison to the highly planar bithiophene imide, ^[10] this distortion is likely caused by the heterogeneous bond length of benzene, thiophene, and imide moiety. The high degree backbone distortion typically is detrimental to intramolecular charge delocalization, the molecule packing, and intermolecular charge hopping. To our delight, in spite of its twisted backbone, TBDI-C₈ shows a close face-to-face π-πstacking with a remarkable distance of

which should compensate the disadvantage of backbone distortion and facilitate intermolecular charge transfer between distinct molecules.

Density Functional Theory Calculation

To investigate the electronic structures and geometries, density functional theory (DFT) calculations were performed at the B3LYP/6-311G*level using the Gaussian 09 program. ^[70] TBDI-based polymers (TBDI-T, TBDI-DT and TBDI-DFDT for example) having 2.5 repeat units with N-methyl substituent were used to reduce the calculation time. The optimized geometries for TBDI-based polymers are shown in Figure 3. All polymers exhibit slightly twisted geometries, which mainly originate from the torsion of TBDI unit. TBDI-DFDT exhibits improved planarity versus TBDI-DT and TBDI-T, which is contributed to the additional F atom, resulting in intramolecular conformation locking through non-covalent F-S interaction. ^[71, 72] The twisted backbone geometries typically are unfavorable for intramolecular charge delocalization and intermolecular charge transfer. The calculated frontier molecular orbitals (Figure 4) indicate that both HOMO and LUMO distribute across the entire backbones of polymers. Although all TBDI-based polymers show twisted backbone geometries, the well delocalized conjugated systems should be beneficial to intramolecular charge carrier delocalization.

Thermal Properties and Materials Crystallinity

The thermal stability and materials crystallinity are investigated by thermal gravimetric analysis (TGA, Figure 5) and differential scanning calorimetry (DSC, Figure 6) , respectively. All the TBDI-based polymers show excellent stability with the decomposition temperature (T_d) of 420, 426, and 402 ℃ for TBDI-T, TBDI-DT, and TBDI-DFDT, respectively. The TGA results indicate that the stability of all TBDI-based polymers is sufficient for OTFT fabrication. The DSC curves reveal that there are no evident thermal transitions for three polymers. It might be caused by high crystallinity that the transition peak much higher than 300 ℃ (instrument limits) .

Optoelectronic Properties

The ultraviolet-visible (UV-vis) absorption spectra of polymers in chloroform solution and in film states are shown in Figure 7, and the corresponding absorption data and optical bandgaps are summarized in Table 1. All polymers show broad absorption band with absorption onset (λ_onset) around 600-640 nm in solution state. Compared to that of BBIT-T, the λ_onsets of TBDI-DT and TBDI-DFDT exhibit a red-shift of 40 and 20 nm, respectively, which is attributed to the stronger electron-donating ability of bithiophene than monothiophene. However, the difluorobithiophene-based polymer TBDI-DFDT shows blue-shifted absorption versus TBDI-DT, which is consistent with the other fluorinated polymer semiconductors. ^[72, 73] The absorption spectra of polymer films are highly similar with those of polymer solution, showing a red-shift of λ_max less than 10 nm. In addition, distinctive absorption shoulders are observed for all polymers, especially for TBDI-DFDT film, which are attributed to enhanced polymer backbone planarity and stronger inter-chain interaction. These results clearly indicate that all TBDI-based polymer semiconductors exhibit strong aggregation in both solution and thin films, which should be beneficial to charge carrier transport. The temperature-dependent UV-vis absorption of polymers in o-DCB solution was also measured and the relevant spectra are shown in Figure 8-10. The absorption of all polymer solutions shows limited blue-shift as the temperature is increased up to 100 ℃, accompanied by gradually reduced absorption shoulder. The results indicate that all polymers possess strong intermolecular interaction even at high temperature, which is consistent with the ¹H NMR measurement. The aggregation leads to NMR signal broadening. The optical bandgaps (E_g ^opt) derived from the absorption onsets of polymer films are 2.05, 1.94, and 2.01 eV for TBDI-T, TBDI-DT, and TBDI-DFDT, respectively.

