Annulated Thienyl-Vinylene-Thienyl Building Blocks for π-Conjugated

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Annulated Thienyl-Vinylene-Thienyl Building Blocks for π‑Conjugated Copolymers: Ring Dimensions and Isomeric Structure Effects on π‑Conjugation Length and Charge Transport Atsuro Takai,†,∥ Zhihua Chen,‡ Xinge Yu,† Nanjia Zhou,† Tobin J. Marks,*,† and Antonio Facchetti*,†,‡ †

Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States S Supporting Information *

ABSTRACT: A series of annulated thienyl-vinylene-thienyl (ATVT) building blocks having varied ring sizes, isomeric structures, and substituents was synthesized and characterized by spectroscopic, electrochemical, quantum chemical, and crystallographic methods. It is found that ATVT ring size and isomeric structure critically affect the planarity, structural rigidity, optical absorption, and redox properties of these new π-units. Various solubilizing substituents can be introduced on the annulated hydrocarbon fragments, preserving the ATVT planarity and redox properties. The corresponding πconjugated copolymers comprising ATVT units and electron-deficient units were also synthesized and characterized. The solubility, redox properties, and carrier transport behavior of these copolymers also depend remarkably on the annulated ring size and the ATVT unit isomeric structure. One of the copolymers composed of an ATVT with five-membered rings (1), (E)4,4′,5,5′-tetrahydro-6,6′-bi(cyclopenta[b]thiophenylidene), and a naphthalenediimide (NDI) unit exhibits a broad UV−vis−NIR absorption with an onset beyond 1100 nm both in solution and in the film state, and thin films exhibit n-type semiconducting properties in field-effect transistors. These results are ascribed to the extended main chain π-conjugation length and the low HOMO−LUMO bandgap. Other π-conjugated copolymers containing unit 1 also exhibit characteristic red-shifted UV−vis−NIR absorption. A diketopyrrolopyrrole-based copolymer with unit 1 serves as an electron donor material in organic photovoltaic devices, exhibiting broad-range external quantum efficiencies from the UV to beyond 1000 nm.

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INTRODUCTION Solution-processable organic semiconductors with high electron/hole mobilities and optimal optical absorption offer applications in diverse optoelectronic technologies such as organic field-effect transistors (FETs) and organic photovoltaic cells (OPVs).1−7 Copolymers composed of electron donor (for hole transport) and electron acceptor (for electron transport) units, linked via π-bridging units, have become the conventional design rule for such materials because of (1) tunable charge transport characteristics depending on the balance between donor/acceptor/bridge polymer components, (2) systematic tunability of the optical bandgap and the electronic structure, and (3) large flexibility in materials design.8−32 Thus, extensive studies have addressed how the chemical structure of each unit affects copolymer properties as well as chemical/air stability.25−32 Oligothiophene units have been used extensively in the construction of π-conjugated polymers as weak electron donors as well as π-bridging units to enhance the electronic coupling between electron-donor/acceptor units, enabling both enhanced backbone planarity and film crystallinity. For example, bithiophene (T2) and thienyl-vinylene-thienyl (TVT) building © 2016 American Chemical Society

blocks, combined with electron-donating units such as thienoacene (TA) or electron-deficient units such as naphthalenediimide (NDI) and diketopyrrolopyrrole (DPP), afford prominent p-channel, n-channel, or ambipolar semiconducting polymers (Scheme 1).8−14,33−47 These copolymers may also exhibit a donor−acceptor charge-transfer band typically located in the vis−NIR region. A prerequisite for technological applications of π-conjugated polymers is to achieve adequate solubility in common organic solvents, which is typically achieved by core alkyl functionalization. Specifically, for T2and TVT-containing polymers, substitution at the thiophene ring 3-positions (Scheme 2a) has been used for this purpose; however, this strategy frequently induces backbone distortion, reducing the π-conjugation length.38,48−50 Such backbone distortion can be decreased when alkoxy groups are employed in place of alkyl ones (Scheme 2b),48,51,52 since backbone planarity is then locked through oxygen···sulfur interactions.53,54 For example, we recently reported that an ethoxyReceived: May 18, 2016 Revised: July 19, 2016 Published: July 19, 2016 5772

