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Cyanated isoindigos for n-type and ambipolar organic thin film transistors† Wan Yue, Tao He, Matthias Stolte, Marcel Gsa¨nger and Frank Wu ¨ rthner*
Received 18th October 2013, Accepted 5th November 2013 DOI: 10.1039/c3cc48037b www.rsc.org/chemcomm
A set of three core-cyanated isoindigos was synthesized by palladiumcatalyzed cyanation of core-brominated isoindigos. With decreased LUMO level to 3.88 eV, the dicyanated isoindigo 5 showed ambientstable electron mobility up to 0.044 cm2 V1 s1 in OTFTs with SAMs of TPA, while it exhibited ambipolar charge transport behaviour (0.11 cm2 V1 s1 for electrons and 0.045 cm2 V1 s1 for holes) on FOPA-modified substrates.
Due to their low power dissipation, high operating speed, and good signal to noise margin, organic complementary circuits comprising both p- and n-channel organic thin film transistors (OTFTs) have received increasing interest for technological as well as scientific reasons.1 In order to achieve organic complementary inverters, reliable organic n-type transistors that can be integrated with p-type transistors are required because the vast majority of organic transistors so far are p-channel OTFTs, for which good ambient stability is easier to realize.2 Hence, the need to build complementary circuits has fuelled research efforts in n-channel transistors. One of the major challenges of fabricating n-channel transistors is the injection of electrons into the LUMO level of the semiconductor from a suitable electrode. This can be achieved by lowering the LUMO level,3 e.g. by incorporation of strong electron-withdrawing substituents such as chlorine, fluorine, cyano, or diimide moieties to the core of intrinsic organic semiconductors. While exploring this concept for several classes of compounds, an ambipolar transport behaviour was observed where a single molecule exhibited both n- and p-channel transport properties. Such behaviour is of interest not only for the development of complementary circuits based on a one-component material
¨t Wu ¨rzburg, Institut fu ¨r Organische Chemie & Center for Nanosystems Universita ¨rzburg, Germany. E-mail: [email protected]; Chemistry, 97074 Wu Fax: +49 931 3184756; Tel: +49 931 3185340 † Electronic supplementary information (ESI) available: Synthetic detail, 1H NMR and 13C NMR spectra, HRMS, UV-vis spectra, cyclic voltammogram, device information, and crystallographic data of 5. CCDC 966365. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc48037b
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(as e.g. given with silicon), but also for new types of devices such as organic light-emitting transistors.4 Isoindigo (further denoted as IID), a structural isomer of indigo, has a symmetrical lactam structure with a moderate electron deficient character. Recently, a parent indigo has been reported to show ambipolar charge transport in OTFTs and circuits (balanced hole and electron about 0.01 cm2 V1 s1).5 Thiophene-functionalized IID dyes bearing donor substituents with absorption in the NIR region have been reported by our group.6 Small molecules based on IID have already been applied as electron donors in solution-processed organic photovoltaics.7 Meanwhile, donor–acceptor polymers based on IID have been synthesized and they could be implemented as active layers to fabricate OTFTs and solar cells.8 However, OTFTs based on IID small molecules have rarely been reported.9 With the idea in mind of revealing the potential of IID small molecules with regard to p-/n-type transport behaviour in OTFTs we have designed new IID derivatives by the introduction of strong electron withdrawing cyano groups to the IID core to lower the LUMO level, intending to improve the electron affinity and modulate the electronic properties of the molecules. Herein we report for the first time that dicyanated IID-based TFTs indeed show electron mobility up to 0.044 cm2 V1 s1 on n-tetradecylphosphonic acid (TPA), and more interestingly they exhibit ambipolar behaviour (0.11 cm2 V1 s1 for electrons and 0.045 cm2 V1 s1 for holes) on 12,12,13,13,14,14,15,15,16,16,17,17,18,18,18-pentadecylfluorooctadecylphosphonic acid (FOPA) self-assembled monolayers (SAMs) under ambient conditions. The synthesis of cyano-functionalized IIDs starts with monobromo-IID 1, which was prepared according to the procedure of Reynolds.7a Palladium-catalyzed cyanation of 1 was carried out with copper(I) cyanide, 1,1 0 -bis(diphenylphosphino)-ferrocene (dppf) and bis(dibenzylideneacetone)palladium(0) [Pd(dba)2] in 1,4-dioxane to afford the monocyano-isoindigo 2 in 55% yield.10 The same approach was applied to synthesize dicyanated IID 5 from dibromo-IID 3 known in the literature (ref. 7a) (Scheme 1). Two compounds, the monocyanated 4 and dicyanated IID 5 formed in the palladium-catalyzed reaction of 3 can be successfully
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Scheme 1 Synthesis of isoindigo derivatives bearing bromo and cyano substituents.
