Naphthodithiophene Diimide (NDTI)-Based Semiconducting

Jan 16, 2015 - A series of naphthodithiophene diimid (NDTI) based semiconducting polymers with various comonomer units, i.e., thienylenevinylene (TV),...
20 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Naphthodithiophene Diimide (NDTI)-Based Semiconducting Copolymers: From Ambipolar to Unipolar n‑Type Polymers Masahiro Nakano, Itaru Osaka, and Kazuo Takimiya* Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: A series of naphthodithiophene diimid (NDTI) based semiconducting polymers with various comonomer units, i.e., thienylenevinylene (TV), naphtho[1,2-b:5,6-b′]dithiophene (NDT), vinylene (V), benzo[c][1,2,5]thiadiazole (BTz), and naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NTz), were synthesized by Stille coupling or copper iodide-assisted Stille coupling using 2,7-dibromo-NDTI or 2,7-bis(trimethylstannyl)-NDTI, respectively. Their HOMO and LUMO energy levels were estimated by cyclic volttammetry and photoelectron spectroscopy. The HOMO energy levels of the NDTI-based polymers were strongly affected by the electronic nature of the comonomer units. In contrast, their LUMO energy levels were almost identical, likely owing to their localized LUMOs on the NDTI moiety. All the polymers showed air-stable electron transport in the field-effect transistors (FETs), thanks to their low-lying LUMO (∼−4.0 eV), as ambipolar (PNDTI-TV, -NDT, ∼0.082 cm2 V−1 s−1, μe = ∼0.029 cm2 V−1 s−1) or unipolar n-channel materials (PNDTI-V, -BTz, PNDTI-NTz, μe = ∼0.21 cm2 V−1 s−1), depending on their HOMO energy levels. These results indicate that the type of active carrier in the NDTI-based polymers can be controlled by the nature of comonomer units. Moreover, the two-dimensional grazing incidence X-ray diffraction (2D GIXD) indicated that the polymers with linear backbone structures and large space between the branched alkyl chains tend to afford well-organized crystalline thin film with the edge-on orientation, consistent with good transport characteristics in their FET devices.

■

π-building blocks with different electronic natures into the conjugated backbone. In fact, so-called donor−acceptor (D−A) polymers, where electron rich π-building blocks (donor, D) and electron deficient π-building blocks (acceptor, A) are alternately incorporated into the conjugated backbone, can afford facile tunability of EHONOs and ELUMOs as well as Egs by choosing and combining suitable π-building blocks.2,6 Compared to a wide variety of electron rich π-building blocks, including various thiophene-based oligomeric or fused systems, electron deficient π-building blocks applicable to semiconducting polymers are relatively limited,7 and thus it is essential to develop new electron deficient π-building blocks in order to promote further possibility of semiconducting polymer-based materials and devices.8 Recently, we have reported on the synthesis and characterization of naphthodithiophenediimide (NDTI) as a promising electron deficient πbuilding block for the development of new materials with lowlying ELUMOs.9 The characteristic features of NDTI include its planar molecular structure enabling efficient π−π intermolecular interaction in the solid state, low-lying ELUMO (∼−4.0 eV

INTRODUCTION π-Conjugated semiconducting polymers have been regarded as one of the key materials for realizing printed electronics devices such as organic field-effect transistors (OFETs) and organic photovoltaics (OPVs).1 In such device applications, it is crucially important to control the electronic structures of polymers to have desirable electronic properties and thus functions. For example, p- or n- type semiconducting materials for OFET devices require sufficiently high-lying energy level of highest occupied molecular orbital (HOMO) or low-lying energy level of lowest unoccupied molecular orbital (LUMO), respectively. Furthermore, to ensure environmental stability of respective semiconducting characteristics, fine-tuning of the HOMO and LUMO energy levels (EHONO and ELUMO, respectively) is indispensable.2 On the other hand, for the OPV applications, it is important to control not only the EHONO or ELUMO in p- or n- type materials, respectively, but also HOMO−LUMO gaps or energy gaps (Egs) to ensure efficient absorption of sunlight.3 Although the electronic structures of conjugated polymers could be tuned by adding side groups on the conjugated backbone, for example, strong electron-withdrawing fluoro-4 or cyano-5 substituents that can lower both EHONO and ELUMO, a more effective, probably essential way is to incorporate various © XXXX American Chemical Society

Received: November 14, 2014 Revised: December 20, 2014

A

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. NDTI-based semiconducting polymers.

Figure 2. Calculated HOMO and LUMO of triad-type model compounds of NDTI-based copolymers.

