Incorporation of Cyano-Substituted Aromatic Blocks into Naphthalene Diimide-Based Copolymers: Toward Unipolar n-Channel Field-Effect Transistors

further permitting effective electron injection. Different from the introduction of the sp 2 -hybridized nitrogen atoms and ﬂ uorine atoms, cyano-substituted aromatic blocks are synthesized and further copolymerized with naphthalene diimide (NDI) unit, affording a series of copolymers of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN. The photophysical, electrochemical, and thermal properties of all the copolymers are systematically investigated, and their semiconducting performance is studied by fabricating ﬁ eld-effect transistors and tested under atmosphere. All the polymers exhibit unipolar n-type semiconducting performance because of the synergetic effect of strong electron-withdrawing NDI units and cyano-substituted aromatic blocks. The highest mobility of 0.20 cm 2 V (cid:2) 1 s (cid:2) 1 is obtained. Moreover, theoretical simulation and thin- ﬁ lm characterization are conducted to reveal the difference in semiconducting performance among the three polymeric materials. Device Fabrication and Characterization : FETs were fabricated with a TG/BC con ﬁ guration to investigate the semiconducting properties of all the synthesized copolymers. Highly doped n þþ -Si/SiO 2 substrates (300nm) were used and the and drain (Au) were pre-pared by the photolithography technique. Then the substrates were soaked in acetone for 3 h and further treated with UV-ozone for 20 min. Afterward, the treated substrates were subject to Piranha solution (H 2 SO 4 : H 2 O 2 ¼ 3:1) for 5 min to obtain hydroxylated surface and then modi ﬁ ed with octadecyltrichlorosilane (OTS) in vacuum at 120 (cid:3) C to form a self-assembled layer. The modi ed substrates were washed with hexane, eth- anol, and chloroform, and a solution of copolymer in o -dichlorobenzene (8 mg mL (cid:2) ) was deposited on the substrates by spin-coating method, forming the semiconducting layer. Thermal annealing treatment at various temperatures was proceeded to optimize the semiconducting performance. Polymethyl methacrylate (PMMA) solution in n -butyl acetate (60 mg mL (cid:2) 1 ) was spin-coated on the top of the semiconducting layer, which was annealed at 85 (cid:3) C to remove the residual and act as the dielectric layer. The device was completed by evaporation of a thin layer of aluminum (Al) as the gate electrode (70 nm). The FET perform- ances of all the fabricated devices were characterized directly in air with a Keithley 4200 SCS semiconductor parameter analyzer. All the mobilities were calculated in the saturated regime according to the following equation

DOI: 10.1002/smsc.202100016 The unipolar n-type polymeric semiconductors are crucial for the development of complementary inverters and complementary logic circuits. To achieve this target, the polymer skeleton should be electron-deficient, which guarantees the energylevel alignment between the lowest unoccupied molecular orbital energy level of polymeric materials and the work function of electrode, further permitting effective electron injection. Different from the introduction of the sp 2 -hybridized nitrogen atoms and fluorine atoms, cyano-substituted aromatic blocks are synthesized and further copolymerized with naphthalene diimide (NDI) unit, affording a series of copolymers of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN. The photophysical, electrochemical, and thermal properties of all the copolymers are systematically investigated, and their semiconducting performance is studied by fabricating fieldeffect transistors and tested under atmosphere. All the polymers exhibit unipolar n-type semiconducting performance because of the synergetic effect of strong electron-withdrawing NDI units and cyano-substituted aromatic blocks. The highest mobility of 0.20 cm 2 V À1 s À1 is obtained. Moreover, theoretical simulation and thin-film characterization are conducted to reveal the difference in semiconducting performance among the three polymeric materials.
that of the bithiophene analogue P(NDI2OD-T2). [24] And thus the corresponding transistors exhibited electron mobility of up to 0.85 cm 2 V À1 s À1 . By changing the substitution position of sp 2 -hybridized nitrogen atoms, the copolymerization between 5,5 0 -bithiazole and NDI affords PNDI2OD-BiTz. [25,26] This polymer also showed a LUMO energy level of around À4.0 eV and the fabricated devices displayed unipolar n-type semiconducting performance. Apart from the modification of the 2,2 0 -bithiophene, (E)-1,2-di(thiophen-2-yl)ethene was also decorated with fluorine atoms at the 3,3 0 -positions. The obtained copolymer of PNDI2OD-FTVT has decreased LUMO energy level, and the electron-dominated ambipolar semiconducting performance was achieved. [27] The mentioned strategies of substitution of sp 2 -hybridized nitrogen atoms or fluorine atoms provide effective approaches for lowing LUMO energy levels ( Figure 1). However, these strategies are insufficient in the development of unipolar n-type polymeric molecules.