separated in moderate yields (4: 36%; 5: 32%). The IID derivatives 2, 4 and 5 have good solubility in common organic solvents such as dichloromethane, chloroform and tetrahydrofuran. The newly synthesized cyanated IID derivatives were characterized using 1H NMR, 13C NMR, high resolution mass spectrometry, and IID 5 additionally using X-ray analysis (see ESI†). To determine the molecular structure and investigate the packing behaviour of the cyanated IID, a suitable single crystal for X-ray diffraction analysis of compound 5 was obtained by slow evaporation of a solution of 5 in dichloromethane and methanol at room temperature (for details and crystallographic data, see ESI† and CCDC 966365). As shown in Fig. 1a, the crystal structure reveals that 5 has a planar core and belongs to a crystal system of the triclinic space group P1% . Indole-2-one heterocycles are interconnected by the central CQC double bond, which has a bond length of 1.375 Å that is longer than typical C(sp2)QC(sp2) double bonds (1.34 Å). Dicyanated IID 5 adopts an interesting packing with faceto-face p–p contacts along the a-axis with a minimum interplanar spacing of only 3.22 Å (Fig. 1b), and hydrogen bonding between cyano and CH units (Fig. 1c). The remarkable capability of cyano groups to interact with aromatic CH groups has recently been recognized as an important supramolecular motif to direct organic semiconductor molecules into desirable layer arrangements.11
Fig. 1 Molecular structure of 5 (a) and solid state packing (b, c). In (b) the one-dimensional face-to-face p–p stacking and in (c) the hydrogen bonding between cyano and CH groups are shown (the details for the C–H N H-bonding are: H N 2.40 Å, C N 3.3339(15) Å, and C–H N 1681). Hydrogen atoms are partially omitted for clarity.
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The electrochemical properties of cyanated isoindigos 2, 4, and 5, along with those of core-unsubstituted N,N 0 -(1-octyl)isoindigo (C8-IID)7a as a reference, were investigated by cyclic voltammetry (Fig. S1 and Table S1, ESI†). All of those compounds exhibit two reversible reduction waves. In contrast to reference C8-IID, which shows a half-wave potential E1/2 at 1.34 and 1.73 V (vs. Fc/Fc+), the half-wave reduction potentials of monocyano compound 2 is less negative at 1.14 and 1.54 V. Monocyano–monobromo IID 4 shows two reduction waves E1/2 at 1.06 and 1.45 V, and a further shift to 0.92 and 1.29 V is observed for 5 with two electron-withdrawing cyano groups which implies the strongest electron accepting ability within this series (Fig. S1 and S2, ESI†). It is indeed remarkable that from 4 to 5 the reduction potential has crossed the ‘‘magic value’’ given by the most famous n-type semiconductor scaffolds of C60-fullerene, and perylene and naphthalene diimide.12 The LUMO levels of C8-IID, 2, 4, and 5 are estimated to be 3.46, 3.66, 3.74, and 3.88 eV, respectively, based on the onset potential of the first reduction wave (Table S1, ESI†). From these data it is obvious that the number of cyano groups on the core of IID has a strong influence on the LUMO energy level. The low LUMO level of compound 5 suggests potential application in n-channel OTFTs. The optical properties of 2, 4 and 5 in chloroform solution and thin film of compound 5 were characterized using UV-vis spectroscopy. The absorption spectra of 2, 4 and 5 in chloroform solution do not change significantly (Fig. S2, ESI†), while the absorption maximum of evaporated thin films spectra of 5 in the visible region features a bathochromic shift of about 50 nm and broadening compared to the corresponding solution spectrum (Fig. S3, ESI†), which