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Chemistry of Materials Scheme 1. Structures of T2, TVT, TA, NDI, and DPP Molecules

Figure 1. Structures of ATVT building blocks: 1, 1EH, 2, and 3. Annulated rings are outlined in red.

substituted TVT yields highly conjugated NDI copolymers which exhibit high carrier mobilities under ambient conditions.55,56 Covalently rigidified bithiophene units also afford planar building blocks particularly useful in OPV applications (Scheme 2c).57−61 Despite the success of the aforementioned strategies, developing new weak electron-donor/π-bridging oligothiophene-like units for π-conjugated copolymer semiconductors offers an attractive opportunity to explore and quantify the scope of such phenomena. We therefore envisioned that TVT units rigidified by annulation with two hydrocarbon rings (Scheme 2d) would be a versatile platform to test new interesting/informative building blocks in organic electronics for the following reasons: (1) The planarity of the building blocks can be modulated by the annulating ring dimensions. (2) Solubilizing substituents can be introduced without affecting core electronic and structural characteristics.62−64 (3) TVT annulation enables the creation of structural isomers and investigating how they affect the corresponding polymerization and product polymer optoelectronic properties.29,62−66 Here, we report a series of annulated thienyl-vinylene-thienyl (ATVT) building blocks varied in annulation ring size (5- and 6-membered rings), vinylene linkages (2- and 3-position of the thiophene moieties), and substituents (none and alkyl), as shown in Figure 1. New π-conjugated copolymers comprising ATVT and naphthalenediimide and diketopyrrolopyrrole are reported as well as related small molecules. The structures and the electronic properties of the ATVT compounds and of the

corresponding small molecules and copolymers are investigated by optical absorption spectroscopy, electrochemical measurements, X-ray crystallography, and DFT calculations. It will be seen that the solubility and HOMO−LUMO gap of this series vary dramatically with annulated ring dimensions, regiochemistry, and alkyl functionalization. Specifically, it is found that the optimal five-membered ATVT unit 1 affords polymers with remarkably red-shifted absorption, independent of the comonomer unit. Finally, the charge transport and photovoltaic characteristics of several derivatives are reported and demonstrate their promise for further optimization.

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EXPERIMENTAL SECTION

Materials. Reagents and solvents were obtained from commercial sources (Sigma-Aldrich, Matrix Scientific, or Tokyo Chemical Industry) and used as received unless otherwise noted. Solvents were freshly distilled prior to use.67 Ferrocene and tetrabutylammonium hexafluorophosphate (TBAPF6) for electrochemistry were purified by recrystallization.67 Detailed synthetic procedures and the characterization by 1H NMR, 13C NMR, and elemental analysis are provided in the Supporting Information. General Procedures. NMR spectra were recorded on an Agilent DD MR-400 (400 MHz for 1H NMR), a Varian Inova 500 (500 MHz), or a Bruker Avance III (500 MHz) spectrometer. Chemical shifts are expressed in ppm relative to chloroform (7.26 ppm for 1H NMR, 77.36 ppm for 13C NMR) or 1,1,2,2-tetrachloroethane (5.95 ppm for 1H NMR) at 25 °C. Absorption spectra were recorded on a PerkinElmer LAMBDA 1050 spectrophotometer. IR spectra were acquired on a Bruker Tensor 37 FTIR spectrometer equipped with an

Scheme 2. π-Bridging Unit Design Strategies for Solution-Processable Conjugated Copolymers

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used for electrical characterization. The area of all devices was 6 mm2, and a 7 mm2 aperture was placed on top of the cells during all measurements. EQEs were characterized using an Oriel model QE-PVSI instrument equipped with a NIST-certified Si diode. Monochromatic light was generated from an Oriel 300 W lamp source.