below the vacuum level), and chemical flexibility that can afford a chance to be integrated into various π-conjugated systems. In fact, the NDTI core can be used in developing a molecular semiconductor (C8−NDTI) for n-type OFETs and a semiconducting polymer with the bithiophene (BT) comonomer unit (PNDT-BT, Figure 1), which afforded not only air-stable ambipolar OFETs with balanced hole (0.10 cm2 V−1 s−1) and electron (0.27 cm2 V−1 s−1) mobilities, but also all-polymer solar cells (all-PSCs) showing decent power conversion efficiency of 2.6% by combining with PTB7 as the donor material.10 It should be emphasized that in the all-PSCs, PNDTI-BT acts as the nonfullerene acceptor as well as a nearinfrared absorber enabling “panchromic” absorption in combination with PTB7, which can complementarily absorb the visible light. These results nicely demonstrated that NDTI-based semiconducting polymers are promising, and we have thus designed several new semiconducting polymers with various comonomer units with different electronic properties, e.g., thienylenevinylene (TV),11 naphtho[1,2-b:5,6-b′]dithiophene (NDT),12 vinylene (V), benzo[c][1,2,5]thiadiazole (BTz),6b and naphtho[1,2c:5,6-c′]bis[1,2,5]thiadiazole (NTz) (Figure 1).13 In this article, we describe the synthesis, electronic structures, thin film structures, and OFET characteristics of a series of new NDTIbased semiconducting copolymers together with PNDT-BT already reported.

carried out theoretical calculations on a triad-type model compound of each polymer, where two comonomer units sandwich the NDTI core, to predict the electronic effects from the comonomer units by using the DFT methods with B3LYP/ 6-31G(d) level.14 The calculated EHOMOs, ELUMOs, and electron density distributions are depicted in Figure 2. Compared to the NDTI monomer (calculated EHOMO: −6.05 eV),9 the calculated EHOMOs of the triads are largely affected by the comonomer unit combined. With the electron rich thiophene-based comonomers, e.g., BT, TV, and NDT, the EHOMOs are largely shifted upward by 0.61−0.92 eV, whereas, with the neutral (V) or electron deficient (BTz and NTz) units, the extent of shift of EHOMO are limited, less than 0.25 eV. In sharp contrast, the ELUMOs of the triads do not significantly change from the NDTI monomer (ELUMO: −3.47 eV); with the electron donating thiophene-based comonomers, the maximum shift is +0.14 for the BT unit, and with electron deficient comonomers, −0.11 eV for the NTz unit. Although these insensitive ELUMOs toward incorporation of various comonomer units, at a glance, are surprising, this can be explained by considering the geometry of LUMOs of the NDTI-based triads. Even with the strong electron-deficient NTz unit, the LUMO coefficients mainly localize on the NDTI part, making the ELUMOs inert toward extension of π-system with additional comonomer units.15 In contrast, the HOMO coefficients are delocalized over the whole triads, indicating that the electronic nature of comonomer units are significantly influential to EHOMOs. In fact, the electron-donating thiophene-based ones (i.e., BT, TV, and NDT) push largely EHOMOs upward, whereas the neutral and electron-deficient ones (V, BTz, and NTz) only slightly. These distinct effects from the comonomer units to the EHOMOs and ELUMOs of resulting systems can afford an interesting and useful tunability of the electronic structure of NDTI-based materials in combination with the inherently low-

■

RESULTS AND DISCUSSION Theoretical MO Calculations. In the design of new NDTI-based polymers, we have selected comonomer units with different electronic structures; TV and NDT as a thiophenebased unit with a simply π-extended and a ring-fused system, respectively, V as a short and electronically neutral (or slightly electron-withdrawing owing to the sp2 carbons) unit, and BTz and NTz as electron deficient comonomer units. We first B

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthesis of PNDTI-Xs

Table 1. Electrochemical and Optical Properties of PNDTI-Xs Evaluated on the Thin Film PNDTI-X

Eoxonset/Va

EHOMO/eVb

Eredonset/Va

ELUMO/eVb

λedge/nm

Egopt/eVc

IP/eVd

TV NDT V BTz NTz BTe

+1.05 +1.35 − − − +1.19

−5.4 −5.7 − − − −5.6

−0.43 −0.39 −0.15 −0.26 −0.13 −0.33

−4.0 −4.0 −4.2 −4.1 −4.2 −4.1

950 810 850 830 800 950

1.3 1.5 1.5 1.5 1.5 1.3

5.5 5.7 6.2 6.0 6.2 5.6

V vs Ag/AgCl. bElectrochemical EHOMO and ELUMO were estimated from E (eV) = −4.4 − Eonset. cEstimated from the absorption edge. dEvaluated by photoelectron spectroscopy in air. eSee ref 9 and Figure S3 for the cyclic voltammogram. a

Scheme 2. Synthesis of Stannylated NDTI (2) and PNDTI-BTz and -NTz via Stille Coupling

averaged molecular weight, Mw), evaluated by GPC at 140 °C against the polystyrene standards, indicating that the polymerization reaction proceeded properly (Table 1, Figure S1, Supporting Information). Note that the PDI of PNDTI-TV is very large, which can be ascribed to aggregation of the polymers even in hot o-dichlorobenzene solution (Figure S1a) as similar to PNDTI-BT previously reported.9 For the synthesis of PNDTI-BTz and -NTz, the Suzuki− Miyaura polymerization between 1 and bis(pinacoleborate)s of BTz (BTz-BPin) and NTz (NTz-BPin)16 were first examined, since the pinacoleborates were readily available. However, the polymerizations did not proceed smoothly affording only low molecular weight products (Mn ∼7 kDa) for both cases (Scheme 1, see Supporting Information), likely owing to a concomitant deborylation reaction. To circumvent the poor reactivity of BTz-BPin and NTz-BPin, we converted 1 into 2,7bis(trimethylstannyl)-NDTI (2) by simply heating 1 with