might be attributed to intermolecular interactions in the solid state. To explore the viability of our design concept, the semiconducting properties of dicyanated IID 5 with the lowest LUMO level in the present series was further evaluated in OTFTs. For this purpose, OTFTs with 5 as the active layer were fabricated by vacuum sublimation in a bottom-gate, top-contact configuration, and all these devices were tested under ambient conditions (for details see the ESI†). Substrates were prepared from Si/SiO2 (100 nm) wafers by atomic layer deposition of an 8 nm thick layer of AlOx and additional modification by SAMs of TPA or FOPA (Ci = 32.4 nF cm2). The typical transfer curve on TPA-modified substrate processed at a substrate temperature of Ts = 80 1C are shown in Fig. 2a (for device characteristics at various Ts values, see Table S2, ESI†). The devices on bare SiO2 showed almost no transistor behaviour and all the devices on the TPA substrate exhibited electron transport behaviour when operated in the saturation regime (VDS = 50 V). The highest electron mobility of 0.044 cm2 V1 s1 (calculated from the slope of the square root of the drain current (IDS)1/2 vs. the gate voltage (VGS)) was obtained for thin films deposited at a substrate temperature Ts = 80 1C (Fig. 2a). For this device the threshold voltage was 8 V and the on/off current ratio of 106. From the AFM image (Fig. 2b), we can see crystalline grains with a typical lamellar growth with layer heights of about 1.5 0.2 nm. Interestingly, all the devices on the FOPA-modified substrate exhibit ambipolar TFT behaviour with very good on/off ratios of
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Fig. 2 OTFTs transfer curves of 5 (Ts = 80 1C) on (a) TPA-modified substrates and (b) corresponding AFM image; (c) FOPA-modified substrates and (d) corresponding AFM image.
105 to 106 for both n- and p-type operations (Fig. 2c and Table S3, ESI†). The highest charge carrier mobilities are again achieved at Ts = 80 1C. The corresponding transfer curve is shown in Fig. 2c. Again, we observed typical lamellar growth with layer heights of about 1.6 0.2 nm in the AFM image (Fig. 2d). The charge carrier mobilities of ambipolar OFETs are 0.11 cm2 V1 s1 for electrons and 0.045 cm2 V1 s1 for holes with an appreciable ambient stability. The observed ambipolar behaviour of 5 on FOPA-modified substrates might be rationalized in terms of the polarization of the active layer by the electronegative fluorine substituents of the SAM,13 pinpointing the crucial role of the gate dielectric. In summary, we have reported here a straightforward synthesis of cyanated isoindigo derivatives and revealed the significant impact of cyano groups on the electronic properties, i.e. LUMO levels, and packing behaviour of these compounds. As a consequence, with only two cyano groups the isoindigo scaffold acquires an electron affinity similar to that of the most common n-channel organic semiconductor materials fullerene, and naphthalene and perylene diimide. Meeting our expectation, good performing n-channel and even ambipolar transistors with excellent on/off current ratio could be fabricated. An interesting influence of different modified dielectric surfaces by SAMs was revealed that warrants further investigation. Likewise further synthetic work on this class of dyes by variation of the substituents and addition of more cyano groups14 to the isoindigo scaffold is motivated by the observation of n-channel transport for the dicyano-isoindigo derivative. We thank Ute Zschieschang and Hagen Klauk (MPI for Solid State Research, Stuttgart) for providing us with TPA- and FOPA-modified substrates.
