■ INTRODUCTION

In recent years organic semiconductors (OSCs) are investigated extensively because of the potential applications to the low-cost and simple fabrication of large-area electronic devices for photovoltaics, 1,2 light-emitting diodes, 3 and organic fieldeffect transistors (OFET). [4][5][6][7][8] There remains, however, a lot of room for improving n-type OSCs, where the development of device performance and stability in air have delayed in comparison with p-type OSCs. Many of n-type OSCs are still unstable when operated in air; this is ascribed to the lowest unoccupied molecular orbital (LUMO) levels which are not sufficiently low. 9 Therefore, enormous efforts have been dedicated to develop air-stable n-type OSCs. 10,11 Among ntype OSCs, diketopyrrolopyrrole, 12,13 naphthalene diimide, [14][15][16][17][18] benzothiadiazole, 19,20 and isoindigo are representa-Scheme 1. (a) Fused 26 and (b) Bridged Bisisatines; 27 (c) Prepared Thienobisisatin and (d) Dicyanomethylene Derivatives, Where R = n-Propyl (Pr), n-Hexyl (Hex), and 2-Ethylhexyl (EH) tive units used not only in acceptor parts of donor-acceptor polymers but also in small-molecule semiconductors. [21][22][23][24] Like these units, introduction of strong electron-withdrawing groups, such as carbonyl groups, 12-1 8, 21 -2 5 diimide groups, [14][15][16][17][18] thiadiazole groups, 19,20 and cyano groups, [26][27][28] is an effective approach for designing novel n-type OSCs.

Bisisoindigo is a representative electron-deficient unit used in n-type OSCs. 29 Bisisatin (Scheme 1) is a synthetic precursor of bisisoindigo and also a promising n-type organic material due to the planar framework with four electron-withdrawing carbonyl groups. Moreover, when the β-position carbonyl group is modified by a dicyanomethylene group, we can further lower the LUMO levels to obtain potentially air-stable n-type materials. [26][27][28] To date, two types of bisisatin derivatives have been reported: one has a fused ring system with naphthalene core (Scheme 1a), 26 and another is bridged by a central single bond (Scheme 1b). 27 However, the latter takes a nonplanar structure twisted around the central single bond when the β-■ Scheme 2. Synthetic Scheme to BTI and BTICN carbonyl groups are replaced with dicyanomethylene groups similarly to Scheme 1d, and this nonplanar framework is not preferable for OFET devices. Therefore, we have designed bisthienoisatin (BTI, Scheme 1c), where the benzene ring of bisisatin is replaced with a thiophene to maintain the perfect planarity. Herein, along with BTI, we have newly synthesized BTICN, in which β-carbonyl groups of BTI are substituted with dicyanomethylene groups (Scheme 1). BTI and BTICN are expected to make intermolecular S•••O and S•••S interactions suitable for carrier transport in OFETs, similarly to our previously reported thienoisoindigo. 25 We have prepared BTI and BTICN bearing different alkyl chains (R) at the N-position to investigate the effect to the molecular packing and the OFET performance. Because we have found difficulty in preparing R = methyl and ethyl derivatives, we have examined R = n-propyl (Pr), n-hexyl (Hex), and 2ethylhexyl (EH) derivatives to improve solubility and crystallinity in addition to close intermolecular interactions realized by alkyl chain arrangement at the same time. In this paper, we report preparation and OFET properties of these BTI and BTICN materials.

■ EXPERIMENTAL SECTION

BTI and BTICN were prepared according to Scheme 2. From 3bromothiophene, thienoisatin (2a 2c) was prepared according to a previous report. 28 By oxidative coupling with silver fluoride as an oxidizing reagent and palladium acetate as a catalyst, BTIs were synthesized using the optimized condition in refs 30 and 31. Finally, BTIs were treated with malononitrile in ethanol via the Knoevenagel condensation reaction to afford BTICNs as the desired compounds. 26,27 All compounds were purified by sublimation before device fabrication.

