Charge Mobility Enhancement for Conjugated DPP-Selenophene

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Charge mobility enhancement for conjugated DPPselenophene polymer by simply replacing one bulky branching alkyl chain with linear one at each DPP unit Zhijie Wang, Zitong Liu, Lu Ning, Mingfei Xiao, Yuanping Yi, Zhengxu Cai, Aditya Sadhanala, Guanxin Zhang, Wei Chen, Henning Sirringhaus, and Deqing Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01007 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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Chemistry of Materials

Charge mobility enhancement for conjugated DPP-selenophene polymer by simply replacing one bulky branching alkyl chain with linear one at each DPP unit Zhijie Wang,†‡ Zitong Liu,*† Lu Ning,†‡ Mingfei Xiao,Δ Yuanping Yi,†‡ Zhengxu Cai,§ Aditya Sadhanala, Δ Guanxin Zhang,† Wei Chen,ǁ Henning Sirringhaus,Δ Deqing Zhang*†‡ † Beijing National Laboratories for Molecular Sciences, CAS Key Laboratories of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China E-mail: [email protected]; [email protected] ‡ University of Chinese Academy of Sciences Beijing 100049, P. R. China Δ Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK § Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China ǁ Institute for Molecular Engineering and Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois, 60439, United States ǁ Institute for Molecular Engineering, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois, 60637, United States ABSTRACT: We demonstrate a simple but efficient approach for improving the semiconducting performances of DPP-based conjugated D-A polymers. This approach involves the replacement of one bulky branching alkyl chain with the linear one at each DPP unit in regular polymer PDPPSe-10 and PDPPSe-12. The UV-vis absorption, Raman spectra, PDS data and theoretical calculations support that the replacement of bulky branching chains with linear ones can weaken the steric hindrance, and accordingly conjugated backbones become more planar and rigid. GIWAXS data show that the incorporation of linear alkyl chains as in PDPPSe-10 and PDPPSe-12 is beneficial for side chain interdigitation and interchain dense packing, leading to improvement of interchain packing order and thin film crystallinity by comparing with PDPPSe, which contains branching alkyl chains. On the basis of field-effect transistor (FET) studies, charge mobilities of PDPPSe-10 and PDPPSe-12 are remarkably enhanced. Hole mobilities of PDPPSe-10 and PDPPSe-12 in air are boosted to 8.1 and 9.4 cm2V-1s-1, which are about 6 and 7 times respectively than that of PDPPSe (1.35 cm2V1 -1 s ). Furthermore, both PDPPSe-10 and PDPPSe-12 behave as ambipolar semiconductors under nitrogen atmosphere with increased hole/electron mobilities up to 6.5/0.48 cm2V-1s-1 and 7.9/0.79 cm2V-1s-1, respectively.

INTRODUCTION Conjugated donor (D)-acceptor (A) polymers have been extensively investigated for solution-processable semiconductors with high charge mobilities which are highly demanding for applications in E-paper, wearable electronics and other devices.1-6 Diketopyrrolopyrrole (DPP) with two flanking thiophenes has been widely utilized as electron acceptor to connect with various electron donors to form the respective conjugated D-A polymers.7-30 Two alkyl groups can be incorporated into the DPP unit, which will endow the DPP-based conjugated D-A polymers with good solubilities in organic solvents.7 In order to enhance the solubilities of these DPP-based polymers, two bulky branching alkyl groups are usually connected to each DPP unit in the polymers.8,9 The presence of bulky branching groups along the polymer backbone can affect

the planarity of conjugated backbone and prevent dense packing of polymer chains due to steric crowding of branching alkyl chains.7-9 Electron donor units with different conjugation lengths such as thiophene, bithiophene, terthiophene and quaterthiophene (Scheme 1a) were incorporated into DPPbased conjugated D-A polymers.24-27 The results reveal that long electron donors not only strengthen the interchain π-π interactions, but also are able to separate the neighbouring branching alkyl chains and thus weaken the steric hindrances, resulting in enhancement of charge mobilities for conjugated polymers.24-27 Alternatively, new branching alkyl chains and siloxaneterminated side chains (Scheme 1b) were devised and connected to DPP, isoindigo and other acceptors.28-33 Because the branching points of these side chains are away from the polymer backbones, the steric hindrance due to alkyl chains can be

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Scheme 1. Chemical structures of DPP-based conjugated polymers: a) containing electron donor units with increasing lengths to weaken the steric hindrances; b) containing branching bulky alkyl chains that are away from polymer backbones; c) containing both branching bulky and linear alkyl chains in a random manner.

reduced and as a result the polymer can be more orderly packed with short π-π stacking distance. Such side-chain engineering has significantly boosted charge mobilities for conjugated D-A polymers with DPP17,28 and isoindigo31,32 as electron acceptors. However, the respective alkyl halogens, precursors to these modified side chains, are not commercially available, and their syntheses involve three or more reaction steps and purifications.34-36 This will surely add additional cost for the scalable preparation of these conjugated D-A polymers for future device applications. Some of us have recently discovered that charge mobilities of DPP-based terpolymers can be enhanced by just partial replacement of bulky branching alkyl chains with linear ones which are expected to reduce the steric hindrance and promote interdigitation of linear alkyl chains (Scheme 1c).37 These terpolymers were prepared by co-polymerization of the 5,5'bis(trimethylstannyl)-2,2'-bithiophene and two DPP monomers, one of which entails two branching alkyl chains and the other contains two linear alkyl chains. Although these two DPP monomers can be easily prepared with the commercially available 11-(bromomethyl)tricosane and 1-bromododecane, branching and linear alkyl chains are randomly arranged within the resulting terpolymers and thus batch-to-batch variation of polymer samples cannot be avoided. In this paper, we report a new and simple approach to reduce steric hindrance and thus remarkably enhance charge mobilities for DPP-based conjugated D-A polymers by replacing one bulky branching alkyl chain with linear one at each DPP unit. The DPP monomers flanked with thiophene moieties (5 and 6 in Scheme 2) were easily synthesized with 1-