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1 (a) E. J. Meijer, D. M. de Leeuw, S. Setayesh, E. van Veenendaal, B.-H. Huisman, P. W. M. Blom, J. C. Hummelen, U. Scherf and T. M. Klapwijk, Nat. Mater., 2003, 2, 678–682; (b) H. Klauk, U. Zschieschang, J. Pflaum and M. Halik, Nature, 2007, 445, 745–748. 2 (a) M. M. Payne, S. R. Parkin, J. E. Anthony, C.-C. Kuo and T. N. Jackson, J. Am. Chem. Soc., 2005, 127, 4986–4987; (b) H. Ebata, T. Izawa, E. Miyazaki, K. Takimiya, M. Ikeda, H. Kuwabara and T. Yui, J. Am. Chem. Soc., 2007, 129, 15732–15733; (c) S. Subramanian, S. K. Park, S. R. Parkin, V. Podzorov, T. N. Jackson and J. E. Anthony, J. Am. Chem. Soc., 2008, 130, 2706–2707; (d) P. Gao, D. Beckmann, H. N. Tsao, X. Feng, ¨llen, Adv. Mater., V. Enkelmann, M. Baumgarten, W. Pisula and K. Mu 2009, 21, 213–216; (e) R. Hamilton, J. Smith, S. Ogier, M. Heeney, J. E. Anthony, I. McCulloch, J. Veres, D. D. C. Bradley and T. D. Anthopoulos, Adv. Mater., 2009, 21, 1166–1171. ¨nemann, H. W. Ho ¨ffken, 3 (a) J. H. Oh, S.-L. Suraru, W.-Y. Lee, M. Ko ¨ger, R. Schmidt, Y. Chung, W.-C. Chen, F. Wu ¨rthner and C. Ro Z. Bao, Adv. Funct. Mater., 2010, 20, 2148–2156; (b) R. Schmidt, J. H. Oh, Y.-S. Sun, M. Deppisch, A.-M. Krause, K. Radacki, ¨nemann, P. Erk, Z. Bao and F. Wu ¨rthner, H. Braunschweig, M. Ko J. Am. Chem. Soc., 2009, 131, 6215–6228; (c) B. A. Jones, J. A. Michael, Y. Myung-Han, F. Antonio, J. M. Tobin and R. W. Michael, Angew. Chem., Int. Ed., 2004, 116, 6523–6526; (d) B. A. Jones, A. Facchetti, T. J. Marks and M. R. Wasielewski, Chem. Mater., 2007, 19, 2703–2705; (e) Z. Wang, C. Kim, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2007, 129, 13362–13363; ( f ) J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 107, 1296–1323; ( g) Y. Zhao, Y. Guo and Y. Liu, Adv. Mater., 2013, 25, 5372–5391. 4 J. Zaumseil, R. H. Friend and H. Sirringhaus, Nat. Mater., 2006, 5, 69–74. 5 M. Irimia-Vladu, E. D. Głowacki, P. A. Troshin, G. Schwabegger, L. Leona, D. K. Susarova, O. Krystal, M. Ullah, Y. Kanbur, M. A. Bodea, V. F. Razumov, H. Sitter, S. Bauer and N. S. Sariciftci, Adv. Mater., 2012, 24, 375–380. ¨rthner, Org. Biomol. 6 D. Bialas, S.-L. Suraru, R. Schmidt and F. Wu Chem., 2011, 9, 6127–6132. 7 (a) J. Mei, K. R. Graham, R. Stalder and J. R. Reynolds, Org. Lett., 2010, 12, 660–663; (b) W. Ying, F. Guo, J. Li, Q. Zhang, W. Wu, H. Tian and J. Hua, ACS Appl. Mater. Interfaces, 2012, 4, 4215–4224; (c) A. Cassin, P. Leriche, M. Allain and J. Roncali, New J. Chem., 2013, 37, 502–507. 8 (a) T. Lei, Y. Cao, Y. Fan, C. J. Liu, S. C. Yuan and J. Pei, J. Am. Chem. Soc., 2011, 133, 6099–6101; (b) T. Lei, J. H. Dou, Z. J. Ma, C. H. Yao, C. J. Liu, J. Y. Wang and J. Pei, J. Am. Chem. Soc., 2012, 134, 20025–20028; (c) J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney and Z. Bao, J. Am. Chem. Soc., 2011, 133, 20130–20133; (d) E. Wang, Z. Ma, Z. Zhang, K. Vandewal, P. Henriksson, O. Inganas, F. Zhang and M. R. Andersson, J. Am. Chem. Soc., 2011, 133, 14244–14247; (e) G. Zhang, Y. Fu, Z. Xie and Q. Zhang, Macromolecules, 2011, 44, 1414–1420. 9 T. Wu, C. Yu, Y. Guo, H. Liu, G. Yu, Y. Fang and Y. Liu, J. Phys. Chem. C, 2012, 116, 22655–22662. 10 S. Chopin, F. Chaignon, E. Blart and F. Odobel, J. Mater. Chem., 2007, 17, 4139–4146. 11 R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, E. Reinold, ¨rner, H. Ziehlke, C. Elschner, K. Leo, M. Riede, M. Weil, C. Ko ¨uerle, J. Am. Chem. Soc., 2012, 134, M. Pfeiffer, C. Uhrich and P. Ba 11064–11067. ¨rthner and M. Stolte, Chem. Commun., 2011, 47, 5109–5115. 12 F. Wu 13 (a) I. N. Hulea, S. Fratini, H. Xie, C. L. Mulder, N. N. Iossad, G. Rastelli, S. Ciuchi and A. F. Morpurgo, Nat. Mater., 2006, 5, 982–986; (b) M. Wang and I. G. Hill, Org. Electron., 2012, 13, 498–505. 14 Multiple cyanation of aromatic cores provided a plethora of interesting functional molecules, see ref. 3 and J. Mareda and S. Matile, Chem.–Eur. J., 2009, 15, 28–37.
