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Tuning Energy Levels and Film Morphology in Benzodithiophene− Thienopyrrolodione Copolymers via Nitrogen Substitutions Teck Lip Dexter Tam* and Ting Ting Lin Agency of Science, Technology and Research (A*STAR), Institute of Materials Research and Engineering (IMRE), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore S Supporting Information *

ABSTRACT: We report the synthesis of the benzo[1,2-d:4,5d′]bisthiazole−thieno[3,4-c]pyrrole-4,6-dione copolymer pBBTzTPD, a nitrogen-substituted version of the highly efficient organic photovoltaic donor copolymer benzo[1,2-d:4,5-d′]dithiophene−thieno[3,4-c]pyrrole-4,6-dione pBDTTPD. This polymer was achieved via a novel and facile synthesis of 2,6diaminobenzo[1,2-d:4,5-d′]bisthiazoledione (1) using inexpensive and commercially available bromanilic acid and thiourea precursors in a one-pot two-step reaction. 1 can be easily converted to 2,6dibromo-4,8-di(2-ethylhexyloxy)benzo[1,2-d:4,5-d′]bisthiazole 6, which can then be copolymerized with n-octylthieno[3,4-c]pyrrole-4,6-dione (TPD) using direct arylation polymerization. The pBBTzTPD copolymer was characterized and compared with pBDTTPD in terms of molecular and electronic structure.

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INTRODUCTION Recent synthetic efforts for developing high-performance organic photovoltaic (OPV) donor materials and field-effect transistor (OFET) n-type materials include the incorporation of fluorine atoms in the backbone of known conjugated polymers with the goal to lower the energy of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO).1−12 Several fluorinated polymers have shown enhanced OPV and OFET performances as compared to their non-fluorinated analogues.1−8,10−12 A deeper HOMO for the donor material in OPV would enhance the open circuit voltage (Voc) of OPV cells and thus has the potential to increase the power conversion efficiency (PCE) of the corresponding devices.13 Furthermore, a deeper HOMO for the donor material would result in reduced air sensitivity and thus increased device stability.14 On the other hand, lower LUMO for n-type material in OFET would provide higher stability against oxygen and moisture trapping and thus result in a more ambient-stable n-channel devices.15,16 Lower LUMO also reduces the amount of electron traps that arise from chemical defects during synthesis and thus higher electron mobilities can be achieved.15 While considerable advantages are observed in both OPV and OFET, the syntheses of fluorinated materials are generally tedious and require the use of hazardous fluorinating reagents. Another approach to lower both HOMO and LUMO energy of conjugated polymers is the substitution of a sp2 carbon with a sp2 nitrogen.20−22 The higher electronegativity of nitrogen versus carbon and the possibility to incorporate it into the polymer backbone allows stabilization of both HOMO and LUMO. However, the substitution of carbon with nitrogen requires different, and typically more challenging, synthetic © XXXX American Chemical Society

approaches for achieving nitrogen-containing building blocks. A classical pair of examples is the chemistry of pyridine as compared to benzene.23 A more relevant pair of examples in the field of conjugated organic materials is thiophene vesus thiazole and benzo[1,2-d:4,5-d′]dithiophene BDT versus benzo[1,2-d:4,5-d′]bisthiazole BBTz, where derivatives of BDT have been used in high performance OPV1,2,13,24,25 and derivatives of BBTz have shown high mobility and stability in organic field effect transistor (OFET).19,26−28 Figure 1 reports representative structures high performance polymers in OPV and OFET and numbering systems for BDT and BBTz. As with the previous example, the synthetic routes of BDT and BBTz based materials are largely different. In particular, the extension of conjugation along the 2- and 6-positions builds on the chemistry of the BDT core for achieving BDT based materials,29−31 while de novo synthesis is required for BBTz based materials.26,27 Regarding functionalization at the 4- and 8-positions, the chemistry for BDT is well-documented since 198630,32 while only one recent example has been reported to date for BBTz, which involves bromination of the BBTz core using bromine in DMF at 100 °C, followed by Suzuki coupling on the 4- and 8-positions and direct arylation on the 2- and 6positions (Scheme 1).33 In this paper, we demonstrate the syntheses of a novel 4,8difunctionalized BBTz building block and, for the first time, its incorporation into a conjugated polymer. Using a rational synthetic approach, the BBTz monomer was synthesized using commercially available precursors in only three steps and high Received: December 21, 2015 Revised: February 7, 2016

