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Tetracyano isoindigo small molecules and their use in n-channel organic field-effect transistors† Raghunath R. Dasari,a Amir Dindar,b Chi Kin Lo,a Cheng-Yin Wang,b Cassandre Quinton,‡a Sanjeev Singh,b Stephen Barlow,a Canek Fuentes-Hernandez,b John R. Reynolds,a Bernard Kippelenb and Seth R. Marder*a N,N 0 -Dihexyl-6,6 0 -dicyanoisoindigo, N,N 0 -didecyl-5,5 0 ,6,6 0 -tetracyanoisoindigo, N,N 0 -dihexyl-5,5 0 ,6,6 0 tetracyanoisoindigo, and N,N 0 -dihexyl-5,5 0 ,6,6 0 -tetracyanothienoisoindigo have been synthesised in moderate yields by the reaction of corresponding di and tetrabromo species with CuCN, with microwave heating leading to higher yields and fewer side products for the tetrasubstituted species. Di- and tetracyano substitution anodically shifts the molecular reduction potential relative to the unsubstituted cores by ca. 0.4 and 0.8 V, respectively, with the resultant values for the tetracyano derivatives (0.58 to 0.67 V vs. FeCp2+/0) suggesting the possibility of air-stable electron transport. All
Received 2nd June 2014, Accepted 29th July 2014
the synthesised cyano derivatives operate in n-channel OFETs, while the tetrabromothienoisoindigo derivative functions in a p-channel transistor. The tetracyanothienoisoindigo derivative exhibits the
DOI: 10.1039/c4cp02427c
highest field-effect electron mobility values – up to 0.04 and 0.09 cm2 V1 s1 in spin-coated and inkjet-printed devices respectively – and OFETs incorporating this compound have been shown to oper-
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ate in air without significant degradation of their mobility values in the saturation regime.
Introduction Solution-processable, air-stable, organic electron-transporting semiconductors have a wide variety of potential applications,1–9 which include n-channel organic field-effect transistors (OFETs).1,6–9 Several classes of electron-poor compounds have been explored for potential use in n-channel OFETs;7–15 however, their transistor device performance is generally still not on par with that of p-channel OFET devices.12,16 Isoindigo has attracted considerable attention recently as an acceptor moiety in donor–acceptor copolymers for organic solar cells.17–19 It is also an interesting candidate for incorporating into materials for transistor applications due to its extended delocalised planar20 aromatic p-system, which may facilitate p–p stacking and enable high charge-carrier mobility. Copolymers based on the isoindigo moiety have been developed for transistor applications; however, due to the moderate electron
a
School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA. E-mail: [email protected] b School of Electrical and Computer Engineering, Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia 30332-0250, USA † Electronic supplementary information (ESI) available: Details of synthesis, electrochemistry and field-effect transistors characterisation. See DOI: 10.1039/ c4cp02427c ‡ Permanent address: PPSM, CNRS UMR8531, ENS Cachan, UniverSud, 61 Avenue ´sident Wilson, 94235 Cachan, France. du Pre
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affinity (EA) of the isoindigo moiety (estimated at ca. 3.5 eV for an unsubstituted small molecule) and the choice of other comonomers, those polymers showed either predominantly hole transport21,22 or ambipolar transport properties.23,24 A critical requirement for n-channel OFETs that are operable under ambient conditions is that the EAs of the active materials approach the work-functions of environmentally stable metal electrodes, in order to facilitate electron injection at moderate gate voltages.7–9 Moreover, materials with high EAs also offer the possibility of stable electron transport under ambient conditions.7–9 One of the effective approaches to attain organic small molecules with high EAs is incorporation of strong electron-withdrawing substituents, such as cyano groups, onto their cores.7 Here, we report on an investigation of the extent to which di- and tetracyano substitution influences the redox potentials and FET behaviour of the isoindigo and thienoisoindigo moieties. Very recently, during the course of our work, N,N0 -dioctyl-6,6 0 -dicyanoisoindigo has been reported and used in vacuum-processed OFETs.25 Here we compare the analogous dihexyl compound to tetracyano derivatives of isoindigo and thienoisoindigo (Fig. 1).
Results and discussion Synthesis N,N 0 -Dihexyl-6,6 0 -dicyanoisoindigo, 1, was synthesised in 39% yield from the corresponding 6,6 0 -dibromoisoindigo, which was
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Fig. 1 Chemical structures of electron-poor small-molecule isoindigo derivatives considered in this work.
Scheme 2
Synthesis of 7.
Finally, 7 was obtained from the reaction of copper(I) cyanide with 6 under microwave irradiation in a modest yield of 36%. All the final products and new intermediates were fully characterised by 1H and 13C NMR spectroscopy, mass spectrometry and elemental analysis (see ESI†). Electrochemical properties
Scheme 1
Synthesis of 4 and 5.