Electrochemical properties

The electrochemical properties of polymer films were investigated using cyclic voltammetry (CV) versus a ferrocene/ferrocium (Fc/Fc⁺) internal standard. The CV curves are shown in Figure 11 and the corresponding data are compiled in Table 1. All polymers show pronounced reversible oxidation and reduction peaks. The HOMO and LUMO energy levels of polymers are calculated from the onset of oxidation and reduction potentials using the equation of E_HOMO ＝ - (4.40 + E_onset ^ox) eV and E_LUMO ＝ - (4.40 + E_onset ^red) eV, respectively. The calculated HOMOs of TBDI-T, TBDI-DT, and TBDI-DFDT are -5.84， -5.57， and -5.74 eV, respectively. The relative higher-lying HOMO of TBDI-DT (versus that of TBDI-T) is originated from the stronger electron-donating ability of bithiophene than thiophene. Due to the electron-withdrawing ability of F atom on the bithiopehne unit, the TBDI-DFDT HOMO (-5.74 eV) is greatly downshifted compared to that (-5.57 eV) of TBDI-DT. The low-lying HOMOs of TBDI-based polymers should suppress p-channel performance in OTFTs. The CV-derived LUMOs of TBDI-T, TBDI-DT, and TBDI-DFDT are -3.11， -3.11， and -3.20 eV, respectively. Among the polymer series, TBDI-DFDT shows the lowest-lying LUMO, which should facilitate electron injection and stabilize the generated organic anion, and hence leads to improved n-channel OTFT performance. The LUMO energy level can also be calculated from HOMO energy level and E_g ^opt using the equation: E_LUMO ＝ E_HOMO + E_g ^opt. The LUMO energy levels derived from this method are -3.78， -3.65， and -3.76 eV for TBDI-T, TBDI-DT, and TBDI-DFDT, respectively. The low-lying LUMO in combination with the suppressed HOMO should promote n-channel performance and suppress p-channel performance in OTFTs and afford the device with large current modulation ratios (I_on/I_offs) .

Device Fabrication and Characterization

Bottom Gate/Top Contact (BGTC) thin film transistor was fabricated in order to investigate the mobility of TBDI-based polymers. P-doped silicons with 200nm SiO₂ substrate were sonicated in acetone and isopropanol followed by SC-1 cleaning, which the solution made up of 100mL H₂O, 20mL ammonium hydroxide and 20mL hydrogen peroxide at the temperature of 120 ℃. 3mM octadecyltrimethoxysilane (OTS) solution with trichloroethylene (TCE) solvent was spin-coated after UV-ozone and plasma treatment. Exposing the substrates in ammonium vapor for 15h, and treating them in toluene, isopropanol and deionized water subsequently, the contact angles on these substrates are 108°-110°. The polymer layer from 3mg/mL CF solution was spin coated in on substrate at 3000rmp. After that, they were annealed at 250 ℃, 220 ℃ and 190 ℃ for 10 minutes, and then cooled to room temperature. The device without annealing was also fabricated to compare with others. Eventually, 40nm Au was evaporated on the top of substrate as source-drain electrodes. The mobility of polymers were extracted from saturated region based on the equation

W the width of channel (1000μm) ， L the length of channel (100μm) . I_DS the source-drain saturation current， C_i the capacitance per unit area of the insulator， μ the mobility, V_G the gate voltage, and V_th the threshold voltage.

