Rational Design of a Narrow-Bandgap Conjugated Polymer Using the

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Rational Design of a Narrow-Bandgap Conjugated Polymer Using the Quinoidal Thieno[3,2‑b]thiophene-Based Building Block for Organic Field-Effect Transistor Applications Jun Huang,† Shuo Lu,‡ Ping-An Chen,§ Kai Wang,† Yuanyuan Hu,§ Yong Liang,*,‡ Ming Wang,*,† and Elsa Reichmanis∥

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†

Center for Advanced Low-dimension Materials, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China ‡ State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China § Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education & Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha 410082, China ∥ School of Chemical and Biomolecular Engineering, School of Chemistry and Biochemistry, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States S Supporting Information *

ABSTRACT: A new quinoidal acceptor building block (namely IQTT) is designed with directed diradical character to control and narrow the bandgap while good chemical stability is maintained with the assistance of quantum mechanics simulations. Then IQTT-based monomer and donor−acceptor (D−A) polymer (PIQTT) are synthesized as well as the isoindigo-based (IID) monomer and D−A polymer (PITT). It is found that the IQTT building block displays stronger electron-withdrawing ability and more planar backbone conformation than IID in the copolymers. Both IQTT-based monomer and PIQTT exhibit significantly redshifted absorption compared to their IID-based counterparts. In the solid state, PIQTT exhibits the superior thin film order and a decent field-effect transistor mobility. This work not only shows that IQTT is an excellent building block for organic electronics but also indicates that the quantum chemical simulation tool that developed in diradicaloid studies is a powerful approach to design novel quinoidal narrow-bandgap D−A conjugated polymers for organic electronics.

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INTRODUCTION

various A structures, a few A building blocks contain quinoidal subunits, and the resulting copolymers display quinoidal character, shown in Figure 1d.11 These structures are attractive due to their advantages of narrowing the bandgap in the resulting copolymers as well as the OFET device applications. The quinoidal character of CPs can be correlated and represented by the bond length alternation (BLA), which is defined as the average of the difference in length between adjacent carbon−carbon bonds in a polyene chain.6a When the quinoidal contribution increases in CPs, the carbon−carbon single bonds between two adjacent aromatic rings exhibit more double-bond character and the BLA decreases; therefore, the band gap decreases.11b,c,12 However, designing novel quinoidal A units is challenging due to the synthetic difficulty and

Narrow-bandgap conjugated polymers (CPs) are under intensive studies due to their applications in organic electronics,1 such as polymer solar cells (PSCs),2 organic field-effect transistors (OFETs),3 organic photodetectors (OPDs),4 and bioelectronics.5 In the past few decades, the donor−acceptor (D−A) CPs have been well-known as the most powerful approach to manipulate the bandgap and achieve superb performance in electronic devices.6 D−A CPs consist of electron-rich moieties (donor, D) and electrondeficient moieties (acceptor, A), and the bandgap can be finetuned by adjusting the electron-donating ability of the D and the electron-withdrawing ability of the A units.7 Therefore, developing suitable A units plays a crucial role in organic electronics. As shown in Figure 1a−c, a great number of A units have been reported, such as (a) benzothiadiazole (BT)based units,8 (b) diimide-based units,9 and (c) diketopyrrolopyrrole (DPP)- and isoindigo (IID)-based units.10 Among © XXXX American Chemical Society

Received: February 20, 2019 Revised: May 22, 2019

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DOI: 10.1021/acs.macromol.9b00370 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Typical acceptor units used in low-bandgap D−A conjugated polymers: (a) BT-based A units, (b) diimide-based A units, (c) DPP- and IID-based A units, and (d) quinoidal A units.

quinoidal structure stability.13 Therefore, it is necessary to spend more efforts developing new quinoidal A units since their D−A copolymers are promising in organic electronics, especially in the near-infrared (NIR) applications.4 In this contribution, we design new quinoidal A units with the assistance of the diradicaloid theory since this theory is extremely significative to design new quinoidal structures but has not been utilized in developing CPs for organic electronics.14 Similar to those structures in Figure 1d, diradicaloids could be drawn as the resonance of open-shell (aromatic form) and closed-shell (quinoidal form), and their diradical characters correlate to the ratio of the closed-shell to open-shell population. Two important parameters are critical in diradicaloid studies.14b,c The first is the diradical character index (y0, in the range of 0−1), which is defined as the weight of the open-shell resonance form in the ground state; i.e., 1 means 100% open-shell. The second is the ΔES−T, which indicates the energy difference between spin-paired singlet state and spin-parallel triplet state (ΔES−T = ES − ET). For larger absolute values of ΔES−T (in the case of ΔES−T < 0), the molecule has a greater tendency to occupy the spin-paired singlet state as the ground state and is more chemically stable. To demonstrate our strategy, we use the IID as the basic structure to design new quinoidal A units, since it is a famous dye with decent electron-withdrawing ability and the center CC bond of IID could be tailored by inserting a quinoidal