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Cyanated isoindigos for n-type and ambipolar organic thin film transistors† Wan Yue, Tao He, Matthias Stolte, Marcel Gsa¨nger and Frank Wu ¨ rthner*
Received 18th October 2013, Accepted 5th November 2013 DOI: 10.1039/c3cc48037b www.rsc.org/chemcomm
A set of three core-cyanated isoindigos was synthesized by palladiumcatalyzed cyanation of core-brominated isoindigos. With decreased LUMO level to 3.88 eV, the dicyanated isoindigo 5 showed ambientstable electron mobility up to 0.044 cm2 V1 s1 in OTFTs with SAMs of TPA, while it exhibited ambipolar charge transport behaviour (0.11 cm2 V1 s1 for electrons and 0.045 cm2 V1 s1 for holes) on FOPA-modified substrates.
Due to their low power dissipation, high operating speed, and good signal to noise margin, organic complementary circuits comprising both p- and n-channel organic thin film transistors (OTFTs) have received increasing interest for technological as well as scientific reasons.1 In order to achieve organic complementary inverters, reliable organic n-type transistors that can be integrated with p-type transistors are required because the vast majority of organic transistors so far are p-channel OTFTs, for which good ambient stability is easier to realize.2 Hence, the need to build complementary circuits has fuelled research efforts in n-channel transistors. One of the major challenges of fabricating n-channel transistors is the injection of electrons into the LUMO level of the semiconductor from a suitable electrode. This can be achieved by lowering the LUMO level,3 e.g. by incorporation of strong electron-withdrawing substituents such as chlorine, fluorine, cyano, or diimide moieties to the core of intrinsic organic semiconductors. While exploring this concept for several classes of compounds, an ambipolar transport behaviour was observed where a single molecule exhibited both n- and p-channel transport properties. Such behaviour is of interest not only for the development of complementary circuits based on a one-component material
¨t Wu ¨rzburg, Institut fu ¨r Organische Chemie & Center for Nanosystems Universita ¨rzburg, Germany. E-mail: [email protected]; Chemistry, 97074 Wu Fax: +49 931 3184756; Tel: +49 931 3185340 † Electronic supplementary information (ESI) available: Synthetic detail, 1H NMR and 13C NMR spectra, HRMS, UV-vis spectra, cyclic voltammogram, device information, and crystallographic data of 5. CCDC 966365. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc48037b
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(as e.g. given with silicon), but also for new types of devices such as organic light-emitting transistors.4 Isoindigo (further denoted as IID), a structural isomer of indigo, has a symmetrical lactam structure with a moderate electron deficient character. Recently, a parent indigo has been reported to show ambipolar charge transport in OTFTs and circuits (balanced hole and electron about 0.01 cm2 V1 s1).5 Thiophene-functionalized IID dyes bearing donor substituents with absorption in the NIR region have been reported by our group.6 Small molecules based on IID have already been applied as electron donors in solution-processed organic photovoltaics.7 Meanwhile, donor–acceptor polymers based on IID have been synthesized and they could be implemented as active layers to fabricate OTFTs and solar cells.8 However, OTFTs