attenuated total reflection (ATR) attachment. Polymer molecular weights were determined on a Varian PL-GPC 220 using 1,2,4trichlorobenzene as the eluent at 150 °C vs polystyrene standards. Single Crystal X-ray Diffraction Analysis. Single crystals of 1, 2, and 3 were obtained by crystallization from chloroform with vapor diffusion of methanol. All measurements were performed on a Bruker APEX2 CCD diffractometer at 100 K with Cu Kα radiation (λ = 1.54178 Å). The structures were solved by direct methods and expanded using Fourier techniques. All nonhydrogen atoms were refined anisotropically. Structure refinements were made by using SHELXL.68 Electrochemical Measurements. Cyclic voltammetry (CV) was performed with an EC epsilon potentiostat with a C-3 cell stand from BASi in deaerated solvents containing 0.10 M TBAPF6 as a supporting electrolyte at 25 °C. A conventional three-electrode cell was used with a platinum working electrode and a platinum wire as a counter electrode. Redox potentials were measured with respect to a reference electrode: Ag/AgNO3 (1.0 × 10−2 M). Experiments were conducted in deaerated acetonitrile with a Pt working electrode coated with a thin polymer film by drop-casting a chloroform solution. The oxidation potential of ferrocene as an external standard is 86 mV (vs Ag/ AgNO3) in acetonitrile (MeCN). Quantum Chemical Calculations. Optimized equilibrium geometries were obtained in QChem 4.069 using DFT (B3LYP/6-31G** level) methods. Field-Effect Transistor Fabrication and Characterization. FETs with a bottom-gate bottom-contact configuration were fabricated on p+-Si wafers with 300 nm SiO2 insulator. The substrates were dipped in a hexane solution of octadecyltrichlorosilane (OTS) for 1 h, followed by washing with hexane and isopropyl alcohol to form an OTS self-assembled monolayer on the SiO2 surface. Gold films (40 nm) as drain and source electrodes were then deposited through a shadow mask. Polymer thin films as the semiconducting layer were fabricated by spin-coating chloroform solutions. The drain-source channel length (L) and width (W) were 50 μm and 1.0 mm, respectively. FET measurements were conducted in vacuum using a probe station with a Keithley 6430 subfemtoamp meter (drain) and a Keithley 2400 (gate) source meter. Field effect mobility (μFET) was calculated in the saturation drain current (Id) regime using the equation: Id = (WCi/2L) μFET (Vg − Vth)2, where Ci is the capacitance of the SiO2 insulator (11.6 nF cm−2) and Vg and Vth are the gate and threshold voltages, respectively. Characterization of Thin Films. Thin films of the present compounds were fabricated using spin-coating techniques. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) used a Veeco Dimension Icon Scanning Probe Microscope in tapping mode and a JEOL JEM-2100F, respectively. X-ray diffraction (XRD) was carried out on a Rigaku ATX-G Thin Film Diffraction Workstation using Cu Kα radiation coupled to a multilayer mirror. Solar Cell Fabrication and Characterization. Prepatterned ITO-coated glass (Thin Film Devices, Inc.) with a sheet resistance of ∼10 Ω/□ was used as the substrate. It was cleaned by sequential sonication in hexane, deionized water, methanol, isopropanol, and acetone. After UV/ozone treatment (Jelight Co. Inc.) for 20 min, a ∼20 nm ZnO electron transport/hole blocking layer was prepared by spin-coating at 8000 rpm a precursor solution prepared from 0.5 M zinc acetate dehydrate in 0.5 M monoethanolamine and 2methoxyethanol under air. After cleaning the electrical contacts, the ZnO substrates were immediately baked in air at 170 °C for 10 min and then transferred into a glovebox. Active layer solutions were prepared from 15 mg mL−1 solutions of DPP polymer in CHCl3. For optimum device performance, active layers were spun-cast at 4000 to 5000 rpm. Thin 7.5 nm layers of MoOx and 120 nm of Ag were then thermally evaporated through a shadow mask at ∼10−6 Torr. For device characterization, J−V characteristics were measured under AM1.5G light (100 mW cm−2) using the Xe arc lamp of a SpectraNova Class A solar simulator. The light intensity was calibrated using an NREL-certified monocrystalline Si diode coupled to a KG3 filter to bring spectral mismatch to unity. A Keithley 2400 source meter was