The charge transport characteristics of the TBDI-based polymer semiconductors are investigated by fabricating OTFTs with a bottom-gate/top-contact (BGTC) device configuration: Au (or Al) /polymer/SAM/SiO₂/Si (doped) . The trimethoxyoctadecylsilane was used as the self-assembly monolayer (SAM) . ^[74] The semiconducting layers are deposited on the dielectric layer via spin-coating polymer solution. The OTFT devices were optimized through thermal annealing at various temperatures. The optimized OTFT data and the corresponding transfer and output characteristics are shown in Table 2 and Figure 12. During device optimization it was found that the annealing temperature shows profound impacts on OTFT performance. The as-casted thin films of TBDI-based polymers exhibit electron-dominating transistor performance with electron mobilities (μ_es) of 0.031 -0.096 cm² V^-1 s^-1) . After thermal annealing at the optimal condition, the mobility of TBDI-T, TBDI-DT, and TBDI-DFDT OTFTs was greatly increased. TBDI-T OTFTs exhibit an improved n-channel transport characteristic with an average μ_e value of 0.07 cm² V^-1 s^-1, V_T of 36 V, and I_on/I_off of 10⁴ after thermal annealing at 250 ℃. The bithiophene-based polymer TBDI-DT shows ambipolar transport characteristics with an average μ_e of 0.13 cm² V^-1 s^-1 and an average hole mobility (μ_h) of 0.011 cm² V^-1 s^-1 after thermal annealing at 220 ℃. In Comparison to TBDI-T, the transistor performance variation of TBDI-DT is in good accord with the evolution of FMO energy levels. The replacement of bithiophene leads to elevated HOMO (-5.57 eV) and more facile hole injection, hence TBDI-DT OTFTs show ambipolar transport characteristics. The slightly increased μ_e of TBDI-DT is likely attributed to its higher degree of film crystallinity as revealed by the X-ray study. Compared to TBDI-DT, the F addition on the bithiophene co-unit leads to TBDI-DFDT with substantially improved n-channel transport characteristic under optimal fabrication condition, which is attributed to its lower-lying LUMO due to the electron-withdrawing F addition and increased backbone planarity enabled by intramolecular F-S non-covalent interaction. As the thermal annealing temperature is increased, TBDI-DFDT OTFTs exhibit gradually improved electron mobility, and the highest μ_e of 0.40 cm² V^-1 s^-1 with a V_T of 49 V and an I_on/I_off of 10⁶ was obtained after annealing at 250 ℃. The influence of thermal annealing on transistor performance will be investigated by AFM and GIXD (vide infra) . As shown in Table 2, all TBDI-based polymers exhibit relative high V_Ts, which are correlated with large electron injection barrier. Compared to TBDI-DT, the V_Ts of TBDI-DFDT are slightly reduced due to its lower-lying LUMO, resulting in decreased electron injection barrier.

Considering the large electron injection barrier between the Fermi level of Au electrode and the LUMO level of TBDI-based polymers, low work function Al is employed as the source/drain electrodes in TBDI-DFDT-based OTFTs. The reduced electron injection barrier results in a greatly decreased V_T of 22 V (Table 2) . Hence the TBDI-DFDT OTFTs exhibit unipolar n-channel transport characteristics with nearly ideal transfer and output curves (Figure 13d and 13h) . The OTFTs show an average μ_e of 0.27 cm² V^-1 s^-1 with minimal off-current (I_off) of 10^-12 -10^-11 A and a remarkable I_on/I_off ratio of ～10⁷. High mobility n-type polymer semiconductors typically show narrow bandgaps with high-lying HOMOs, which lead to facile hole injection and pronounced p-channel performance, hence suffering from low I_on/I_off ratios of 10³-10⁴. Using TBDI as the building block, the resulting polymer semiconductor TBDI-DFDT shows a wide bandgap (2.01 eV) with a very low-lying HOMO (-5.74 eV) , which in combination with device engineering affords the OTFTs with unipolar n-channel with a substantial μ_e of 0.34 cm² V^-1 s^-1, a remarkable I_on/I_off ratio of ～10⁷, and a moderate V_T of 22 V. Such unipolar n-channel OTFTs are highly desired for application in real electrical devices.

Table 2 Bottom-Gate/Top-Contact (BGTC) OTFT Device Performance Parameters of the TBDI-based Polymer Semiconductors.

^a Data represent the best mobilities. The average mobilities from > 5 devices are shown in parentheses. ^b Al was applied as source-drain electrodes.