based on IID small molecules have rarely been reported.9 With the idea in mind of revealing the potential of IID small molecules with regard to p-/n-type transport behaviour in OTFTs we have designed new IID derivatives by the introduction of strong electron withdrawing cyano groups to the IID core to lower the LUMO level, intending to improve the electron affinity and modulate the electronic properties of the molecules. Herein we report for the first time that dicyanated IID-based TFTs indeed show electron mobility up to 0.044 cm2 V1 s1 on n-tetradecylphosphonic acid (TPA), and more interestingly they exhibit ambipolar behaviour (0.11 cm2 V1 s1 for electrons and 0.045 cm2 V1 s1 for holes) on 12,12,13,13,14,14,15,15,16,16,17,17,18,18,18-pentadecylfluorooctadecylphosphonic acid (FOPA) self-assembled monolayers (SAMs) under ambient conditions. The synthesis of cyano-functionalized IIDs starts with monobromo-IID 1, which was prepared according to the procedure of Reynolds.7a Palladium-catalyzed cyanation of 1 was carried out with copper(I) cyanide, 1,1 0 -bis(diphenylphosphino)-ferrocene (dppf) and bis(dibenzylideneacetone)palladium(0) [Pd(dba)2] in 1,4-dioxane to afford the monocyano-isoindigo 2 in 55% yield.10 The same approach was applied to synthesize dicyanated IID 5 from dibromo-IID 3 known in the literature (ref. 7a) (Scheme 1). Two compounds, the monocyanated 4 and dicyanated IID 5 formed in the palladium-catalyzed reaction of 3 can be successfully
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Scheme 1 Synthesis of isoindigo derivatives bearing bromo and cyano substituents.
separated in moderate yields (4: 36%; 5: 32%). The IID derivatives 2, 4 and 5 have good solubility in common organic solvents such as dichloromethane, chloroform and tetrahydrofuran. The newly synthesized cyanated IID derivatives were characterized using 1H NMR, 13C NMR, high resolution mass spectrometry, and IID 5 additionally using X-ray analysis (see ESI†). To determine the molecular structure and investigate the packing behaviour of the cyanated IID, a suitable single crystal for X-ray diffraction analysis of compound 5 was obtained by slow evaporation of a solution of 5 in dichloromethane and methanol at room temperature (for details and crystallographic data, see ESI† and CCDC 966365). As shown in Fig. 1a, the crystal structure reveals that 5 has a planar core and belongs to a crystal system of the triclinic space group P1% . Indole-2-one heterocycles are interconnected by the central CQC double bond, which has a bond length of 1.375 Å that is longer than typical C(sp2)QC(sp2) double bonds (1.34 Å). Dicyanated IID 5 adopts an interesting packing with faceto-face p–p contacts along the a-axis with a minimum interplanar spacing of only 3.22 Å (Fig. 1b), and hydrogen bonding between cyano and CH units (Fig. 1c). The remarkable capability of cyano groups to interact with aromatic CH groups has recently been recognized as an important supramolecular motif to direct organic semiconductor molecules into desirable layer arrangements.11
Fig. 1 Molecular structure of 5 (a) and solid state packing (b, c). In (b) the one-dimensional face-to-face p–p stacking and in (c) the hydrogen bonding between cyano and CH groups are shown (the details for the C–H N H-bonding are: H N 2.40 Å, C N 3.3339(15) Å, and C–H N 1681). Hydrogen atoms are partially omitted for clarity.
