Conjugated Donor–Acceptor Polymers Entailing Pechmann Dye

Aug 12, 2016 - We present four conjugated donor–acceptor (D–A) polymers PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe entailing a new electron acceptor P...
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Conjugated Donor−Acceptor Polymers Entailing Pechmann DyeDerived Acceptor with Siloxane-Terminated Side Chains Exhibiting Balanced Ambipolar Semiconducting Behavior Si-Fen Yang,†,‡ Zi-Tong Liu,*,† Zheng-Xu Cai,† He-Wei Luo,† Peng-Lin Qi,† Guan-Xin Zhang,† and De-Qing Zhang*,†,‡ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: We present four conjugated donor−acceptor (D−A) polymers PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe entailing a new electron acceptor Pechmann dye framework, i.e., bipyrrolylidene-2,2′(1H,1′H)-dione (BPD), and thiophene and selenophene as the respective electron donors. PBPDT-Si and PBPDSe-Si contain siloxane-terminated side chains, while PBPDT and PBPDSe bear branching alkyl chains. The respective HOMO energies of PBPDT-Si and PBPDSe-Si are slightly higher than those of PBPDT and PBPDSe, whereas the respective LUMO energies of PBPDT-Si and PBPDSe-Si are slightly lower than those of PBPDT and PBPDSe. The results reveal that (i) thin films of all four polymers show ambipolar semiconducting performances under a nitrogen atmosphere, (ii) hole and electron mobilities are more balanced for PBPDT-Si and PBPDSe-Si, and (iii) the employment of siloxane-terminated side chains is beneficial for improving charge transporting compared to branching alkyl side chains. Thin film hole and electron mobilities of PBPDT-Si can reach 0.74 and 0.87 cm2 V−1 s−1, respectively.

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INTRODUCTION For the development of new optoelectronic materials, alternating conjugated electron donor−acceptor (D−A) polymers have been intensively studied in recent years.1−3 These studies have yielded a lot of polymeric semiconductors with high charge mobilities.4−6 These conjugated polymers with broad absorptions and suitable frontier orbital energies have been also studied as donors or acceptors for photovoltaic cells with high power conversion efficiencies.7,8 Different approaches are under investigations in order to further enhance semiconducting performances of conjugated D−A polymers while retaining their stabilities and solution processabilities, so that they can be practically used in large-area, low-cost, and flexible electronic devices.9 Normally, the molecular design strategy is to devise new electron-rich and -deficient frameworks and vary the combination of donor and acceptor moieties and the linkages as well to optimize the electronic structures of the polymers and tune the interchain π−π interactions.10 In this aspect, various electron donors including thiophene, selenophene, 2,2′-bithiopehene, and thieno[3,2b]thiophene have been incorporated into the polymer backbones.3a,10,11 In comparison, the electron acceptors available for construction of conjugated donor−acceptor polymers are still limited. Bis-imide (e.g., naphthalenediimide (NDI) and © XXXX American Chemical Society

perylenediimide (PDI)) and bis-amide-based electron acceptors (e.g., isoindigo and diketopyrrolopyrrole (DPP)) have been intensively investigated for the past decades.10 Lots of conjugated DPP or isoindigo-based donor−acceptor polymers have been prepared recently, and they show high hole, electron, or ambipolar charge carrier mobilities.4,10a,d,12 We have recently disclosed a new bis-amide acceptor, bipyrrolylidene2,2′(1H,1′H)-dione (BPD)a derivative of Pechmann dye and successfully utilized BPD to synthesize conjugated polymers.13 Being dependent on the electron donors, the resulting polymers show either p-type or ambipolar semiconducting properties. But, their electron mobilities are relatively lower than hole mobilities, and thus the hole and electron transporting is unbalanced. Alternatively, side alkyl chains can not only improve the solubilities of conjugated polymers but also affect their interchain packing and backbone conformation; thus, side alkyl chains can indirectly influence thin film microstructures and charge transporting behaviors.14,15 One prominent example is the large mobility enhancement for isoindigo-based polymers Received: July 5, 2016 Revised: August 3, 2016

A

DOI: 10.1021/acs.macromol.6b01440 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Structures of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe

Scheme 2. Synthetic Routes to PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSea

a

Reagents and conditions: (i) 5-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)pentan-1-amine, CH2Cl2, rt, 12 h; HCl (2.0 M); (ii) lithium diisopropylamide, −78 °C, 30 min; (iii) 1,2-dibromotetrachloroethane, −78 °C, 60 min; (iv) Pd2(dba)3, P(o-tol)3, toluene, 90 °C, 24 h.

semiconducting properties with electron and hole mobilities up to 0.87 and 0.74 cm2 V−1 s−1, respectively. Synthesis and Characterization. Synthetic routes to PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe are shown in Scheme 2. The detailed synthetic procedures were described in the Experimental Section and Supporting Information. Compound 1 was prepared according to the literature.16 Reaction of 1 with 5-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3yl)pentan-1-amine yielded 2 in 12.9% yield. The successive reactions of 2 with LDA and 1,2-dibromotetrachloroethane led to 3 in 57.6% yield. The Stille coupling of 3 with 2,5bis(trimethylstannyl)thiophene or 2,5-bis(trimethylstannyl)selenophene led to PBPDT-Si and PBPDSe-Si, respectively. The resulting polymers were precipitated with methanol. Finally, Soxhlet extraction with various solvents was performed to eliminate the respective monomers and oligomers to afford PBPDT-Si and PBPDSe-Si in 83.3% and 90.5% yields, respectively. In a similar way, PBPDT and PBPDSe were obtained in 89.3% and 80.6% yields, respectively. The chemical structures of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe

after replacement of side alkyl chains with siloxane-terminated side chains.14a Later, Yang, Oh, and co-workers also observed the charge mobility increase for DPP-based polymers by using siloxane-terminated side chains.14b,c Charge mobilities were also reported to be enhanced for isoindigo- and DPP-based polymers by varying the branching alkyl chains via separation of the branching point far away from the conjugated mainchain.15a,b Herein we report our new exploration of BPD-based conjugated D−A polymers, PBPDT-Si and PBPDSe-Si, entailing thiophene and selenophene as the respective electron donors with siloxane-terminated side chains (see Scheme 1). For comparison, the corresponding polymers, PBPDT and PBPDSe, containing the same electron acceptor and donor moieties, but with the branching alkyl chains, were also prepared as shown in Scheme 1. The results reveal that the replacement of the branching alkyl chains with siloxaneterminated side chains is beneficial for improving the semiconducting performances of the BPD-based polymers. PBPDT-Si and PBPDSe-Si exhibit more balanced ambipolar B

DOI: 10.1021/acs.macromol.6b01440 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Photophysical Properties and HOMO/LUMO Energies of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe λmaxa (nm) polymer PBPDT-Si PBPDSe-Si PBPDT PBPDSe

solution 443, 436, 436, 432,

900 842 854 791

exptc (calcd)d

film 443, 448, 444, 451,

890 868 890 857

b Eonset ox

(V)

0.21 0.02 0.26 0.08

b Eonset red

(V)

−1.15 −1.11 −1.18 −1.15

HOMO (eV) −5.01 −4.82 −5.06 −4.88

(−4.65) (−4.64) (−4.68) (−4.70)

LUMO (eV) −3.65 −3.69 −3.62 −3.65

(−2.96) (−2.97) (−2.94) (−2.95)

Eg,cve (eV)

Eg,optf (eV)

Eg,optg (eV)

1.36 1.13 1.44 1.23

1.12 1.12 1.13 1.14

1.10 1.13 1.22 1.24

Absorption maxima in 1,2-dichlorobenzene solution (10 μM) and thin film. bOnset potentials (V vs Fc/Fc+). cEstimated with the following d e f onset equations: EHOMO = −(Eonset oxl + 4.8) eV, ELUMO = −(Eredl + 4.8) eV. Based on DFT calculations. Eg,cv = ELUMO − EHOMO (eV). Based on thin film g onset absorption. Based on the onset absorption data of solutions. a

Figure 1. Absorption spectra of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe in 1,2-dichlorobenzene solution (10 μM) (A) and thin films (B).