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DOI: 10.1021/acs.macromol.5b02742 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 2. Structures of the polymers discussed in this work.

triethylamine and heating at 80 °C overnight to complete the reaction. During the reaction the dark red solution became clear and a light orange followed by precipitation of an orange solid. Upon addition of triethylamine, the reaction mixture first turned a little purplish, and then, after heating at 80 °C overnight, the 1 separated as a dark maroon solid in 85% yields and it was used in the next step without further purification. We also attempted the reaction using chloranilic acid and bromanil instead of bromanilic acid, but no product could be recovered. This result indicates higher reactivity of the C−Br bond and regio-directing effect of the hydroxyl group are crucial for product formation. The proposed mechanism for the formation of 1 is shown in Scheme 3. The “soft” sulfur from thiourea attacks the “soft” carbon bearing the bromine, which is analogous to the first step in the Hantzsch thiazole synthesis mechanism,34 followed by the elimination of HBr. The “hard” nitrogen attacks the nearby “hard” carbon bearing the hydroxyl group instead of the “hard” carbonyl carbon because of higher reactivity in the former. Elimination of water followed by aromatization yields intermediate A. The mechanism repeats on the other side of A to form 1. We stress that the two thiazole rings formation may not be sequential; that is, both ring formation may occur at the same time. Deamination of 1 via a procedure developed by Doyle et al.35 using tert-butyl nitrite in dry DMF yielded 2. The functionalization of 2 at the 4,8-positions proceeds in a similar fashion as benzodithiophenedione. However, attempts to stannylate, borylate, or brominate the 2,6-positions of the 4,8disubstituted BBTz derivatives 3 and 4 using similar conditions as 4,8-disubstituted BDT did not yield the desired products (Scheme 2). We suspect that deprotonation of the BBTz resulted in simultaneous ring-opening of the thiazole rings.36,37 Thus, an alternate route was used by reduction and in situ alkylation of 1 to yield 5. Because of the presence of the amino groups, N-alkylation occurs as side reaction, and thus its yield was found to be lower than that of the corresponding reaction using 2. The resulting 5 was also found to be slightly unstable due to oxidation of the amino groups. The diamine can be easily converted to 6 via the Sandmeyer reaction using tertbutyl nitrite in the presence of cupric bromide in 43% yield. The new polymer based on the BBTz core, as well as the corresponding BDT one, was synthesized via direct arylation of the dibromo precursors (6 or 7) with TPD (Scheme 4), using a procedure adopted from the Leclerc’s group.38 Herrmann’s catalyst and pivalic acid were used as the catalyst system, with tris(o-methoxyphenyl)phosphine as the ligand and cesium carbonate as the base, in toluene at 120 °C. Both polymers were purified via precipitation in methanol, followed by Soxhlet extraction using acetone, hexanes, and then 1,2-dichloro-

Figure 1. Structures and numbering system of BDT and BBTz and examples of high performing BDT and BBTz based polymers in OPV17,18 and OFET,19 respectively.

Scheme 1. Functionalization of BBTz at the 4- and 8Positions According to the Literature33

yields. The BBTz copolymer with 5-octylthieno[3,4-c]pyrrole4,6-dione TPD was synthesized using direct arylation polymerization (DArP) and characterized. As a comparison, DArP version of a high performance OPV copolymer of TPD and BDT was also synthesized. The effects of nitrogen substitutions on the molecular and electronic structures of these polymers are also discussed.