subunit in between two indolinone subunits.10d As shown in Figure 2, four basic quinoidal subunits are introduced as modifications to the IID chemical structure, including quinoidal benzene (QB), quinoidal thiophene (QT), quinoidal bithiophene (QBT), and quinoidal thieno[3,2-b]thiophene (QTT), and the resulting model molecules are namely IQB, IQT, IQBT, and IQTT, respectively. Their structures are resonances between aromatic (open-shell, highlighted in red in Figure 2) and quinoidal (closed-shell, highlighted in blue in Figure 2) forms. Because they are diradicaloids, stability becomes a key factor that determines whether these structures could be obtained and applied in organic electronics.11c,14 Two crucial topics addressed are how the quinoidal subunit variations affect the stability and how effective this approach is at narrowing the bandgap. To investigate the two topics listed above, we quantitatively evaluated four model molecules’ diradical characters through theoretical calculation and concluded that the unreported structure of IQTT is stable. Then IQTT was chosen in the synthesis of a new D−A CP (PIQTT) to compare with the IID-based reference polymer PITT. Both polymers’ absorption, energy levels, thermal properties, OFET mobilities, and thin film orientations were measured to understand the new CP design strategy. B


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Macromolecules

Figure 2. Quinoidal design strategy and four quinoidal model molecules.

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RESULTS AND DISCUSSION Diradical Character Simulation. As shown in Table 1, four model molecules’ diradical characters were evaluated by

Because IQT is stable, one can further extend the center quinoidal thiophene to increase the molecular conjugation length. Then IQBT was designed, which had been reported for a long time with a formal name of indophenine19 but without disclosing any diradical character information. Herein, it showed the y0 of 0.30 and ΔES−T of −13.4 kcal/mol, which suggested that IQBT gained a significantly improved diradical character than IQT. However, there was a synthetic difficulty of IQBT in the purification due to its six possible stereoisomers,19d−g and no pure IQBT was reported in the literature. Based on the above studies, a new molecule of IQTT with a QTT center core was then designed. IQTT was pseudolinear20 and had an enlarged π-plane relative to IQT and had less possible stereoisomers in the synthesis relative to IQBT. Though there was no suggested reference value of ΔES−T to determine whether a diradicaloid was stable or not, we were able to use the values of ΔES−T (IQB), ΔES−T (IQT), and ΔES−T (IQBT) as references to judge the stability in designing similar IID-based diradicaloids. For IQTT, the y0 was 0.24 and ΔES−T was −19.0 kcal/mol. The diradical character of IQTT was in between that of IQT and IQBT, which suggested that the IQTT should be stable since both IQT and IQBT had been obtained in the literature. In addition, the fact that both IQBT and IQTT had greater diradical character than IQT agreed well with the finding in the literature that diradical character increased with the extension of center quinoidal structure,18 which would be beneficial to further reduce the bandgap. Synthesis. We then synthesized the IQTT-based monomer and copolymer to verify the above design shown in Scheme 1. 1-(5-Decylheptadecyl)-6-bromoisatin and 6,6′-dibromo-N,N′(5-decylheptadecyl)isoindigo (IIDBr) were synthesized according to reported methods;10b−e other compounds were obtained from commercial resources. First, n-butyllithium was

Table 1. y0 and ΔES−T Values of Four Model Molecules model molecule

IQB

IQT

IQBT

IQTT

y0 ΔES−T (kcal/mol)