OFETs were fabricated by thermal deposition of organic active layers (50 nm) onto tetratetracontane (C 44 H 90 , TTC, ε = 2.5)modified (20 nm) SiO 2 (300 nm, C = 11.5 nF/cm 2 )/n-doped Si substrates, 32 34 where the calculated overall capacitance of the gate dielectrics was 10.4 nF/cm 2 . 35 Then, the top-contact Au electrodes were thermally deposited using a shadow metal mask; the channel length (L) and width (W) were 100 and 1000 μm, respectively. The OFET properties were measured under the vacuum of 10 3 Pa and in air by using a Keithley 4200 semiconductor parameter analyzer. The field-effect mobility (μ) and threshold voltage (V T ) were calculated in the saturation regime by using the equation 2 , where I DS and V G are the drain current and gate voltage, respectively. Then, μ was extracted from the slope where the I DS vs V G plot was straight.

The density functional calculations were performed with the B3LYP* functional and TZP basis set using ADF software. 36 RESULTS AND DISCUSSION Electronic Properties. The frontier orbital energy levels were investigated by cyclic voltammetry and the absorption spectra (Figure 1). BTIs and BTICNs in dichloromethane (DCM) exhibited two reversible reduction waves, and the LUMO levels were estimated from the first reduction half-wave potentials (E 1/2 ). 37 As shown in Table 1, the LUMO levels of BTIs are located at 3.80 eV or slightly below and the LUMO levels of BTICNs are located below 4.25 eV, indicating the electron-withdrawing ability of the dicyanomethylene part is strong enough to achieve air-stable n-type OFET properties. 9,38 The optical gaps (E g opt ) are estimated from λ onset of the DCM solution absorption spectra. A bathochromic shift of 140 nm is observed from BTIs to BTICNs due to the electron-withdrawing dicyanomethylene groups (Figure 1c,d). Also, in comparison with the solution spectra, the absorption bands of thin films are shifted to long wavelengths because of the intermolecular interactions in the solid state (Figure S1). From the LUMO levels and the optical gaps, the highest Molecular overlap mode viewed perpendicular to the molecular plane. the b axis with the interplanar distance of 3.32 Å (Figure 3b).

g Table 1. Summary of Electrochemical and Optical Properties

occupied molecular orbital (HOMO) levels of BTIs and BTICNs are estimated to be slightly above 6.0 eV (Table 1). Crystal Structures. Crystal data of BTI-Pr, BTI-Hex, and BTICN-EH are summarized in Table 2, and lattice constants of BTICN-Hex are described in the Supporting Information. BTI-Pr crystallizes in a triclinic system with the space group P 1, and the half molecule is crystallographically independent. BTI-Hex and BTICN-EH make basically isostructural crystals having a monoclinic system with the space group C2/c, in which the half molecule is crystallographically independent. These molecules are located on inversion centers, and the molecular cores adopt a perfectly planar geometry.

In BTI-Pr, the alkyl chains are extending approximately perpendicular to the BTI plane (Figure 2a,b). The molecules form a uniform stacking structure along the a axis with the interplanar spacing of 3.36 Å (Figure 2b). The stacked molecules are slipped along the molecular long axis (Figure 2c), so that a thienoisatin group is located in between the two thienoisatin groups of the adjacent molecule. Because the sulfur atoms are oriented alternately upward and downward, there is no short S•••S contact in the stacks. The adjacent columns are parallel to each other, so that all molecules are parallel (Figure 2d). Along the intercolumnar (b) axis, an intermolecular S•••S interaction with the distance of 3.44 Å is observed (Figure 2d). In addition, a hydrogen bond O•••H with the distance of 2.31 Å is observed between the oxygen The molecular overlap mode is similar to BTI-Pr (Figure 3c). The adjacent columns are, however, tilted in opposite directions by making an angle of 82.6° (Figure 3d). Nonetheless, the adjacent chains are connected by an S•••S short contact with the distance of 3.47 Å (Figure 3e). In addition, the adjacent chains are connected by hydrogen bonds with the distance of 2. BTICN-EH is basically isostructural to BTI-Hex, though the alkyl chains are more tilted (29.8°) from the molecular plane (Figure 4a). The molecules are uniformly stacked along the b axis with the interplanar distance of 3.40 Å (Figure 4b). The stacked molecules are slipped not only along the molecular long axis but also along the molecular short axis. Accordingly, the thiophene sulfur sides of thienoisatin groups overlap with each other in an eclipsed manner (Figure 4c). In addition, a nitrogen atom of dicyanomethylene groups is placed on the top of a thiophene ring. Similarly to BTI-Hex, the adjacent columns are tilted in opposite directions by making an angle of 59.9° (Figure 4d). However, BTICN-EH does not have any short S•••S contact, though the adjacent chains are connected by hydrogen bonds with the distance of 2.51 Å for C N•••H thiophene (Figure 4e). The absence of any short S•••S contact is related to the introduction of the dicyanomethylene groups, which block the side-by-side contacts. As a result, the conduction path is confined in the one-dimensional columnar direction.