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RESULTS AND DISCUSSION Synthesis and Characterization of ATVT Monomers. The ATVT series were obtained by McMurry coupling of the corresponding thienyl ketones. Unsubstituted thienyl ketones (4,5-dihydro-6H-cyclopenta[b]thiophen-6-one,62 5,6-dihydro4H-cyclopenta[b]thiophen-4-one,70 and 6,7-dihydrobenzo[b]thiophen-4(5H)-one)71 affording 1, 2, and 3, respectively, were synthesized according to the literature. Compound 1EH was synthesized starting from thiophene-3-carboxaldehyde as shown in Scheme 3. Each step proceeds in good yield. After Scheme 3. Synthetic Routes to 1EH and Stannylated Compound 1EH-tin

the reaction with the Grignard reagent of 2-ethylhexyl bromide (EH−Br), the resulting alcohol was oxidized by CrO3, followed by a Wittig−Horner reaction with phosphonate carbanions and hydrolysis with NaOH to afford the corresponding carboxylic acid in 84% yield. After hydrogenation, the carboxylic acid was converted to the acyl chloride, which was then subjected to an intramolecular Friedel−Crafts acylation to afford the corresponding annulated thienyl ketone as a racemic mixture in 73% yield. Note that various substituents can be introduced on the annulated hydrocarbon positions of the thienyl ketones by using different Grignard reagents. Indeed, Roncali and coworkers used a similar approach to access methyl- and butylsubstituted ATVT.62 Here, we introduce 2-ethylhexyl side chains to increase the solubility of the ATVT units as well as the corresponding polymers. McMurry coupling of the thienyl ketones gave mainly (E)-isomers as indicated by the olefinic 1H NMR coupling constant. The (E)-isomers were isolated by recrystallization from dichloromethane (CH2Cl2) and excess methanol (MeOH). Compound 1EH was obtained as a mixture 5774

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Figure 2. Single crystal X-ray structures of: (a) TVT obtained from The Cambridge Crystallographic Data Centre (CSD entry TDTHEY),73 (b) 1, (c) 2, and (d) 3. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) are (a) C2−C3, 1.395(1); C4−C1, 1.349(5); C2−C5, 1.457(8); C5−C5′, 1.309(7); C3−C2−C5, 123.5(1); S1−C2−C5, 124.5(2); (b) C2−C3, 1.369(3); C4−C1, 1.363(3); C2−C5, 1.439(3); C5−C5′, 1.343(4); C3−C2−C5, 113.4(2); S1−C2−C5, 134.8(2); C2−C5−C6, 105.4(2); (c) C2−C3, 1.370(6); C4−C1, 1.355(6); C3−C5, 1.462(6); C5−C5′, 1.344(6); C2−C3−C5, 110.0(3); C4−C3−C5, 137.6(4); C3−C5−C6, 106.4(3); (d) C2−C3, 1.371(11); C4−C1, 1.328(12); C3−C5, 1.469(11); C5−C5′, 1.350(9); C2−C3−C5, 121.5(6); C4−C3−C5, 127.8(7); C3−C5−C6, 112.4(6).

of diastereomers. Finally, lithiation of ATVTs followed by the addition of Me3SnCl affords the stannylated derivatives in excellent yields (Scheme 3) which were used without further purification for the polymerization (see the Experimental Section in the Supporting Information for details). Figure 2 shows the single crystal X-ray structures of three ATVT building blocks (1−3) as well as that of TVT.72−75 The TVT units annulated with five-carbon atoms (1 and 2) have planar structures (Figure 2b,c), a characteristic not observed in conventional 3,3′-alkyl-substituted bithiophene and TVT derivatives.29,55,56,76,77 In contrast, building block 3 is significantly distorted (Figure 2d), owing to the larger sixmembered rings and the tendency to achieve an armchair conformation.78 The six-membered ring annulation also results in an asymmetric structure of the thiophene rings of 3 (C2− C3, 1.371(11) Å; C4−C1, 1.328(12) Å), while the fivemembered ring annulation gives more symmetric structures of 1 (C2−C3, 1.369(3) Å; C4−C1, 1.363(3) Å) and 2 (C2−C3, 1.370(6) Å; C4−C1, 1.355(6) Å). The torsional angles between the thiophene π-planes are 0° (for 1), 0° (for 2), and 61.3° (for 3), respectively. This result clearly indicates that ATVT planarity depends on the annulated ring size. The vinylene length (−CHCH−) of each ATVT increases in the order: 4.22 Å (1) < 4.27 Å (2) < 4.28 Å (3). The vinylene length of 1 is even shorter than in TVT (4.23 Å),73−75 indicating the effective π-conjugation of 1. In contrast, the distorted structure of 3 should decrease the electronic coupling in the corresponding π-conjugated small molecules and polymers.79 The optical spectra of the ATVT units and TVT in chloroform (CHCl3) are shown in Figure 3. The absorption band at the longest wavelength of 1 (369 nm) is significantly red-shifted compared to that of the other derivatives, which are located at 358 nm for TVT, 309 nm for 2, and 301 nm for 3. This result indicates considerable π-conjugation between the two thiophene units in 1. Note also that the absorption bands of 1 are much sharper than those of T2, TVT, and even noncovalently rigidified TVT units (Figure S1).55,56 This result indicates that the conformation of 1 is very rigid not only in the solid state but also in solution. The 1EH unit also exhibits sharp peaks at the same absorption maximum (370 nm) as unit 1 (Figure S1), meaning that substituents on the annulated