itself synthesised according to the reported procedure,26 using copper(I) cyanide in N,N 0 -dimethylformamide (DMF). 5,5 0 ,6,6 0 Tetrabromoisoindigo (10) was obtained in a similar way to 6,6 0 -dibromoisoindigo from 5,6-dibromooxindole (8) and 5,6dibromoisatin (9), which themselves were synthesised from commercially available 6-bromooxindole and 6-bromoisatin, respectively, according to modified literature procedures,27,28 as shown in Scheme 1. To solubilise the tetrabromoisoindigo core in common organic solvents, compound 10 was N,N 0 -dialkylated in refluxing DMF in the presence of K2CO3 using 1-bromodecane or 1-bromohexane. The resultant n-decyl and n-hexyl substituted tetrabromoisoindigos, 2 and 3, respectively, were purified via recrystallisations and obtained in 72–74% yield. To synthesise the desired tetracyanoisoindigo final products (4–5), the alkylsubstituted tetrabromoisoindigos were initially reacted with an excess of copper(I) cyanide in DMF under conventional heating conditions; however, only low yields (10–12%) of target products were obtained. 4 and 5 were successfully synthesised in higher yields under microwave irradiation conditions, which also led to fewer byproducts, aiding the facile isolation of the N,N 0 -dialkyl-5,5 0 ,6,6 0 -tetracyanoisoindigo final products. The synthesis of N,N 0 -dihexyl 5,5 0 ,6,6 0 -tetracyanothienoisoindigo (7) began with an Ullmann coupling of 3-bromothiophene29 and 1-hexylamine, followed by reaction with oxalyl chloride to give the thiophene-based analogue of N-hexyl isatin, 11, as shown in Scheme 2. Compound 12 was obtained by the reaction of 11 with bromine in 63% yield, and was dimerised using Lawesson’s reagent to afford the tetrabromothienoisoindigo derivative, 6, in an analogous manner to the synthesis of thienoisoindigo.29
19346 | Phys. Chem. Chem. Phys., 2014, 16, 19345--19350
The redox behaviour of di- and tetracyano-substituted isoindigo and thienoisoindigo compounds was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using 0.1 M Bu4NPF6 in CH2Cl2 as the electrolyte solution. The redox potentials discussed in this section are all quoted relative to the ferrocenium/ferrocene reference couple. The tetrabromo intermediates in the syntheses of the tetracyano compounds were also investigated to compare the effects of bromo and cyano substituents on the redox potentials. The tetrabromo isoindigos 2 and 3 showed two reversible reduction events within the accessible solvent window, assigned to successive reduction to the corresponding mono- and dianions. However, their thienoisoindigo analogue 6 showed three reduction and two oxidation waves, although even the first oxidation and reduction processes were not fully reversible, precluding facile assignment of the subsequent processes (see Fig. 2). Half-wave potentials obtained from CV measurements for the first molecular reductions of 2, 3 and 6, are both anodically
Fig. 2 Cyclic voltammograms of 3, 5, 6 and 7. In all cases the feature at E1/2 = 0 V is due to internal ferrocene (FeCp2) reference.
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shifted by ca. 0.25 V relative to that of analogous unsubstituted (thieno)isoindigo derivatives (see Table 1), indicating that the bromo substituents have a moderately electron-withdrawing influence. The di- and tetracyano-substituted isoindigo and thienoisoindigo derivatives all show two reversible reductions at more anodic potentials than the corresponding unsubstituted or brominated species, and no molecular oxidations within the solvent window. The first reduction of the dicyanoisoindigo 1 is anodically shifted by ca. 0.39 V relative to unsubstituted isoindigo; this is very similar to what has recently been reported for the analogous dioctyl species.25 Those of tetracyano compounds 4 and 5 are shifted by another 0.36 V, indicating that the cyano groups have a roughly additive effect on the redox potentials. Tetracyano-substitution has a similar effect on the thienoisoindigo core: the first reduction of 7 is anodically shifted by 0.87 V relative to unsubstituted derivatives. The first reduction potentials of new tetracyano(thieno)isoindigo compounds are within the range in which, according to empirical studies reported in the literature,30,31 electron transport is expected to be stable under ambient conditions.
Fig. 3 UV-vis spectra of 5 in THF solution and 4 & 5 in thin-films (spin-coated from 1,1,2,2-tetrachloroethane).
Optical properties UV-vis spectra of 1–7 were obtained in dichloromethane and are summarised in Table 1. The UV-Vis spectral features of the bromo and cyano substituted isoindigos were characterised by strong absorption bands in the UV and a weaker low-energy band in the visible regions. However, the lower energy bands of the tetrabromo and tetracyano thiophene analogues, 6 and 7, showed considerably higher molar absorptivities (see Table 1) than those of 1–5. All compounds have exhibited comparable optical gaps (estimated from the onset of absorption) of ca. 2.0 eV in solution, with slightly lower values for thienoisoindigo compounds. Fig. 3 displays the UV-Vis absorption spectra of 4 and 5, which have different alkyl chain lengths, in thin-films and 5 in solution (compound 4 showed an identical solution spectra to 5). The absorption features of the thin-film spectra of both compounds are similar to one another and slightly red-shifted and broadened compared to those seen in solution. For compound 7 (Fig. 4) the low-energy band of the thin-film showed slightly more broadening and red-shifting relative to solution than seen for 4 and 5.
Table 1
Fig. 4 UV-vis spectra of 7 in solution and film (spin-coated from 1,1,2,2tetrachloroethane).
