Low Band Gap DonorâAcceptor Conjugated Polymers with Indanone

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

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Low Band Gap Donor−Acceptor Conjugated Polymers with Indanone-Condensed Thiadiazolo[3,4‑g]quinoxaline Acceptors Huajie Chen,*,† Guosheng Cai,† Ankang Guo,‡ Zhiyuan Zhao,‡ Junhua Kuang,‡ Liping Zheng,† Lingli Zhao,† Jinyang Chen,‡ Yunlong Guo,*,‡ and Yunqi Liu*,‡ †

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Key Laboratory of Environmentally Friendly Chemistry and Application of the Ministry of Education, and Key Laboratory for Green Organic Synthesis and Application of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan 411105, China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Donor−acceptor (D−A) conjugated polymers with the band gaps below 1.0 eV can exhibit unique near-infrared (NIR) activities and multiple functional applications. However, it is still a big challenge to develop such materials because of the scarcity of effective synthetic strategies and strong acceptor building blocks. Herein, we report the design, synthesis, and application of two novel indanone-condensed thiadiazolo[3,4-g]quinoxaline (TQ) acceptor units, which display high electron affinities and low-lying lowest unoccupied molecular orbital (LUMO) levels because of the incorporation of auxiliary electron-deficient carbonyl or cyano groups into the TQ core. Moreover, two low band gap D−A conjugated polymers are synthesized via Stille condensation reactions between the newly developed TQ acceptor units and 2,5-bis(3-(2-decyltetradecyl)thiophen-2-yl)thieno[3,2-b]thiophene donor units. The effect of the substitute groups (carbonyl and cyano groups) on the geometry, optical property, electronic structure [highest occupied molecular orbital (HOMO)/ LUMO levels and band gap], film organization, and charge transport of the polymers are discussed carefully. The resulting polymers exhibit very broad NIR absorptions extended to around 1880 nm, deep-lying LUMO energy levels ( 1.0 eV) D−A conjugated polymers, which led by the polymers containing imide- or amide-containing dyes,27 such as isoindigo,28−30 diketopyrrolopyrrole,31−33 and naphthalenediimide.34−37 Nevertheless, it is still a big challenge to reduce the band gaps of the D−A conjugated polymers below 1.0 eV because of the limited availability of synthetic strategies and strong acceptor units. So far, the widely used strong acceptors are several thiadiazole- and pryazine-containing oquinoidal heterocycles, such as thieno[3,4-c][1,2,5]thiadiazole (TT),11,38,39 thieno[3,4-b]pyrazine,20−26,40 benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazole (BBT), 41−44 thiadiazolo[3,4-g]q u i n o x a l i n e (T Q ) , 8 , 9 , 4 4 − 5 0 a n d p y r a z i n o [ 2 , 3 -g ] quinoxaline.51−53 Consequently, the continued exploration of strong acceptor building blocks and their low band gap D−A conjugated polymers is still worthy of being studied for synthetic chemists. Because of its highly electron-deficient nature and large πconjugation backbone, TQ and its derivatives are regarded as the strong acceptor units and widely utilized to develop D−A conjugated polymers with low band gaps and various functional applications, especially in ambipolar FETs.8,9,46−48 Compared with BBT analogues, the unique advantage for the B

DOI: 10.1021/acs.macromol.9b00834 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