Synthesis and Thermal Properties
Scheme 1 shows molecular structures and the synthetic routes of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN. Compound SVS-Br was synthesized according to methods reported by Cheng et al. [31] Then SVSCN was obtained under the condition of Zn(CN) 2 and Pd(PPh 3 ) 4 . Compound SVSCN was treated with lithium diisopropylamide (LDA, 2 M) and quenched with trimethyltin chloride (1 M), affording monomer of SVSCN-Sn successfully. The Stille polymerization was applied between monomer NDI-C 10 C 12 and BTCN-Sn (TVTCN-Sn or SVSCN-Sn) to achieve copolymers of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN. The molecular weights of polymers were evaluated with hightemperature gel permeation chromatography (GPC) at 150 C, calibrated with a polystyrene standard. The number-average   Figure S1, Supporting Information), we also studied the thermal properties of all the copolymers, and the results indicate that the three copolymers show excellent thermal stability with the decomposition temperature up to 400 C. In addition, PNDI-TVTCN shows a couple of peaks at 273 and 303 C, which correspond to the freezing and melting points, respectively. However, there are no apparent peaks for PNDI-BTCN and PNDI-SVSCN, indicating both the polymers do not proceed obvious melting and solidification process during this temperature range.

Optical Properties and Energy-Level Evaluation
We investigated the photophysical properties of this series of polymers by conducting UV-vis absorption characterization, and the results are shown in Figure 2. Obviously, all copolymers display similar absorption profiles. PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN exhibit typical dual absorption bands. And this dual-band absorption profiles are typical features of NDIbased polymers. The high-energy absorption band is attributed to πÀπ* transition, whereas the low-energy absorption band corresponds to the interaction between the NDI section and the cyano-substituted aromatic section. In addition, there is an obvious absorption bathochromic-shift from the solution to the thin film for all the three polymers, especially for the low-energy  According to the working principle of OFET, the carrier transport type is closely related to the frontier orbital energy levels of molecule. Therefore, we conducted ultraviolet photoelectron spectroscopy (UPS) characterization of polymer thin films on silica substrates and the results are shown in Figure 2. Apparently, the E cutoff and E H,onset of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN are 16.85/2.49, 16.84/2.25, and 16.76/2.13 eV, respectively. By adopting the equation of IP ¼ hv À (E cutoff À E H,onset ) eV, the ionization potential (IP) was determined to be 6.86 eV for PNDI-BTCN, 6.63 eV for PNDI-TVTCN, and 6.59 eV for PNDI-SVSCN. Based on the optical energy gap obtained earlier, the LUMO energy levels of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN are calculated to be À4.88, À4.74, and À4.81 eV, respectively. We also conducted cyclic voltammetry measurements to detect the frontier orbital energy levels with a three-electrode system, in which a glass carbon electrode, a wire of platinum, and an Ag/AgCl electrode act as the working, the counter, and the reference electrodes, www.advancedsciencenews.com www.small-science-journal.com respectively. The cyclic voltammetry curve indicates that all the polymers exhibit obvious reduction peak, and there is no discernible oxidation peak ( Figure S2, Supporting Information). The emergence of the reduction peak indicates the obtained copolymers undergo n-doping facilely, and the absence of the oxidation peaks suggests that all the three polymers cannot go on p-doping effectively. Based on the onset of the first reduction peak, the LUMO energy levels of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN are estimated to be À4.09, À4.16, and À4.15 eV, respectively. Compared with the bithiophene derivative of P(NDI2OD-T2), PNDI-BTCN exhibits lower LUMO energy level by 0.09 eV, attributing to the introduction of the strong electron-withdrawing cyano group. Moreover, the further decreased LUMO energy levels of PNDI-TVTCN and PNDI-SVSCN are possibly attributed to the enhanced intramolecular interaction due to the insertion of double bond section. The LUMO energy levels calculated based on IPs and optical gaps are deeper than those estimated from the reduction peaks and the large difference between the two methods is ascribed to the hole-electron binding energy. The frontier orbital energy levels imply that all the polymers would perform as n-type semiconductors ( Table 1).