Si likely exhibit more planar backbones because steric repulsions among side chains are minimized.1g,14b,c As shown in Figure 1, both solutions and thin films of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe exhibit wide absorptions extending to 1200 nm. The maximum absorption in solution is bathochromically shifted from 791 nm for PBPDSe to 842 nm for PBPDSe-Si. Such an absorption redshift also occurs for PBPDT-Si by comparing with PBPDT. These absorption red-shifts after replacement of branching alkyl chains with siloxane-terminated side chains may be attributed to (i) the conjugated backbones of PBPDSe-Si and PBPDT-Si are more planar and (ii) aggregation of the polymer chains of PBPDSe-Si and PBPDT-Si may occur in solutions because of their low solubilities. Interestingly, the absorption spectral differences for thin films of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe become minor. This may be caused by the fact that red-shifts of PBPDT and PBPDSe after transformation from their solutions to the thin films are expected to be large, while the spectral differences between the respective solutions and thin films are minor for PBPDT-Si and PBPDSe-Si. The optical bandgaps were estimated to be 1.12, 1.12, 1.13, and 1.14 eV for PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe, respectively, based on the respective onset absorptions of their thin films. DFT calculations (B3LYP/6-31G(d,p)) were performed by using two repeating units of each polymer. To be simplified, the 2-octyldodecyl side chains were replaced with 2-methylbutyl groups for PBPDT and PBPDSe, while pentyl groups were used to replace the siloxane-terminated chains for PBPDT-Si and PBPDSe-Si. The calculated energy levels are shown in Table 1. It should be noted that HOMO levels of PBPDT-Si and PBPDSe-Si are higher than those of PBPDT and PBPDSe, while the respective LUMO energies of PBPDT-Si and PBPDSe-Si are lower than those of PBPDT and PBPDSe. The results agree with the experimental energies levels

were verified by proton nuclear magnetic resonance and elemental analysis. PBPDT-Si and PBPDSe-Si are only soluble in 1,2-dichlorobenzene (ODCB) and 1,1,2,2-tetrachloroethane (TCE), while PBPDT and PBPDSe can be dissolved in CHCl3 and TCE. The molecular weights for PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe were determined with high temperature gel permeation chromatography. Mws of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe are 68, 28, 84, and 160 kg mol−1 with polydispersities of 2.4, 2.2, 2.4, and 3.1, respectively. The decomposition temperatures of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe are 317, 358, 367, and 294 °C at 5% weight loss based on the thermal gravimetric analysis data (see Figure S1). HOMO/LUMO Energies. Thin films of PBPDT-Si and PBPDSe-Si were measured with cyclic voltametry. For comparison, cyclic voltammograms of PBPDT and PBPDSe were also measured. As shown in Figure S2, both PBPDT-Si and PBPDSe-Si show one quasi-reversible reduction wave and quasi-reversible oxidation wave. LUMO and HOMO energies of all polymers were estimated based on the respective onset potentials (oxidation and reduction), in reference to Fc/Fc+ (for details, see Supporting Information).17 As shown in Table 1, the respective HOMO levels of PBPDSe-Si and PBPDSe are higher than those of PBPDT-Si and PBPDT. This can be ascribed to the fact that selenophene shows stronger electrondonating ability than thiophene. The respective HOMO energies of PBPDT-Si and PBPDSe-Si are slightly higher than those of PBPDT and PBPDSe, whereas the respective LUMO energies of PBPDT-Si and PBPDSe-Si are slightly lower than those of PBPDT and PBPDSe. This is likely caused by the steric interactions among the neighboring branched octyldodecyl chains which may lead to more twisted backbones for PBPDT and PBPDSe; in comparison, PBPDT-Si and PBPDSeC

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Figure 2. Calculated molecular orbitals of two repeated units of PBPDT-Si (A), PBPDSe-Si (B), PBPDT (C), and PBPDSe (D).