0.33 −10.3

0.19 −24.3

0.30 −13.4

0.24 −19.0

calculating the y0 and ΔES−T values with quantum mechanics simulations14b,15 (see the Supporting Information, pp 3−13). For IQB, the computed y0 was 0.33 and ΔES−T was −10.3 kcal/mol. The y0 value indicated that IQB had a relatively large tendency to be open-shell. Moreover, the relatively small |ΔES−T| suggested that the energy barrier from the ground state singlet to triplet could be easily overcome; consequently, two electrons of the open-shell IQB were highly reactive through spin-unpaired triplet state, and the molecule might be difficult to synthesize. This was further confirmed by several failed attempts of synthesizing similar QB-based molecules, which underwent oligomerization spontaneously during preparation.16 On the contrary, the IQT-based small molecules and polymers were reported without any stability issue.17 Our calculations showed that the y0 of IQT decreased to 0.19 and ΔES−T of IQT changed to −24.3 kcal/mol, indicating IQT had a smaller diradical character as compared to IQB while the relatively large energy barrier (|ΔES−T|) guaranteed good stability. It is well-known that the aromaticity of benzene is significantly greater than that of thiophene, which also accounts for the much stronger diradical character of IQB than IQT.18 C


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Macromolecules Scheme 1. Synthesis of IQTTBr, PIQTT, and PITT

Geometry, Orbital, and Bond Length Simulation. To gain a general structural insight into the IQTT-based polymer, we calculated molecular geometries, the highest occupied molecular orbital (HOMO) distribution, and the lowest unoccupied molecular orbital (LUMO) distribution through density functional theory (DFT) using the CAM-B3LYP/631G(d,p) method.21 We performed the calculation by using the closed-shell (restricted) model trimers and extracted data from the central repeat unit to better reflect molecular structure in the polymer system. Figure 3a shows the

added dropwise into 2,5-dibromothieno[3,2-b]thiophene solution at −78 °C, and then the mixture was added into the 1(5-decylheptadecyl)-6-bromoisatin solution at −78 °C for 30 min. The reaction was kept stirring overnight at room temperature to produce 2,5-bis[6-bromo-1-(5-decylheptadecyl)indolin-3-hydroxy-2-one-3-yl]thieno[3,2-b]thiophene (ITT-OH), and then ITT-OH was reduced by using stannous chloride in chloroform to give the final product of (3Z,3′Z)-3,3′-(thieno[3,2-b]thiophene-2,5-diylidene)bis[6bromo-1-(5-decylheptadecyl)indolin-2-one] (IQTTBr) for polymerization. The structure of IQTTBr was confirmed by two-dimensional 1H−1H nuclear Overhauser effect spectroscopy (2D 1H−1H NOSEY) as there was a cross-talking between H (highlighted in red, Scheme 1) on the QTT and H (highlighted in blue, Scheme 1) on the indolinone (see Figure S7). The other two stereoisomers of IQTTBr were difficult to purify through chromatography and not isolated in this study. Then the polymer PIQTT was synthesized by Stille polycondensation of 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene and IQTTBr under the presence of Pd(PPh3)4 using a mixed solvent of o-xylene/DMF. The polymerization was performed for 24 h at 140 °C. The reference polymer PITT was synthesized under similar conditions. Molecular weights of PIQTT and PITT were measured at 150 °C by gelpermeation chromatography by using polystyrene as the standard and 1,2,4-trichlorobenzene as the eluent, which gave a number of average molecular weight (Mn) of 13.6 kDa and a polydispersity index (PDI) of 3.5 for PIQTT and a Mn of 51.6 kDa and a PDI of 1.2 for PITT, respectively. We tried several polymerization conditions but could not further increase the molecular weight of PIQTT. Thermogravimetric analysis showed that both polymers were stable below 395 °C (Figure S17). Differential scanning calorimeter measurements were performed, but no obvious thermal transitions were found in the temperature range 30−330 °C for both polymers (Figure S18).

Figure 3. DFT calculations of PITT and PIQTT (cropped images of the center repeat unit): (a) geometries, (b) HOMO, and (c) LUMO.

optimized geometries. For PITT, the dihedral angles labeled 1 and 2 were 6° and 27°, respectively; for PIQTT, the dihedral angles labeled 3 and 4 were about zero, and the dihedral angle of 5 was 29°. These results suggested that the IQTT was slightly more planar than IID, a trend that propagated in PIQTT vs PITT. Figure 3b shows the HOMO distribution, and both polymers’ HOMO were delocalized along the backbone. Figure 3c displayed the LUMO distribution. For PITT, one could clearly see that the LUMO were mainly localized on the IID unit while there were still a small amount LUMO distributed on the TT. Interestingly, for PIQTT, it was found that most of the LUMO were localized on the IQTT D


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Macromolecules unit, which suggested that IQTT was a very strong acceptor unit7,10b,c and had a greater capacity to accept electrons relative to IID in this D−A system. Besides the D−A effect, the BLA of the above two trimers was also theoretically calculated by using the same method to investigate the bond length changes when QTT subunits were inserted into PITT polymer chain to produce PIQTT. As shown in Figure 4, we chose the similar part in both PITT and

Figure 5. (a) Normalized absorption profiles of IIDBr and IQTTBr solutions (0.01 mg/mL in chloroform) and PITT and PIQTT in solution (0.01 mg/mL in chlorobenzene) and in thin film. (b) Cyclic voltammetry measurements of PITT and PIQTT thin films.