Theoretical Simulation
Molecular simulation on model trimers of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN was conducted to investigate molecular geometries and the electronic structures with a hybrid B3LYP correlation functional and 6-31 þ G (d) basis set. The alkyl chains were replaced by methyl groups for simplicity. Figure S3, Supporting Information, shows the simulated molecular geometries and Figure 3 displays the frontier orbitals of model trimers. Apparently, the molecule structures of all the synthesized copolymers are distorted, resulting from the large dihedral angle between NDI units and the cyano-substituted aromatic blocks (60.36 for PNDI-BTCN, 56.09 for PNDI-TVTCN, and 86.93 for PNDI-SVSCN). In addition, the nonplanar BTCN unit causes the molecular skeleton of PNDI-BTCN to be more distorted than those of PNDI-TVTCN and PNDI-SVSCN.
The twisted molecular backbone contributes to the localization of frontier molecular orbitals as revealed by Figure 3. In other words, the NDI section dominates the LUMO energy level and the cyano-substituted aromatic blocks determine the highest occupied molecular orbital (HOMO) energy level. The HOMO/ LUMO energy levels of model trimers of PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN are À6.64/À4.22, À6.32/ À4.24, and À6.24/À4.20 eV, respectively, consistent with the experimental results. According to the experimental and calculated energy levels, we speculate that all the copolymers will behave as unipolar n-type semiconductors because such energy levels block hole injection and facilitate electron injection at the same time.

Charge Transport Characteristics
To investigate the semiconducting performance, we fabricated OFETs with a top-gate/bottom-contact (TG/BC) configuration based on the synthesized polymeric materials. The detailed fabrication and measurement process are provided in the   Experimental Section. Figure 4 shows the typical output and transfer curves of the fabricated OFET devices. The OFETs based on PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN show representative n-type carrier transport characteristics and the corresponding semiconducting performances are shown in Table 2. The exclusive n-type semiconducting properties are attributed to the low frontier orbital energy levels, merely facilitating the electron injection, which is consistent with the results of ultraviolet photoelectron spectroscopy and cyclic voltammetry characterizations. For OFETs, thermal annealing treatment is beneficial for molecular rearrangement into a crystalline state, and further elevating the carrier mobility. The optimal annealing temperature was found to be 200 C ( Figure S4, Supporting Information), and the corresponding highest mobility are 0.12, 0.16, and 0.20 cm 2 V À1 s À1 for PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN, respectively.

Film Morphology and Microstructural Analysis
The molecular order and crystallinity of polymer thin-films play a significant role in the semiconducting performance. [32,33] Therefore, we performed 2D grazing incidence X-ray diffraction (2D-GIXRD) of the polymer thin films to study the crystallinity and the molecular orientation relative to substrate. As shown in Figure 5, the pristine films of PNDI-BTCN show weak diffraction peak in the out-of-plane direction, while PNDI-TVTCN and PNDI-SVSCN thin film exhibit obvious (h00) diffraction peaks up to three orders. The arc-shaped of (100) peak indicates that the crystallites are oriented randomly, possibly attributing to the twisted polymer skeleton. In addition, a weak (010) peak also appears in the out-of-plane direction ( Figure 5 and Figure S5, Supporting Information), representing the face-on orientation. After thermal annealing treatment at 200 C, the diffraction intensity gets enhanced. Moreover, as for PNDI-TVTCN and PNDI-SVSCN, one more diffraction peak appears, demonstrating the enhanced crystallinity after annealing treatment, partially contributing to the slightly higher mobility than that of PNDI-BTCN. Based on the (010) peaks, we calculated the π-π stacking distances. The π-π stacking distances for pristine and annealed films are 4.22/4.20 Å for PNDI-BTCN, 3.80/3.75 Å for PNDI-TVTCN, 3.83/3.80 Å for PNDI-SVSCN ( Figure S5, Supporting Information). Compared with PNDI-BTCN, the denser arrangement of PNDI-TVTCN and PNDI-SVSCN could be another reason for their slightly higher semiconducting performance.  In addition, we also performed pole figure analysis to determine the orientation distribution of the crystallites with respect to the substrate ( Figure S6, Supporting Information). The results demonstrate that all the thin films adopt edge-on arrangement predominately. After thermal annealing treatment, the orientation distributions have no obvious change, suggesting the thermal annealing process almost has no impact on the orientation distributions for this series materials. We also explored the surface morphology of polymer thin films annealed at 200 C by conducting atomic force microscope (AFM) characterizations. Figure 6 shows the corresponding AFM images. Apparently, the introduction of BTCN, TVTCN, and SVSCN contributes to different surface morphologies for the final copolymers. For PNDI-BTCN, the annealed film has compact fibrous structure. In contrast, PNDI-TVTCN film shows amorphous structure. Compared with PNDI-BTCN and PNDI-TVTCN, PNDI-SVSCN film exhibits a more obvious fibrous structure, benefiting the carrier transporting. The aforementioned investigations confirm that both the crystallinity and surface morphology affect the final semiconducting performance. Thin film of PNDI-SVSCN shows good crystallinity and surface morphology simultaneously, and contributing to the best semiconducting performance of the series of polymers.