OFET characterization To evaluate the electron-transport properties of the tetrabromo-, dicyano-, and tetracyano-substituted (thieno)isoindigo compounds, 1–7, OFETs were fabricated in top-gate, bottom-contact geometry with a CYTOP/Al2O3 bilayer gate dielectric and Au source/drain electrodes. Materials 1–7 were spin-coated onto glass substrates from 1,1,2,2-tetrachloroethane (TCE) in a nitrogen-filled glove box. OFETs based on the dicyanoisoindigo 1 showed typical n-channel characteristics (see ESI†) with electron-mobility
Electrochemicala and optical datae of isoindigo compounds
Compound
E1/2+/0/V
E1/20//V
E1/2/2/V
lmax/nm (emax/104 M1 cm1)
IP f/eV
EAh/eV
Opticali gap/eV
1 2 3 4 5 6 7 iIc TiId
b
0.94 1.08 1.05 0.58 0.58 1.24 0.67 1.33 1.54
1.33 1.47 1.44 1.01 1.03 1.58 1.21 1.71 2.03
396 293 293 292 292 316 349 — —
6.0 5.8 5.9 6.2 6.2 5.5 (5.6)g 6.0 (5.8)g (5.4)g
3.9 3.7 3.8 4.2 4.2 3.6 4.1 3.5 3.3
2.1 2.1 2.1 2.0 2.0 1.9 1.9 — —
b b b b
+0.76 b
+1.01 +0.58
(2.10), (4.11), (4.14), (4.48), (4.66), (0.97), (2.47),
512 385 385 369 369 405 575
(0.61) (2.09), (2.10), (2.42), (2.48), (1.39), (1.32)
402 402 519 519 427
(2.10), 514 (0.48) (2.11), 514 (0.48) (0.45) (0.47) (1.34), 579 (1.51)
a 0.1 M nBu4NPF6/CH2Cl2 vs. FeCp2+/0. b Oxidation was not observed within potential range. c N,N 0 -Dihexyl-isoindigo. d N,N 0 -Diethylhexyl. g Alternative estimate from IP = eE1/2+/0 + 4.8 eV. thienoisoindigo. e Obtained in CH2Cl2. f Solid-state IP estimated from IP = EA + Eoptical g h Solid-state EA estimated from EA = eE1/20/ + 4.8 eV. i From the low energy band absorption edge.
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Table 2
PCCP OFET properties of isoindigo and thienoisoindigo compounds
Cpd
Processing method
S/D electrode
Solvent
mh a/cm2 V1 s1
me a/cm2 V1 s1
VTh b/V
Ion/Ioff c
1 4 4 5 5 6 7 7
Spin-coating Spin-coating Inkjet-printingd Spin-coating Inkjet-printingd Spin-coating Spin-coating Inkjet-printingd
Au Au pAge Au pAg Au Au Ag
TCE TCE DCB TCE DCB TCE TCE DCB
NA NA NA NA NA 0.023 (1.4 (1.2) 102) NA NA
0.01 (3.8 (3.0) 103) 3.3 103 (2.7 (0.3) 103) 0.01 (8.5 (3.9) 103) 4.9 104 (3.1 (1.6) 104) 2.3 103 (1.7 (0.48) 103) NA 0.043 (3.0 (1.1) 102) 0.086 (7.3 (1.8) 102)
7.4 1.0 1.9 2.9 2.7 5.5 0.5 1.0
1.7 103 2 103 8 104 1.5 102 1.1 104 3 103 8.7 102 5.9 104
a
Average mobility values (over 2–8 devices) in parenthesis. silver.
b
Threshold voltage. c Current on/off ratio.
d
(1.5) (0.2) (0.5) (0.4) (0.3) (0.3) (0.1) (0.1)
On polyethersulfone substrate. e Printed
values up to 0.01 cm2 V1 s1 and a threshold voltage of 7.4 V when operated in the saturation regime. The recently reported dioctyl analogue of 1 was reported to show a mobility value of 0.044 cm2 V1 s1 in a device with Au source/drain electrodes, and ambipolar behaviour with even higher electron mobility value of 0.11 cm2 V1 s1 and a threshold voltage of 35 V using a fluoroalkyl phosphonic acid modification of the gate dielectric.25 However, these results are not directly comparable to those reported here since they are based on bottom-gate, topcontact device geometry, and the active layer was deposited by vacuum sublimation. Tetrabromo isoindigos 2 and 3 yielded few working devices, however they did not exhibit characteristic OFET behaviour; hence mobility values are not reported. The tetrabromo-substituted thienoisoindigo, 6, showed p-channel transfer characteristics with hole-mobility values
up to 0.02 cm2 V1 s1. In contrast, tetracyano-substituted isoindigos, 4 and 5, which the electrochemical data suggest have higher EAs than 6, showed only n-channel FET behaviour, with moderate electron mobility values in the saturation regime (see Table 2). The tetracyano-substituted thienoisoindigo, 7, exhibited n-channel OFET characteristics with ideal shapes of transfer and output curves (see Fig. 5) and also, yielding a maximum electron mobility value of 0.043 cm2 V1 s1 with a lower threshold voltage of 0.5 V. It is interesting to note that, the threshold voltage values measured for OFET devices of 7 are much lower than those in devices of 1, consistent with a lower electron-injection barrier
Fig. 5 Representative (a) transfer and (b) output characteristics of OFET devices of 7 fabricated with spin-coating.
Fig. 6 Representative (a) transfer and (b) output characteristics of OFET devices of 7 fabricated with inkjet-printing.
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between Au electrodes and 7, as would be expected from the electrochemical data. OFETs based on 4, 5 and 7 were also fabricated via inkjetprinting in top-gate, bottom-contact geometry with a CYTOP/ Al2O3 bilayer gate dielectric and Ag or printable silver ink (pAg) source/drain electrodes. 1,2-Dichlorobenzene (DCB) solutions of 4, 5 and 7 were inkjet-printed onto glass and flexible plastic polyethersulfone (PES) substrates in ambient atmosphere. However, on the glass substrate dewetting of film was observed. Inkjet-printed OFETs based on 4 and 5 showed moderate electron mobility values, with 4 showing slightly higher mobility values of up to 0.01 cm2 V1 s1 (see Table 2). Tetracyano-substituted thienoisoindigo, 7, (see Fig. 6 and Table 2), showed the highest electron mobility among all the compounds examined in this study, electron mobility value of up to 0.086 cm2 V1 s1 for the OFET devices on a PES substrate. To investigate the environmental stability of operation for OFETs based on 7, expected on the basis of the electrochemical measurements, spin-coated OFETs using 7 were exposed to air for up to one week and OFET device measurements were carried out under ambient conditions. OFET devices operated normally in air (see Fig. 7). Values of electron mobility, in the saturation regime, and threshold voltage of devices measured in air showed negligible variations compared to those measured in N2 (Table S1 in ESI†).