Figure 3. UV−vis absorption spectra of 1 (red line), TVT (green line), 2 (blue line), and 3 (purple line) in CHCl3 at 25 °C.

hydrocarbon positions minimally affect the planarity and rigidity of 1. CV measurements in CH2Cl2 were next employed to determine how TVT building block annulation affects the redox properties (Figure S2). The one-electron oxidation processes of all ATVTs are irreversible, so that the first oxidation potential was determined from the onset of the oxidation wave (E1oxonset). The first oxidation potentials vs Fc/ Fc+ increase in the order: 1 (+0.23 V) ∼ 1EH (+0.27 V) < 2 (+0.39 V) < 3 (+0.55 V). The oxidation potential of 1 is even lower than that of TVT62 because of the core planarization and the electron-donating properties of the five-membered hydrocarbon rings. This trend is consistent with the red-shifted absorption band of 1 compared to that of TVT, 2, and 3 (Figure 3). Note also that the oxidation potential of 1EH is nearly identical to that of 1, despite the electron-donating characteristics of the 2-ethylhexyl substituents. This result indicates that alkyl-substitution of ATVT, in contrast to that of T2 and TVT,64 does not significantly raise the core HOMO energy. Thus, the oxidation potential is primarily determined by the annulated core ring size and regiochemistry. Synthesis and Physicochemical Properties of Naphthalenediimide + ATVTs Copolymers. To further assess the properties of the new ATVT building blocks, several πconjugated small molecules and polymers were synthesized using NDI units as testbeds (Figure 4). The NDI-based small molecules and polymers were obtained by Stille coupling 5775

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Figure 4. Structures of the ATVT copolymers and the corresponding small molecules.

(see the Experimental Section in the Supporting Information for details). Table 1 summarizes relevant physicochemical data for this series. The solubility of P(NDI-1) and that of the highMw P(NDI-2) in CHCl3 is lower than that of the other copolymers, probably because their planar structures result in stronger interpolymer aggregation. Thus, the 1H NMR was measured in 1,1,2,2-tetrachloroethane for P(NDI-1), and a lowMW sample of P(NDI-2) (MW ∼ 10 kDa) had to be synthesized for this investigation by decreasing the reaction time of the Stille polymerization. The low-MW P(NDI-2) is sufficiently soluble in CHCl3 and 1,2,4-trichlorobenzene for full characterization. Figure 5 shows the UV−vis−NIR spectra of the NDI-ATVT copolymers in CHCl3 solution. The absorption transitions located beyond 500 nm are assigned to the charge-transfer (CT) excitations between the electron donor (ATVT) and the acceptor (NDI).38 The CT band of P(NDI-1), with an onset beyond 1100 nm (λmax = 951 nm), is substantially red-shifted compared to the other ATVT as well as to other NDIthienylene polymers.38,80−83 In contrast, the absorption maximum (λmax = 598 nm) of P(NDI-3) occurs at the shortest wavelength as the result of the nonoptimal π-connectivity and the aforementioned distorted structure of 3 (Figure 2d).78 These results indicate that the effective π-conjugation length of the NDI-ATVT copolymers falls in the order: P(NDI-3) < P(NDI-2) < P(NDI-1). The effective π-conjugation trend between the NDI unit and 1 is also supported by comparing the UV−vis−NIR spectra of the small molecules M(NDI-1) and M(NDI-2) (Figure S3a). The CT band of M(NDI-1) is significantly red-shifted (730 nm) and its absorption coefficient is larger (1.1 × 104 M−1 cm−1) than that of M(NDI-2) (631 nm and 5 × 103 M−1 cm−1, respectively), owing to the stronger interaction between the electron donor (1) and acceptor (NDI) units. The optical absorption bands of P(NDI-2) and P(NDI-3) thin films annealed at 150 °C are red-shifted versus those in