bromodecane or 1-bromododecane and 11(bromomethyl)tricosane, all of which are commercially available. We report two conjugated DPP-selenophene polymers PDPPSe-10 and PDPPSe-12 (Scheme 2) which contain n-decyl and n-dodecyl as the respective linear alkyl chains linked to each DPP unit. For comparison, PDPPSe, which possesses the same conjugated backbone as for PDPPSe-10 and PDPPSe-12 with two bulky branching chains, was also prepared. The results reveal that i) conjugated backbones of PDPPSe-10 and PDPPSe-12 become more planar in comparison with that of PDPPSe based on the UV-vis absorption, Raman spectra, photothermal deflection (PDS) and theoretical calculations; ii) polymer chains adopt edge-on orientations and are more orderly packed within thin films of PDPPSe-10 and PDPPSe-12 based on the GIWAXS data; and iii) hole mobilities are boosted to 8.1 and 9.4 cm2V-1s-1 in air for PDPPSe-10 and PDPPSe12, respectively. It is also noted that both PDPPSe-10 and PDPPSe-12 exhibit ambipolar semiconducting behavior under N2 atmosphere, and both hole and electron mobilities are remarkably increased to 6.5/0.48 cm2V-1s-1 and 7.9/0.79 cm2V-1s-1, in comparison with those of PDPPSe (1.18/0.19 cm2V-1s-1). It is noted that DPP units with two different side chains have been reported and introduced into small molecules and polymers.38-43 However, these side chains entail functional group such as oligo(ethylene glycol) chains,38-40 siloxane terminated chains,40 perylene bisimide terminated chains,41 and aryl group terminated chains.42-43 Most of these DPP-containing conjugated polymers have been employed for organic photovoltaic cells.41-42


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Figure 1. (a) Distribution of the dihedral angles between the thiophene and selenophene units (S-C-C-Se) and between the thiophene and DPP units (S-C-C-N) in the polymer conjugated backbones obtained by molecular dynamics simulations; (b) illustration of the corresponding dihedral angles and conceived configurations of the polymer backbones according to the simulated results. mation for PDPPSe-12 because the S-C-C-N dihedral angles RESULTS AND DISCUSSION are smaller than 90°. In comparison, the S-C-C-N dihedral angles are larger than 90° for PDPPSe, and the thiophene and Design rationale based on theoretical calculations. In order to DPP units are in “trans” conformation. support our design rationale and reveal the influence of side Synthesis and Characterization. The synthesis of PDPPSe-10 alkyl chains on the structures of the backbones of conjugated and PDPPSe-12 is outlined in Scheme 2. The synthesis of monD-A polymers, we carried out molecular dynamics simulations 44-46 omers 3 started from compound 1 which reacted with 11To be (see Supporting Information for simulation details). (bromomethyl)tricosane and 1-bromodecane sequentially to simplified, five-repeated units of PDPPSe-12 and PDPPSe afford 3 in acceptable yield. Similarly, compound 4 was yielded. were calculated. The twist angles between the thiophene and Compounds 3 and 4 were subjected to bromination with NBS, selenophene units (the S-C-C-Se dihedral angles in Figure 1) leading to 5 and 6 in good yields.43 Stille polycondensation of and between the thiophene and DPP units (the S-C-C-N di2,5-bis(trimethylstannyl)selenophene with monomers 5 and 6, hedral angles) are employed to gauge the planarity of the polrespectively, yielded PDPPSe-10 and PDPPSe-12 in high ymer backbones. The obtained distributions of the twist angles yields, after precipitation and Soxhlet extraction (see Experibetween different conjugated units are shown in Figure 1. For mental section). The reference polymer PDPPSe was prepared PDPPSe-12, the S-C-C-N dihedral angles (between the thioaccording to the reported procedure.39 The chemical structures phene and DPP units) are in the range of 0-47o with 18o at the of these polymers were verified by 1H NMR, solid-state 13C maximum distribution, while the S-C-C-Se dihedral angles NMR, and elemental analysis (see Experimental section). (between the thiophene and selenophene units) are in the o o o PDPPSe-10 and PDPPSe-12 possess similar solubility as range of 127 -180 with 158 at the maximum distribution. In PDPPSe in o-dichlorobenzene, 1,1,2,2-tetrachloroethane and comparison, the S-C-C-N and S-C-C-Se dihedral angles are in other halogenated solvents, which enables them to be solution the range of 108o-180o with 143o at the maximum distribution processible for fabrication of field effect transistors (FETs). and 107o-180o with 153o at the maximum distribution, respecMolecular weights (Mn) of PDPPSe-10, PDPPSe-12 and tively, for PDPPSe. These simulation results indicate that the PDPPSe were measured to be 55.1, 60.7 and 76.6 kDa, with twists between these conjugated units are substantially smaller PDI of 2.52, 2.60 and 2.76, respectively, with gel permeation for PDPPSe-12 with respect to PDPPSe. That is to say, rechromatography. As shown in Figure S4 of thermogravimetric placement of the branched alkyl groups with straight linear analysis (TGA) curves, the temperatures at 5% weight loss alkyl groups will result in better planarity for the backbone of were 393 oC for PDPPSe-10, 397 oC for PDPPSe-12 and 393 PDPPSe-12, which is beneficial for increasing inter-chain π-π o C for PDPPSe. These illustrate that all polymers are thermally interaction for the polymer. In addition, as shown in Figure 1, stable below 300 oC. DSC curves in Figure S5 show that there as the S-C-C-Se dihedral angles are larger than 90° for both were no thermal transitions below 250 oC for PDPPSe-10, PDPPSe-12 and PDPPSe, the thiophene and selenophene PDPPSe-12 as well as PDPPSe. units tend to adopt “trans” conformation for both polymers. The thiophene and DPP units prefer to adopt “cis” confor-



Scheme 2. Chemical structures of PDPPSe-10, PDPPSe-12 and PDPPSe, and synthetic routes to PDPPSe-10 and PDPPSe-12. C12H25 N S