Film Morphology and the Correlation with Device Performance

The film morphology and microstructure of BDTI-based polymer semiconductors was investigated by atomic force microscopy (AFM) and grazing incident X-ray diffraction (GIXD) . The AFM height and phase images of as-casted TBDI-based polymers are depicted in Figure 13, all polymers exhibit fiber like interwoven networks with distinct structural features, which indicate that all polymers have good crystallinity. Compared to that (0.67 nm) of TBDI-T, the root-mean-square (RMS) surface roughness of TBDI-DT and TBDI-DFDT is greatly increased to 2.45 and 1.74 nm, respectively, which demonstrates that the crystallinity of TBDI-DT and TBDI-DFDT is significantly improved. The height and phase images of TBDI-T, TBDI-DT, and TBDI-DFDT films after annealing at 250, 220, and 250 ℃ are depicted in Figure 14. The RMS roughness of annealed films of TBDI-T, TBDI-DT, and TBDI-DFDT films is increased to 0.77, 4.16, and 3.83 nm, respectively. The results indicate that thermal annealing can promote film crystallinity, which is in good accord with the improved OTFT mobility.

The two-dimensional GIXD images of the diffraction patterns and the corresponding linecuts of polymer films are shown in Figure 15 and Figure 16, and the relevant crystal packing parameters are summarized in Table 3 and Table 4. On the basis of the as-cast film GIXD patterns (Figure 15) , polymer TBDI-T displays isotropic diffraction along the out-of-plane and in-plane up to 200 diffractions peak with the corresponding inter-lamellar digitation distances of ～3.0 nm and strong out-of-plane dominated π-stacking at

which corresponds to a π-stacking distance of ～0.37 nm with a face-on backbone orientation. Interestingly the as-cast films of TBDI-DT and TBDI-DFDT polymers display remarkable anisotropic diffraction along the out-of-plane up to 500 diffraction peak with the corresponding interlamellar digitation distances of 2.4-2.8 nm and strong in-plane dominated π-stacking at

which corresponds a π-stacking distance of ～0.36 nm. The diffraction patterns indicate an edge-on dominating backbone orientation for polymer TBDI-DT and TBDI-DFDT, which is beneficial to charge transport in OTFTs. ^[75] The film crystallinity and polymer backbone orientation well explain the improved OTFT performance of TBDI-DT and TBDI-DFDT compared to TBDI-T. It is remarkable to note that the twisted TBDI moiety can enable the resulting polymer semiconductors with high-degree of film crystallinity with remarkable compact polymer chain packing (0.36-0.37 nm) .

After thermal annealing under the optimal OTFT fabrication condition, all TBDI-based polymers exhibit further improved film crystallinity (Figure 16) , as revealed by the increased diffraction intensity and the diffraction peaks progressing to higher orders. On the basis of the GIXD image and the corresponding linecuts, thermal annealing shows minimal impact on the polymer chain orientation, and TBDI-DT and TBDI-DFDT maintain the edge-on-dominating orientation versus the face-on-dominating orientation of TBDI-T. Replacement of thiophene with bithiophene unit, TBDI-DT and TBDI-DFDT show improved film crystallinity versus TBDI-T. According to the Scherrer equation, ^[76] the crystalline coherence lengths (CCL) of both out-of-plane and in-plane directions are estimated for all films. Thermal annealing leads to increased CCLs of all polymers, which indicates improved microstructural ordering. Therefore thermal annealing results in improved carrier transport properties in OTFTs.

Table 3. d-Spacings and crystalline correlation lengths (calculated via Scherrer analysis) for TBDI-based polymers as cast films.

Table 4 d-Spacings and crystalline correlation lengths (calculated via Scherrer analysis) for TBDI-based polymers under thermal annealed films.

In summary, we have developed a novel imide-functionalized building block, dithienylbenzodiimide (TBDI) , which features a non-planar backbone but a close π-stacking of

as revealed by single crystal structure study. The twisted backbone and intrinsic electrical property of TBDI leads to the TBDI-based polymer semiconductors with wide bandgaps and low-lying FMO levels. In spite of its twisted backbone, all TBDI-based polymers show high-degree of film crystallinity with close π-stackings of

specifically TBDI-DT and TBDI-DFDT. Attributed to the strong electron withdrawing imide group, all TBDI-based polymers show low-lying HOMOs and LUMOs, which facilitate electron injection but suppress hole injection. OTFT characterizations show that TBDI-DT-based OTFTs exhibit ambipolar characteristics with electron and hole mobility of 0.15 and 0.015 cm² V^-1 s^-1, respectively. TBDI-T and TBDI-DFDT OTFTs can achieve unipolar n-channel OTFT performance with electron mobility of 0.11 and 0.34 cm² V^-1 s^-1, respectively. Our study reveals that dithienylbenzodiimide is a highly promising building block for enabling high-performance n-channel organic thin-film transistors and non-planar building block also can be used for constructing high mobility polymer semiconductors.