The difference of the conduction path is investigated by calculating the transfer integrals for electrons (Table 3). 39,40 The stacking direction is the a axis in BTI-Pr, whereas the b axis in BTI-Hex and BTICN-EH. BTI-Pr has comparable intrachain (//a) and interchain (//b) transfers, and the anisotropy is less than three in BTI-Hex. However, BTICN-EH has a large transfer only along the stacks (//b). This is entirely in agreement with the absence of interchain S•••S interactions. Nonetheless, BTICN-EH has a very large intrachain transfer because of the characteristic overlap mode (Figure 4c). The reorganization energy (λ) of BTICN (221 meV) is considerably smaller than that of BTI (338 meV). The resulting calculated mobility of BTICN-EH is significantly larger than those of BTI-Pr and BTI-Hex. 41 Thin Film Properties. To evaluate thin-film surface morphologies and microstructures, X-ray diffraction (XRD) and grazing-incidence wide-angle X-ray scattering (GIWAXS) were performed (Figure 5). BTIs and BTICNs show sharp XRD peaks (Figure 5a,b), where the extracted d values are summarized in Table 4. The out-of-plane d-spacings observed in GIWAXS (Figure 5c-h) are in good agreement with the XRD results. Many sharp GIWAXS peaks are observed particularly in BTICN-R, reflecting the highly crystalline nature of the thin films. We have observed π-π stacking peaks at around 3.7 Å (q xy = 1.69-1.72 Å -1 ) and 4.1 Å (q xy = 1.52-1.55 Å -1 ). Another peak appears around 7.5 Å (q xy = 0.83-0.86 Å -1 ) in BTI-Pr, BTI-Hex, BTI-EH, and BTICN-EH as well as around 9.2 Å (q xy = 0.68-0.69 Å -1 ) in BTICN-Pr and BTICN-Hex (Table 4), which are attributable to the interchain periodicity. The π-π peaks around 3.7 Å coincide In BTI-Pr, the out-of-plane d-spacing (12.1 Å, q z = 0.52 Å 1 ) is very close to c sin α sin β(11.9 Å), indicating the molecules are standing perpendicular to the substrate. The interchain peak observed at 7.35 Å is consistent with crystallographic b axis (7.65 Å). These observations indicate that the crystallographic ab plane is aligned parallel to the substrate (Figure S5). The molecular tilt angle is 45.8° from the substrate normal. The crystal structure of BTICN-Pr is unknown, but the XRD assumed to be approximately perpendicular to the substrate, the molecular length is estimated to be 19.4 Å (Figure S6).

BTI-Hex

This is in good agreement with the observed XRD, but the molecular core has to take the side-on arrangement. This packing pattern is entirely different from the other compounds, so we could not exclude the possibility that the thin-film structure is different from the crystal structure. In such a case, pattern suggests the thin-film structure is basically the same as BTI-Pr.