lying EHOMO of the NDTI unit itself. For developing ambipolar polymers with high-lying EHOMOs and low-lying ELUMOs, the electron-donating comonomer units should be chosen, whereas for unipolar n-type polymers with both low-lying EHOMOs and ELUMOs, the electron-deficient or neutral comonomer units seem to be promising. From these calculations on the model triads, the targeted polymers seem to be attractive as ambipolar or n-type polymers. Synthesis. Among the NDTI-based polymers (Figure 1), PNDTI -TV, -NDT, and -V were synthesized via the Stille polymerization between 2,7-dibromo-NDTI (1) and the bis(trialkylstannyl) derivative of thienylene-vinylene (TVSn),10 naphthodithiophene (NDT-Sn),11 and vinylene (VSn), which are easily available, in the presence of tris(dibenzylideneacetone)dipalladium (0) as the catalyst (Scheme 1). These polymers have molecular weights lager than 15 kDa (number-averaged molecular weight, Mn) or 20 kDa (weightC

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 3. Cyclic voltammograms (a), photoelectron spectroscopy in air (b), and absorption spectra (c) of PNDTI-Xs.

Table 2. FET Characteristicsa of PNDTI-Xs-Based OFETs p-channel operation

n-channel operation

PNDTI-X

μholeb/cm2 V−1 s−1

Ion/Ioff

Vth/V

μelectronb/cm2 V−1 s−1

TV NDT V BTz NTz BTc

4.3 × 10−4 0.082 − − − 0.10

5.0 × 102 6.0 × 104 − − − 5.0 × 103

1.1 −24.6 − − − −34

3.1 × 10−3 0.029 9.5 × 10−4 0.1 0.21 0.27

Ion/Ioff

Vth/V

× × × × × ×

23.1 29.5 26.2 19.6 15.0 16

6.0 5.0 5.0 1.0 1.3 5.0

104 103 104 104 104 102

Devices with bottom gate, top-contact configuration with L = 40 μm and W = 1500 μm fabricated on Si/SiO2 substrates modified with octadecyltrichlorosilane (ODTS) self-assembled monolayer (SAM) were used. bExtracted from the saturation regime (Vg = Vd = |60| V) under ambient conditions. cSee ref 9. a

window, indicative of rather n-type nature of these polymers. The electrochemical ELUMOs of PNDTI -V, -BTz, and -NTz are expected to be −4.2, −4.1, and −4.2 eV, respectively. In accordance with the theoretical calculations on the triad model compounds of polymers (Figure 2), the electrochemical ELUMOs are less sensitive to the comonomer units and in the range of −4.0 to −4.2 eV, which meet the criteria for the air-stable operation of n-type OFETs. EHOMOs estimated by the PESA measurements are also listed in Table 1. For PNDTI -TV and -NDT, the EHOMOs are in good agreement with the electrochemical EHOMOs. On the other hand, the EHOMOs of PNDTI -V, -BTz, and -NTz are −6.2, −6.0, and −6.2 eV, respectively, being qualitatively consistent with the trend demonstrated by the theoretical calculations. These EHOMOs and ELUMOs experimentally determined in fact indicate that PNDTI -TV and -NDT should behave as ambipolar semiconductors similar to PNDTI-BT, whereas PNDTI -V, -BTz, and -NTz as unipolar n-type semiconductors in actual OFET devices. Absorption spectra of the polymer thin films show intensive absorption bands in the visible to nearinfrared region depending on the comonomer units. The optical energy gap (Eg) estimated from the absorption onsets are 1.3−1.5 eV (Table 1). Polymer FETs and Thin-Film Structure. Field-effect transistors with the bottom-gate, top-contact (BGTC)

hexamethylditin in the presence of palladium catalyst (87% isolated yield, Scheme 2).17 The Stille polymerization using 2 and dibromo-BTz or -NTz under the standard reaction conditions, however, ended up in similar results, affording only low molecular-weight products (Scheme 2, see Supporting Information). We then tested the copper iodide-assisted Stille coupling conditions, which have been known to accelerate the trans-metalation reaction in the catalytic cycle. In fact, the reactions were effective to give PNDTI-BTz and -NTz with relatively high molecular weights (Mn ∼ 15 kDa) (Scheme 2).18 Physicochemical Properties and Electronic Structures. In order to evaluate electronic structures of the polymers, cyclic voltammetry (CV), photoelectron yield spectroscopy in air (PESA), and absorption spectra using spin-coated thin films were measured (Figure 3). From the oxidation and/or reduction onsets in the voltammograms, electrochemical EHOMO and ELUMO, respectively, were estimated (Table 2). Among the NDTI-based polymers, those with thiophene-based comonomer units, e.g., PNDTI-TV and -NDT show both the oxidation and reduction waves, indicative of the amphoteric nature of these polymers. Their electrochemical EHOMOs are estimated to be −5.4 and −5.7 eV, respectively, whereas the ELUMOs are both −4.0 eV. On the other hand, the other polymers with the V, BTz, and NTz comonomer units only showed reduction waves in the ordinary electrochemical D

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. Transfer and output characteristics of PNDTI -TV (a, b, c, d), -NDT (e, f, g, h), -V (i, j), -BTz (k, l), and -NTz (m, n)-based devices.