polymers at their optimal annealing temperatures for 2.0 h. Clearly, these four conjugated D−A polymers show ambipolar semiconducting behavior under an inert atmosphere. Table 2 summarizes the respective data of mobilities (μh and μe) before and after thermal annealing at different temperatures. After thermal annealing, hole and electron mobilities were incremented for each polymer. For example, the average μh and μe of the as-prepared FETs of PBPDT-Si were 0.07 and 0.04 cm2 V−1 s−1, and they were enhanced to 0.22 and 0.10 cm2 V−1 s−1 after thermal annealing at 120 °C; further annealing at 200 °C boosted the average μh and μe to 0.62 and 0.69 cm2 V−1 s−1, respectively. But, they started to decrease after increasing annealing temperature to 240 °C. PBPDT-Si and PBPDSe-Si exhibited higher hole and electron mobilities than PBPDT and PBPDSe. For instance, after thermal annealing at 200 °C, μh and μe for PBPDT-Si were enhanced to 0.74 and 0.87 cm2 V−1 s−1, respectively, whereas the maximum μh and μe of PBPDT were 0.22 and 0.16 cm2 V−1 s−1, respectively, after thermal annealing at 160 °C. Thus, the results hint that replacement of branching alkyl chains (in

determined with cyclic voltammetry. The calculated HOMO/ LUMO energies also indicate that bandgaps of PBPDT and PBPDSe are higher than those of PBPDT-Si and PBPDSe-Si. This also agrees with the experimental bandgaps determined with either cyclic voltammetry or absorption spectrometry. These results reveal that (i) the bandgaps of conjugated polymers can be affected by bulky side chains and (ii) the bandgaps can be slightly lowered by moving the side-chain branching position away from the conjugated mainchain. Additionally, DFT calculations reveal that these four polymers’ HOMO and LUMO orbitals are distributed evenly over their conjugated main chains (see Figure 2). Such structural features are favorable for ambipolar charge transport.13b Thin Film FETs. Thin-film bottom-gate/bottom-contact field-effect transistors (BGBC) of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe were fabricated. The experimental details are provided in the Supporting Information. The fabrication of FETs and measurement were taken under inert atmosphere (nitrogen). Figure 3 displays the film characteristics of FETs (transfer and output curves) of these conjugated D

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Figure 3. FET characteristics (transfer and output curves) with thin films of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe (VDSs are −100 and 100 V for the transfer characteristics of p- and n-channels, respectively) after thermal annealing at the optimal temperatures.

PBPDT and PBPDSe) with siloxane-terminated side chains (in PBPDT-Si and PBPDSe-Si) seems beneficial for charge transporting. Such charge mobility enhancement observed for PBPDT-Si and PBPDSe-Si is likely owing to the fact the siloxane-terminated side chain’s branching position is moved away from the conjugated mainchains, and accordingly polymer backbones may become more planar and thus the interactions between conjugated backbones can be enhanced. The ratio of average hole mobility and electron mobility (μhave/μeave) is 0.9 for PBPDT-Si and 1.3 for PBPDSe-Si; thus, hole and electron mobilities of PBPDT-Si and PBPDSe-Si are well balanced. In comparison, hole mobilities of PBPDT and PBPDSe are much higher than their electron mobilities, and the ratio of μhave/μeave

reaches 2.5 for PBPDT and 5.3 for PBPDSe. This can be ascribed to the result that the LUMO energies of PBPDT-Si and PBPDSe-Si are relatively lower than those of PBPDT and PBPDSe, since a low LUMO level is expected to be favorable for electron transporting (see Table 2). For understanding the variation of their charge mobilities by replacing branching alkyl chains with siloxane-terminated side chains, thin films of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe were characterized with AFM and GIXRD. AFM images of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe thin films before and after thermal annealing are shown in Figure 4. The as-prepared thin films of PBPDT and PBPDSe-Si were found to contain unevenly-distributed molecular domains. The E

DOI: 10.1021/acs.macromol.6b01440 Macromolecules XXXX, XXX, XXX−XXX

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Table 2. Thin Film FET Parameters (Electron and Hole Mobilities, On/Off Ratios (Ion/off), and Threshold Voltages (Vth)) for PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe compd PBPDT-Si

PBPDSe-Si

PBPDT

PBPDSe

temp (°C) 25 120 160 200 240 25 120 160 200 25 120 160 200 25 120 160 200

μha (cm2 V−1 s−1)

Vth (V)

Ion/off

0.07/0.12 0.22/0.31 0.53/0.71 0.62/0.74 0.23/0.38 0.06/0.09 0.22/0.34 0.39/0.52 0.16/0.26 0.03/0.05 0.14/0.20 0.15/0.22 0.11/0.15 0.02/0.03 0.15/0.21 0.16/0.22 0.07/0.11

