Letter Cite This: Org. Lett. 2018, 20, 40−43
pubs.acs.org/OrgLett
Di-2-(2-oxindolin-3-ylidene)malononitrile Derivatives for N‑Type AirStable Organic Field-Effect Transistors Attrimuni P. Dhondge, Jian-You Chen, Ta Lin, Feng-Ming Yen, Kan-Wei Li, Hsin-Chun Hsieh, and Ming-Yu Kuo* Department of Applied Chemistry, National Chi Nan University, No. 1 University Road, 54561 Puli, Nantou, Taiwan S Supporting Information *
ABSTRACT: The nitrogenization of phenyl rings on DIM derivatives not only enhances molecular coplanarity but also stabilizes molecular LUMO levels, favoring charge transfer and improving air stability. Therefore, n-type organic field-effect transistors (OFETs) that are based on DIM-N2C8 with nitrogen atoms on both sides of the phenyl rings exhibit a moderate electron mobility of 0.059 cm2 V−1 s−1 under ambient conditions. ir-stable n-type organic field-effect transistors (OFETs) have attracted much attention owing to the necessity to combine them with air-stable p-type OFETs to fabricate organic complementary circuits with a large noise margin and low power consumption.1 Unlike the performance of air-stable p-type OFETs, the performance of most n-type OFETs decays rapidly under ambient conditions because of their relatively high-lying lowest unoccupied molecular orbital (LUMO) levels, which are responsible for the high susceptibility of radical anions.2 The introduction of electron-withdrawing groups, such as cyano, nitrogen, carbonyls, and halogens, into organic semiconductors (OSCs) is an effective means of stabilizing LUMO levels and realizing n-type air-stable OFETs.3 In the past decade, air-stable p-type OFETs with hole mobilities that are competitive with those of amorphous silicon FET (∼1 cm2 V−1 s−1) have been realized.4 Despite this noteworthy advancement, only a few ntype OFETs with electron mobilities that exceed 1 cm2 V−1 s−1 under ambient conditions have been produced.5−9 Rylene diimides, such as perylene diimides (PDIs) and naphthalene diimides (NDIs), are the most attractive n-type air-stable OSCs owing to their low-lying LUMO levels and high electron mobilities.5,6 A single-crystal FET (SCFET) that is based on 2,6dichloro-NDI with two fluoroalkyl side chains (−CH2C3F7) attached to the imide nitrogens exhibits an impressive electron mobility of 8.6 cm2 V−1 s−1 in air.5a Pei et al. reported on a series of benzodifurandione-based oligo(p-phenylenevinylene) (BDOPV) derivatives with record-breaking electron mobilities from 2.6 to 12.6 cm2 V−1 s−1 under ambient conditions.7 5,7,12,14-Tetrachloro-6,13-diazapentacene (TCDAP) and tricyanovinyldihydrofuran derivative, DBOB-DTCF, are two other n-type OSCs that can be used in OFETs with electron mobility of more than 1 cm2 V−1 s−1 under ambient conditions.8 Recently, dicyanomethylene-substituted thienoquinoidal (DCMTQ) compounds have been widely explored as n-type OSCs because of the strong electron-withdrawing ability of their cyanovinyl groups, which stabilize their LUMO levels.9,10 Thieno[3,4-c]pyrrole-4,6-dione- and furan-containing
A
© 2017 American Chemical Society
DCMTQs exhibit electron mobilities as high as 3.0 and 7.7 cm2 V−1 s−1, respectively, as measured in air.9 To the best of our knowledge, these are the only four OSCs with electron mobilities of above 1 cm2 V−1 s−1 under ambient conditions, besides PDI, NDI, and BDOPV derivatives. Therefore, the design and synthesis of novel air-stable n-type OSCs with high performance in air are still required. 2-(2-Oxindolin-3-ylidene)malononitrile (MN) is a widely used and easily prepared motif that is used in the synthesis of natural products and bioactive compounds.11 However, MNcontaining OSCs for use in OFETs have not been examined. This study reports on the synthesis and molecular design of three di-2-(2-oxindolin-3-ylidene)malononitrile (DIM) derivatives with various numbers of nitrogen substituents on the phenyl rings of DIM for use in n-type OFETs (Figure 1a). Their
Figure 1. (a) Chemical structures and notations, (b) optimized structures, (c) calculated LUMO levels of DIM derivatives. Values in parentheses are obtained from cyclic voltammetry (CV). Received: October 23, 2017 Published: December 8, 2017 40
DOI: 10.1021/acs.orglett.7b03284 Org. Lett. 2018, 20, 40−43
Letter
Organic Letters
were elucidated by cyclic voltammetry (CV) in a three-electrode cell using a Ag/AgCl electrode as a reference electrode. The LUMO levels were calculated using the relation ELUMO = 4.8 − (Ered1/2 − Eox, ferrocene) eV. The LUMO levels of DIM-NC8 and DIM-N2C8 are −4.11 and −4.16 eV, which are 0.09 and 0.14 eV, respectively, lower than that of DIM-C8, consistent with the calculated LUMO values (Figure 1c). The low-lying LUMO levels of DIMs indicate their potential for use in n-type air-stable OFETs.2 Notably, the splitting of first reduction potential indicates the interaction between two identical reduction sites (i.e., malononitriles), consistent with the LUMO orbitals of DIM derivatives delocalized over the whole π-conjugated framework.13 Single crystals of all DIM derivatives were successfully grown using the solvent vapor diffusion method (chloroform/ isopropanol).14 All three DIMs exhibit face-to-face column packing with a very short π−π distance of 3.31 Å (Figure 3),