Fig. 7 Representative (a) transfer and (b) output characteristics of spincoated OFET devices of 7 measured in nitrogen and air.
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Conclusions In summary, new high electron-affinity tetracyanoisoindigo and tetracyanothienoisoindigo small molecules were synthesised. Electron-withdrawing cyano groups anodically shift the reduction potential of isoindigo small molecules more than bromo substituents. OFET measurements showed tetracyano thienoisoindigo 7 exhibits the highest electron mobility values, with values approaching 0.1 cm2 V1 s1 for inkjet-printed OFETs on a PES substrate. Spin-coated OFET devices based on this molecule were shown to exhibit stable mobility values, in the saturation regime, when operated in ambient conditions.
Acknowledgements This work was supported in part by Solvay S.A., and by the Office of Naval Research under agreement No N00014-11-1-0313, N00014-14-1-0126 and N00014-14-1-0173. C.Q. thanks the French Ministry of Research for a doctoral fellowship.
Notes and references 1 H. Klauk, U. Zschieschang, J. Pflaum and M. Halik, Nature, 2007, 445, 745. 2 C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater., 2002, 14, 99. 3 G. Gelinck, P. Heremans, K. Nomoto and T. Anthopoulos, Adv. Mater., 2010, 22, 3778. 4 J. E. Anthony, Chem. Mater., 2011, 23, 583. 5 A. C. Arias, J. D. Mackenzie, I. McCulloch, J. Rivnay and A. Salleo, Chem. Rev., 2010, 110, 3. 6 J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 107, 1296. 7 H. Usta, A. Facchetti and T. J. Marks, Acc. Chem. Res., 2011, 44, 501. 8 J. E. Anthony, A. Facchetti, H. Martin, S. R. Marder and X. Zhan, Adv. Mater., 2010, 22, 3876. 9 X. Zhan, A. Facchetti, S. Barlow, T. J. Marks, M. R. Wasielewski and S. R. Marder, Adv. Mater., 2011, 23, 268. 10 H. Dong, C. Wang and W. Hu, Chem. Commun., 2010, 46, 5211. 11 L. E. Polander, S. P. Tiwari, L. Pandey, B. M. Seifried, ´das, B. Kippelen Q. Zhang, S. Barlow, C. Risko, J. L. Bre and S. R. Marder, Chem. Mater., 2011, 23, 3408. 12 S. Holiday, J. E. Donaghey and I. McCulloch, Chem. Mater., 2014, 26, 647. 13 X. Gao and Y. Hu, J. Mater. Chem. C, 2014, 2, 3099. 14 X. Zhao and X. Zhan, Chem. Soc. Rev., 2011, 40, 3728. 15 Y. Lin and X. Zhan, Mater. Horiz., 2014, DOI: 10.1039/ c4mh00042k. 16 H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai and T. Hasegawa, Nature, 2011, 475, 364. 17 J. Mei, K. R. Graham, R. Stalder and J. R. Reynolds, Org. Lett., 2010, 12, 660. 18 R. Stalder, J. Mei, K. R. Graham, L. A. Estrada and J. R. Reynolds, Chem. Mater., 2014, 26, 664. 19 E. Wang, W. Mammo and M. R. Andersson, Adv. Mater., 2014, 26, 1801.
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20 DFT calculations show almost planar gas phase structures for isoindigo & its thiophene analogue (see ESI†). 21 J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney and Z. Bao, J. Am. Chem. Soc., 2011, 133, 20130. 22 M. S. Chen, J. R. Niskala, D. A. Unruh, C. K. Chu, O. P. Lee ´chet, Chem. Mater., 2013, 25, 4088. and J. M. J. Fre 23 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. 24 R. S. Ashraf, A. J. Kronemeijer, D. I. James, H. Sirringhaus and I. McCulloch, Chem. Commun., 2012, 48, 3939. ¨nger and F. Wu ¨rthner, Chem. 25 W. Y. Tao, M. Stolte, M. Gsa Commun., 2014, 50, 545.
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26 L. A. Estrada, D. Y. Liu, D. H. Salazar, A. L. Dyer and J. R. Reynolds, Macromolecules, 2012, 45, 8211. ¨f and Y. Xue, WO 03/082853 A1, 2003. 27 S. Berg, S. Hellberg, M. Nylo 28 K. L. Vine, J. M. Locke, M. Ranson, S. G. Pyne and J. B. Bremmer, Bioorg. Med. Chem., 2007, 15, 931. 29 G. W. P. Van Pruissen, F. Gholamrezaie, M. M. Wienk and R. A. J. Janssen, J. Mater. Chem., 2012, 22, 20387. 30 D. M. deLeeuw, M. M. J. Simenon, A. R. Brow and R. E. F. Einerhand, Synth. Met., 1997, 87, 53. 31 H. Usta, C. Risko, Z. Wang, H. Huang, M. K. Deliomeroglu, A. Zhukhovitskiy, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2009, 131, 5586.