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RESULTS AND DISCUSSION The BBTz monomer syntheses (Scheme 2) began by reacting the inexpensive and commercially available bromanilic acid with thiourea in DMF at room temperature, followed by addition of B


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Macromolecules Scheme 2. Syntheses of the BBTz Monomers

Scheme 3. Proposed Mechanism for the Formation of 1

Scheme 4. Syntheses of pBBTzTPD and pBDTTPD Copolymers via DArP

and Stille protocols, with λmax located at 610 and 624 nm for solution and thin film, respectively.25,38 The optical bandgaps of the solution and thin film are ∼1.85 eV. On the other hand, the new polymer pBBTzTPD exhibit a larger optical bandgap of ∼2.00 eV with solution and thin film λmax values of 470 and 473 nm, respectively. A pronounced shoulder is observed in both solution and thin film at ∼570 nm. Cyclic voltammetry (CV) experiments were carried out to probe redox processes and quantify the frontier molecular orbital (FMO) energies. Both polymers exhibit quasi-reversible oxidation and reduction features (Figure 4a). The oxidation and reduction onsets of pBDTTPD thin film are positioned at +1.06 and −0.87 V, respectively. The FMO energy levels were estimated by using the HOMO of ferrocene as the reference (4.8 eV, +0.40 V vs Ag/AgCl). Accordingly, the HOMO and LUMO values of pBDTTPD derived from the CV measurements are −5.46 and −3.53 eV, respectively. In comparison, the oxidation and reduction onsets of pBBTzTPD thin film are at +1.21 and −0.77 V, respectively, corresponding to HOMO and

benzene. pBDTTPD was obtained as a dark blue solid while pBBTzTPD was obtained as a dark brown solid. The photophysical (UV), electrochemical (CV), and morphological (XRD) characterizations of pBBTzTPD and pBDTTPD are shown in Figures 3 and 4 and tabulated in Table 1. The UV−vis absorption of the parent polymer pBDTTPD is similar to those previously synthesized by DArP C


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distances of 18.7 and 15.8 Å. No observable π−π stacking peak at 2θ ∼ 30° could be observed for pBBTzTPD. Density functional theory (DFT) for pBDTTPD and pBBTzTPD repeating units (Figure 5) indicate shorter BDTTPD inter-ring C−C bond (1.446 Å vs 2.36°) and larger dihedral angle (1.448 Å vs 0.13°) than for BBTz-TPD. This result can be easily understood by (1) a stronger donor− acceptor interaction between BDT and TPD than between BBTz and TPD and (2) existence of a larger steric repulsion between C−H of BDT and S of TPD than between N of BBTz and S of TPD. Another clear difference between these repeating units is the orientation of the methoxy groups on BDT and BBTz. For pBDTTPD, the methoxy groups are pointing out-ofplane of BDT (92.15°), while for pBBTzTPD, the methoxy groups are pointing in-plane of BBTz (5.16°). Therefore, there is likely a steric repulsion between the C−H and methoxy groups in BDT while there are some form of favorable interaction between nitrogen and methoxy group. The orientation of the methoxy groups suggests the more planar pBBTzTPD unit would promote π−π stacking and thus lower solubility than pBDTTPD, which prevents formation of higher molecular weight polymers during polymerization. The frontier molecular orbitals of both (BDTTPD)n and (BBTzTPD)n repeating units (n = 1−4), mimicking the electronic structural evolution into the corresponding polymers, are shown in Figure 6. pBDTTPD shows delocalized HOMO and LUMO while pBBTzTPD shows localized HOMO and delocalized LUMO. The LUMO energy values decrease as n increases for both (BDTTPD)n and (BBTzTPD)n as the result of LUMO delocalization. pBBTzTPD shows lower LUMO values than pBDTTPD due to the replacement of carbon with the more electronegative nitrogen. In contrast, the HOMO energy values increase as n increases for pBDTTPD due to delocalization of HOMO, while the HOMO energy values slightly decrease for pBBTzTPD due to localization of HOMO. For n ≥ 3, the HOMO energy of pBBTzTPD becomes lower than that of pBDTTPD. The calculated HOMO and LUMO energy trends are consistent with the polymer limit experimental HOMO and LUMO trends. A plot of the HOMO−LUMO gap versus 1/n is reported in Figure 7a for both pBDTTPD and pBBTzTPD. A good linear fit with R2 values of 0.997 for pBDTTPD and 0.995 for pBBTzTPD were obtained. The extrapolated HOMO−LUMO

Figure 3. Solution (in 1,2-dichlorobenzene) and thin film spin-coated from 1,2-dichlorobenzene UV−vis spectra of pBDTTPD and pBBTzTPD.