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The electrochemical properties of cyanated isoindigos 2, 4, and 5, along with those of core-unsubstituted N,N 0 -(1-octyl)isoindigo (C8-IID)7a as a reference, were investigated by cyclic voltammetry (Fig. S1 and Table S1, ESI†). All of those compounds exhibit two reversible reduction waves. In contrast to reference C8-IID, which shows a half-wave potential E1/2 at 1.34 and 1.73 V (vs. Fc/Fc+), the half-wave reduction potentials of monocyano compound 2 is less negative at 1.14 and 1.54 V. Monocyano–monobromo IID 4 shows two reduction waves E1/2 at 1.06 and 1.45 V, and a further shift to 0.92 and 1.29 V is observed for 5 with two electron-withdrawing cyano groups which implies the strongest electron accepting ability within this series (Fig. S1 and S2, ESI†). It is indeed remarkable that from 4 to 5 the reduction potential has crossed the ‘‘magic value’’ given by the most famous n-type semiconductor scaffolds of C60-fullerene, and perylene and naphthalene diimide.12 The LUMO levels of C8-IID, 2, 4, and 5 are estimated to be 3.46, 3.66, 3.74, and 3.88 eV, respectively, based on the onset potential of the first reduction wave (Table S1, ESI†). From these data it is obvious that the number of cyano groups on the core of IID has a strong influence on the LUMO energy level. The low LUMO level of compound 5 suggests potential application in n-channel OTFTs. The optical properties of 2, 4 and 5 in chloroform solution and thin film of compound 5 were characterized using UV-vis spectroscopy. The absorption spectra of 2, 4 and 5 in chloroform solution do not change significantly (Fig. S2, ESI†), while the absorption maximum of evaporated thin films spectra of 5 in the visible region features a bathochromic shift of about 50 nm and broadening compared to the corresponding solution spectrum (Fig. S3, ESI†), which might be attributed to intermolecular interactions in the solid state. To explore the viability of our design concept, the semiconducting properties of dicyanated IID 5 with the lowest LUMO level in the present series was further evaluated in OTFTs. For this purpose, OTFTs with 5 as the active layer were fabricated by vacuum sublimation in a bottom-gate, top-contact configuration, and all these devices were tested under ambient conditions (for details see the ESI†). Substrates were prepared from Si/SiO2 (100 nm) wafers by atomic layer deposition of an 8 nm thick layer of AlOx and additional modification by SAMs of TPA or FOPA (Ci = 32.4 nF cm2). The typical transfer curve on TPA-modified substrate processed at a substrate temperature of Ts = 80 1C are shown in Fig. 2a (for device characteristics at various Ts values, see Table S2, ESI†). The devices on bare SiO2 showed almost no transistor behaviour and all the devices on the TPA substrate exhibited electron transport behaviour when operated in the saturation regime (VDS = 50 V). The highest electron mobility of 0.044 cm2 V1 s1 (calculated from the slope of the square root of the drain current (IDS)1/2 vs. the gate voltage (VGS)) was obtained for thin films deposited at a substrate temperature Ts = 80 1C (Fig. 2a). For this device the threshold voltage was 8 V and the on/off current ratio of 106. From the AFM image (Fig. 2b), we can see crystalline grains with a typical lamellar growth with layer heights of about 1.5 0.2 nm. Interestingly, all the devices on the FOPA-modified substrate exhibit ambipolar TFT behaviour with very good on/off ratios of
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Notes and references
Fig. 2 OTFTs transfer curves of 5 (Ts = 80 1C) on (a) TPA-modified substrates and (b) corresponding AFM image; (c) FOPA-modified substrates and (d) corresponding AFM image.
105 to 106 for both n- and p-type operations (Fig. 2c and Table S3, ESI†). The highest charge carrier mobilities are again achieved at Ts = 80 1C. The corresponding transfer curve is shown in Fig. 2c. Again, we observed typical lamellar growth with layer heights of about 1.6 0.2 nm in the AFM image (Fig. 2d). The charge carrier mobilities of ambipolar OFETs are 0.11 cm2 V1 s1 for electrons and 0.045 cm2 V1 s1 for holes with an appreciable ambient stability. The observed ambipolar behaviour of 5 on FOPA-modified substrates might be rationalized in terms of the polarization of the active layer by the electronegative fluorine substituents of the SAM,13 pinpointing the crucial role of the gate dielectric. In summary, we have reported here a straightforward synthesis of cyanated isoindigo derivatives and revealed the significant impact of cyano groups on the electronic properties, i.e. LUMO levels, and packing behaviour of these compounds. As a consequence, with only two cyano groups the isoindigo scaffold acquires an electron affinity similar to that of the most common n-channel organic semiconductor materials fullerene, and naphthalene and perylene diimide. Meeting our expectation, good performing n-channel and even ambipolar transistors with excellent on/off current ratio could be fabricated. An interesting influence of different modified dielectric surfaces by SAMs was revealed that warrants further investigation. Likewise further synthetic work on this class of dyes by variation of the substituents and addition of more cyano groups14 to the isoindigo scaffold is motivated by the observation of n-channel transport for the dicyano-isoindigo derivative. We thank Ute Zschieschang and Hagen Klauk (MPI for Solid State Research, Stuttgart) for providing us with TPA- and FOPA-modified substrates.
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