Figure 4. Calculated bond lengths of 15 bonds in PITT and PIQTT trimers (see Figure S16).

nm for PIQTT, which corresponded to thin film optical bandgaps (Egopt) of 1.61 and 1.28 eV, respectively. In short, the introduction of QTT unit in the PITT backbone reduced the Egopt by ca. 0.33 eV, which supported that IQTT was a stronger A unit relative to IID since their D units were the same, as well as the BLA of PIQTT was decreased relative to that of PITT. The ionization potential (IP) and electron affinity (EA) were measured from thin films by cyclic voltammetry.23 As shown in Figure 5b, PITT displayed an oxidation onset at 0.44 V corresponding to an IP of 5.24 eV and a reduction onset at −1.44 V corresponding to an EA of 3.36 eV. The electrochemical bandgap (EgEC) was 1.88 eV. PIQTT showed a slightly lower oxidation onset at 0.39 V, corresponding to an IP of 5.19 eV, but the reduction onset was significantly different than that of PITT, which was −1.17 V, corresponding to an EA of 3.63 eV. The EgEC was 1.56 eV. The main difference lying in EA (0.27 eV) again pointed out that IQTT had a greater ability to pull electrons relative to IID. OFET Fabrication and Performance. Because the IIDbased D−A CPs exhibited excellent mobility in OFET applications,10b,c we then measured the OFET mobility of PIQTT side by side with PITT. The bottom-gate, top-contact (BG-TC) device configuration was used in this study. Their average mobility, on/off ratio, and threshold voltage (Vt) under different annealing conditions are summarized in Table 2. The average mobilities were calculated from three devices, and their standard deviations were negligible. Both polymers displayed the best device performance under the annealing condition of 300 °C, and their transfer I−V curves are shown in Figure 6. More device fabrication and other experiment details are provided in the Supporting Information. As shown in Table 2, when films were annealed at 200 °C, PITT exhibited a relatively low mobility of 0.077 cm2/(V s).

PIQTT trimers (see Figure S16) to learn the influence of introducing QTT in PITT on bond lengths. Bonds 1 and 15 in PIQTT were single bonds and adjacent to QTT. Their bond lengths were significantly reduced relative to those bonds in PITT by ca. 0.017 Å. Bonds 2 and 14 in PIQTT were also influenced by QTT. Their bond lengths were reduced relative to those bonds in PITT by ca. 0.005 Å. Other bond lengths in PIQTT were almost identical to their counterparts in PITT since they were too far to QTT. Overall, these results suggested that the introduction of QTT did decrease the BLA as well as the bond lengths, which should reduce the bandgap of PIQTT in comparison of PITT.11b,c,12 Absorption and Energy Levels. UV−vis−NIR absorption measurements were then performed. First, we measured the two monomers’ absorption in chloroform solution. As shown in Figure 5a, IQTTBr displayed a substantially redshifted absorbance relative to the IIDBr unit, which was attributed to the diradical character.14b,18 For polymers’ solution absorption, PITT showed a weak π−π* transition at ca. 460 nm and an intramolecular charge transfer (ICT) transition at ca. 650 nm together with a transition at ca. 716 nm corresponding with the well-ordered chain aggregation.10c,22 Interestingly, for PIQTT solution, the π−π* transition was not pronounced around 505 nm. The ICT transition was found at ca. 789 nm together with a transition at ca. 876 nm that was correlated to the well-ordered chain aggregation.10c,22 In addition, the main absorption peak of PIQTT was significantly broader relative to that of PITT. In the thin film, both PITT and PIQTT showed similar absorptions relative to those in solution, which indicated that both polymers were preaggregated in their solutions.10d The film absorption onsets were about 770 nm for PITT and 969 E


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Macromolecules Table 2. Mobility, On/Off Ratio, and Vt of PITT and PIQTT OFETs device PITT

PIQTT

annealing temp (°C)

μsat (cm2/(V s))

on/off

Vt (V)

200 250 300 200 250 300

0.077 0.20 0.20 0.024 0.047 0.13

103−104 104 105 104 104−105 104−105

−28.2 −16.9 −3.8 −26.7 −25.4 −10.4

Figure 7. (a) 2D-GIXD images of PITT and PIQTT thin films annealed at 300 °C. (b) Cake-cut 2D-GIXD curves of PITT and PIQTT thin films annealed at 300 °C.