assignable to the lamella-like structure were observed on the qz axis up to the third order (qz = 0.16, 0.30, 0.48 Å−1; d = 39.3 Å), only very weak peak on the qxy axis (qxy = 1.74 Å−1; d = 3.61 Å) was detected, strongly implying that, although the polymer forms a lamellar-like edge-on orientation, ordering nature in the in-plane direction, i.e., the transverse direction on the substrate surface was rather poor, which must be detrimental for OFETs requiring the in-plane carrier transport. The poor ordering of PNDTI-TV in the thin film state could be ascribed to the largely curvy backbone structure, compared to that of PNDTIBT (Figure 5f), with the additional vinylene unit between the two thiophenes, affording structural disorders in the polymer backbone. In contrast, PNDTI-NDT with a rigid ring-fused system afforded decent ambipolar OFETs with μh (0.082 cm2 V−1 s−1), which is almost comparable to that for PNDTI-BT. This can be understood by taking the crystalline order in the thin film state into account (Figure 5b); similar to the 2D GIXD of the PNDTI-BT thin-film (Figure 5f), the PNDTINDT thin-film affords a series of lamella peaks on the qz axis up

configuration were fabricated by spin-coating the polymer solutions (3 g L−1 in chlorobenzene) onto octadecyltrichlorosilane (ODTS)-treated SiO2 substrates. The polymer thinfilms were annealed at 320 °C, and then the source and drain electrodes (Au: 80 nm) were deposited on top of the polymer thin films. The devices were evaluated under ambient conditions, and the mobilietes were extracted from the saturation regime. As expected from the electronic structures of polymers, the OFETs based on PNDTI -TV and -NDT showed ambipolar behaviors (Figure 4). However, the extracted hole and electron mobilities (μh and μe, respectively) for the present polymer-based OFETs are lower than those of PNDTIBT-based ones (μh, 0.10 cm2 V−1 s−1; μe, 0.27 cm2 V−1 s−1); in particular significantly lower mobilities by more than 2 orders of magnitude as well as nonidealistic output characteristics with poor saturation behaviors were observed for PNDTI-TV. This can be explained by the less-ordered polymer structure in the thin film as demonstrated by two-dimensional grazing incidence X-ray diffraction (2D GIXD) (Figure 5a).19 Although the peaks E

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 5. 2D GIXDs of spin-coated thin films of (a) PNDTI-TV, (b) -NDT, (c) -V, (d) -BTz, (e) -NTz, and (f) -BT.

to the forth order (qz = 0.25, 0.50, 0.75, 1.01 Å−1; d = 24.9 Å), together with a clear peak on qxy axis, reasonably assignable to the π-stacking ordering (qxy = 1.79 Å−1; d = 3.51 Å), indicative of the formation of a well-ordered edge-on structure with the lamella motif. On the contrary, the μe of the PNDTI-NDTbased OFET (0.029 cm2 V−1 s−1) is much lower by 1 order of magnitude than that for PNDTI-BT. This can be rationalized by the different trends in the HOMO and LUMO of the polymer; as discussed in the previous section for the theoretical calculations, the HOMO geometry of the NDTI-based polymers can be expressed as “delocalized” over the polymer backbone direction, whereas the LUMO geometry as “localized” on the NDTI core part (Figure 2). With a structurally large comonomer unit such as NDT, the LUMO on the polymer backbone can be “diluted”, making rather poor intermolecular LUMO−LUMO overlap in the thin film state, which may in turn result in the reduced electron mobility. It should be mentioned that the surface morphology of the thin films elucidated by atomic force microscopy (AFM) can not give any vital information on the difference in the mobility of the OFETs based on PNDTI -BT, -TV, and -NDT, since their AFM images were quite similar (Figure S2). As expected from the low-lying EHOMOs, other polymers, PNDTI -V, -BTz, and -NTz afforded unipolar n-type OFETs,20 except for PNDTI-BTz, which afforded ambipolar OFETs, though the p-type character was very weak and only observed under the largely negative bias conditions with Vth of −53 V (Figure S4). For the electron transport, it is expected that the small comonomer unit seems to be advantageous, because, as we discussed above, the LUMO tends to localize on the NDTI core and is hard to steep out to the adjacent units. In other words, the polymers with smaller comonomer units likely afford effective intermolecular overlap of LUMOs. Contrary to this naive picture, however, PNDTI-V possessing the smallest comonomer unit among the present polymers afforded n-type OFETs with μe of 9.5 × 10−4 cm2 V−1 s−1, which is the lowest among the present polymers. This is again explained by the thin film structure; the 2D GIXD profile of PNDTI-V clearly represents its amorphous nature with the characteristic halo at around q = 1.5 Å−1 (Figure 5c). Poor structural ordering of PNDTI-V can come from nonplanar backbone structure owing