−5 to 22 −10 to 2 −13 to −4 −25 to 7 −19 to −7 8 to 13 −10 to −2 −27 to −2 −36 to −16 12 to 33 −7 to −2 −18 to 4 −17 to −5 5 to 11 5 to 11 −17 to −6 −17 to −5

10 −10 103 103 102 102 102 102−103 103 103 102−103 103−104 103−104 102−103 103 103−104 104 103 2

μea (cm2 V−1 s−1) 3

0.04/0.06 0.10/0.15 0.39/0.57 0.69/0.87 0.37/0.45 0.05/0.09 0.06/0.08 0.30/0.35 0.11/0.25 0.001/0.002 0.002/0.003 0.06/0.16 0.05/0.1 0.002/0.003 0.008/0.01 0.03/0.05 0.02/0.04

Vth (V) 15 12 32 49 56 27 44 27 37 32 41 70 28 35 47 66 70

to to to to to to to to to to to to to to to to to

40 24 54 60 61 52 64 65 48 44 48 82 74 40 50 79 77

Ion/off 10 10 10 102−103 102−103 10 10 102−103 102 10 10 102 10 10 10 102−103 102

a The FET data of four polymers were collected on the basis of more than 10 devices, and the mobilities were presented with “average/maximum” form.

Figure 4. AFM images of thin films of PBPDT-Si (A, E), PBPDSe-Si (B, F), PBPDT (C, G), and PBPDSe (D, H) before and after thermal annealing.

root-mean-square roughnesses (RRMS) were 0.70 and 1.74 nm for thin films of PBPDT and PBPDSe-Si, respectively. In comparison, the as-prepared PBPDT-Si and PBPDSe thin films were uniform. After thermal annealing polymer chains of PBPDT-Si and PBPDSe-Si were aggregated into interconnected thin fibers, and simultaneously their RRMS increased to 1.27 and 1.14 nm, respectively. Large domains were found within thermally annealed thin film of PBPDT, while short nanofibers emerged and further aggregated into relatively large domains for PBPDSe as shown in Figure 4. These morphological changes partially explain (i) both μh and μe of PBPDT-Si and PBPDSe-Si are higher than those of PBPDT and PBPDSe and (ii) the hole and electron mobilities increase after thermal annealing for PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe. The X-ray diffraction patterns (in-plane and out-of-plane) of thin films of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe before and after thermal annealing are shown in Figure 5. Thin films of these polymers exhibit only weak diffractions owing to arrangement of alkyl chains and no diffractions due to interchain π−π stacking.18 Two weak diffraction signals, corresponding to d-spacings of 24.5 Å (2θ = 3.6° (in-plane)) and 22.6 Å (2θ = 3.9° (out-of-plane)), were detected for the as-

Figure 5. GIXRD patterns of PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe.

prepared thin film of PBPDT-Si. Their intensities were slightly enhanced after thermal annealing at 200 °C. Similarly, the asprepared thin film of PBPDSe-Si shows two weak diffraction signals, corresponding to d-spacing of 23.8 Å (2θ = 3.7° (outof-plane)) and 26.0 Å (2θ = 3.4° (in-plane)). Also, their F

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Macromolecules intensities increased slightly after thermal annealing at 160 °C. According to these diffraction data, PBPDT-Si and PBPDSe-Si on the OTS-modified substrate may exhibit both face-on and edge-on intermolecular arrangements. In comparison, one diffraction signal at 2θ = 3.8° (in-plane), which corresponds to d-spacing of 23.2 Å, was detected for the thin film of PBPDT only after thermal annealing at 160 °C. For thin film of PBPDSe, one diffraction signal at 2θ = 4.2° (out-of-plane), which corresponds to d-spacing of 21.0 Å, was observed after thermal annealing at 160 °C. The GIXRD data as shown in Figure 5 indicate that main chains of the two polymers are arranged in face-on mode for PBPDT and edge-on mode for PBPDSe on the substrates after thermal annealing. These results demonstrate that the incorporation of siloxaneterminated side chain can alter the arrangements of these polymer chains on the substrate. This may explain the observation that PBPDT-Si and PBPDSe-Si show higher charge mobilities than PBPDT and PBPDSe.14a−f Apart from thiophene and selenophene, 2,2′-bithiophene and thieno[3,2-b]thiophene were also utilized to generate two BPDbased polymers PBPDBT and PBPDTT (Scheme 3) as

