Embedding electron-deficient nitrogen atoms in polymer backbone towards high performance n-type polymer field-effect transistors

The low LUMO level and the conformation-locked planar backbone provide polymer AzaBDOPV-2T with electron mobilities over 3.22 cm2 V–1 s–1 tested in air.

Conjugated polymers have attracted great interests in low-cost, exible, and large-area electronic applications due to their solution-processability, good mechanical properties, and tunable electronic properties. 1 In the past few years, the development of novel building blocks for conjugated polymers, such as benzothiadiazole (BT), 2 diketopyrrolopyrrole (DPP), 3 isoindigo (II), 4 and naphthalene diimide (NDI), 5 has led to significant progress in the carrier mobilities of polymer semiconductors. Nevertheless, only a few of these polymers can exhibit high electron mobilities when operated under ambient conditions, 5c thus limiting their application. It has been an intriguing research topic to develop high-performance airstable n-type polymer semiconductors in organic electronics.
Low LUMO levels are indeed desirable to facilitate electron injection and enhance the electrochemical stability of n-type polymer semiconductors. 6 The introduction of electron-withdrawing groups (such as F or Cl) onto polymer conjugated backbones has been shown to be an effective way to lower their LUMO energy levels, thus leading to enhanced n-type transport. 6c,d However, compared to the extensive efforts spent on halogen substitution, embedding electron-decient sp 2nitrogen atoms in polymer backbones has been somewhat overlooked. 7 Incorporation of electron-decient pyridine units into polymer backbones could also lower the energy levels of the resultant conjugated polymers. In addition, the replacement of benzene rings with pyridine units is also benecial to improving the planarity of polymer backbones because sp 2 -nitrogen atoms are less sterically demanding than CH units. 8 Therefore, such a sp 2 -nitrogen embedding strategy may provide new opportunities to develop decent n-type semiconductors.
Recently, benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV), which has four electron-withdrawing carbonyl groups on the backbone and a low LUMO level of À4.24 eV, was developed as an excellent electron-accepting moiety for a large number of semiconductors in eld-effect transistors (FETs). 9 Specially, the D-A conjugated polymer BDOPV-2T showed a high electron mobility up to 1.74 cm 2 V À1 s À1 . 9c To further improve the electron transport property of BDOPV-based polymers, herein, we embed sp 2 -nitrogen atoms in BDOPV, resulting in a stronger electron-decient building block diaza-BDOPV (AzaBDOPV) (Fig. 1). The D-A conjugated polymer AzaBDOPV-2T shows a more planar backbone and a lower LUMO level (down to À4.37 eV) compared with BDOPV-2T. As a consequence, AzaBDOPV-2T exhibits higher electron mobilities of over 3.22 cm 2 V À1 s À1 for devices tested under ambient conditions, which is among the highest in n-type polymer FETs. 5c, 10 To achieve the monomer AzaBDOPV, 6-bromo-7-azaisatin was synthesized from a commercial available precursor, 7-azaindole. 11 As illustrated in Scheme 1, 7-azaindole (1) was converted into N-oxide-7-azaindole (2) by m-CPBA in high yield. Benzoyl bromide was used as the brominating reagent to afford a Reissert-Henze salt. 12 Hence, the bromination reaction only occurred at the ortho-position relative to the nitrogen atom on pyridine. The benzoyl group on the nitrogen atom of the pyrrole was subsequently removed by the base treatment, giving 6bromo-7-azaindole (3) in 56% yield for the two steps. Then, the alkylation of 3 gave the corresponding alkylated azaindole (4) in 82% yield, which was further oxidated through pyridinium chlorochromate (PCC) to afford 6-bromo-7-azaisatin in 68% yield. 13 Interestingly, 6,6 0 -dibromo-7,7 0 -azaisoindigo was also observed in 10% yield in this oxidation reaction. 13a Further investigation revealed that 6,6 0 -dibromo-7,7 0 -azaisoindigo was afforded exclusively in 36% yield when using dry PCC and excess AlCl 3 . The generation of azaisoindigo was expected as a result of the Lewis-acid-catalyzed aldol condensation between the azaisatin and azaindole, with subsequent dehydration and oxidation as shown in Fig. S1. † An excess amount of AlCl 3 and a non-water environment may promote this process, resulting in selectively affording compound 6. 13b Considering the potential optoelectronic applications of azaisoindigo derivatives, our method provides a shorter and simpler strategy to obtain azaisoindigo compared with the more common method that requires the coupling of oxindole with isatin.
The absorption spectra of the polymer BDOPV-2T in dilute solution, and the polymer AzaBDOPV-2T both in dilute solution and in a thin lm, are shown in Fig. 2. The bandgap of AzaB-DOPV-2T calculated from the onset of lm absorption is 1.32 eV. AzaBDOPV-2T displayed a dual-band absorption both in solution and in lm with obvious 0-0 and 0-1 vibrational peaks. Compared with the absorption of BDOPV-2T in dilute solution, the 0-0 vibrational peak of AzaBDOPV-2T was clearly observed in solution, indicating that AzaBDOPV-2T has a more planar backbone structure relative to BDOPV-2T. No signicant redshi of the absorption was observed for AzaBDOPV-2T from the solution to the lm, which indicates that the polymer probably forms some preaggregates in solution due to strong intermolecular interactions. Interestingly, computational analysis of the AzaBDOPV-2T fragment reveals that the sulfur and pyridinic nitrogen atoms were favorable to be on the same side. The calculated S-N distance (2.27Å) is signicantly shorter than the sum of the S-N van der Waals radii (3.35Å), indicating a typical S-N interaction, 14 which may "lock" the polymer conformation. As a result, the calculated pyridyl-thienyl rotational angle in AzaBDOPV-2T is decreased from 21.5 to 0.5 , revealing that the introduction of nitrogen atoms provides the polymer with a more planar backbone and better p-conjugation, which is consistent with the absorption features.