optimized geometries, calculated using density functional theory (DFT at B3LYP/6-31G** level with the GAUSSIAN 09 Program),12 demonstrate that nitrogen substituents on phenyl rings not only enhance molecular coplanarity (Figure 1b) but also stabilize the LUMO and HOMO levels (Figures 1c and S1). These properties favor charge transfer and improve the air stability of n-type OFETs. The maximum electron mobility of an OFET that is based on DIM-N2C8 exhibits a moderate electron mobility of 0.059 cm2 V−1 s−1 in air, and it is accompanied by a high on/off ratio of 1.3 × 104. Scheme 1 shows the synthesis of DIM derivatives; detailed synthetic procedures are elucidated in Scheme S1 (Supporting Scheme 1. Synthetic Routes, Reagents, and Conditions to DIM Derivatives
Figure 3. Molecular packings and dimer type neighbors (Dn) of (a) DIM-C8, (b) DIM-NC8, and (c) DIM-N2C8 viewed along the short axes of backbones. For clarification, the backbones and side chains of molecules are presented by scaled ball and stick and line molecular models, respectively.
Information). 6-Bromoindoline (1) and 7-bromoazaindole (2) were N-alkylated with 1-bromooctane in the presence of sodium hydride in DMF to give N-alkylated indole compounds 3 and 4 in excellent yields. Subsequent palladium-catalyzed borylation of 3 with bis(pinacolato)diboron and stannylation of 4 with trimethyltin chloride at the 6-position yielded 5 and 6, respectively. The dimerization reaction was followed by the well-known palladium-catalyzed Suzuki−Miyaura and Stille cross-coupling reaction to give 7−9. Compounds 7−9 were easily oxidized with chromium trioxide in acetic acid to form desired bisisatins 10−12. Finally, the bisisatins were treated with malononitrile via a Knoevenagel condensation reaction to generate the target products in satisfactory yields. The decomposition temperatures of all target molecules exceeded 350 °C (Figure S2), as determined by thermogravimetric analysis (TGA). The UV−vis absorption spectra of DIM derivatives in dichloromethane (10−5 M) exhibit three main bands at 280−290, 360−402, and 530−548 nm as shown in Figure S3. DIM-N2C8 is slightly red-shifted compared with DIM-C8 and DIM-NC8. The electrochemical behaviors of DIM derivatives (Figure 2)
favoring charge transfer between molecules. Columns are connected by the interactions of the nitrogen in cyano groups with hydrogen on six-membered rings (Figure S4). DIM-N2C8 is coplanar, consistent with the optimized structure. However, the torsion angles of DIM-C8 (0°, 1°, 5°) and DIM-NC8 (0°) between the six-membered rings are smaller than those in the gas phase (Figure 1b), owing to the π−π interactions in the solid state. Both DIM-NC8 and DIM-N2C8 have only one kind of π−π dimer (D1) while DIM-C8 has three types of π−π dimer (D1−D3) in the order D1D2D2D1D3. The electron transfer integral (t−) for each π−π dimer can be estimated as half of the energy splitting between the LUMO and LUMO+1 levels of the dimer, using the HyperChem Professional Release 7 with intermediate neglect of differential overlap (INDO) level.15 The t− values of DIM-C8 along D1, D2, and D3 are 37, 78, and 9 meV, respectively. The t− values of DIM-NC8 and DIM-N2C8 are 43 and 52 meV, respectively. Along the π−π column of DIMC8, the smallest t− value would determine its macroscopic electron mobility. Therefore, we believe that the electron mobilities of DIM derivatives would increase with the number of nitrogen substituents on their phenyl rings. To study the influence of the nitrogen substituent on the electron-transporting properties of DIM derivatives, top-contact bottom-gate OFETs were fabricated on octadecyltrichlorosilane (ODTS)-modified SiO2 substrates at various substrate temperatures (t). The X-ray diffraction (XRD) spectra of deposited films of DIM derivatives revealed layered structures on the substrates with sharp (00l) reflections (Figure 4). The
Figure 2. Cyclic voltammograms of DIM derivatives in CH2Cl2 containing 0.1 M n-Bu4NPF6. 41
DOI: 10.1021/acs.orglett.7b03284 Org. Lett. 2018, 20, 40−43
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Organic Letters
Table 1. Summary of Electrical Characteristics of OFETs Based on DIM Derivatives device DIM-C8
t (°C)
ave μea (cm2 V−1 s−1)
Vth (V)
on/off
50
8.9 × 10−3 (1.3 × 10−2) 1.4 × 10−2 (1.5 × 10−2) 6.7 × 10−3 (7.9 × 10−3) 4.6 × 10−3 (5.3 × 10−3) 1.1 × 10−2 (1.2 × 10−2) 5.5 × 10−2 (5.9 × 10−2)
47.6
6.1 × 101
42.1
2.3 × 102
37.4
2.8 × 102
33.6
3.6 × 102
34.6
2.4 × 101
27.4
1.3 × 104
65 DIM-NC8
50 65
DIM-N2C8
50 65
a
Average electron (μe) mobilities were obtained from 10 devices.