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Tetracyano isoindigo small molecules and their use in n-channel organic field-effect transistors† Raghunath R. Dasari,a Amir Dindar,b Chi Kin Lo,a Cheng-Yin Wang,b Cassandre Quinton,‡a Sanjeev Singh,b Stephen Barlow,a Canek Fuentes-Hernandez,b John R. Reynolds,a Bernard Kippelenb and Seth R. Marder*a N,N 0 -Dihexyl-6,6 0 -dicyanoisoindigo, N,N 0 -didecyl-5,5 0 ,6,6 0 -tetracyanoisoindigo, N,N 0 -dihexyl-5,5 0 ,6,6 0 tetracyanoisoindigo, and N,N 0 -dihexyl-5,5 0 ,6,6 0 -tetracyanothienoisoindigo have been synthesised in moderate yields by the reaction of corresponding di and tetrabromo species with CuCN, with microwave heating leading to higher yields and fewer side products for the tetrasubstituted species. Di- and tetracyano substitution anodically shifts the molecular reduction potential relative to the unsubstituted cores by ca. 0.4 and 0.8 V, respectively, with the resultant values for the tetracyano derivatives (0.58 to 0.67 V vs. FeCp2+/0) suggesting the possibility of air-stable electron transport. All
Received 2nd June 2014, Accepted 29th July 2014
the synthesised cyano derivatives operate in n-channel OFETs, while the tetrabromothienoisoindigo derivative functions in a p-channel transistor. The tetracyanothienoisoindigo derivative exhibits the
DOI: 10.1039/c4cp02427c
highest field-effect electron mobility values – up to 0.04 and 0.09 cm2 V1 s1 in spin-coated and inkjet-printed devices respectively – and OFETs incorporating this compound have been shown to oper-
www.rsc.org/pccp
ate in air without significant degradation of their mobility values in the saturation regime.
Introduction Solution-processable, air-stable, organic electron-transporting semiconductors have a wide variety of potential applications,1–9 which include n-channel organic field-effect transistors (OFETs).1,6–9 Several classes of electron-poor compounds have been explored for potential use in n-channel OFETs;7–15 however, their transistor device performance is generally still not on par with that of p-channel OFET devices.12,16 Isoindigo has attracted considerable attention recently as an acceptor moiety in donor–acceptor copolymers for organic solar cells.17–19 It is also an interesting candidate for incorporating into materials for transistor applications due to its extended delocalised planar20 aromatic p-system, which may facilitate p–p stacking and enable high charge-carrier mobility. Copolymers based on the isoindigo moiety have been developed for transistor applications; however, due to the moderate electron
a
School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA. E-mail: [email protected] b School of Electrical and Computer Engineering, Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia 30332-0250, USA † Electronic supplementary information (ESI) available: Details of synthesis, electrochemistry and field-effect transistors characterisation. See DOI: 10.1039/ c4cp02427c ‡ Permanent address: PPSM, CNRS UMR8531, ENS Cachan, UniverSud, 61 Avenue ´sident Wilson, 94235 Cachan, France. du Pre
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affinity (EA) of the isoindigo moiety (estimated at ca. 3.5 eV for an unsubstituted small molecule) and the choice of other comonomers, those polymers showed either predominantly hole transport21,22 or ambipolar transport properties.23,24 A critical requirement for n-channel OFETs that are operable under ambient conditions is that the EAs of the active materials approach the work-functions of environmentally stable metal electrodes, in order to facilitate electron injection at moderate gate voltages.7–9 Moreover, materials with high EAs also offer the possibility of stable electron transport under ambient conditions.7–9 One of the effective approaches to attain organic small molecules with high EAs is incorporation of strong electron-withdrawing substituents, such as cyano groups, onto their cores.7 Here, we report on an investigation of the extent to which di- and tetracyano substitution influences the redox potentials and FET behaviour of the isoindigo and thienoisoindigo moieties. Very recently, during the course of our work, N,N0 -dioctyl-6,6 0 -dicyanoisoindigo has been reported and used in vacuum-processed OFETs.25 Here we compare the analogous dihexyl compound to tetracyano derivatives of isoindigo and thienoisoindigo (Fig. 1).
Results and discussion Synthesis N,N 0 -Dihexyl-6,6 0 -dicyanoisoindigo, 1, was synthesised in 39% yield from the corresponding 6,6 0 -dibromoisoindigo, which was
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Fig. 1 Chemical structures of electron-poor small-molecule isoindigo derivatives considered in this work.
Scheme 2
Synthesis of 7.
Finally, 7 was obtained from the reaction of copper(I) cyanide with 6 under microwave irradiation in a modest yield of 36%. All the final products and new intermediates were fully characterised by 1H and 13C NMR spectroscopy, mass spectrometry and elemental analysis (see ESI†). Electrochemical properties
Scheme 1
Synthesis of 4 and 5.