to close proximity of the large 2-decyltetradecyl groups on the NDTI core. In fact, simple model consideration suggests that the alkyl groups are so large that they can not allow the polymer’s conjugated backbone to be planar owing to the significant steric congestion between the neighboring alkyl groups (Figure S5). On the other hand, PNDT-BTz and -NTz with moderately large comonomer units possessing the electron-deficient nature afforded OFETs with μes higher than 0.1 cm2 V−1 s−1. In particular, the highest μe and the unipolar n-type behaviors were observed for the PNDT-NTzbased OFETs. This is also well correlated to the thin film structure elucidated by 2D GIXD (Figure 5e), in which the typical lamella structure with the edge-on orientation (qz = 0.23, 0.47, 0.69 Å−1; d = 26.9 Å) and the in-plane π-stacking structure (qxy = 1.78 Å−1; d = 3.53 Å) are observed. Rather puzzling correlation between the μes and thin film structure was obtained for PNDT-BTz (Figure 5d). Although the μe of the PNDT-BTz-based OFET is as high as 0.1 cm2 V−1 s−1, its crystalline order in the thin film state seems to be quite poor, because only one spot on the qz axis, nondetectable spots on the qxy axis, and a weak amorphous halo were observed. At the moment we do not have any clear experimental data that explain this puzzling correlation. However, one possible explanation is that the BTz core affords the polymer backbone with an almost straightly conjugated structure, which can facilitate effective packing in the thin film state. Furthermore, the BTz core is non centrosymmetric, but the C2v axissymmetric, and in combination with the C2h symmetric NDTI core in the straight backbone structure, the BTz parts can arbitrarily flip without interfering the effective packing, implying that even the polymer has the effectively packed structure, the ordering periodicity is rather poor, resulting in the amorphouslike nature in the 2D GIXD profile.

■

CONCLUSION To understand the potential of the NDTI unit as the building unit for semiconducting polymers, we have synthesized several NDTI-based polymers with various comonomer units via the Stille polymerization using 2,7-dibromo-NDTI (1) or the copper iodide-assisted Stille coupling using 2,7-bis(trimethylstannyl)-NDTI (2), the latter of which was F

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

irradiated at a fixed incident angle on the order of 0.12° through a HUBER diffractometer and the GIXD patterns were recorded on a 2D image detector (PILATUS 300 K) with an X-ray energy of 12.39 keV (λ = 1 Å). Samples for the GIXD measurements were prepared by spin-casting the polymer on the octadecyltrichlorosilane- (ODTS-) modified Si/SiO2 substrates. Synthesis. N,N′-Bis(2-decyltetradecyl)-2,7-bis(trimethylstanyl)naphtho[2,3-b:6,7-b′]dithiophene-4,5,9,10-diimide (2). A solution of N,N′-bis(2-decyltetradecyl)-2,7-dibromo-NDTI 1 (242 mg, 0.2 mmol) and hexamethylditin (328 mg, 1.0 mmol) in dry toluene (25 mL) was deoxygenated for 3 min. Tetrakis(triphenylphosphine)palladium(0) (24 mg, 0.02 mmol) was added, and the reaction mixture was heated to 95 °C for 12 h. After cooling, the reaction mixture was poured into methanol to precipitate a solid, which was collected by filtration. Recentralization of the solid from dichloromethane/ethanol afforded 2 (240 mg, 87%) as a dark green solid; mp 78−79 °C. 1H NMR (400 MHz): δ 0.57 (s, JSn−H = 29.2 Hz, 18H), 0.84 (t, J = 6.8 Hz, 6H), 0.86 (t, J = 6.8 Hz, 6H), 1.18−1.55 (m, 80H), 2.17 (quin, J = 8.0 Hz, 2H), 4.31 (d, J = 8.0 Hz, 4H), 9.14 (s, 2H). 13C NMR (100 MHz): δ −7.9 (JSn−C = 185.1 Hz), 14.2, 22.8, 26.6, 29.4, 29.5, 29.8, 30.2, 31.7, 32.0, 26.6, 45.3, 116.2, 117.7, 131.6, 143.3, 148.7, 159.5, 163.46, 163.49. HRMS (ESI+): calcd for C72H118N2O4S2Sn2, 1379.6700 [MH+]; found, 1379.6695. General Procedures of Stille Polymerization. To a 0.5−2.0 mL microwave pressurized vial equipped with a stirring bar were added N,N′-bis(2-decyltetradecyl)-2,7-dibromo-NDTI 1 (0.05 mmol), stannylated copolymer unit (0.05 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.8 mg, 0.9 μmol), tri(o-tolyl)phosphine (1.1 mg, 3.5 μmol), and toluene (1.8 mL). Then, the tube was sealed, refilled with argon, put into microwave reactor, and then heated at 180 °C for 60 min. After cooling to room temperature, the reaction mixture was poured into methanol (25 mL) containing hydrochloric acid (1 M, 0.5 mL) and stirred for 6 h. The resulting precipitate was collected by filtration and was subjected to sequential Soxhlet extraction with methanol, hexane, and dichloromethane to remove low molecular weight fractions. The residue was extracted with chloroform, and then the concentrated fraction was precipitated in 20 mL of methanol to yield PNDTI-Xs; PNDTI-TV: 92% yield. Anal. Calcd for (C76H108N2O4S4)n: C, 73.50; H, 8.77; N, 2.26. Found C, 73.22; H, 8.64; N, 2.13. PNDTI-NDT: 85% yield. Anal. Calcd for (C80H108N2O4S4)n: C, 74.37; H, 8.58; N, 2.17. Found C, 74.05; H, 8.58; N, 2.06. PNDTI-V: 45% yield. Anal. Calcd for (C68H104N2O4S4)n: C, 75.79; H, 9.73; N, 2.60. Found C, 75.54; H, 10.23; N, 2.48. Copper Iodide Assisted Stille Polymerization for PNDTI-BTz and -NTz. To a 0.5−2.0 mL microwave-pressurized vial equipped with a stirring bar, N,N-bis(2-decyltetradecyl)-2,7-bis(trimethylstannyl)NDTI 2 (0.05 mmol), brominated copolymer unit (0.05 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.8 mg, 0.9 μmol), copper iodide (0.57 mg, 3.0 μmol), triphenylarsine (1.1 mg, 3.5 μmol), and toluene (1.8 mL) were added. Then, the tube was sealed, refilled with argon, put into a microwave reactor, and then heated at 180 °C for 60 min. After cooling to room temperature, the reaction mixture was poured into methanol (50 mL) containing hydrochloric acid (1 M, 1 mL) and stirred for 6 h. The resulting precipitate was collected by filtration and was subjected to sequential Soxhlet extraction with methanol, hexane, and dichloromethane to remove low molecular weight fractions. The residue was extracted with chloroform, and then the concentrated fraction was precipitated in 50 mL of methanol to yield PNDTI-X: PNDTI-BTz: 66% yield. Anal. Calcd for (C72H104N2O4S3)n: C, 72.93; H, 8.84; N, 4.72. Found C, 72.49; H, 8.79; N, 4.55. PNDTI-NTz: 41% yield. Anal. Calcd for (C76H104N6O4S4)n: C, 70.55; H, 8.10; N, 6.50. Found C, 66.48; H, 7.39; N, 6.60. Device Fabrication and Characterization. OFET devices were fabricated in a “top-contact” configuration on heavily doped n+-Si (100) wafers with 200 nm thick thermally grown SiO2 (Ci = 17.3 nF/ cm2). The SiO2 surface was modified with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (ODTS) by immersing the clean