with branching alkyl chains, were also synthesized and characterized. These four polymers exhibit ambipolar semiconducting performance under a nitrogen atmosphere. PBPDT-Si shows higher hole and electron mobilities than PBPDT. The same holds true for PBPDSe-Si and PBPDSe. Furthermore, hole and electron mobilities are more balanced for PBPDT-Si and PBPDSe-Si. Therefore, the replacement of branching alkyl chains with siloxane-terminated side chains is beneficial for charge transporting for BPD-based conjugated D−A polymers. Among the polymers, PBPDT-Si exhibits the highest mobilities up to 0.87 cm2 V−1 s−1 (electron mobility) and 0.74 cm2 V−1 s−1 (hole mobility). It is planned to design new BPD-based polymers by variation of electron donors and optimize thin film morphologies with the end to further boost the charge mobilities.

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EXPERIMENTAL SECTION

Materials. If not specified elsewhere, the starting materials and reagents were commercially available and used directly. 1, 4, and 5(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)pentan-1-amine were prepared by following the reported procedures.13b,14,16 The synthesis and characterization of compounds 2 and 3 are provided in the Supporting Information. General Synthetic Procedures for PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe. To 10 mL of anhydrous toluene in a Schlenk tube, compounds 3 or 4 (1.0 equiv) and bis-tin compounds (selenophene or thiophene) (1.0 equiv) were added under nitrogen. Then, Pd2(dba)3(0) (0.01 equiv) and P(o-tol)3 (0.08 equiv) were introduced to the above solution in one portion. Through a freeze− pump−thaw cycle, the tube was charged with nitrogen for three times. After that the reaction system was stirred at 90 °C for 24 h under an inert atmosphere. Then it was cooled down, leading to a viscous black gel-like solution. The mixture was poured into CH3OH and filtered. Each polymer was purified by Soxhlet extraction with various solvents (CH3OH, acetone, hexane, and chloroform sequentially) to remove the existing oligomers, monomers, and other impurities. Finally, polymer products were collected and dried under vacuum. Synthesis of PBPDT-Si. 2,5-Bis(trimethylstannyl)thiophene (58 mg, 0.14 mmol), compound 3 (150 mg, 0.14 mmol), Pd2(dba)3 (1.3 mg), and P(o-tol)3 (3.4 mg) were used. PBPDT-Si was obtained as dark blue solid (115 mg, 83.3%). 1H NMR (400 MHz, CDCl2CDCl2, δ): 7.44 (m, br, 2H), 7.27 (m, br, 6H), 3.95 (m, br, 4H), 1.76 (m, br, 4H), 1.48−1.37 (m, br, 8H), 0.58 (m, br, 4H), 0.19−0.09 (m, br, 42H). GPC: Mn = 28 kDa, Mw = 68 kDa, PDI = 2.4. Anal. Calcd for C44H70N2O6S3Si6: C, 53.50; H, 7.14; N, 2.84; S, 9.74. Found: C, 53.46; H, 7.19; N, 2.94; S, 9.65. Synthesis of PBPDSe-Si. 2,5-Bis(trimethylstannyl)selenophene (51.6 mg, 0.11 mmol), compound 3 (120 mg, 0.11 mmol), Pd2(dba)3 (1.0 mg), and P(o-tol)3 (2.75 mg) were used. PBPDSe-Si was obtained as dark blue solid (103 mg, 90.5%). 1H NMR (400 MHz, CDCl2CDCl2, δ): 7.43 (m, br, 4H), 7.27−7.24 (m, br, 4H), 3.95 (m, br, 4H), 1.42−1.38 (m, br, 12H), 0.58 (m, br, 4H), 0.18−0.10 (m, br, 42H). GPC: Mn = 13 kDa, Mw = 28 kDa, PDI = 2.2. Anal. Calcd for C44H70N2O6S2SeSi6: C, 51.08; H, 6.82; N, 2.71, S, 6.20. Found: C, 49.60; H, 6.58; N, 2.79; S, 6.22. Synthesis of PBPDT. 2,5-Bis(trimethylstannyl)thiophene (45 mg, 0.11 mmol), compound 4 (110 mg, 0.11 mmol), Pd2(dba)3 (1.0 mg), and P(o-tol)3 (2.68 mg) were used. PBPDT was obtained as dark blue solid (95 mg, 89.3%). 1H NMR (400 MHz, CDCl2CDCl2, δ): 7.43 (m, br, 2H), 7.24 (m, br, 6H), 3.87 (m, br, 4H), 1.86 (m, 2H), 1.50−1.31 (m, 64H), 0.94 (m, 12H). GPC: Mn = 37 kDa, Mw = 84 kDa, PDI = 2.4. Anal. Calcd for C60H90N2O4S3: C, 74. 48; H, 9.38; N, 2.90, S, 9.94. Found: C, 74.32; H, 9.21; N, 2.59; S, 10.68. Synthesis of PBPDSe. 2,5-Bis(trimethylstannyl)selenophene (50 mg, 0.11 mmol), compound 4 (110 mg, 0.11 mmol), Pd2(dba)3 (1.0 mg), and P(o-tol)3 (2.68 mg) were used. PBPDSe was obtained as dark blue solid (90 mg, 80.6%). 1H NMR (500 MHz, CDCl2CDCl2, δ): 7.42 (m, br, 4H), 7.26−7.23 (m, br, 4H), 3.88 (m, br, 4H), 1.89