for two steps). The melting point cannot be observed at >270 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.20 (d, J = 8.0 Hz, 1H), 8.01 (d, J = 8.0 Hz, 1H), 7.86 (t, J = 8.0 Hz, 1H), 7.72 (t, J = 8.0 Hz, 1H), 1.36−1.26 (m, 42H). 13C NMR (100 MHz, CDCl3), δ (ppm): 188.08, 155.93, 155.51, 154.47, 152.21, 141.09, 139.09, 137.08, 133.60, 124.74, 123.58, 118.21, 115.21, 112.81, 109.77, 100.48, 100.16, 18.89, 18.80, 11.53, 11.51. FT-IR data: νCO, 1735 cm−1. HRMS m/z: [M]+ calcd for (C37H46N4OSSi2), 650.2925; found, 650.2927. Synthesis of TQCN-TIPS. Under nitrogen, 0.6 mL of TiCl4 was slowly added into a mixed solution of TQ-TIPS (200 mg, 0.31 mmol), malononitrile (79 mg, 1.2 mmol), 30 mL of dichloromethane, and 1 mL of pyridine. This mixture was heated up to 60 °C. After 12 h, the reaction solution was extracted with dichloromethane and then dried over magnesium sulfate. Finally, dichloromethane was removed by reduced pressure to afford a dark green solid, which was further purified by column chromatography (petroleum ether and dichloromethane, v/v = 2:1) (121 mg, 56%). The melting point cannot be observed at >270 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.65 (d, J = 8.0 Hz, 1H), 8.22 (d, J = 4.0 Hz, 1H), 7.84 (t, J = 8.0 Hz, 1H), 7.75 (t, J = 8.0 Hz, 1H), 1.40−1.24 (m, 42H). 13C NMR (100 MHz, CDCl3), δ (ppm): 155.47, 155.37, 153.63, 153.57, 151.74, 138.14, 137.09, 136.10, 133.75, 126.57, 124.09, 118.09, 115.31, 114.54, 113.30, 110.15, 100.33, 100.24, 79.21, 77.22, 18.88, 11.52, 11.37. FTIR data: ν −CN , 2225 cm −1 . HRMS m/z: [M] + calcd for (C40H46N6SSi2), 700.3194; found, 700.3192. Synthesis of BTTQ. Under nitrogen, compound 7 (1.0 g, 0.94 mmol), iron powder (0.63 g, 11.2 mmol), and 30 mL of acetic acid were added into a 200 mL flask. The mixed solution was stirred at 75 °C for 5 h and then extracted with dichloromethane. The organic phase was washed with 5% aqueous NaOH until a pH value of 6−7 and then dried over magnesium sulfate. After removal of dichloromethane, a deep red diamine 8 was obtained in high yields and used directly to synthesize the target compound BTTQ. Subsequently, a condensation coupling reaction between diamine 8 and compound 5 (460 mg, 2.58 mmol) was performed at 80 °C in alcohol (15 mL). After 2 h, the organic phase was separated by extraction with dichlormethane and then dried over magnesium sulfate. Finally, dichlormethane was removed under reduced pressure to yield a deep green solid, which was further purified by column chromatography (petroleum ether/dichloromethane, v/v = 2:1) (848 mg, 80% for two steps). mp = 82−83 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.77 (s, 1H), 8.74 (s, 1H), 8.18 (d, J = 8.0 Hz, 1H), 7.98 (d, J = 8.0 Hz, 1H), 7.82 (t, J = 8.0 Hz, 1H), 7.68 (t, J = 6.0 Hz, 1H), 7.28 (s, 1H), 7.13 (s, 1H), 2.68−2.63 (m, 4H), 1.73 (br, 2H), 1.46−1.32 (m, 16H), 1.32−1.20 (m, 64H), 0.86 (t, J = 8.0 Hz, 12H). 13C NMR (100 MHz, CDCl3), δ (ppm): 188.08, 153.04, 152.60, 151.70, 149.28, 141.59, 141.31, 140.84, 138.96, 136.96, 136.47, 135.79, 135.57, 135.07, 135.03, 132.77, 130.45, 128.13, 124.75, 124.39, 124.01, 121.70, 39.01, 38.96, 34.95, 33.42, 31.95, 30.21, 29.83, 29.80, 29.77, 29.72, 29.70, 29.41, 29.39, 26.76, 22.71, 14.13 (some peaks are overlapped). FT-IR data: νCO, 1724 cm−1. HRMS m/z: [M]+ calcd for (C71H106N4OS3), 1126.7523; found, 1126.7517. Synthesis of BTTQ-2Br. Under an ice bath, to a solution of BTTQ (0.53 g, 0.47 mmol) and chloroform (20 mL), a mixed solution of NBS (210 mg, 1.17 mmol) and DMF (5 mL) was added slowly. After being stirred for 5 h at 30 °C, the organic phase was extracted with dichlormethane and dried over MgSO4. The obtained filtrate was subjected to reduced distill and then column chromatography (petroleum ether/dichloromethane, v/v = 3:1) to yield a dark green solid (429 mg, 71%). mp = 114−115 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.61 (s, 1H), 8.56 (s, 1H), 7.94 (d, J = 8.0 Hz, 1H), 7.91 (d, J = 4.0 Hz, 1H), 7.80 (t, J = 6.0 Hz, 1H), 7.67 (t, J = 6.0 Hz, 1H), 2.58 (d, J = 8.0 Hz, 2H), 2.44 (d, J = 8.0 Hz, 2H), 1.79−1.72 (br, 2H), 1.44−1.32 (m, 16H), 1.32−1.18 (m, 64H), 0.87−0.84 (m, 12H). 13C NMR (100 MHz, CDCl3), δ (ppm): 186.90, 152.85, 151.64, 150.78, 148.91, 140.92, 140.46, 140.10, 138.83, 136.58, 135.21, 134.90, 134.78, 134.47, 134.00, 133.06, 124.43, 124.12, 123.16, 122.26, 120.09, 119.48, 38.58, 38.51, 34.12, 33.37, 31.94, 30.26, 30.23, 29.83, 29.79, 29.76, 29.74, 29.71, 29.41,