N

O

S

N n-C10H21

C10H21

C10H21

S

O

C12H25

C12H25

C10H21

Se

O

n

PDPPSe-10

N

O

S S

N n-C12H25

Se

O

n

O S

N

Se

C10H21 PDPPSe

PDPPSe-12

C10H21

C10H21 H N S O

C10H21 Br

O

C12H25

O

S

K2CO3, DMF

S

C12H25

C12H25 N O

N H

R-Br S

N H

1

n

C12H25

K2CO3, DMF

2 C10H21

N S

O

S N R -C 3 R = n 10H21 -C 4 R = n 12H25

O

C12H25 NBS CHCl3

Br

N S O

O

+ S

N R

Br

Sn

Se Sn

Pd2(dba)3,P(o-tol)3

PDPPSe-10 93% yield

Toluene, 110 oC

PDPPSe-12 92% yield

7

-C 5 R = n 10H21 -C 6 R = n 12H25

Absorption spectra and HOMO/LUMO levels. Figure 2 shows the solution absorption spectra of PDPPSe-10 and PDPPSe-12 as well as PDPPSe and their thin films, and the absorption spectral data are listed in Table 1. In comparison with that of PDPPSe, the absorption maxima of PDPPSe-10 and PDPPSe12 in solutions are red-shifted by 26 nm and 24 nm, respectively. The 0-0/0-1 absorptions around 884 nm and 808 nm were detected for thin films of PDPPSe-10 and PDPPSe-12. Similarly, absorption spectra of thin films of PDPPSe-10 and PDPPSe-12 are red-shifted by comparing that of PDPPSe. Such absorption spectral red shifts can be induced by i) the fact that the conjugated backbones of PDPPSe-10 and PDPPSe-12 become more planar than PDPPSe after replacing one branching chain with linear one for each DPP unit; and ii) the interchain π-π stacking within thin films of PDPPSe-10 and PDPPSe-12 and their pre-aggregates in solutions.47,48 The pre-aggregation effect in solution can be excluded by either heating the solution or lowering the concentration. As shown in Figure S6, the solution absorption spectra of PDPPSe-10 and PDPPSe-12 as well as PDPPSe were blue-shifted when the respective solutions (1×10-5 mol/L) were heated to 100 oC. Moreover, the absorption spectra of these three polymers at 100 oC were almost identical with the respective ones when their concentrations were decreased to 1×10-6 mol/L. Thus, the pre-aggregates formed in solutions of these polymers can be disassembled upon heating. The absorption maxima for solutions (1×10-5 mol/L) at 100 oC PDPPSe-10 and PDPPSe12 appear at 848 nm and 846 nm, which are red-shifted in comparison with that of PDPPSe (832 nm). Such spectral redshifts imply that the conjugated backbones of PDPPSe-10 and PDPPSe-12 are more planar than PDPPSe. In fact, more planar conjugated backbones facilitate the interchain π-π interactions and thus the aggregation of polymer chains.49 This is well consistent with the observation that the red-shifts of absorption maxima for PDPPSe-10 (22 nm) and PDPPSe-12 (22 nm) are larger than that of PDPPSe (12 nm) when the respective solutions were cooled from 100 oC to room temperature. HOMO/LUMO levels of PDPPSe-10 and PDPPSe-12 as well as PDPPSe were estimated with the respective onset oxidation and reduction potentials, which were determined with

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their cyclic voltammograms in the form of thin films (Figure S7). HOMO levels of PDPPSe-10 (-5.20 eV) and PDPPSe-12 (-5.21 eV) are slightly higher than that of PDPPSe (-5.28 eV). The LUMO level of PDPPSe-10 (-3.64 eV) is the same as PDPPSe (-3.64 eV), while the LUMO level of PDPPSe-12 (3.66 eV) becomes slightly lower. The bandgaps of PDPPSe-10 (1.56 eV) and PDPPSe-12 (1.55 eV) based on their HOMO/LUMO levels are lower than that of PDPPSe (1.64 eV). This is consistent with the fact that conjugated backbones of PDPPSe-10 and PDPPSe-12 are more planar than that of PDPPSe on the basis of their absorption spectral studies and theoretical calculations as discussed above. Raman and Photothermal Deflection Spectra. In order to provide further support for the fact that the conjugated backbones of PDPPSe-10 and PDPPSe-12 are more planar than that of PDPPSe, their Raman spectra were carefully examined. Raman spectral data support the conclusion that the backbone planarity is improved for PDPPSe-10 and PDPPSe-12. Figure S8 shows the Raman spectra of as-prepared and thermallyannealed thin films of PDPPSe-10, PDPPSe-12 and PDPPSe upon excitation at 532 nm. These thin films were prepared in the same way as for FET (field-effect transistor) studies. The Raman shifts around 1366 and 1420 cm-1 correspond to the intraunit and interunit C-C, C-N stretches involving DPP unit, while the Raman shift around 1446 cm-1 is attributed to the intraunit backbone C=C symmetric stretch according to previous report.50 As shown in Figure S8, the intensities of Raman shifts at 1446 cm-1 are higher for thin films of PDPPSe-10 and PDPPSe-12 than that for PDPPSe before and after thermal annealing, indicating that π-electrons are more delocalized across the thiophene and selenophene units within the conjugated backbones of PDPPSe-10 and PDPPSe-12.51

Figure 2. Normalized absorption spectra of PDPPSe-10, PDPPSe-12 and PDPPSe in 1,2-dichlorobenzene solution (1.0 × 10-5 M) and their thin films on a quartz plate.


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Table 1. Molecular weights, absorption maxima, redox potentials, HOMO/LUMO energies and bandgaps of PDPPSe-10, PDPPSe-12 and PDPPSe.

Polymer

Mn(kDa)/PDI

λmaxa (nm) (εmax, M-1 cm-1) solution film

Eredonset (V)b

ELUMO (eV) c

Eoxonset (V)b

EHOMO (eV) c

Eg, cv (eV) d

Eg, opt (eV) e

PDPPSe-10

55.1/2.52

870 (68000)

808 884

-1.16

-3.64

0.40

-5.20

1.56

1.27

PDPPSe-12

60.7/2.60

868 (66000)

808 884

-1.14

-3.66

0.41

-5.21

1.55

1.26

PDPPSe

76.6/2.76

844 (61000)

800 868

-1.16

-3.64

0.48

-5.28

1.64

1.30

a Absorption maxima in 1,2-dichlorobenzene solution (1.0 × 10-5 M) and thin film. b Onset potentials (V vs Fc/Fc+) for reduction (Eredonset) and oxidation (Eoxonset). c HOMO and LUMO energies were estimated with the following equations: EHOMO = - (Eoxonset + 4.8) eV, ELUMO = - (Eredonset+ 4.8) eV. d Eg,cv = ELUMO - EHOMO (eV). e Based on thin film onset absorption.