between mono- and dibrominated NDIs, respectively, and distannylated ATVTs (Scheme 4). The polymers were Scheme 4. Synthetic Routes to NDI-ATVT Small Molecules and Polymers

obtained in 54−88% yields after Soxhlet purification, and the molecular masses (Mw) and polydispersity indices (PDI) of P(NDI-1), P(NDI-1EH), P(NDI-2), and P(NDI-3) by GPC are Mw = 54.1 kDa (PDI = 2.72), Mw = 21.6 kDa (PDI = 3.25), Mw = 10.7 kDa (PDI = 2.51), and Mw = 23.8 kDa (PDI = 3.20), respectively. The small molecules M(NDI-1) and M(NDI-2) were purified by silica-gel column chromatography and reprecipitation from CH2Cl2 and excess MeOH and isolated in 85% and 75% yields, respectively. All compounds were characterized by 1H NMR spectroscopy and elemental analysis 5776

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Chemistry of Materials Table 1. Physicochemical Properties of ATVT Compounds and the Corresponding NDI Copolymers

a

compound

E1oxonset, Va

1 1EH 2 3 M(NDI-1) M(NDI-2) P(NDI-1) P(NDI-1EH) P(NDI-2) P(NDI-3)

+0.23 +0.27 +0.39 +0.55 +0.48 +0.52 +0.50 +0.54 +0.58 +0.71

E1red, Va

EHOMO/ELUMO, eVb

c λsol max, nm

Mw, kDa

PDI

−1.11 −1.10 −1.03 −1.07 −1.07 −1.07

−5.03 −5.07 −5.19 −5.35 −5.28/−3.69 −5.32/−3.70 −5.30/−3.77 −5.34/−3.73 −5.38/−3.73 −5.51/−3.73

369 370 309 301 727 631 951 842 674 596

0.244 0.468 0.244 0.272 1.222 1.222 54.1 21.6 10.7 23.8

2.72 3.25 2.51 3.20

Measured in MeCN vs Fc/Fc+. bCalculated as EHOMO = −(E1oxonset + 4.8 eV) and ELUMO = −(E1red + 4.8 eV). cMeasured in CHCl3 at 25 °C.

The cyclic voltammograms of the NDI-ATVT copolymers and small molecules are shown in Figures S6 and S7, and data are summarized in Table 1. The two reversible reduction processes at −1.03 and −1.33 V vs Fc/Fc+ of P(NDI-1) are assigned to the reduction of the NDI moieties (NDI/NDI•− and NDI•−/NDI2−).33 The oxidation process at +0.50 V assigned to the oxidation of the ATVT unit was determined from the onset of the oxidation wave (E1oxonset). The oxidation potential of P(NDI-1) is positively shifted versus that of 1, because of the electron-deficient NDI moieties adjacent to the unit 1. A comparison of the HOMO−LUMO levels of the NDI-ATVT small molecules and polymers are visualized in Figure 6. The first reduction potentials (E1red), and thus the Figure 5. UV−vis−NIR absorption spectra of P(NDI-3) (purple line), P(NDI-2) (blue line), P(NDI-TVT) (green line), P(NDI-1EH) (pink line), and P(NDI-1) (red line) in CHCl3 at 25 °C. The spectra are normalized to the absorption maximum of the CT bands beyond 500 nm.