particularly useful for the synthesis of the NDTI-based polymers with the electron deficient comonomer units, such as BTz and NTz. The electronic structures of polymers were then elucidated by means of physicochemical characterization including cyclic voltammetry. It should be noted that the HOMO energy levels of the NDTI-based polymers were found to be fairly affected by the electronic nature of comonomer units; in sharp contrast, the LUMO energy levels were almost identical, not significantly dependent on the comonomer units. This tendency can be explained by the electronic structures of HOMOs and LUMOs of the NDTI-based compounds, where the LUMO tends to localize on the NDTI part, whereas the HOMO to delocalized over the adjacent comonomer units. As a result, the copolymers with thiophene-based units showed ambipolar characteristics, whereas ones with the electron deficient units behaved as the n-type organic semiconductor in the OFET devices. This characteristic electronic structure of NDTI can afford interesting tunability of the resulting materials in combination with the inherently low-lying LUMO energy level of NDTI. On the other hand, the hole and electron mobilities seem to largely depend on the shape of polymer backbone structures, which dictates the effectiveness of close packing and crystallinity of the polymers in the thin film state. From these results, we must say that in order to design desirable polymers using the NDTI core, several different factors, such as electronic nature, size, and shape of the commonomer units and relative bulkiness of the alkyl groups on the NDTI core should be taken into account. With these knowledge, we are currently working on the development of new NDTI-based materials by combining different π-units at the thiophene α-positions and alkyl groups on the nitrogen atoms of NDTI structure.

■

EXPERIMENTAL SECTION

General Data. All chemicals and solvents are of reagent grade unless otherwise indicated. N,N′-Bis(2-decyltetradecyl)-2,7-dibromoNDTI (1) were synthesized as reported.9 Bis-stannylated and bisborylated compounds were prepared according to the reported procedure, 12b,16,21 except for commercially available 1,2-bis(tributylstannyl)ethene and 2,5-bis[(1,1,2,2-tetramethylpinacolato)boryl]benzo[c][1,2,5]-thiadiazole, which were used as received. Melting points were uncorrected. All reactions were carried out under nitrogen atmosphere. Nuclear magnetic resonance spectra were obtained in deuterated chloroform (CDCl3) or 1,1,2,2-tetrachloroethane-d2 with TMS as the internal reference unless otherwise stated; chemical shifts (δ) are reported in parts per million. IR spectra were recorded using a KBr pellet for solid samples. Elemental analysis was measured by Yanaco MT-6 CHN CORDER at the Materials Characterization Support Unit in RIKEN, Advanced Technology Support Division. UV−vis absorption spectra were measured using a Shimadzu UV-3600 spectrometer. Cyclic voltammograms (CVs) were recorded on an ALS Electrochemical Analyzer Model 612D in benzonitrile containing tetrabutylammonium hexafluorophosphate (Bu4NPF6, 0.1 M) as supporting electrolyte at a scan rate of 100 mV/s. Counter and working electrodes were made of Pt, and the reference electrode was Ag/AgCl. All the potentials were calibrated with the standard ferrocene/ferrocenium redox couple (Fc/Fc+: E1/2 = +0.46 V measured under identical conditions). Ionization potential (IP) was determined from the onset of photoelectron spectra measured by using a photoelectron spectrometer MODEL AC-2 or AC-3 in air (Riken Keiki CO., LTD). Differential scanning calorimetry (DSC) was carried out under nitrogen on a SII DSC 7020 at a scanning rate of 10 °C min−1. AFM images were obtained on a Nanotechnology, Inc. scanning probe microscope Nanocute system. Grazing incidence X-ray diffraction (GIXD) experiments were conducted at the SPring-8 on beamline BL46XU. The sample was G