and DIM-NC8, which would increase on-current and thus enhance the on/off current ratio of DIM-N2C8. The electron mobilities of OFETs based on DIM-C8 and DIM-N2C8 increase with the rise in substrate temperature because of the increases in crystallinity (Figure 4a and 4c) and grain size (Figures 6 and S6). However, the electron mobilities of DIM-
Figure 4. XRD patterns of DIM derivative films at different substrate temperatures (a) DIM-C8, (b) DIM-NC8, and (c) DIM-N2C8.
corresponding d-spacings of thin films of DIM-C8, DIM-NC8, and DIM-N2C8 are 17.32, 17.32, and 16.50 Å, respectively, which are consistent with the crystallographic b-axis of DIM-C8 (16.90 Å), c-axis of DIM-NC8 (16.09 Å), and c-axis of DIMN2C8 (16.32 Å). These XRD results suggest that the π-stacking DIM derivatives have end-on molecular orientations on the substrate, favoring charge transport between source and drain electrodes. All of the OFETs that are based on DIM derivatives exhibit typical n-type semiconducting behavior under ambient conditions. Figure 5 plots the output and transfer characteristics
Figure 6. AFM images (3 μm × 3 μm) of DIM-N2C8 films based on ODTS-modified SiO2 at (a) t = 50 °C and (b) t = 65 °C.
NC8-based OFETs drop slightly as the substrate temperature increases from 50 to 65 °C, consistent with the decreases in crystallinity (Figure 4 b) and grain size (Figure S6). The electron mobility of DIM-NC8 is lower than that of DIM-C8, which is inconsistent with the result predicted from electron transfer integrals, indicating the fabrication condition of DIM-NC8based OFETs is not optimized yet. The nitrogen substituents on the phenyl rings not only enhance molecular coplanarity but also stabilize the LUMO levels, which benefit the charge transfer between molecules and electron injection from the electrode into the LUMO level, respectively. Accordingly, among DIM derivatives, the DIM-N2C8-based OFET has the highest electron mobility of 0.059 cm2 V−1 s−1 and lowest threshold voltage of 27.4 V. Although the electron mobilities of DIM derivatives are lower than 1 cm2 V−1 s−1, it is easy to tune their conjugation length and energy levels by inserting π-linkers such as thieno[3,2-b]thiophene and naphthalene between 2-(2oxindolin-3-ylidene)malononitriles. Therefore, we believe this framework has potential for use in high-performance air-stable ntype and ambipolar OFETs. In conclusion, this work developed a series of novel DIM derivatives for use in high-mobility air-stable n-type OFETs. The energy levels and conformation of DIM derivatives can be simply tuned by introducing nitrogen atoms on connected phenyl rings. The nitrogenization of phenyl rings on DIM derivatives not only enhances molecular coplanarity but also stabilizes molecular
Figure 5. (a) Output and (b) transfer characteristics of DIM-N2C8 OFETs (t = 65 °C) on ODTS-modified SiO2.
of the DIM-N2C8-based OFET that is fabricated at t = 65 °C. Figure S5 presents the outputs and transfer characteristics of the other OFETs. Table 1 reports the electron mobility (μe), on/off current ratio, and threshold voltage (Vth) of all OFET devices. The on/off current ratio of DIM-N2C8 is higher than those of DIM-C8 and DIM-NC8 probably owing to the larger grain size and lower LUMO level of DIM-N2C8 compared with DIM-C8 42
DOI: 10.1021/acs.orglett.7b03284 Org. Lett. 2018, 20, 40−43
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Organic Letters
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LUMO levels, favoring charge transfer and improving air stability. Therefore, n-type OFETs that are based on DIMN2C8 with nitrogen atoms on both sides of the phenyl rings exhibit a moderate electron mobility of 0.059 cm2 V−1 s−1 under ambient conditions. The insights that are offered by the present work will benefit the development of n-type OFETs and nonfullerene acceptors for use in organic solar cells.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03284. Complete ref 12, experimental procedures, characterization data, and computational coordinates (PDF) Accession Codes
CCDC 1581685−1581687 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ming-Yu Kuo: 0000-0002-2787-0554 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from Ministry of Science and Technology of Taiwan under Contract Number NSC 102-2113-M-260-007-MY3 and MOST 105-2119-M-260005-MY3. We also thank Prof. S. L. Wang and Ms. P. L. Chen (National Tsing Hua University, Taiwan) for the X-ray structure analyses and the National Center for High-performance Computing for providing computational support.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b03284 Org. Lett. 2018, 20, 40−43