CHCl3, as shown in Figure S3b,c. Such red-shifted absorption for solid films is commonly observed in π-conjugated polymers and reflects a combination of interpolymer π−π stacking and increased polymer backbone planarity.80−82 Interestingly, the NIR absorption band of the P(NDI-1) thin film (λmax = 910 nm) is blue-shifted compared to that in CHCl3 (λmax = 951 nm), as shown in Figure S3d. This result is presumably due to a combination of backbone rigidity, thus minimal conformational planarization on going from solution to the solid state, and inefficient interpolymer π−π interactions, probably because of the stereoirregular 2-octyldodecyl solubilizing side chains.11,29,84 DFT molecular orbital computations (B3LYP/6-31G** level) on NDI-ATVT-NDI segments (ATVT = 1 and 2) further explain the experimental results (Figure S4). There are two local minimum conformations where the hydrogen and sulfur atoms of the thiophene units are oriented toward the carbonyl oxygens of the NDI units. The former case is the global minimum conformation for both M(NDI-1) and M(NDI-2). The torsional angles between 1 (2) and the NDI units in the two conformers are 43° (42°) and 131° (129°), respectively, which are similar to those estimated in related NDI-oligothiophene copolymers.11,80−82,85−88 The HOMOs of M(NDI-1) and M(NDI-2) are primarily localized on the 1 and 2 moieties, while the LUMOs are primarily localized on the NDI units. The HOMO level of M(NDI-1) is computed to lie at −5.14 eV, which is slightly above that of M(NDI-2), −5.22 eV.89

Figure 6. HOMO−LUMO orderings of the NDI-ATVT polymers and the small molecules.

electrochemically derived LUMO energies, are pinned regardless of the TVT building blocks. In contrast, the first oxidation potentials of the polymers increase from +0.50 to +0.71 V in the order: P(NDI-1) ∼ P(NDI-1EH) < P(NDI-2) < P(NDI-3), as were similarly observed in the CV of the ATVT building blocks.90 These redox behaviors are consistent with the DFT calculation results indicating that the HOMO level is dominated by the donor unit (ATVT), while the LUMO level is controlled by the acceptor unit (NDI). The HOMO level of 1 is the highest among the TVT building blocks.55,80 This result is also consistent with the HOMO−LUMO bandgaps determined from the optical absorption onset (Figures 5 and S1), which were found to be P(NDI-1) (1.06 eV) ∼ P(NDI-1EH) (1.08 eV) < P(NDI-2) (1.36 eV) < P(NDI-3) (1.36 eV).91 Despite the similar conformations of the NDI-1 and NDI-2 sequences, such differences in the optical and the redox properties are logically the result of different 5777

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Chemistry of Materials effective π-conjugation lengths in the 2- vs 3-position of the thiophene.92 ATVT Structural Effects on Semiconductor Film Morphologies and Charge Transport. The NDI-based polymer series comprising all the ATVT building blocks can be used as testbeds, in combination with the small molecular systems, to evaluate how the ATVT structure affects charge transport characteristics. Since the effective π-conjugation length of P(NDI-1) is greater than those of the other NDIATVT polymers, it is expected that this polymer will outperform the others. The charge transport characteristics of the present polymers were evaluated in bottom-gate bottom-contact FET devices.93 Polymer thin films as the semiconducting layer were spincoated on the octadecyltrichlorosilane (OTS) treated SiO2 substrates. The p+-Si substrate and Au films were used as a common gate electrode and source-drain electrodes, respectively (see Experimental Section for details). Figure 7a shows

compounds are also shown in Table 2 and Figure S8. The electron mobilities and Ion/Ioff of P(NDI-1) are higher than Table 2. FET Device (Bottom-Gate Bottom-Contact Structure) Performance of NDI-ATVT Copolymers compound

μe, cm2 V−1 s−1

Ion/Ioff

M(NDI-1) M(NDI-2) P(NDI-1) P(NDI-1EH) P(NDI-2) P(NDI-3)

1.6 × 10−4 inactive 9.1 × 10−3 1 × 10−5 inactive inactive

3.7 × 102 5.5 × 102