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules wafer substrate in 0.1 M ODTS in toluene at 60 °C for 20 min. Polymer layers were then spin-coated from warm (∼140 °C) chlorobenzene solution (4 g/L) with a spinning rate of 6000 rpm for 45 s. After annealing the polymer thin film at 320 °C under nitrogen atmosphere, Au drain and source electrodes (thickness 80 nm) were vacuum deposited on top of the polymer thin films through a shadow mask, where the drain-source channel length (L) and width (W) are 40 μm and ca. 1.5 mm, respectively. Current−voltage characteristics of the OFET devices were measured at room temperature in air with a Keithly 4200-SCS semiconductor characterization system. Field-effect mobilities were calculated in the saturation regime (Vd = Vg = |60 V|) of the Id using the following equation:

Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc. 2011, 133, 4625−4631. (c) Xu, Y. X.; Chueh, C. C.; Yip, H. L.; Ding, F. Z.; Li, Y. X.; Li, C. Z.; Li, X.; Chen, W. C.; Jen, A. K. Adv. Mater. 2012, 24, 6356−6361. (d) Lei, T.; Dou, J.-H.; Ma, Z.-J.; Yao, C.-H.; Liu, C.-J.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2012, 134, 20025−20028. (e) Schroeder, B. C.; Ashraf, R. S.; Thomas, S.; White, A. J.; Biniek, L.; Nielsen, C. B.; Zhang, W.; Huang, Z.; Tuladhar, P. S.; Watkins, S. E.; Anthopoulos, T. D.; Durrant, J. R.; McCulloch, I. Chem. Commun. 2012, 48, 7699−7701. (f) Bronstein, H.; Frost, J. M.; Hadipour, A.; Kim, Y.; Nielsen, C. B.; Ashraf, R. S.; Rand, B. P.; Watkins, S.; McCulloch, I. Chem. Mater. 2013, 25, 277−285. (g) Park, J. H.; Jung, E. H.; Jung, J. W.; Jo, W. H. Adv. Mater. 2013, 25, 2583−2588. (h) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.; Yang, Y. Nat. Commun. 2013, 4, 1446. (5) (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498−500. (b) Fujita, H.; Tsuboi, K.; Michinobu, T. Macromol. Chem. Phys. 2011, 212, 1758−1766. (6) (a) Facchetti, A. Chem. Mater. 2011, 23, 733−758. (b) Boudreault, P.-L. T.; Najari, A.; Leclerc, M. Chem. Mater. 2011, 23, 456− 469. (7) (a) Suraru, S.-L.; Würthner, F. Angew. Chem., Int. Ed. 2014, 53, 7428−7448. (b) Liu, Z.; Zhang, G.; Cai, Z.; Chen, X.; Luo, H.; Li, Y.; Wang, J.; Zhang, D. Adv. Mater. 2014, 26, 6965−6977. (8) (a) Guo, X.; Facchetti, A.; Marks, T. J. Chem. Rev. 2014, 114, 8943−9021. (b) He, Y.; Hong, W.; Li, Y. J. Mater. Chem. C 2014, 2, 8651−8661. (9) Fukutomi, Y.; Nakano, M.; Hu, J.-Y.; Osaka, I.; Takimiya, K. J. Am. Chem. Soc. 2013, 135, 11445−11448. (10) Zhou, E.; Nakano, M.; Izawa, S.; Cong, J.; Osaka, I.; Takimiya, K.; Tajima, K. ACS Macro Lett. 2014, 872−875. (11) (a) Fuchigami, H.; Tsumura, A.; Koezuka, H. Appl. Phys. Lett. 1993, 63, 1372−1374. (b) Kim, J.; Lim, B.; Baeg, K.-J.; Noh, Y.-Y.; Khim, D.; Jeong, H.-G.; Yun, J.-M.; Kim, D.-Y. Chem. Mater. 2011, 23, 4663−4665. (12) (a) Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Org. Chem. 2010, 75, 1228−1234. (b) Osaka, I.; Abe, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Am. Chem. Soc. 2010, 132, 5000−5001. (13) (a) Wang, M.; Hu, X.; Liu, P.; Li, W.; Gong, X.; Huang, F.; Cao, Y. J. Am. Chem. Soc. 2011, 133, 9638−9641. (b) Osaka, I.; Shimawaki, M.; Mori, H.; Doi, I.; Miyazaki, E.; Koganezawa, T.; Takimiya, K. J. Am. Chem. Soc. 2012, 134, 3498−3507. (c) Osaka, I.; Kakara, T.; Takemura, N.; Koganezawa, T.; Takimiya, K. J. Am. Chem. Soc. 2013, 135, 8834−8837. (14) MO calculations were carried out with the DFT method at the B3LYP/6-31G(d) level using the Gaussian 09 program package: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian09; Gaussian, Inc.: Wallingford, CT, 2009. (15) Suraru, S.-L.; Zschieschang, U.; Klauk, H.; Wurthner, F. Chem. Commun. 2011, 47, 11504−11506. (16) Kawashima, K.; Miyazaki, E.; Shimawaki, M.; Inoue, Y.; Mori, H.; Takemura, N.; Osaka, I.; Takimiya, K. Polym. Chem. 2013, 4, 5224−5227.