Figure 3. GIWAXS patterns of PDPPSe (a), PDPPSe-10 (b) and PDPPSe-12 (c) deposited on OTS-modified SiO2/Si substrates after thermal annealing at 180 oC. Schematic illustration for stacking orientation of PDPPSe (d, face-on) and PDPPSe-10, PDPPSe12 (e, edge-on). In addition, photothermal deflection spectra (PDS) of the PDPPSe-12. Thin films of PDPPSe-10 and PDPPSe-12 as well spin-coated thin films of PDPPSe-10, PDPPSe-12 and as PDPPSe were prepared with conventional spin-coating PDPPSe were measured52,53 and shown in Figure S9. On the technique, and they were subjected to GIWAXS (grazing incibasis of the PDS data, the respective Urbach energies were dence wide angle X-ray scattering) analysis.54 Figure S10 and Figure 3 show 2D GIWAXS patterns for the as-prepared thin estimated to be 33, 33.5 and 36.5 meV for PDPPSe-10, films of PDPPSe, PDPPSe-10 and PDPPSe-12 and those after PDPPSe-12 and PDPPSe, respectively, according to the literathermal annealing at 180 oC. The respective scattering peaks ture reported method.52,53 The Urbach energies of PDPPSe-10 and PDPPSe-12 are slightly smaller than that of PDPPSe. This for the lamellar stacking of alkyl chains and interchain π-π is a significant difference, as the error in extracting the Urbach stacking were collected in Table S1. energy from our measurements is on the order of 5%. Thus, In order to investigate the ratios of the edge-on and face-on these PDS data demonstrate that PDPPSe-10 and PDPPSe-12 populations in the thin films before and after thermal annealshow comparatively lower degree of energetic disorder than ing at 180 oC, pole figures for the (100) scattering signals were PDPPSe. It is known that more planar and rigid backbones of constructed and shown in Figure S11, and the area ratios of the conjugated polymers are expected to possess low Urbach enertwo regions (χ = 0o-45o and 45o-90o) were calculated according gies according to previous report. 52 Thus, the comparison of to previous reports.55,56 The edge-on/face-on ratios were calcuUrbach energies among PDPPSe, PDPPSe-10 and PDPPSe-12 lated to be 63.4/36.6, 96.5/3.5 and 96.5/3.5 for the ashints that torsion angles among the conjugated moieties are prepared thin films of PDPPSe, PDPPSe-10 and PDPPSe-12, reduced for PDPPSe-10 and PDPPSe-12 after replacing one respectively. The respective edge-on/face-on ratios were branching alkyl chain with the linear one at each DPP unit. changed to 72.3/27.3, 97.7/2.3 and 97.8/2.2 after thermal Interchain packing and thin film morphology. In the following, annealing at 180 oC. These results demonstrate that polymer we show that the significant effect of the replacement of one chains of PDPPSe-10 and PDPPSe-12 adopt mainly edge-on bulky branching chain with the linear one at each DPP unit on packing mode on the substrate, whereas polymer chains of the interchain packing for thin films of PDPPSe-10 and PDPPSe take both edge-on and face-on orientations. The edge-



on contents were slightly enhanced after thermal annealing at 180 oC for all three polymers. As expected, more scattering signals were observed and signal intensities were enhanced for PDPPSe-10 and PDPPSe12 as well as PDPPSe after thermal annealing at 180 oC (see Figure 3). The thermally annealed thin films of PDPPSe-10 and PDPPSe-12 show more scattering signals with higher intensities in comparison with those of PDPPSe. Therefore, it can be concluded that thin films of PDPPSe-10 and PDPPSe12 show better crystallinities than that of PDPPSe. As shown in Figure 3 and Table S1, the thermally annealed thin film of PDPPSe-12 shows four orders of the out-of-plane scattering signals that result from the lamellar packing of side chains at qz = 0.29, 0.57, 0.84 and 1.13 Å-1, corresponding to a d-spacing of 21.9 Å. Similarly, the d-spacing due to the lamellar packing of alkyl chains was determined to be 21.2 Å based on the out-ofplane scattering signals at 0.30, 0.58, 0.87, 1.16 Å-1 for the thermally annealed thin film of PDPPSe-10. In comparison, thin film of PDPPSe after thermal annealing shows only two orders of scattering signals in the out-of-plane direction with a d-spacing of 23.5 Å, and moreover their intensities were much weaker than those of PDPPSe-10 and PDPPSe-12. Clearly, the d-spacing owing to the lamellar packing of side alkyl chains is shortened for PDPPSe-10 and PDPPSe-12. This can be attributed to the interdigitation of side alkyl chains which is facilitated by replacing the branching chains with linear ones as in PDPPSe-10 and PDPPSe-12. The thermally annealed thin films of PDPPSe-10 and PDPPSe-12 also exhibit in-plane scattering signals at 1.69 and 1.71 Å-1 (see Figure 3 and Table S1), which result from the respective interchain π-π stacking with packing distances of 3.69 and 3.67 Å. However, thin film of PDPPSe shows a diffused out-of-plane diffraction arc at qz = 1.71 Å-1, corresponding to a π-π stacking distance of 3.68 Å. Furthermore, on the basis of the full width at hall-maximum (FWHM) of scattering signals due to π-π stacking (see Figure S12), the correlation lengths (LC) along the π-π stacking directions were estimated to be 2.26, 3.74 and 3.76 nm for PDPPSe, PDPPSe-10 and PDPPSe-12, respectively. It is known that LC is the distance over which crystalline order is preserved.57-58 Hence, thin films of PDPPSe-10 and PDPPSe-12 entail longer ordering of interchain π-π stacking than that of PDPPSe (see Figure 3). As listed in Table S1, not only the FWHMs of (010) signals, but also those of (x00) ones become smaller for thin films of all three polymers after thermal annealing at 180 oC. In particular, FWHMs of scattering signals for the thermally annealed thin films of PDPPSe-10 and PDPPSe-12 are smaller than those of PDPPSe thin film, indicating that the thermally annealed thin films of PDPPSe-10 and PDPPSe-12 show improved thin film crystallinity. Thin film morphologies were characterized with AFM. Figure 4 and Figure S13 and show AFM images of thin films of PDPPSe-10 and PDPPSe-12 as well as PDPPSe before and after thermal annealing at 180 oC. The thin film morphologies for each polymer before and after thermal annealing are similar, but root-mean-square (RMS) roughnesses increase after thermal annealing. PDPPSe-10 and PDPPSe-12 exhibit similar thin film morphology in which nanofibers were inter-connected as in thin film of PDPPSe. But, the root-mean-square roughnesses were found to be higher for thin films of PDPPSe-10 and