Embodiments of the invention have been described above and, obviously, modifications and alterations will occur to others upon the reading and understanding of this specification. The invention and any claims are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.

All of the following reference documents are incorporated herein by reference.

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Claims

An imide-functional unit of Formula I,

wherein

R is a straight or branched alkyl, and

X is H or F atom.
The imide-functional unit of claim 1, wherein R is a straight or branched alkyl having 5-15 carbon atoms.
The imide-functional unit of claim 1, wherein R is a straight or branched alkyl having 7-23 carbon atoms.
A copolymer comprising the imide-functional unit of any of claims 1-3 having Formula II,

wherein

is an aromatic unit, and

n is an integer ranging from 5 to 100, inclusive.
The copolymer according to claim 4, wherein
is selected from the group consisting of the following moieties:

wherein

.
Preparation method of the imide-functional unit of any of claims 1-3, comprising the following steps:

(1) adding dimethyl 2, 5-dibromoterephthalate, methyl 2- (trimethylstannyl) thiophene -3-carboxylateθand an organic solvent into a reaction vessel, and purging the mixture with an inert gas；

(2) adding Pd (PPh₃) ₄, then blowing an inert gas and heating the mixture to 80-140 ℃ ℃；

(3) adding alkali carbonate, an organic solvent or water to the mixture and refluxing the mixture at 40-90 ℃；

(4) adding SOCl₂ into the reaction vessel；

(5) adding alkyl amine and heating the mixture to 140-160 ℃；

(6) adding Br₂ and FeCl₃ and heating the mixture to 40-90 ℃；

(7) extracting the reaction mixture and washing； and

(8) concentrating the organic layer and purifying to obtain the imide-functional unit.
The preparation method according to claim 6, wherein

in step (1) ,

the mole ratio of the dimethyl 2, 5-dibromoterephthalate to methyl 2-(trimethylstannyl) thiophene-3-carboxylate is 1: 2-4, preferably 1: 2.2-3, more preferably 1: 2.5；

preferably, the organic solvent is selected from THF, EtOH, dioxane, DMF, toluene or a mixture thereof and the adding amount of the organic solvent is 2-10 mL per mmol of the 2, 5-dibromoterephthalate, preferably 3-7 mL per mmol of the 2,5-dibromoterephthalate；

preferably, the purging is maintained for more than 10 minutes, preferably more than 20 minutes, more preferably about 30 minutes；

preferably, the inert gas is selected from Ar, N₂, He, Ne, or a mixture thereof.

preferably, in step (2) ,

the mole ratio of the Pd (PPh₃) ₄ to 2, 5-dibromoterephthalate is 1: 5-20, preferably 1:8-15, more preferably 1: 10；

preferably, the inert gas is selected from Ar, N₂, He, Ne, or a mixture thereof；

preferably, the blowing is maintained for more than 5 minutes, preferably more than 10 minutes, more preferably about 20 minutes；

preferably, the mixture is heated to 80-140 ℃； preferably to 111 ℃；

preferably, the heating is conducted under microwave irradiation；

preferably, in step (3) ,

preferably, the alkali carbonate is selected from K₂CO₃, Na₂CO₃, Li₂CO₃, LiOH, NaOH, KOH or a mixture thereof；

preferably, the organic solvent is selected from THF, EtOH, dioxane, DMF or a mixture of at least two of them；

preferably, the mixture is refluxed at 70 ℃；

preferably, in step (4) , the ratio of SOCl₂ is 1-5 mL/mmol, preferably 2-4 mL/mmol；

preferably, the ratio of alkyl amine to 2, 5-bis (3- (chlorocarbonyl) thiophen-2-yl) terephthaloyl dichloride is1: 2-4, preferably 1: 2.2-3, more preferably 1: 2.5；

preferably, the mixture is heated to 140-160 ℃； preferably to 150 ℃；

preferably, in step (6) , the mixture is heated to 40-90 ℃； preferably to 60 ℃；

preferably, in step (7) , the reaction mixture is extracted with an organic solvent, preferably with DCM；

preferably, the washing is conducted with water and brine；

preferably, in step (8) , the concentrating is conducted under reduced pressure；

preferably, the purifying is conducted by column chromatography using petroleum ether as an eluent.
Preparation method of the copolymer of claim 4 or 5, comprising:

(1) adding the imide-functional unit of claim 1, an aromatic unit material, tris (di-benzylideneacetone) dipalladium (0) (Pd₂ (dba) ₃) , and tris (o-tolyl) phosphine (P(o-tolyl) ₃) into a reaction vessel, and subjecting the reaction vessel and the mixture to an inert gas；

(2) adding an organic solvent； sealing the reaction vessel under an inert gas flow and then stirring while heating；

(3) adding 2- (tributylstanny) thiophene and stirring the reaction mixture while heating；finally, adding 2-bromothiophene and stirring the reaction mixture while heating；

(4) after cooling to room temperature, dripping the reaction mixture into methanol containing hydrochloric acid；

(5) drying the solid precipitate obtained in step (4) to obtain a crude product, and then extracting the crude product；

(6) after extracting, concentrating the polymer solution, and then dripping into methanol, collecting the solid and drying to obtain the copolymer.
The preparation method of according to claim 7, wherein in step (1) ,

the mole ratio of the imide-functional unit of claim 1to the aromatic unit material is 1:0.5-2, preferably 1: 0.8-1.5, more preferably 1: 1；

preferably, the mole ratio of the tris (dibenzylideneacetone) dipalladium (0) (Pd₂ (dba) ₃) to tris (o-tolyl) phosphine (P (o-tolyl) ₃) is 1: 4-15, preferably 1: 6-10, more preferably 1: 8； the Pd loading is 0.005-0.1 equiv, preferably 0.01-0.06；

preferably, the reaction vessel and the mixture are subjected to 1-5 pump/purge cycles with Ar；

preferably, the inert gas is selected from Ar, N₂, He, Ne, or a mixture thereof；

preferably, in step (2) , the inert gas is selected from Ar, N₂, He, Ne, or a mixture thereof；

preferably, the organic solvent is selected from any one of anhydrous toluene, benzene, chlorobenzene, DMF, or a mixture thereof；

preferably, the ratio of the organic solvent to the electron-donating unit is 10-75 mL/mmol, preferably 5-50 mL/mmol；

preferably, the heating is conducted at 50-170 ℃ for 1-72h, preferably at 80-150 ℃ for 3-50h；

preferably, the heating is conducted under microwave irradiation；

preferably, the heating is conducted by 80 ℃ for 10 minutes, 100 ℃ for 10 minutes, and 140 ℃ for 3 h under microwave irradiation；

preferably, in step (3) , the heating is conducted at 80-170 ℃ for more than 0.2 h, preferably at 100-160 ℃ for more than 0.4 h；

preferably, the heating is conducted under microwave irradiation；

preferably, the heating is conducted under microwave irradiation at 140 ℃ for 0.5 h, then adding 2-bromothiophene and stirring the reaction mixture at 140 ℃ for another 0.5 h；

preferably, the mole ratio of the 2- (tributylstanny) thiophene to the electron-donating unit is 0.1-0.5: 1, preferably 0.2: 0.4-1；

preferably, the mole ratio of the 2-bromothiophene to the electron-donating unit is 0.2-1.5: 1, preferably 0.4: 0.8-1；

preferably, in step (4) , the methanol contains 0.5-10mL hydrochloric acid，preferably 0.5-10mLof 5-20 mol/L hydrochloric acid；

preferably, the dripping is conducted under vigorous stirring, preferably is conducted for at least 0.5 h, preferably at least 1 h；

preferably, in step (6) , the dripping is conducted under vigorous stirring；

preferably, the collecting is conducted by filtration；

preferably, the drying is conducted under reduced pressure.
Use of the copolymer according to claim 4 or 5 in thin-film transistor.