outstanding advantage for both novel acceptors is that the ITQ acceptor can be directly used to produce a much stronger acceptor ITQCN via functionalization with cyano groups, which offers a great convenience to further enhance the electron affinity and reduce the band gap of the target polymers. As revealed by density functional theory (DFT) calculations, both ITQ and ITQCN units exhibit much lower LUMO energy levels relative to the reported TQ analogues (TTQ, BTQ, BDTTQ, PhTQ, and APhTQ, Figure 2). This reduced LUMO values can be ascribed to the introduction of the auxiliary carbonyl and cyano groups into the parent TQ core. In particular, the remarkably low-lying LUMO value (−3.8 eV) of ITQCN suggests that it should have much stronger electron affinity compared to that of highly electronwithdrawing acceptor BBT (−3.6 eV, Figure 2). Moreover, the calculated HOMO values of both ITQ and ITQCN units are 0.2−0.4 eV lower than that of BBT, which may enable their corresponding copolymers to obtain reduced HOMO values relative to the BBT-containing analogues. On the basis of the newly developed TQ acceptor units, we reported the synthesis and charge transport properties of two novel low band gap D−A conjugated polymers (PBTTQ-TT and PBTTQCN-TT, Figure 2), in which 2,5-bis(3-(2decyltetradecyl)thiophen-2-yl)thieno[3,2-b]-thiophene units were used as the electron donor segment. Moreover, we investigated the effect of substitute groups (carbonyl and cyano groups) on the absorption, band gap, film organization, as well as charge transport of all the TQ-based derivatives. The results indicate that both polymers achieve broad NIR absorption bands extended to ca. 1880 nm and low optical band gap (Eopt g ). Compared with the carbonyl-containing PBTTQ-TT (Eopt g = 0.75 eV), the incorporation of cyano groups into the backbone of PBTTQCN-TT leads to a reduced Eopt g of ca. 0.66 eV, which is one of the lowest Eopt values among the reported g TQ-based polymers, such as PTTQT (1.27 eV),46 PTTQ (0.64 eV),8 PBDTTQ-3 (0.76 eV),9 PPhTQ (0.80 eV),48 PAPhTQ (0.99 eV),9 and PTTTQ (0.95 eV)54 (Figure 2). Moreover, such narrow Eopt g even can be comparable to the classical BBT-containing polymer (PBBTTT, 0.56 eV).7