LUMO values of −5.61 and −3.63 eV, respectively. This result is consistent with the greater electronegativity of nitrogen reducing both HOMO and LUMO energies. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were also performed on these polymers (see Supporting Information). According to TGA, both polymers exhibit less than 2% weight loss at 300 °C under N2, with pBDTTPD showing similar profile as literature reports.39,40 DSC shows no sizable phase transitions for both polymers for scans from room temperature to 300 °C, although Stille polymerized pBDTTPD were reported to exhibit a phase transition at 138 °C.39 Out-of-plane XRD of pBDTTPD thin films (Figure 4b) shows weak diffraction peaks located at 2θ ∼ 5° corresponding to lamellar spacings of 21.1 and 15.9 Å and a very weak π−π stacking peak at 2θ ∼ 25° corresponding to 3.6 Å. Leclerc’s and Fréchet’s groups have reported similar lamellar and π−π stacking distance of ∼21 and 3.6 Å, respectively, for Stille polymerized pBDTTPD, with the absence of the second lamellar stacking.25,39 For pBBTzTPD, lamellar diffraction peaks were also observed at 2θ ∼ 5° but correspond to smaller

Figure 4. Thin film (a) CV and (b) out-of-plane XRD of pBDTTPD and pBBTzTPD. D


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Macromolecules Table 1. Characterization of pBDTTPD and pBBTzTPDa pBDTTPD pBBTzTPD

HOMO (eV)

LUMO (eV)

CV Eg (eV)

solution opt Eg (eV)

film opt Eg (eV)

Mw (g mol−1)

PDI

−5.46 −5.61

−3.53 −3.63

1.93 1.98

1.85 1.99

1.84 1.97

57127 14330

2.79 2.46

a HOMO and LUMO values by the onset of the oxidation and reduction sweeps from the thin film CV. Solution UV−vis were measured in 1,2dichlorobenzene. Mw was determined by GPC in 1,2,4-trichlorobenzene against polystyrene standard.

consistent with the observed smaller optical bandgap of pBDTTPD. We thus performed time-dependent DFT (TDDFT) computations for oligomeric segments (n = 4) for both polymers to determine their theoretical UV−vis absorption (Figure 7b) by calculating the first 12 excited states. The results show the lowest energy transition of (BBTzTPD)4 has a λ = 650.72 nm and weak oscillator strength of 0.6888 as compared to that of (BDTTPD)4, which has a λ = 607.83 nm and a strong oscillator strength of 2.7955. Interestingly, this lowest energy transition of (BBTzTPD)4 cannot be observed in the calculated UV−vis spectra. Only the 13th transition (λ = 496.64, oscillator strength = 2.6846) can be observed for (BBTzTPD)4. The lowest energy transition of (BDTTPD)4 can easily be seen in the calculated UV−vis spectra. The small oscillator strength for the lowest energy transition can be explained by the fact that the HOMO and LUMO of pBBTzTPD do not have much overlap for n ≥ 2. As for pBDTTPD, a good HOMO and LUMO overlap result in the large oscillator strength for the lowest energy transition. The shape of the calculated UV−vis spectra for both (BDTTPD)4 and (BBTzTPD)4 matches with the experimental UV−vis of the polymers, indicating the lowest energy transition for pBBTzTPD is also not observable experimentally.

Figure 5. DFT geometry-optimized structures showing bond distance and dihedral angle of (a) pBDTTPD and (b) pBBTzTPD repeating unit, respectively. The alkyl groups were reduced to methyl groups for simplicity. Gray: carbon; white: hydrogen; yellow: sulfur; red: oxygen; blue: nitrogen. Level of theory B3LYP 6-31G(d,p), gas phase.