itself synthesised according to the reported procedure,26 using copper(I) cyanide in N,N 0 -dimethylformamide (DMF). 5,5 0 ,6,6 0 Tetrabromoisoindigo (10) was obtained in a similar way to 6,6 0 -dibromoisoindigo from 5,6-dibromooxindole (8) and 5,6dibromoisatin (9), which themselves were synthesised from commercially available 6-bromooxindole and 6-bromoisatin, respectively, according to modified literature procedures,27,28 as shown in Scheme 1. To solubilise the tetrabromoisoindigo core in common organic solvents, compound 10 was N,N 0 -dialkylated in refluxing DMF in the presence of K2CO3 using 1-bromodecane or 1-bromohexane. The resultant n-decyl and n-hexyl substituted tetrabromoisoindigos, 2 and 3, respectively, were purified via recrystallisations and obtained in 72–74% yield. To synthesise the desired tetracyanoisoindigo final products (4–5), the alkylsubstituted tetrabromoisoindigos were initially reacted with an excess of copper(I) cyanide in DMF under conventional heating conditions; however, only low yields (10–12%) of target products were obtained. 4 and 5 were successfully synthesised in higher yields under microwave irradiation conditions, which also led to fewer byproducts, aiding the facile isolation of the N,N 0 -dialkyl-5,5 0 ,6,6 0 -tetracyanoisoindigo final products. The synthesis of N,N 0 -dihexyl 5,5 0 ,6,6 0 -tetracyanothienoisoindigo (7) began with an Ullmann coupling of 3-bromothiophene29 and 1-hexylamine, followed by reaction with oxalyl chloride to give the thiophene-based analogue of N-hexyl isatin, 11, as shown in Scheme 2. Compound 12 was obtained by the reaction of 11 with bromine in 63% yield, and was dimerised using Lawesson’s reagent to afford the tetrabromothienoisoindigo derivative, 6, in an analogous manner to the synthesis of thienoisoindigo.29
19346 | Phys. Chem. Chem. Phys., 2014, 16, 19345--19350
The redox behaviour of di- and tetracyano-substituted isoindigo and thienoisoindigo compounds was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using 0.1 M Bu4NPF6 in CH2Cl2 as the electrolyte solution. The redox potentials discussed in this section are all quoted relative to the ferrocenium/ferrocene reference couple. The tetrabromo intermediates in the syntheses of the tetracyano compounds were also investigated to compare the effects of bromo and cyano substituents on the redox potentials. The tetrabromo isoindigos 2 and 3 showed two reversible reduction events within the accessible solvent window, assigned to successive reduction to the corresponding mono- and dianions. However, their thienoisoindigo analogue 6 showed three reduction and two oxidation waves, although even the first oxidation and reduction processes were not fully reversible, precluding facile assignment of the subsequent processes (see Fig. 2). Half-wave potentials obtained from CV measurements for the first molecular reductions of 2, 3 and 6, are both anodically
Fig. 2 Cyclic voltammograms of 3, 5, 6 and 7. In all cases the feature at E1/2 = 0 V is due to internal ferrocene (FeCp2) reference.
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shifted by ca. 0.25 V relative to that of analogous unsubstituted (thieno)isoindigo derivatives (see Table 1), indicating that the bromo substituents have a moderately electron-withdrawing influence. The di- and tetracyano-substituted isoindigo and thienoisoindigo derivatives all show two reversible reductions at more anodic potentials than the corresponding unsubstituted or brominated species, and no molecular oxidations within the solvent window. The first reduction of the dicyanoisoindigo 1 is anodically shifted by ca. 0.39 V relative to unsubstituted isoindigo; this is very similar to what has recently been reported for the analogous dioctyl species.25 Those of tetracyano compounds 4 and 5 are shifted by another 0.36 V, indicating that the cyano groups have a roughly additive effect on the redox potentials. Tetracyano-substitution has a similar effect on the thienoisoindigo core: the first reduction of 7 is anodically shifted by 0.87 V relative to unsubstituted derivatives. The first reduction potentials of new tetracyano(thieno)isoindigo compounds are within the range in which, according to empirical studies reported in the literature,30,31 electron transport is expected to be stable under ambient conditions.
Fig. 3 UV-vis spectra of 5 in THF solution and 4 & 5 in thin-films (spin-coated from 1,1,2,2-tetrachloroethane).
Optical properties UV-vis spectra of 1–7 were obtained in dichloromethane and are summarised in Table 1. The UV-Vis spectral features of the bromo and cyano substituted isoindigos were characterised by strong absorption bands in the UV and a weaker low-energy band in the visible regions. However, the lower energy bands of the tetrabromo and tetracyano thiophene analogues, 6 and 7, showed considerably higher molar absorptivities (see Table 1) than those of 1–5. All compounds have exhibited comparable optical gaps (estimated from the onset of absorption) of ca. 2.0 eV in solution, with slightly lower values for thienoisoindigo compounds. Fig. 3 displays the UV-Vis absorption spectra of 4 and 5, which have different alkyl chain lengths, in thin-films and 5 in solution (compound 4 showed an identical solution spectra to 5). The absorption features of the thin-film spectra of both compounds are similar to one another and slightly red-shifted and broadened compared to those seen in solution. For compound 7 (Fig. 4) the low-energy band of the thin-film showed slightly more broadening and red-shifting relative to solution than seen for 4 and 5.
Table 1
Fig. 4 UV-vis spectra of 7 in solution and film (spin-coated from 1,1,2,2tetrachloroethane).