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EXPERIMENTAL SECTION

Materials. The starting materials, including 4,7-dibromo-5,6dinitro-2,1,3-benzothiadiazole (2), 2,2-dihydroxy-1H-indene1,3(2H)-dione (5), and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene, were purchased from commercial corporations (Chem Great Wall or Alfa Aesar) and used directly. Compounds 1trimethylstannyl-2-triisopropylsilylethyne (1),57 5,6-dinitro-4,7-bis((triisopropylsilyl)ethynyl)-benzo[c][1,2,5]thiadiazole (3),58 2-trimethylstannyl-4-(2-decyltetradecyl)thiophene (6),9 and 4,7-bis(4-(2decyltetradecyl)thiophen-2-yl)-5,6-dinitrobenzo-[c][1,2,5]thiadiazole (7)9 were synthesized and purified using the reported methods. Synthesis of TQ-TIPS. Under nitrogen, a mixed solution of dinitro compound 3 (1.5 g, 2.6 mmol), iron powder (1.7 g, 30.4 mmol), and 60 mL of acetic acid was stirred at 75 °C. After 5 h, the resultant solution was extracted with dichloromethane and washed with 5% aqueous NaOH until a pH value of 6−7. The collected organic layer was then dried over magnesium sulfate. After removal of dichloromethane, a yellow oil (diamine 4) was obtained, which was then used directly to prepare compound TQ-TIPS without further purification. Subsequently, compound 4 was added into a solution of 15 mL of alcohol and compound 5 (460 mg, 2.58 mmol). Under nitrogen, this reaction solution was refluxed for 2 h, then extracted with dichlormethane, and finally dried over magnesium sulfate. The obtained crude product was further purified by column chromatography (dichloromethane and petroleum ether, v/v = 1:1) (1.02 g, 60% C


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Scheme 1. Synthetic Routes to the TQ-Based Model Molecules (TQ-TIPS and TQCN-TIPS) and Copolymers (PBTTQ-TT and PBTTQCN-TT)

29.39, 26.64, 26.61, 22.70, 14.13 (some peaks are overlapped). FT-IR data: ν C  O , 1726 cm − 1 . HRMS: m/z: [M] + calcd for (C71H104Br2N4OS3), 1282.5733; found, 1282.5724. Elm. Anal. Calcd for C71H104Br2N4OS3: C, 66.33; H, 8.15; N, 4.36. Found: C, 66.51; H, 8.24; N, 4.46. Synthesis of BTTQCN-2Br. Malononitrile (0.1 g, 1.51 mmol), BTTQ-2Br (0.5 g, 0.39 mmol), dichloromethane (50 mL), and pyridine (3 mL) were added into a 200 mL flask and then protected by nitrogen. After that, 0.5 mL of TiCl4 was dropped into this reaction solution slowly at 0 °C. The resultant solution was then stirred at 40 °C for 15 h. The resulting solution was extracted with dichlormethane and dried over magnesium sulfate. After removing dichlormethane, the obtained crude product was further subjected to column chromatography (petroleum ether/dichloromethane, v/v = 2:1). A dark green solid was obtained after removing the solvent (432 mg, 83%). mp = 167−168 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.65 (s, 1H), 8.54 (d, J = 8.0 Hz, 1H), 8.32 (s, 1H), 7.94 (d, J = 8.0 Hz, 1H), 7.70 (t, J = 6.0 Hz, 1H), 7.63 (t, J = 6.0 Hz, 1H), 2.59 (d, J = 8.0 Hz, 2H), 2.50 (d, J = 8.0 Hz, 2H), 1.75 (br, 2H), 1.46−1.31 (m, 16H), 1.31−1.13 (m, 64H), 0.87−0.83 (m, 12H). 13C NMR (100 MHz, CDCl3), δ (ppm): 153.63, 151.64, 150.91, 150.21, 149.01, 142.40, 141.06, 136.91, 135.60, 134.48, 134.42, 134.30, 134.20, 133.18, 126.20, 124.60, 123.36, 120.88, 120.38, 120.35, 113.56, 112.79, 78.15, 77.22, 38.54, 34.13, 33.69, 33.38, 33.30, 31.95, 30.26, 30.22, 29.81, 29.78, 29.75, 29.73, 29.70, 29.40, 26.58, 22.70, 14.13 (some peaks are overlapped). FT-IR data: ν−CN, 2225 cm−1. HRMS m/z: [M]+ calcd for C74H104Br2N6S3: 1330.5846; found, 1330.5838. Elm. Anal. Calcd for C74H104Br2N6S3: C, 66.64; H, 7.86; N, 6.30. Found: C, 66.78; H, 8.02; N, 6.47. Stille Polymerization of PBTTQ-TT. Chlorobenzene (CB, 3 mL), 9 mg of Pd 2 (dba) 3 , 15 mg of P(o-tol) 3 , 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (90 mg, 0.19 mmol), and BTTQ-2Br (250 mg, 0.19 mmol) were added into a 20 mL flask and then protected by nitrogen atmosphere. This mixture was heated up to 115 °C and then stirred for 72 h. The reaction mixture was