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PDPPSe-12 than that for thin film of PDPPSe before and after thermal annealing. For instance, the root-mean-square roughnesses were measured to be 1.19, 1.24 and 0.81 nm for PDPPSe-10 and PDPPSe-12 and PDPPSe, respectively, after thermal annealing at 180 oC. This may be attributed to the formation of large domains of polymer chains within thin films of PDPPSe-10 and PDPPSe-12. To conclude, the replacement of one bulky branching side chain with the linear one at each DPP unit in PDPPSe-10 and PDPPSe-12 can weaken the steric hindrance owing to branching alkyl chains in PDPPSe as illustrated in Scheme 3. This results in more orderly interchain packing and improvement of thin film crystallinity, while thin film morphology with interconnected nanofibers is kept. As illustrated in Figure 4, this simple side-chain engineering induces the interchain packing mode on the substrate to be changed from face-on to edge-on and increases the correlation lengths (LC) along the π-π stacking directions. As it will be discussed below, these structural alterations with regard to interchain packing are beneficial for improving the charge mobilities of conjugated D-A polymers. Charge mobility enhancement. In the following, we show that the remarkable enhancement of charge mobilities for thin films of PDPPSe-10 and PDPPSe-12, which is expected on the basis of the improvement of interchain packing order and thin film crystallinity. Charge mobilities of thin films of PDPPSe-10 and PDPPSe-12 as well as PDPPSe were extracted from the transfer curves of bottom-gate/bottom-contact (BGBC) field-effect

Figure 4. AFM height images of the thermally (at 180 oC) annealed thin-films of PDPPSe (a), PDPPSe-10 (b) and PDPPSe-12 (c).

Figure 5. The p-type transfer and output characteristics of FETs with PDPPSe (a, d), PDPPSe-10 (b, e) and PDPPSe-12 (c, f) after thermal annealing at 180 oC, measured under ambient atmosphere. The p-type, n-type transfer and output characteristics of FETs with PDPPSe (g, j, gˊ, jˊ), PDPPSe10 (h, k, hˊ, kˊ) and PDPPSe-12 (i, l, iˊ, lˊ) after thermal annealing at 180 oC.


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Scheme 3. Schematic illustration of the reduction of steric crowding for side chains after the replacement of one branching bulky chain with the linear one at each DPP unit.

Table 2. Mobilities (μh and μe), threshold voltages (VTh) and Ion/Ioff ratios for FETs with thin films of PDPPSe-10, PDPPSe-12 and PDPPSe. Under air Polymer

Temp. μh (oC) (cm2 V-1 s-1) a,c

Under nitrogen

VTh (V)

Ion/Ioff a

μh (cm2 V-1 s-1)

VTh (V)

Ion/Ioff a

μe b, c (cm2 V-1 s-1)

VTh b (V)

Ion/Ioff b

a

b, c

b

RT

2.2/1.8

-10 ~ 2

105~ 106

1.7/1.2

-7 ~ -19

103 ~ 105

0.17/0.11

86 ~ 95

102~ 103

180

8.1/5.8

-8 ~ 3

105~ 106

6.5/4.1

-4 ~ -17

103 ~ 105

0.48/0.36

78 ~ 100

102~ 103

RT

3.1/2.2

-8 ~ 1

105~ 106

2.4/1.6

-5 ~ -16

103 ~ 104

0.26/0.18

83 ~ 94

102~ 103

180

9.4/7.5

-7 - 3

105~ 106

7.9/5.3

-3 ~ -15

102 ~ 104

0.79/0.51

85 ~ 95

102~ 103

RT

0.48/0.30

-9 ~ 1

105~ 106

0.43/0.26

-8 ~ -18

103 ~ 104

0.050/0.036

82 ~ 90

102~ 103

180

1.35/0.89

-10 ~ 0

105~ 106

1.18/0.85

-7 ~ -16

102 ~ 104

0.19/0.14

78 ~ 86

102~ 103

PDPPSe-10

PDPPSe-12

PDPPSe

Measured under ambient atmosphere, and the channel width (W) and length (L) were 1440 and 10 μm, respectively. b Measured under N2, and the channel width (W) and length (L) were 1440 and 50 μm. c The mobilities were provided in “highest/average’’ form, and the performance data were obtained based on more than 10 different FETs. a

transistors (FETs), which were fabricated with conventional procedures (see Supporting Information) by using the respective polymer thin films as the semiconducting layers. Figure 5 shows the transfer and output curves, which were measured in air, for the respective FETs with thin films of PDPPSe-10, PDPPSe-12 and PDPPSe. As expected, all three polymers behave as p-type semiconductors. As listed in Table 2, FETs with the as-prepared thin films of PDPPSe-10. and PDPPSe-12 show similar Ion/off and Vth as that with thin film of PDPPSe. However, hole mobilities of as-prepared thin films of PDPPSe10 (2.2 cm2V-1s-1) and PDPPSe-12 (3.1 cm2V-1s-1) are ca. 5/6 times higher than that of PDPPSe (0.48 cm2V-1s-1). As expected, thermal annealing can enhance hole mobilities for these three polymers. The maxima/average hole mobilities can reach 8.1/5.8 cm2V-1s-1 and 9.4/7.5 cm2V-1s-1 for thin films of PDPPSe-10 and PDPPSe-12 after thermal annealing at 180 o C. In comparison, the maxima/average hole mobilities were measured to be 1.35/0.89 cm2V-1s-1 for the thermally annealed thin film of PDPPSe under the same condition. We note that the transfer characteristics are not ideal and typically exhibit a