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CONCLUSION We have demonstrated a new and facile synthesis of 4,8disubstituted BBTz and its incorporation into a new conjugated polymer. The effects of the nitrogen versus CH substitution on the molecular and electronic structures of BBTz versus BDT based polymers were discussed. Importantly, nitrogen substitution results in a more planar polymeric backbone due to reduced N steric demand as well as it is effective in lowering

Figure 6. HOMO and LUMO of (BDTTPD)n and (BBTzTPD)n repeating units from n = 1 to n = 4. The structures were geometry optimized using DFT B3LYP 6-31G(d,p) level of theory in the gas phase.

energy gaps were found to be 2.12 eV for pBDTTPD and 2.05 eV for pBBTzTPD when n = ∞. However, this result is not

Figure 7. (a) Plot of calculated HOMO−LUMO gap versus 1/n. (b) Calculated UV−vis spectra of (BDTTPD)4 and (BBTzTPD)4 using the TDDFT B3LYP 6-31G(d,p) level of theory in the gas phase. Half-width at half-height = 0.333 eV. E


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Macromolecules both HOMO and LUMO. Stronger π−π stacking interactions due to reduced core−core distance led to low solubility and thus low molecular weight of pBBTzTPD. In view of the importance of good performance and ambient stable benzobisthiazole based organic field-effect transistor materials and many high performance benzodithiophene based organic photovoltaic materials, we believe this synthetic route enabling functionalized BBTz’s provides a route to conjugated polymers for high performance organic electronic applications.

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poly(cyclopenta[2,1-b:3,4-b′]dithiophene-alt-quinoxaline) copolymers. J. Mater. Chem. A 2015, 3, 2960. (9) Nketia-Yawson, B.; Lee, H.-S.; Seo, D.; Yoon, Y.; Park, W.-T.; Kwak, K.; Son, H. J.; Kim, B.; Noh, Y.-Y. A Highly Planar Fluorinated Benzothiadiazole-Based Conjugated Polymer for High-Performance Organic Thin-Film Transistors. Adv. Mater. 2015, 27, 3045. (10) Lei, T.; Dou, J.-H.; Ma, Z.-J.; Yao, C.-H.; Liu, C.-J.; Wang, J.-Y.; Pei, J. Ambipolar Polymer Field-Effect Transistors Based on Fluorinated Isoindigo: High Performance and Improved Ambient Stability. J. Am. Chem. Soc. 2012, 134, 20025. (11) Park, J. H.; Jung, E. H.; Jung, J. W.; Jo, W. H. A Fluorinated Phenylene Unit as a Building Block for High-Performance n-Type Semiconducting Polymer. Adv. Mater. 2013, 25, 2583. (12) Lei, T.; Xia, X.; Wang, J.-Y.; Liu, C.-J.; Pei, J. Conformation Locked” Strong Electron-Deficient Poly(p-Phenylene Vinylene) Derivatives for Ambient-Stable n-Type Field-Effect Transistors: Synthesis, Properties, and Effects of Fluorine Substitution Position. J. Am. Chem. Soc. 2014, 136, 2135. (13) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Polymer solar cells with enhanced open-circuit voltage and efficiency. Nat. Photonics 2009, 3, 649. (14) Beaupré, S.; Leclerc, M. PCDTBT: en route for low cost plastic solar cells. J. Mater. Chem. A 2013, 1, 11097. (15) Nicolai, H. T.; Kuik, M.; Wetzelaer, G. A. H.; de Boer, B.; Campbell, C.; Risko, C.; Brédas, J. L.; Blom, P. W. M. Unification of trap-limited electron transport in semiconducting polymers. Nat. Mater. 2012, 11, 882. (16) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R. Rylene and Related Diimides for Organic Electronics. Adv. Mater. 2011, 23, 268. (17) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photonics 2012, 6, 593. (18) Liu, C.; Yi, C.; Wang, K.; Yang, Y.; Bhatta, R. S.; Tsige, M.; Xiao, S.; Gong, X. Single-Junction Polymer Solar Cells with Over 10% Efficiency by a Novel Two-Dimensional Donor−Acceptor Conjugated Copolymer. ACS Appl. Mater. Interfaces 2015, 7, 4928. (19) Osaka, I.; Takimiya, K.; McCullough, R. D. BenzobisthiazoleBased Semiconducting Copolymers Showing Excellent Environmental Stability in High-Humidity Air. Adv. Mater. 2010, 22, 4993. (20) Yamashita, Y. Development of High-performance n-Type Organic Field-effect Transistors Based on Nitrogen Heterocycles. Chem. Lett. 2009, 38, 870. (21) Ortiz, R. o. P.; Yan, H.; Facchetti, A.; Marks, T. J. Azine- and Azole-Functionalized Oligó and Polythiophene Semiconductors for Organic Thin-Film Transistors. Materials 2010, 3, 1533. (22) Zhuang, W.; Lundin, A.; Andersson, M. R. Computational modelling of donor−acceptor conjugated polymers through engineered backbone manipulations based on a thiophene−quinoxaline alternating copolymer. J. Mater. Chem. A 2014, 2, 2202. (23) Joule, J. A.; Mills, K. Heterocyclic Chemistry, 5th ed.; John Wiley & Sons: 2010. (24) Hou, J.; Chen, H.-Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.; Li, G. Synthesis of a Low Band Gap Polymer and Its Application in Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2009, 131, 15586. (25) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet, J. M. J. Synthetic Control of Structural Order in N-Alkylthieno[3,4-c]pyrrole-4,6-dione-Based Polymers for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 7595. (26) Ahmed, E.; Kim, F. S.; Xin, H.; Jenekhe, S. A. Benzobisthiazole− Thiophene Copolymer Semiconductors: Synthesis, Enhanced Stability, Field-Effect Transistors, and Efficient Solar Cells. Macromolecules 2009, 42, 8615. (27) Ahmed, E.; Subramaniyan, S.; Kim, F. S.; Xin, H.; Jenekhe, S. A. Benzobisthiazole-Based Donor−Acceptor Copolymer Semiconductors for Photovoltaic Cells and Highly Stable Field-Effect Transistors. Macromolecules 2011, 44, 7207.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02742. Detailed syntheses of all the compounds, including MALDI-TOF, 1H NMR, and 13C NMR spectra of the small molecules; DFT calculated HOMO and LUMO of (BDTTPD)8 and (BBTzTPD)3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The DFT calculations were supported by the A*STAR computational resource centre through the use of its high performance computing facilities.