Cyclic voltammetry (CV) was carried out to investigate the energy levels of the monomer AzaBDOPV and the polymer AzaBDOPV-2T (Fig. S4 † and 3a). Aer the introduction of nitrogen atoms, the LUMO level of the monomer AzaBDOPV reaches À4.35 eV, which is 0.11 eV lower than that of BDOPV, suggesting that AzaBDOPV might be a more efficient electron-decient building block for n-type conjugated polymers. The HOMO/LUMO levels of AzaBDOPV-2T measured from CV are À5.80/À4.37 eV, which are consistent with the HOMO/LUMO levels (À5.77/À4.45 eV) estimated from photoelectron spectroscopy (PES) (Fig. S5 †) and the optical bandgap. Note that the LUMO level of AzaBDOPV-2T is also signicantly lower than that of BDOPV-2T (À4.15 eV), indicating that AzaBDOPV-2T might exhibit more efficient and stable electron transporting properties than BDOPV-2T. 9c To our knowledge, polymer AzaBDOPV-2T is among the most electron-decient conjugated polymers reported to date. 15 The HOMO level of AzaBDOPV-2T is nearly the same as that of BDOPV-2T (HOMO level of À5.72 eV), indicating that the introduction of nitrogen at the position near the amide group has larger effect on the LUMO than the HOMO. This result was in accordance with the calculated molecular orbital distribution, where the HOMO of AzaBDOPV-2T is delocalized and the LUMO is localized on the AzaBDOPV cores (Fig. 3b).
Field-effect transistors with a top-gate/bottom-contact (TG/ BC) conguration were then fabricated to characterize the charge transport properties of AzaBDOPV-2T. The semiconducting layer was deposited by spin-coating the polymer solution (3 mg mL À1 in ODCB) on an Au (source-drain)/SiO 2 substrate. n-Octane was then spin-coated onto the as-spun AzaBDOPV-2T lm before thermal annealing for 5 min. Several annealing temperatures were tried, and annealing at 200 C gave the best device performance (Fig. S6 †). Aer thermal annealing, a CYTOP solution was spin-coated on top of the lm as the dielectric layer, and an aluminum layer was thermally evaporated as the gate electrode. All devices were fabricated in a glovebox and tested in air (R H ¼ 50-60%). AzaBDOPV-2T displayed typical n-type transport characteristics (Fig. 4), with the highest electron mobility up to 3.22 cm 2 V À1 s À1 (an average mobility of 1.63 cm 2 V À1 s À1 from seventeen devices and the standard deviation was 0.54 cm 2 V À1 s À1 ), whereas the reference polymer BDOPV-2T showed highest electron mobilities of up to 1.74 cm 2 V À1 s À1 . 9c This result is attributed to the fact that AzaBDOPV-2T has a lower LUMO energy level and less conformational disorder than BDOPV-2T.
The molecular packing and surface morphology of the AzaB-DOPV-2T lm were investigated by grazing incident X-ray diffraction (GIXD) and tapping-mode atomic force microscopy (AFM) as shown in Fig. 5 and S7. † The lm of AzaBDOPV-2T showed a strong out-of-plane diffraction peak (100) at a 2q value of 2.50 , corresponding to a d-spacing of 28.4Å (l ¼ 1.24Å). Another four orders of diffraction peaks attributed to (h00) diffractions and an in-plane diffraction peak of (010) was also observed, indicating a distinct edge-on lamellar packing formed in the lm. Aer the introduction of nitrogen atoms, the p-p stacking distance of AzaBDOPV-2T was measured to be 3.44Å (Fig. S8 †), which was shorter than that of BDOPV-2T (3.55Å), suggesting stronger inter-chain interactions of AzaBDOPV-2T in the solid state compared to BDOPV-2T. The strong crystallinity of AzaBDOPV-2T is also supported by the AFM image, which displays a ber-like intercalating network with obvious crystallized zones. Both the close p-p stacking distance and strong crystallinity are benecial to the high mobilities of AzaBDOPV-2T. Fig. 2 (a) The absorption spectra of the polymer BDOPV-2T in dilute solution (10 À5 M), AzaBDOPV-2T both in dilute solution (10 À5 M) and in thin film; (b) DFT-optimized geometries of AzaBDOPV-2T and BDOPV-2T fragments. Alkyl chains were replaced by methyl groups for simplicity (geometries were optimized at the b3lyp/6-311g(d,p) level).

Conclusions
In conclusion, we have developed a novel electron-decient building block, AzaBDOPV, which shows a much lower LUMO level compared with BDOPV. Based on the AzaBDOPV unit, the D-A conjugated polymer, AzaBDOPV-2T, displays a low LUMO level down to À4.37 eV and therefore typical n-type transport characteristics, with electron mobilities up to 3.22 cm 2 V À1 s À1 in air. These investigations demonstrate that the incorporation of electron-withdrawing sp 2 -nitrogen atoms into a BDOPVbased polymer not only lowers the energy levels of the conjugated polymer, but also optimizes its backbone conformation, leading to improved inter-chain interactions and lm microstructures, which are critical to achieving high device performance.