Conclusion
In this article, three cyano-substituted monomers, i.e., BTCN, TVTCN, and SVSCN, were designed and synthesized and further copolymerized with NDI monomer, providing PNDI-BTCN,  PNDI-TVTCN, and PNDI-SVSCN, respectively. Due to the synergetic effect of the strong electron-deficient NDI unit and cyano-substituted aromatic blocks, all the resulted copolymers possess low LUMO energy levels of blow À4.0 eV, and thus the synthesized polymeric materials exhibit unipolar n-type semiconducting performance. The highest mobilities of devices based on PNDI-BTCN, PNDI-TVTCN, and PNDI-SVSCN are 0.12, 0.16, and 0.20 cm 2 V À1 s À1 , respectively. The theoretical calculations confirm that the molecular backbone of this series of copolymers is twisted severely and such phenomenon is ascribed to the large dihedral angle between NDI unit and the cyanosubstituted aromatic rings, further contributing to the HOMO and LUMO localized in BTCN (TVTCN or SVSCN) and NDI section, respectively. The superior semiconducting performance of PNDI-SVSCN is attributed to its higher crystallinity and better surface morphology compared with those of PNDI-BTCN and PNDI-TVTCN, which are demonstrated by the GIXRD and AFM characterizations. This work proves that cyano-substitution is an efficient strategy in constructing unipolar n-type semiconductors.
(E)-1,2-bis(3-Cyanoselenophene-2-yl)Ethene (SVSCN): To a two-neck flask protected with argon, (E)-1,2-bis(3-bromoselenophene-2-yl)ethene (SVS-Br, 1.00 g, 2.25 mmol), Zn(CN) 2 (582.02 mg, 4.96 mmol), Pd(PPh 3 ) 4 (260 mg, 0.23 mmol), and 10 mL of DMF were added successively. Then the mixture was heated to 110 C and stirred overnight. After cooling the system to room temperature, the mixture was extracted with dichloromethane for three times, and the organic phase was washed with saturated brine and dried with anhydrous Na 2 SO 4 . Filtration was conducted and the solvent was removed on a rotary evaporator. Then column chromatography and recrystallization were further performed and the target compound was achieved as brown needle crystals (469 mg, 62%). 1  (E)-2,2'-(Ethene-1,2-Diyl)bis(5-(Trimethylstannyl)Selenophene-3-Carbonitrile) (SVSCN-Sn): To a two-neck flask charged with argon, lithium diisopropylamine (LDA, 3.28 mmol, 1.64 mL) was added. The system was cooled to À78 C with a liquid nitrogen-acetone bath, and then a THF solution of SVSCN (0.50 g, 1.49 mmol) was added dropwise. The mixture was stirred for 2 h at the same temperature, after which trimethyltin chloride (4.47 mmol, 4.47 mL) was added. Removing the liquid nitrogenacetone bath, the system was stirred at the room temperature for another 2 h. The mixture was extracted with dichloromethane for three times and the organic phase was washed with saturated brine and dried with anhydrous Na 2 SO 4 . Filtration was conducted and the solvent was removed under reduced pressure. Column chromatography was conducted to purification, recrystallization was further performed, and the title compound was obtained as flake crystal (592 mg, 60%). 1  General Procedure for Polymerization and Purification: Monomer NDI-C 10 C 12 (0.10 mmol), BTCN-Sn (TVTCN-Sn or SVSCN-Sn, 0.10 mmol), Pd 2 (dba) 3 (dba ¼ dibenzylideneacetone, 4.50 mg), and tri(o-tolyl)phosphine (12.30 mg) were added to a Schlenk tube charged with argon successively, and then 5 mL of o-dichlorobenzene was syringed into the tube. After which the system went through freeze-pump-thaw cycles at liquid nitrogen bath for three times to remove the oxygen thoroughly. The reaction solution was stirred for 72 h at 120 C. The reaction was stopped and cooled to room temperature; the crude product was poured into 200 mL of methanol containing 2 mL HCl (aq. 6 M) and stirred for another 3 h; the solid was collected by filtration. Extraction was further conducted with methanol, acetone, and hexane to remove oligomers and the residual catalysts. The final product was obtained by using chloroform as the final extraction solvent and the molecular structures were determined by high-temperature 1 H NMR and elemental analysis.