Id = (WCi/2L)μ(Vg − Vth)2 where Ci is the capacitance of the SiO2 dielectric, Id is the source− drain current, and Vg, and Vth are the gate and threshold voltages, respectively. Current on/off ratios (Ion/Ioff) were determined from the minimum current at around Vg = 0−20 V or −20−0 V (Ioff) and the current at Vg = |60 V| (Ion).

■

ASSOCIATED CONTENT

S Supporting Information *

Polymerization method of PNDTI-BTz and -NTz, GPC charts of PNDTI-Xs, AFM images of thin films of PNDTI-Xs on Si/ SiO2 substrate, cyclic voltammogram of PNDTI-BT, transfer and output curves of PNDTI-BTz based transistors, schematic illustration of steric congestion of branched alkyl groups of model compound for PNDTI-V, DSC profiles of PNDTI-Xs, XRD patterns of the thin film of PNDTI-BT, FET characteristics of PNDTI-Xs-based OFETs, and NMR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(K.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by Grants-in-Aid for Scientific Research (Nos. 23245041 and 26810109) from MEXT, Japan. Elemental analysis and HRMS were carried out at the Materials Characterization Support Unit in RIKEN, Advanced Technology Support Division. The DFT calculations were performed by using the RIKEN Integrated Cluster of Clusters (RICC). We also thank Riken Keiki Co. Ltd. for the photoelectron spectroscopy by using a photoelectron spectrometer MODEL AC-3 and Dr. Tomoyuki Koganezawa, JASRI, for his supports in 2D GIXD measurements. M.N. is grateful for the award of a postdoctoral fellowship from the Japan Society for the Promotion of Science (JSPS).

■

REFERENCES

(1) (a) Organic Electronics, Manufacturing and Applications; Klauk, H., Ed.; Wiley-VCH: Weinheim, Germany, 2006. (b) Organic Electronics II: More Materials and Applications; Klauk, H., Ed.; WileyVCH: Weinheim, Germany, 2012. (c) Facchetti, A. Mater. Today 2007, 10, 28−37. (2) Takimiya, K.; Osaka, I.; Nakano, M. Chem. Mater. 2014, 26, 587− 593. (3) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789−794. (4) (a) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649−653. (b) Price, S. C.; H

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (17) Polander, L. E.; Romanov, A. S.; Barlow, S.; Hwang, D. K.; Kippelen, B.; Timofeeva, T. V.; Marder, S. R. Org. Lett. 2012, 14, 918− 921. (18) Farina, V.; Kapadia, S.; Krishnan, B.; Wang, C.; Liebeskind, L. S. J. Org. Chem. 1994, 59, 5905−5911. (19) (a) Rivnay, J.; Mannsfeld, S. C. B.; Miller, C. E.; Salleo, A.; Toney, M. F. Chem. Rev. 2012, 112, 5488−5519. (b) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. Macromolecules 1992, 25, 4364−4372. (20) (a) Babel, A.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13656−13657. (b) Letizia, J. A.; Salata, M. R.; Tribout, C. M.; Facchetti, A.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 9679−9694. (c) Kola, S.; Kim, J. H.; Ireland, R.; Yeh, M.-L.; Smith, K.; Guo, W.; Katz, H. E. ACS Macro Lett. 2013, 2, 664−669. (d) Lei, T.; Dou, J.-H.; Cao, X.-Y.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2013, 135, 12168−12171. (e) Lei, T.; Xia, X.; Wang, J.-Y.; Liu, C.-J.; Pei, J. J. Am. Chem. Soc. 2014, 136, 2135−2141. (f) Vasimalla, S.; Senanayak, S. P.; Sharma, M.; Narayan, K. S.; Iyer, P. K. Chem. Mater. 2014, 26, 4030− 4037. (g) Zhao, Z.; Zhang, F.; Hu, Y.; Wang, Z.; Leng, B.; Gao, X.; Di, C.; Zhu, D. ACS Macro Lett. 2014, 3, 1174−1177. (h) Luzio, A.; Fazzi, D.; Nubling, F.; Matsidik, R.; Straub, A.; Komber, H.; Giussani, E.; Watkins, S. E.; Barbatti, M.; Thiel, W.; Gann, E.; Thomsen, L.; McNeill, C. R.; Caironi, M.; Sommer, M. Chem. Mater. 2014, 26, 6233−6240. (21) (a) Lim, B.; Baeg, K.-J.; Jeong, H.-G.; Jo, J.; Kim, H.; Park, J.-W.; Noh, Y.-Y.; Vak, D.; Park, J.-H.; Park, J.-W.; Kim, D.-Y. Adv. Mater. 2009, 21, 2808−2814. (b) Sonar, P.; Zhuo, J.-M.; Zhao, L.-H.; Lim, K.-M.; Chen, J.; Rondinone, A. J.; Singh, S. P.; Chua, L.-L.; Ho, P. H. H.; Dodabalapur, A. J. Mater. Chem. 2012, 22, 17284−17292.

I

DOI: 10.1021/ma502306f Macromolecules XXXX, XXX, XXX−XXX