region of higher slope at low gate voltage and a region of smaller slope at high gate voltage. In fact, such non-ideal transfer curves were observed for FETs with a number of conjugated D-A polymers,1,59 but its origins are not clearly understood.59 Therefore, we calculated the reliability factors of FETs measured in air according to the reported procedure (for details, see Figure S14).60 The average reliability factors are calculated to be 52%, 51% and 56% for the respective ten devices with PDPPSe-10, PDPPSe-12 and PDPPSe. Clearly, their reliability factors are similar. Therefore, comparison of their charge mobilities should be acceptable. FETs with PDPPSe-10, PDPPSe-12 and PDPPSe were also measured under nitrogen atmosphere. On the basis of transfer and output curves, thin films of PDPPSe-10 and PDPPSe-12 as well as PDPPSe exhibit ambipolar semiconducting properties under nitrogen atmosphere. This is in a good agreement with previous reports on conjugated diketopyrrolopyrrole-selenophene polymer.29 The ambipolar semiconducting performance data are listed in Table 2. In comparison with those of PDPPSe, the respective hole and electron mobilities are obviously incre-



mented, although they are not balanced. For instance, hole and electron mobilities are as high as 6.5 and 0.48 cm2V-1s-1 for thin films of PDPPSe-10 after thermal annealing at 180 oC, which are about 6 and 3 times than those of PDPPSe under the same condition. Similarly, hole and electron mobilities for thin film of PDPPSe-12 are even boosted to 7.9 and 0.79 cm2V-1s-1 after thermal annealing at 180 oC, being about 7 and 4 times than those of PDPPSe under the same condition. Furthermore, it is noted that the enhancement of charge mobilities for PDPPSe10 and PDPPSe-12 does not compromise other semiconducting parameters (VTh and Ion/off) by comparing with those of PDPPSe (see Table 2). We not only extracted the mobilities by fitting linear part of the respective plots of the root square of IDS vs VGS at low voltage region (see above), but also employed “two-points” method to estimate charge mobilities as reported early,9 in order to avoid the mobility overestimation. Two points at the respective VTh and VGS of the plot of IDS1/2 versus VGS were selected, and the mobilities were estimated and shown in Table S2. The details of the mobility extraction with this “two-points” method was provided in Supporting Information and Figure S15. The maxima hole mobilities of PDPPSe-12, PDPPSe-10 and PDPPSe in air were estimated to be 4.7, 4.1, and 0.98 cm2V-1s-1 after thermal annealing, whereas the maxima hole/electron mobilities for PDPPSe-12, PDPPSe-10 and PDPPSe in nitrogen were estimated to be 2.7/0.58, 2.1/0.35 and 0.92/0.16 cm2V-1s-1. Above all, both mobility estimation methods support the conclusion that PDPPSe-12 and PDPPSe-10 exhibit better semiconducting performances than PDPPSe. Oh, Yang and their coworkers reported three siloxane terminated polymers PTDPPSe-SiC4, PTDPPSe-SiC5 and PTDPPSe-SiC629 (see Scheme 4) with the same conjugated backbones as for PDPPSe-10 and PDPPSe-12. They were reported to show high hole and electron mobilities by using solution-sheared technique.61 However, thin films of these siloxane terminated polymers, which were prepared with the conventional spin-coating technique, exhibited lower hole mobilities than those of PDPPSe-10 and PDPPSe-12, whereas their electron mobilities are comparable to those of PDPPSe10 and PDPPSe-12 (see Scheme 4). Therefore, this new sidechain engineering approach for DPP-based conjugated D-A polymers, in which one bulky branching alkyl chain is replaced with the linear one at each DPP unit, is advantageous in terms of boosting charge mobilities. Scheme 4. Chemical structures of PTDPPSe-SiC4/5/6 and thin film charge mobilities measured under nitrogen by spin-coating.29

N S Si O

O

Si O Si O Si

O

S S

N

Se n

Si O Si

Si O

O

Si O Si

PTDPPSe-SiC4 µhmax/cm2V-1s-1 µemax/cm2V-1s-1

Si O Si O Si

2 N O

N S

S N

Se n

Si O

O

Si O Si

2

PTDPPSe-SiC5

Si O Si O Si

3 O S

N

Se n

3

PTDPPSe-SiC6

2.78

2.92

1.69

0.85

1.13

0.20

CONCLUSIONS In this paper, we introduce a new side-chain engineering approach for DPP-based conjugated D-A polymers in which one bulky branching alkyl chain is replaced with the linear one

Page 8 of 13

at each DPP unit. With this new approach in mind two new DPP-based conjugated polymers PDPPSe-10 and PDPPSe-12 were prepared and investigated. It is noted that the corresponding DPP monomers can be simply synthesized from commercially available compounds. The results reveal that i) the replacement of bulky branching chains with linear ones can weaken the steric hindrance owing to branching chains, and accordingly conjugated backbones of PDPPSe-10 and PDPPSe-12 become more planar and rigid; ii) the incorporation of linear alkyl chains as in PDPPSe-10 and PDPPSe-12 is beneficial for side chain interdigitation; and iii) interchain packing order and thin film crystallinity are obviously improved for PDPPSe-10 and PDPPSe-12 by comparing with PDPPSe. These solid state structural changes, which are induced by the replacement of one branching alkyl chain with the linear one at each DPP unit, lead to remarkable enhancement of charge mobilities for PDPPSe-10 and PDPPSe-12. Hole mobilities of thin films of PDPPSe-10 and PDPPSe-12 in air are boosted to 8.1 and 9.4 cm2V-1s-1. It is expected that this simple and efficient approach for improving semiconducting performances is applicable for other conjugated polymers, and further investigations along this vein are underway.