Figure 6. Transfer curves of PITT and PIQTT devices annealed at 300 °C.

chains were the same, it was possible that PIQTT polymer chains were more interdigitated and therefore decreased the dlamella.27 In all, PIQTT exhibited a better thin film order than PITT which agreed with the geometry calculations. Because the molecular weight also played an important role in the charge transport, PITT still showed slightly better OFET performance relative to PIQTT.

Then the mobility increased to 0.20 cm2/(V s) when films were annealed at 250 and 300 °C. For PIQTT BG-TC devices, the mobility was 0.024 cm2/(V s) when the film was annealed at 200 °C and increased to 0.047 cm2/(V s) when it was annealed at 250 °C. The best performance was achieved at the annealing temperature of 300 °C with a maximum mobility of 0.13 cm2/(V s). The mobility of PITTQ was slightly lower than that of PITT, though the DFT calculation suggests that PITTQ backbone was slightly more planar than that of the PITT backbone. One possible reason was that the higher molecular weight of PITT relative to PIQTT could facilitate the charge transport.24 Thin Film Organizations. Two-dimensional grazingincidence X-ray diffraction (2D-GIXD) was used to gain insight into the thin film organization.25 Figure 7a shows the recorded images of thin films annealed at 300 °C. Other diffraction images are provided in Figure S22. As shown in Figure 7a, one could clearly see both polymers’ first (100), second (200), and third (300) order peaks from the lamella stacking, together with the distinguished (010) peaks from the π−π stacking. In addition, it was found that PIQTT film had an edge-on orientation as its (010) peak located at the in-plane direction (qxy) while its (100), (200), and (300) peaks located at the out-of-plane direction (qz), suggesting that PIQTT film formed a desirable in-plane charge transport pathway.3a For the PITT film, there was no preferential orientation. Their corresponding cake-cut curves are provided in Figure 7b, which were integrated from a wide polar angle26 range of 0°− 90° to analyze stacking distances under a better intensity. For PITT, the lamella distance (dlamella) was 2.76 nm according to the (100) peak at Q = 2.28 nm−1, while the π−π stacking distance (dπ−π) was 0.34 nm according to the (010) peak location. For PIQTT, dπ−π was similar to dπ−π (PITT), but dlamella was 2.53 nm according to its (100) peak at Q = 2.48 nm−1, which was shorter than that of PITT. Because their alkyl

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CONCLUSION In summary, a new quinoidal acceptor building block of IQTT is designed with the assistance of quantum mechanics simulations on diradical character. Then the IQTT-based monomer and copolymer (PIQTT) are successfully synthesized. The studies show that IQTT has a moderate diradical character and displays a significantly stronger electronwithdrawing ability relative to IID; hence, the bandgap of PIQTT is successfully minimized relative to PITT. In addition, the geometry simulations suggest that PIQTT has a more planar backbone than PITT. Further mobility and 2D-GIXD measurements both confirm that PIQTT is a promising polymer for OFET applications. Most importantly, we use the quantum chemical simulation tools developed in diradicaloid studies to screen stable quinoidal acceptor build blocks for D− A CPs, demonstrating a powerful approach to develop new narrow-bandgap CPs. This work provides a new aspect for chemist to design novel conjugated molecules for organic electronics.

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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.9b00370. Details of quantum chemical simulations, materials synthesis, NMR characterization, TGA and DSC curves, F


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OFET device fabrication and characterizations, and 2DGIXD images (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(M.W.) E-mail: [email protected]. *(Y.L.) E-mail: [email protected]. ORCID

Yuanyuan Hu: 0000-0001-8511-1401 Yong Liang: 0000-0001-5026-6710 Ming Wang: 0000-0003-4689-6538 Elsa Reichmanis: 0000-0002-8205-8016 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Fundamental Research Fund for the Central Universities, the National Natural Science Foundation of China (No. 21805032), the National Thousand Young Talents Program, the Jiangsu Specially-Appointed Professor Plan, and the Natural Science Foundation of Jiangsu Province (BK20170631) in China. We thank Dr. Chuncheng Liu and Dr. Michael J. Ford for help in finalizing the manuscript.

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