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REFERENCES

(1) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7% Efficiency. Angew. Chem., Int. Ed. 2011, 50, 2995. (2) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. Fluorine Substituted Conjugated Polymer of Medium Band Gap Yields 7% Efficiency in Polymer−Fullerene Solar Cells. J. Am. Chem. Soc. 2011, 133, 4625. (3) Guo, S.; Ning, J.; Körstgens, V.; Yao, Y.; Herzig, E. M.; Roth, S. V.; Müller-Buschbaum, P. The Effect of Fluorination in Manipulating the Nanomorphology in PTB7:PC71 BM Bulk Heterojunction Systems Adv. Energy Mater. 2015, 510.1002/aenm.201401315. (4) Tumbleston, J. R.; Collins, B. A.; Yang, L.; Stuart, A. C.; Gann, E.; Ma, W.; You, W.; Ade, H. The influence of molecular orientation on organic bulk heterojunction solar cells. Nat. Photonics 2014, 8, 385. (5) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Anthracene-Based Medium Bandgap Conjugated Polymers for High Performance Polymer Solar Cells Exceeding 8% PCE Without Additive and Annealing Process Adv. Energy Mater. 2015, 10.1002/ aenm.201500065. (6) Duan, C.; Furlan, A.; van Franeker, J. J.; Willems, R. E. M.; Wienk, M. M.; Janssen, R. A. J. Wide-Bandgap Benzodithiophene− Benzothiadiazole Copolymers for Highly Efficient Multijunction Polymer Solar Cells. Adv. Mater. 2015, 27, 4461. (7) Li, Z.; Lin, H.; Jiang, K.; Carpenter, J.; Li, Y.; Liu, Y.; Hu, H.; Zhao, J.; Ma, W.; Ade, H.; Yan, H. Dramatic performance enhancement for large bandgap thick-film polymer solar cells introduced by a difluorinated donor unit. Nano Energy 2015, 15, 607. (8) Verstappen, P.; Kesters, J.; Vanormelingen, W.; Heintges, G. l. H. L.; Drijkoningen, J.; Vangerven, T.; Marin, L.; Koudjina, S.; Champagne, B. t.; Manca, J.; Lutsen, L.; Vanderzande, D.; Maes, W. Fluorination as an effective tool to increase the open-circuit voltage and charge carrier mobility of organic solar cells based on F