OFET characterization To evaluate the electron-transport properties of the tetrabromo-, dicyano-, and tetracyano-substituted (thieno)isoindigo compounds, 1–7, OFETs were fabricated in top-gate, bottom-contact geometry with a CYTOP/Al2O3 bilayer gate dielectric and Au source/drain electrodes. Materials 1–7 were spin-coated onto glass substrates from 1,1,2,2-tetrachloroethane (TCE) in a nitrogen-filled glove box. OFETs based on the dicyanoisoindigo 1 showed typical n-channel characteristics (see ESI†) with electron-mobility
Electrochemicala and optical datae of isoindigo compounds
Compound
E1/2+/0/V
E1/20//V
E1/2/2/V
lmax/nm (emax/104 M1 cm1)
IP f/eV
EAh/eV
Opticali gap/eV
1 2 3 4 5 6 7 iIc TiId
b
0.94 1.08 1.05 0.58 0.58 1.24 0.67 1.33 1.54
1.33 1.47 1.44 1.01 1.03 1.58 1.21 1.71 2.03
396 293 293 292 292 316 349 — —
6.0 5.8 5.9 6.2 6.2 5.5 (5.6)g 6.0 (5.8)g (5.4)g
3.9 3.7 3.8 4.2 4.2 3.6 4.1 3.5 3.3
2.1 2.1 2.1 2.0 2.0 1.9 1.9 — —
b b b b
+0.76 b
+1.01 +0.58
(2.10), (4.11), (4.14), (4.48), (4.66), (0.97), (2.47),
512 385 385 369 369 405 575
(0.61) (2.09), (2.10), (2.42), (2.48), (1.39), (1.32)
402 402 519 519 427
(2.10), 514 (0.48) (2.11), 514 (0.48) (0.45) (0.47) (1.34), 579 (1.51)
a 0.1 M nBu4NPF6/CH2Cl2 vs. FeCp2+/0. b Oxidation was not observed within potential range. c N,N 0 -Dihexyl-isoindigo. d N,N 0 -Diethylhexyl. g Alternative estimate from IP = eE1/2+/0 + 4.8 eV. thienoisoindigo. e Obtained in CH2Cl2. f Solid-state IP estimated from IP = EA + Eoptical g h Solid-state EA estimated from EA = eE1/20/ + 4.8 eV. i From the low energy band absorption edge.
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Table 2
PCCP OFET properties of isoindigo and thienoisoindigo compounds
Cpd
Processing method
S/D electrode
Solvent
mh a/cm2 V1 s1
me a/cm2 V1 s1
VTh b/V
Ion/Ioff c
1 4 4 5 5 6 7 7
Spin-coating Spin-coating Inkjet-printingd Spin-coating Inkjet-printingd Spin-coating Spin-coating Inkjet-printingd
Au Au pAge Au pAg Au Au Ag
TCE TCE DCB TCE DCB TCE TCE DCB
NA NA NA NA NA 0.023 (1.4 (1.2) 102) NA NA
0.01 (3.8 (3.0) 103) 3.3 103 (2.7 (0.3) 103) 0.01 (8.5 (3.9) 103) 4.9 104 (3.1 (1.6) 104) 2.3 103 (1.7 (0.48) 103) NA 0.043 (3.0 (1.1) 102) 0.086 (7.3 (1.8) 102)
7.4 1.0 1.9 2.9 2.7 5.5 0.5 1.0
1.7 103 2 103 8 104 1.5 102 1.1 104 3 103 8.7 102 5.9 104
a
Average mobility values (over 2–8 devices) in parenthesis. silver.
b
Threshold voltage. c Current on/off ratio.
d
(1.5) (0.2) (0.5) (0.4) (0.3) (0.3) (0.1) (0.1)
On polyethersulfone substrate. e Printed
values up to 0.01 cm2 V1 s1 and a threshold voltage of 7.4 V when operated in the saturation regime. The recently reported dioctyl analogue of 1 was reported to show a mobility value of 0.044 cm2 V1 s1 in a device with Au source/drain electrodes, and ambipolar behaviour with even higher electron mobility value of 0.11 cm2 V1 s1 and a threshold voltage of 35 V using a fluoroalkyl phosphonic acid modification of the gate dielectric.25 However, these results are not directly comparable to those reported here since they are based on bottom-gate, topcontact device geometry, and the active layer was deposited by vacuum sublimation. Tetrabromo isoindigos 2 and 3 yielded few working devices, however they did not exhibit characteristic OFET behaviour; hence mobility values are not reported. The tetrabromo-substituted thienoisoindigo, 6, showed p-channel transfer characteristics with hole-mobility values
up to 0.02 cm2 V1 s1. In contrast, tetracyano-substituted isoindigos, 4 and 5, which the electrochemical data suggest have higher EAs than 6, showed only n-channel FET behaviour, with moderate electron mobility values in the saturation regime (see Table 2). The tetracyano-substituted thienoisoindigo, 7, exhibited n-channel OFET characteristics with ideal shapes of transfer and output curves (see Fig. 5) and also, yielding a maximum electron mobility value of 0.043 cm2 V1 s1 with a lower threshold voltage of 0.5 V. It is interesting to note that, the threshold voltage values measured for OFET devices of 7 are much lower than those in devices of 1, consistent with a lower electron-injection barrier
Fig. 5 Representative (a) transfer and (b) output characteristics of OFET devices of 7 fabricated with spin-coating.
Fig. 6 Representative (a) transfer and (b) output characteristics of OFET devices of 7 fabricated with inkjet-printing.
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between Au electrodes and 7, as would be expected from the electrochemical data. OFETs based on 4, 5 and 7 were also fabricated via inkjetprinting in top-gate, bottom-contact geometry with a CYTOP/ Al2O3 bilayer gate dielectric and Ag or printable silver ink (pAg) source/drain electrodes. 1,2-Dichlorobenzene (DCB) solutions of 4, 5 and 7 were inkjet-printed onto glass and flexible plastic polyethersulfone (PES) substrates in ambient atmosphere. However, on the glass substrate dewetting of film was observed. Inkjet-printed OFETs based on 4 and 5 showed moderate electron mobility values, with 4 showing slightly higher mobility values of up to 0.01 cm2 V1 s1 (see Table 2). Tetracyano-substituted thienoisoindigo, 7, (see Fig. 6 and Table 2), showed the highest electron mobility among all the compounds examined in this study, electron mobility value of up to 0.086 cm2 V1 s1 for the OFET devices on a PES substrate. To investigate the environmental stability of operation for OFETs based on 7, expected on the basis of the electrochemical measurements, spin-coated OFETs using 7 were exposed to air for up to one week and OFET device measurements were carried out under ambient conditions. OFET devices operated normally in air (see Fig. 7). Values of electron mobility, in the saturation regime, and threshold voltage of devices measured in air showed negligible variations compared to those measured in N2 (Table S1 in ESI†).