Table 5. OFET Properties of the Thin-Film Transistors

The out-of-plane d-spacing of BTI-Hex (19.6 Å, q z = 0.32 Å -1 ) is considerably larger than a/2 (16.5 Å) and c (14.9 Å). compound conditions measurements

V T (V) I on /I off 5

In contrast to BTI-Pr, alkyl chains of BTI-Hex are extending not perpendicular to the molecular plane but perpendicular to 5. In general, the n-propyl derivatives tend to show smaller mobilities than the other two, and BTICN exhibits higher performance than BTI. BTI-Pr and BTI-Hex show electron mobilities in the 10 -2 cm 2 V -1 s -1 order (Figure 6a,b), in agreement with the calculated mobility (Table 3). However, the mobility of BTI-EH drops to the 10 -3 cm 2 V -1 s -1 order (Figure 6c). The highest electron mobility (μ max ) is 0.10 cm 2 V -1 s -1 in BTI-Hex.

Upon air exposure, mobilities of the BTIs transistors decrease to typically half (Table 5), and the off current increases compared to the operation under vacuum (Figure 6a-c). In particular, the off current of BTI-Hex increases considerably. However, the transfer characteristics of BTI-Pr and BTI-EH do not largely change in the logarithmic scale. This moderate air stability is consistent with the comparatively high LUMO levels around -3.8 eV (Table 1).

Under vacuum, BTICNs exhibit electron mobilities in the order of 10 -1 -10 -2 cm 2 V -1 s -1 with the on/off ratios up to 10 6 . BTICNs show negative V T , indicating the normally on properties (Figure 6d-f MΩ and similar conductivity. However, the off current in the negative V G region affords 3.3 × 10 -7 S cm -1 . This reminds us transport observed at the interface of two OSCs. 42,43 The mobility increases in the order of BTICN-Pr < BTICN-Hex < BTICN-EH, and μ max is 0.21 cm 2 V -1 s -1 in BTICN-EH. This is consistent with the large calculated mobility (0.82 cm 2 V -1 s -1 in Table 3), which is associated with the large transfer integral (164 meV) as well as the comparatively small reorganization energy. In air, μ and V T do not change largely, so that these transistors are operated stably in air as expected from the LUMO levels. The long-term stability was investigated by measuring the transistor properties after 150-day storage in air (Figure 6a-f and Table 5). Mobilities of BTIs drop evidently after 150 days (Table 5), and the positions of the green and black curves are considerably different from the blue and red ones (Figure 6ac). Those of BTICNs are, however, almost unchanged; this is evident that the green and black curves do not move from the original ones (Figure 6d-f). BTICNs show long-term stability even in the thin-film transistors, but BTIs degrade gradually in many in n-type transistors, 10 but this is associated with the the ambient conditions. Shifts of V T are particularly notable in strong acceptor ability of BTICN with the LUMO levels around -4.28 eV. The normally on property is closely related to the bulk conductivity. The electrical resistance of BTICN-EH, estimated by the two-probe method using a 50 nm thin film, is 16 MΩ, so the resulting conductivity amounts to 1.3 × 10 -3 S cm -1 . In Figure 6f, I D = 10 -5 A at V G = 0 V leads to 6 BTI-Pr and BTICN-Pr. In BTI and BTICN with hexyl and 2ethylhexyl groups, a large shift of V T is not observed after the long-term storage. One of the reasons is the passivation effect of the alkyl chain; 44 the propyl groups are too short to achieve passivation effect, but the hexyl and 2-ethylhexyl groups are considered to work as passivation layers. Although these molecules are largely tilted ( 50°) from the substrate normal, BTIs exhibit moderate electron mobilities, and BTICNs exhibit excellent n-type transistor characteristics with remarkable long-term air stability. In particular, BTICN-EH shows a maximum electron mobility of 0.21 cm 2 V 1 s 1 . This is consistent with the large calculated mobility, which comes from the large transfer integral and the small reorganization energy. BTI is a promising electron-accepting framework for OSC materials, and introduction of dicyanomethylene groups is an efficient way to improve the n-type OFET properties.