dropped into 200 mL of methanol solution with 6 mL of hydrochloric acid (HCl, 12 M) and then stirred for 3 h at 30 °C. The collected precipitate was further purified via Soxhlet extraction method. Ethanol, acetone, hexane, and chlorobenzene were used to purify polymer sample sequently. A dark polymer solid was obtained after removal of chlorobenzene (205 mg, 90%). 1H NMR (400 MHz, CDCl3), δ (ppm): see Figure S20. Molecular weight by GPC: Mn = 15.1 kDa, Mw = 39.9 kDa, PDI = 2.64. FT-IR data: νCO, 1724 cm−1. Elm. Anal. Calcd for (C77H108N4OS5)n: C, 73.05; H, 8.60; N, 4.43. Found: C, 73.29; H, 8.57; N, 4.72. Stille Polymerization of PBTTQCN-TT. Chlorobenzene (5 mL), BTTQCN-2Br (240 mg, 0.18 mmol), 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (84 mg, 0.18 mmol), 9 mg of Pd2(dba)3, and 15 mg of P(o-tol)3 were added into a 20 mL flask. This mixture was protected by nitrogen and then stirred at 115 °C for 72 h. The purification procedure is similar to PBTTQ-TT. A dark polymer solid was obtained from chlorobenzene fraction (220 mg, 93%). 1H NMR (400 MHz, CDCl3), δ (ppm): see Figure S21. Molecular weight by GPC: Mn = 24.9 kDa, Mw = 75.4 kDa, PDI = 3.03. FT-IR data: ν−CN, 2221 cm−1. Elm. Anal. Calcd for (C80H108N6S5)n: C, 73.12; H, 8.28; N, 6.40. Found: C, 73.57; H, 8.31; N, 6.54.

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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 shows the synthetic routes and molecular structures of the newly developed TQ-based derivatives. The synthesis of both model molecules (TQ-TIPS and TQCN-TIPS) started from Stille coupling reaction between 1-trimethylstannyl-2-triisopropylsilylethyne (1) and 4,7-dibromo-5,6-dinitrobenzothiadizole (2) to give compound 3 in high yields >90%. After that, the iron-catalyzed reduction of dinitro compound 3 was performed to afford diamine 2. Finally, the condensation coupling reaction between diamine 2 and compound 5 was D


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Macromolecules readily carried out to give the first model molecule (TQTIPS), which was then converted to another model compound (TQCN-TIPS) via Knoevenagel condensation under the conditions of malononitrile and TiCl4. The chemical structures of TQ-TIPS and TQCN-TIPS were confirmed by three characterization techniques, including 1H and 13C NMR spectra and single crystals. As illustrated in Scheme 1, the synthetic method of the two thiophene-containing monomers is similar to the two model molecules except for different starting reagents. The Stille coupling reaction between 2-trimethylstannyl-4-(2decyltetradecyl)thiophene (6) and compound 2 produced the dinitro compound 7. After reduction of nitro compound 7, the corresponding diamine 8 was obtained and then directly condensed with compound 5 to afford the key thiophenecontaining intermediate BTTQ. Subsequently, dibromination of BTTQ with NBS was conducted to afford the desired monomer BTTQ-2Br in a high yield of 71% and then followed by Knoevenagel condensation of BTTQ-2Br with malononitrile to give another monomer BTTQCN-2Br in a good yield of 83%. Finally, the target copolymers (PBTTQ-TT and PBTTQCN-TT) were prepared via Stille reactions between dibrominated TQ monomers (BTTQ-2Br or BTTQCN-2Br) and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene. Both precipitation and Soxhlet extraction techniques were used to purify two copolymers. Because of the attachment of appropriate solubilizing side chains, both polymers can be easily dissolved in most halogenated solvents such as chloroform, CB, dichlorobenzene, and trichlorobenzene (TCB) at 25 °C. To evaluate the molecular weight and polydispersity indexes (PDIs), gel permeation chromatography (GPC) was conducted at 150 °C, using polystyrene and TCB as the standard and eluent, respectively. The weight-average molecular weights of PBTTQ-TT and PBTTQCN-TT were 39.9 and 75.4 kDa, with the related PDIs of 2.64 and 3.03, respectively (Table1