Fig. 7 Representative (a) transfer and (b) output characteristics of spincoated OFET devices of 7 measured in nitrogen and air.
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Conclusions In summary, new high electron-affinity tetracyanoisoindigo and tetracyanothienoisoindigo small molecules were synthesised. Electron-withdrawing cyano groups anodically shift the reduction potential of isoindigo small molecules more than bromo substituents. OFET measurements showed tetracyano thienoisoindigo 7 exhibits the highest electron mobility values, with values approaching 0.1 cm2 V1 s1 for inkjet-printed OFETs on a PES substrate. Spin-coated OFET devices based on this molecule were shown to exhibit stable mobility values, in the saturation regime, when operated in ambient conditions.
Acknowledgements This work was supported in part by Solvay S.A., and by the Office of Naval Research under agreement No N00014-11-1-0313, N00014-14-1-0126 and N00014-14-1-0173. C.Q. thanks the French Ministry of Research for a doctoral fellowship.
Notes and references 1 H. Klauk, U. Zschieschang, J. Pflaum and M. Halik, Nature, 2007, 445, 745. 2 C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater., 2002, 14, 99. 3 G. Gelinck, P. Heremans, K. Nomoto and T. Anthopoulos, Adv. Mater., 2010, 22, 3778. 4 J. E. Anthony, Chem. Mater., 2011, 23, 583. 5 A. C. Arias, J. D. Mackenzie, I. McCulloch, J. Rivnay and A. Salleo, Chem. Rev., 2010, 110, 3. 6 J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 107, 1296. 7 H. Usta, A. Facchetti and T. J. Marks, Acc. Chem. Res., 2011, 44, 501. 8 J. E. Anthony, A. Facchetti, H. Martin, S. R. Marder and X. Zhan, Adv. Mater., 2010, 22, 3876. 9 X. Zhan, A. Facchetti, S. Barlow, T. J. Marks, M. R. Wasielewski and S. R. Marder, Adv. Mater., 2011, 23, 268. 10 H. Dong, C. Wang and W. Hu, Chem. Commun., 2010, 46, 5211. 11 L. E. Polander, S. P. Tiwari, L. Pandey, B. M. Seifried, ´das, B. Kippelen Q. Zhang, S. Barlow, C. Risko, J. L. Bre and S. R. Marder, Chem. Mater., 2011, 23, 3408. 12 S. Holiday, J. E. Donaghey and I. McCulloch, Chem. Mater., 2014, 26, 647. 13 X. Gao and Y. Hu, J. Mater. Chem. C, 2014, 2, 3099. 14 X. Zhao and X. Zhan, Chem. Soc. Rev., 2011, 40, 3728. 15 Y. Lin and X. Zhan, Mater. Horiz., 2014, DOI: 10.1039/ c4mh00042k. 16 H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai and T. Hasegawa, Nature, 2011, 475, 364. 17 J. Mei, K. R. Graham, R. Stalder and J. R. Reynolds, Org. Lett., 2010, 12, 660. 18 R. Stalder, J. Mei, K. R. Graham, L. A. Estrada and J. R. Reynolds, Chem. Mater., 2014, 26, 664. 19 E. Wang, W. Mammo and M. R. Andersson, Adv. Mater., 2014, 26, 1801.
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20 DFT calculations show almost planar gas phase structures for isoindigo & its thiophene analogue (see ESI†). 21 J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney and Z. Bao, J. Am. Chem. Soc., 2011, 133, 20130. 22 M. S. Chen, J. R. Niskala, D. A. Unruh, C. K. Chu, O. P. Lee ´chet, Chem. Mater., 2013, 25, 4088. and J. M. J. Fre 23 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. 24 R. S. Ashraf, A. J. Kronemeijer, D. I. James, H. Sirringhaus and I. McCulloch, Chem. Commun., 2012, 48, 3939. ¨nger and F. Wu ¨rthner, Chem. 25 W. Y. Tao, M. Stolte, M. Gsa Commun., 2014, 50, 545.
19350 | Phys. Chem. Chem. Phys., 2014, 16, 19345--19350
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26 L. A. Estrada, D. Y. Liu, D. H. Salazar, A. L. Dyer and J. R. Reynolds, Macromolecules, 2012, 45, 8211. ¨f and Y. Xue, WO 03/082853 A1, 2003. 27 S. Berg, S. Hellberg, M. Nylo 28 K. L. Vine, J. M. Locke, M. Ranson, S. G. Pyne and J. B. Bremmer, Bioorg. Med. Chem., 2007, 15, 931. 29 G. W. P. Van Pruissen, F. Gholamrezaie, M. M. Wienk and R. A. J. Janssen, J. Mater. Chem., 2012, 22, 20387. 30 D. M. deLeeuw, M. M. J. Simenon, A. R. Brow and R. E. F. Einerhand, Synth. Met., 1997, 87, 53. 31 H. Usta, C. Risko, Z. Wang, H. Huang, M. K. Deliomeroglu, A. Zhukhovitskiy, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2009, 131, 5586.
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