Scheme 3. Chemical Structures of PBPDTT and PBPDBT with Thieno[3,2-b]thiophene and 2,2′-Bithiophene as the Respective Electron Donors13b

reported by some of us previously.13b In comparison with PBPDT-Si, PBPDT, PBPDBT, and PBPDTT with thiophene, 2,2′-bithiophene, and thieno[3,2-b]thiophene as the respective donors, PBPDSe-Si and PBPDSe entailing selenophene as electron donors exhibit relatively high HOMO levels and low LUMO energies.13b Among the six BPD-based conjugated polymers, PBPDBT with 2,2′-bithiophene as electron donors shows a relatively high LUMO energy.13b These results reveal that HOMO/LUMO levels of these BPD-based polymers can be subtly tuned by varying the electron donors. The HOMO/ LUMO levels of these six-BPD based polymers can explain the fact that PBPDBT with 2,2′-bithiophene exhibits p-type semiconducting property, whereas the other five BPD-based polymers display ambipolar semiconducting performance.13b Thin films of PBPDT, PBPDSe, PBPDBT, and PBPDTT with branching alkyl chains exhibit face-on or edge-on or both face-on and edge-on intermolecular arrangements depending on the respective electron donors. But, for thin films of PBPDT-Si and PBPDSe-Si with siloxane-terminated side chains, face-on and edge-on interchain packing modes coexist. It is interesting to note that the coexistence of face-on and edge-on packing modes leads to relatively high and balanced hole and electron mobilities for these BPD-based polymers.13b

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CONCLUSION BPD-based conjugated donor−acceptor polymers PBPDT-Si and PBPDSe-Si with siloxane-terminated side chains were synthesized, and their semiconducting properties were studied. For comparison, the corresponding conjugated D−A polymers PBPDT and PBPDSe with the same donor and acceptor moieties as for PBPDT-Si and PBPDSe-Si, respectively, but G

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Macromolecules (m, br, 2H), 1.44−1.34 (m, 64H), 0.97−0.64 (m, br, 12H). GPC: Mn = 52 kDa, Mw = 160 kDa, PDI = 3.1. Anal. Calcd for C60H90N2O2S2Se: C, 71. 04; H, 8.94; N, 2.76; S, 6.32. Found: C, 70.62; H, 8.84; N, 2.62; S, 6.68.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01440. Characterization techniques, synthesis of compounds 2 and 3, thermal gravimetric analysis, CV, DFT calculations, fabrication of BGBC FET devices, 1H NMR and 13 C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.-Q.Z.). *E-mail [email protected] (Z.-T.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support of Strategic Priority Research Program of CAS (XDB12010300, XDA09020000) and NSFC (21190032, 21372226). We also gratefully thank the assistance of researchers of 1W1A, Beijing Synchrotron Radiation Facility for measuring GIXRD.

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