Article

Macromolecules (28) Subramaniyan, S.; Kim, F. S.; Ren, G.; Li, H.; Jenekhe, S. A. High Mobility Thiazole−Diketopyrrolopyrrole Copolymer Semiconductors for High Performance Field-Effect Transistors and Photovoltaic Devices. Macromolecules 2012, 45, 9029. (29) Rieger, R.; Beckmann, D.; Mavrinskiy, A.; Kastler, M.; Müllen, K. Backbone Curvature in Polythiophenes. Chem. Mater. 2010, 22, 5314. (30) Pan, H.; Li, Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. LowTemperature, Solution-Processed, High-Mobility Polymer Semiconductors for Thin-Film Transistors. J. Am. Chem. Soc. 2007, 129, 4112. (31) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties. J. Am. Chem. Soc. 2009, 131, 7792. (32) Beimling, P.; Koßmehl, G. Synthesis of benzo[1,2-b′:4,5b′]dithiophene and its 4,8-Dimethoxy and 4,8-dimethyl derivatives. Chem. Ber. 1986, 119, 3198. (33) Bon, J. L.; Feng, D.; Marder, S. R.; Blakey, S. B. A C−H Functionalization Protocol for the Direct Synthesis of Benzobisthiazole Derivatives. J. Org. Chem. 2014, 79, 7766. (34) Hantzsch, A.; Weber, J. H. Ueber Verbindungen des Thiazols (Pyridins der Thiophenreihe). Ber. Dtsch. Chem. Ges. 1887, 20, 3118. (35) Doyle, M. P.; Dellaria, J. F., Jr.; Siegfried, B.; Bishop, S. W. Reductive deamination of arylamines by alkyl nitrites in N,Ndimethylformamide. A direct conversion of arylamines to aromatic hydrocarbons. J. Org. Chem. 1977, 42, 3494. (36) Hilf, C.; Bosold, F.; Harms, K.; Marsch, M.; Boche, G. The Equilibrium Between 2-Lithium-Oxazole(-Thiazole, -Imidazole) Derivatives and Their Acyclic Isomers − A Structural Investigation. Chem. Ber. 1997, 130, 1213. (37) Sánchez, R. S.; Zhuravlev, F. A. Mechanistic Evidence for a Ring-Opening Pathway in the Pd-Catalyzed Direct Arylation of Benzoxazoles. J. Am. Chem. Soc. 2007, 129, 5824. (38) Beaupré, S.; Pron, A.; Drouin, S. H.; Najari, A.; Mercier, L. G.; Robitaille, A.; Leclerc, M. Thieno-, Furo-, and Selenopheno[3,4c]pyrrole-4,6-dione Copolymers: Effect of the Heteroatom on the Electrooptical Properties. Macromolecules 2012, 45, 6906. (39) Zou, Y.; Najari, A.; Berrouard, P.; Beaupré, S.; Aïch, B. R.; Tao, Y.; Leclerc, M. A Thieno[3,4-c]pyrrole-4,6-dione-Based Copolymer for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 5330. (40) Warnan, J.; Cabanetos, C.; Labban, A. E.; Hansen, M. R.; Tassone, C.; Toney, M. F.; Beaujuge, P. M. Ordering Effects in Benzo[1,2-b:4,5-b′]difuran-thieno[3,4-c]pyrrole-4,6-dione Polymers with > 7% Solar Cell Efficiency. Adv. Mater. 2014, 26, 4357.

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