EXPERIMENTAL SECTION The reagents and starting materials were commercially available and used without any further purification, if not specified elsewhere. PDPPSe was prepared according to the reported procedure.39,43 Synthesis of compound 2. Compound 1 (4.0 g, 13.3 mmol) and K2CO3 (3.7 g, 26.7 mmol) were added to a 250 mL double-neck round-bottom flask, and 120 mL of dry DMF were injected by syringe under nitrogen atmosphere. The reaction was heated to 120 oC for 30 min. 11-(Bromomethyl)tricosane (5.6 g, 13.3 mmol) was slowly added dropwise over 4 h, and the reaction mixture was stirred for 12 h at 120 oC. Solvents were removed by rotary evaporation and the residue was purified by column chromatography. Red-brown crude product (2.4 g, 28%) was obtained without further purification. HRMS (MALDI-TOF): calcd. for C38H56N2O2S2 (M+) 637.3855; Found: 637.3856. Synthesis of compound 3. Compound 2 (2.4 g, 3.8 mmol) and K2CO3 (1.04 g, 7.5 mmol) were added to a 250mL doubleneck round-bottom flask, and 100 mL of dry DMF was injected by syringe under nitrogen atmosphere. The reaction was heated to 120 oC for 30 min. 1-Bromodecane (1.25 g, 5.7 mmol) was added and the reaction mixture was stirred for 8 h at 120 oC. Solvents were removed by rotary evaporation and the residue was purified by column chromatography with CH2Cl2 and petroleum ether (1:3, v/v) as the eluent. Compound 3 was obtained as a red-brown solid (2.3 g, 80%). 1H NMR (400 MHz, CDCl3): δ 8.95-8.94 (m, 1H), 8.85-8.84 (m, 1H), 7.63- 7.61 (m, 2H), 7.29-7.25 (m, 2H), 4.08-4.00 (m, 4H), 1.95-1.83 (m, 1H), 1.79-1.71 (m, 2H), 1.42-1.21 (m, 54H), 0.89-0.86 (m, 9H). 13C NMR (CDCl3, 100 MHz): δ161.74, 161.37,140.35, 140.07, 135.34, 135.03, 130.53, 130.42, 129.88, 129.82, 128.60, 128.33, 108.13, 107.62, 46.23, 42.25, 37.75, 31.91, 31.22, 29.99, 29.62, 29.53, 29.35, 29.27, 26.88, 26.23, 22.67, 14.08. HR-MS (MALDI-TOF): calcd. for C48H76N2O2S2 (M+) 777.5421; Found: 777.5421. Anal. calcd


Page 9 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for C48H76N2O2S2: C,74.17; H,9.86; N, 3.60; S,8.25; Found: C,74.24; H,9.78; N, 3.62; S,8.18. Synthesis of compound 4. Compound 2 (2.0 g, 3.1 mmol) and K2CO3 (0.87 g, 6.3 mmol) were added to a 250 mL double-neck round-bottom flask, then dry DMF (100 mL) were injected by syringe under nitrogen atmosphere. The reaction was heated to 120 oC for 30 min. 1-Bromododecane (1.17 g, 4.7 mmol) was added and the reaction mixture was stirred for 8 h at 120 oC. Solvents were removed by rotary evaporation and the residue was purified by column chromatography with CH2Cl2 and petroleum ether (1:3, v/v) as the eluent. Compound 4 was obtained as a red-brown solid (2.0 g, 79%). 1H NMR (400 MHz, CDCl3): δ 8.95-8.94 (m, 1H), 8.85-8.84 (m, 1H), 7.63-7.61 (m, 2H), 7.29-7.25 (m, 2H), 4.08-4.00 (m, 4H), 1.91-1.88 (m, 1H), 1.79-1.71 (m, 2H), 1.44-1.21 (m, 58H), 0.89-0.85 (m, 9H). 13C NMR (CDCl3, 100 MHz): δ 161.75, 161.38, 140.35, 140.08, 135.34, 135.03, 130.54, 130.43, 129.88, 129.82, 128.60, 128.34, 108.13, 107.62, 46.23, 42.26, 37.75, 31.91, 31.22, 29.99, 29.62, 29.54, 29.33, 29.24, 26.89, 26.23, 22.67, 14.09. HR-MS (MALDI-TOF): calcd. for C50H80N2O2S2 (M+) 805.5735; Found: 805.5734. Anal. calcd for C50H80N2O2S2: C,74.57; H, 10.01; N, 3.48; S, 7.96; Found: C, 74.49; H, 10.00; N, 3.37; S, 7.89. Synthesis of compound 5. In a 100 mL round-bottom flask, compound 3 (1.1 g, 1.4 mmol) was dissolved in 50 mL of CHCl3 under nitrogen atmosphere. N-Bromosuccinimide (0.55 g, 3.1 mmol) was added, and the reaction mixture was stirred for 4 h at room temperature. Solvents were removed by rotary evaporation and the residue was purified with column chromatography using CH2Cl2 and petroleum ether (1:3, v/v) as the eluent. Compound 5 was obtained as a red-purple solid (900 mg, 68%). 1H NMR (400 MHz, CDCl3): δ 8.70 (d, J = 4.0 Hz, 1H), 8.60 (d, J = 4.0 Hz, 1H), 7.22 (dd, J = 8.0, 4.0 Hz, 2H), 3.97 (t, J = 8.0 Hz, 2H), 3.91 (d, J= 8.0 Hz, 2H), 1.881.85 (m, 1H), 1.75-1.68 (m, 2H), 1.41-1.21 (m, 54H), 0.890.86 (m, 9H). 13C NMR (CDCl3, 100 MHz): δ 161.38, 161.01,139.29, 139.06, 135.43, 135.11, 131.64, 131.39, 131.23, 131.15, 119.06, 118.92, 108.20, 107.73, 46.37, 42.31, 37.78, 31.91, 31.87, 31.22, 29.96, 29.67, 29.62, 29.53, 29.48, 29.34, 29.26, 29.17, 26.82, 26.21, 22.67, 14.08. HR-MS (MALDITOF): calcd. for C48H74Br2N2O2S2 (M+) 932.3553; Found: 932.3553. Anal. calcd for C48H74Br2N2O2S2: C, 61.66; H, 7.98; N, 3.00; S, 6.86; Found: C, 61.58; H, 8.11; N, 2.86; S, 6.83. Synthesis of compound 6. In a 100 mL round-bottom flask, compound 3 (1.2 g, 1.5 mmol) was dissolved in 50 mL of CHCl3 under nitrogen atmosphere. N-Bromosuccinimide (0.58 g, 3.3 mmol) was added, and the reaction mixture was stirred for 4 h at room temperature. Solvents were removed by rotary evaporation and the residue was purified by column chromatographywith CH2Cl2 and petroleum ether (1:3, v/v) as the eluent. Compound 5 was obtained as a red-purple solid (0.95 g, 66%). 1H NMR (400 MHz, CDCl3): δ 8.68 (d, J = 4.0 Hz, 1H), 8.59 (d, J = 4.0 Hz, 1H), 7.22 (dd, J = 8.0, 4.0 Hz, 2H), 3.98 (t, J = 8.0, 2H), 3.91 (d, J = 8.0 Hz, 2H), 1.88-1.85 (m, 1H), 1.76-1.68 (m, 2H), 1.41-1.22 (m, 58H), 0.90-0.86 (m, 9H). 13C NMR (CDCl3, 100 MHz): δ 161.39, 161.02, 139.29, 139.06, 135.42, 135.10, 131.64, 131.39, 131.23, 131.15, 119.05, 118.91, 108.22, 107.74, 46.38, 42.31, 37.79, 31.92, 31.90, 31.23, 29.95, 29.67, 29.64, 29.61, 29.53, 29.48, 29.34, 29.32, 29.18, 26.83, 26.21, 22.67, 14.07. HR-MS (MALDITOF): calcd. for C50H78Br2N2O2S2 (M+) 960.3867; Found:

960.3866. Anal. calcd for C50H78Br2N2O2S2: C, 62.36; H, 8.16; N, 2.91; S, 6.66; Found: C, 62.07; H, 8.23; N, 2.81; S, 6.70. General synthetic procedures for PDPPSe-10, and PDPPSe-12. Compounds 5 or 6 (1.0 equiv), (dimethyl(5(trimethylstannyl)selenophen-2-yl)stannyl)methylium (1.0 equiv), Pd2(dba)3 (0.01 equiv) and P(o-tol)3 (0.08 equiv) were dissolved in toluene (10 mL) in a Schlenk tube. Through a freeze-pump-thaw cycle, the tube was charged with nitrogen for three times. The reaction mixture was stirred at 110 °C for 3 h. The resulting mixture was poured into methanol and stirred for 3.0 h. 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 and other impurities. The resulting polymer was collected and dried under vacuum at 50 °C for 48 h. Synthesis of PDPPSe-10. Compound 5 (150 mg, 0.16 mmol), compound 7 (73.3 mg, 0.16 mmol), P(o-tol)3 (3.8 mg, 0.0125 mmol), and Pd2(dba)3 (1.5 mg, 0.0016 mmol) were used. The purified PDPPSe-10 was collected to give deep blue solid (135 mg, 93% yield). 1H NMR (500 MHz, 1,1,2,2tetrachloroethane-d2, 100 °C): δ 8.90 (m, 2H), 7.39-7.03 (m, 4H), 4.03 (m, 4H), 2.05-2.02 (m, 3H), 1.41-1.27 (m, 54H), 0.92-0.87 (m, 9H). 13C NMR (100 MHz, solid): δ 160.45, 142.32, 137.41, 128.47, 126.28, 108.81, 45.56, 42.33, 39.39, 32.82, 30.52, 23.50, 14.82. Anal. calcd for (C52H76N2O2S2Se)n: C, 69.05; H, 8.49; N, 3.10; S, 7.09; Found: C, 68.75; H, 8.37; N, 2.76; S, 6.88. Synthesis of PDPPSe-12. Compound 6 (150 mg, 0.156 mmol), compound 7 (71.1 mg, 0.156 mmol), P(o-tol)3 (3.8 mg, 0.0125 mmol), and Pd2(dba)3 (1.4 mg, 0.0016 mmol) were used. The purified PDPPSe-12 was collected to give deep blue solid (134 mg, 92% yield). 1H NMR (500 MHz, 1,1,2,2tetrachloroethane-d2, 100 °C): δ 8.90 (m, 2H), 7.40-7.03 (m, 4H), 4.02 (m, 4H), 2.05-1.99 (m, 3H), 1.41-1.27 (m, 58H), 1.01-0.89 (m, 9H). 13C NMR (100 MHz, solid): δ 160.46, 142.29, 137.31, 127.26, 126.61, 108.77, 45.37, 40.75, 39.09, 32.80, 30.54, 23.49, 14.81. Anal. calcd for (C54H80N2O2S2Se)n: C, 69.55; H, 8.67; N, 3.00; S, 6.88; Found: C, 69.03; H, 8.43; N, 2.91; S, 7.11.

ASSOCIATED CONTENT Supporting Information. TGA, DSC, Raman spectra, cyclic voltammograms, PDS, Fabrication of OFETs, 1H NMR and 13C NMR data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Present Addresses † Beijing National Laboratories for Molecular Sciences, CAS Key Laboratories of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China ‡ University of Chinese Academy of Sciences Beijing 100049, P. R. China Δ Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK



§ Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China ǁ Institute for Molecular Engineering and Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois, 60439, United States ǁ Institute for Molecular Engineering, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois, 60637, United States

Author Contributions D.Z. proposed the study and designed the polymer structures. D.Z. and Z.L. analyzed the data. Z.W. and Z.L. carried out the experiments. Z. W. and G. Z. did structural characterization. L.N. and Y.Y. carried out the theoretical calculations. M.Z. and K.B. carried out the PDS experiments and H. S explained the data. Z.C. and W.C. carried out the GIWAXS experiments. D.Z. and Z.L. prepared the manuscript. All authors discussed, revised, and approved the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the financial support of the Strategic Priority Research Program of the CAS (XDB12010300), the National Key R&D Program of China (2017YFA0204701), NSFC (21661132006). Mingfei Xiao thanks the Cambridge Overseas Trust and Chinese Scholarship Council for Ph.D funding. W.C. gratefully acknowledges financial support from the US Department of Energy, Office of Science, Materials Sciences and Engineering Division. We also thank Dr. Joseph Strzalka and Dr. Zhang Jiang for the assistance with GIWAXS measurements. Use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DEAC02-06CH11357.

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