building blocks, the suitable single crystals for TQ-TIPS and TQCN-TIPS were obtained by slowly evaporating their dichloromethane solution at room temperature. The detailed crystal parameters can be found in Table S1. In TQ-TIPS crystals, a disordered packing was found for the TIPS groups. Fortunately, this negative effect has a negligible influence on the structure analysis.61,62 The solid-state structures were found to be monoclinic and triclinic for TQ-TIPS and TQCNTIPS, respectively. Both molecules exhibit very small twisted dihedral angles of ca. 1.67° for TQ-TIPS (Figure 3a) and ca.

Figure 3. Dihedral angles between the benzene and five-membered carbocycles (blue) and the benzothiazole carbocycles (purple) are ca. 1.67° for TQ-TIPS (a) and 5.92° for TQCN-TIPS (b). Intermolecular interactions of the dimers for TQ-TIPS (c) and TQCN-TIPS (d). van der Waals radius: C = 1.70 Å; N = 1.55 Å; S = 1.80 Å; H = 1.20 Å.

Table 1. Yield, Molecular Weight, and Decomposition Temperature of the Polymers polymer

yield (%)

Mn (kDa)

Mw (kDa)

PDI

Td (°C)

PBTTQ-TT PBTTQCN-TT

90 93

15.1 24.9

39.9 75.4

2.64 3.03

390 368

5.92° for TQCN-TIPS (Figure 3b), indicative of good backbone conjugation and coplanarity. As can be seen from Figure 3c,d, closed head-to-head interactions can be observed between two thiadiazole moieties from each molecules, in which TQ-TIPS shows three types of weak interactions including two N−S (3.20 Å), two S−H (2.91 Å), and two C−H (2.87 Å) interactions, while TQCN-TIPS has only two N−S (3.09 Å) and one N−N (3.06 Å) contacts. Note that all the observed distances are slightly shorter as compared to the combined values of van der Waals radii of two different atoms, such as nitrogen−sulfur (3.35 Å), sulfur−hydrogen (3.00 Å), nitrogen−nitrogen (3.10 Å), and carbon−hydrogen (2.90 Å). As illustrated in Figures S4 and S5, both TQ-TIPS and TQCN-TIPS molecules form the sliped cofacial stacks with different orientations. TQ-TIPS shows strong π−π interactions with very close π−π distances (3.38 and 3.56 Å) (Figure S5). After the attachment of two cyano groups, the π−π distances of TQCN-TIPS slightly increase to 3.40 and 3.66 Å (Figure S5), probably because of its twisted backbone configuration. Optical Properties. Figure 4 illustrates the typical absorption curves of the newly developed TQ-based model molecules, comonomers, and polymers. The absorption data

and Figure S1). The observed PDIs are slightly large, probably because of the strong interchain aggregation in the polymer solutions.59,60 Fourier transform infrared (FT-IR) measurements reveal a typical characteristic band of carbonyl groups (νCO) at ca. 1726 cm−1 for PBTTQ-TT and a clear characteristic band of cyano groups (νCN) at ca. 2223 cm−1 for PBTTQCN-TT, thereby providing a partial evidence to confirm the molecular structure of both polymers (Figure S2). Although 1H NMR spectra of both PBTTQ-TT and PBTTQCN-TT were collected at 373 K, only strong alkyl signals (δ = 3.2−2.5 and 0.5−2.1 ppm) but very weak aromatic signals (δ = 6.5−9.5 ppm) were detected (Figures S20 and S21), indicating that strong interchain aggregations exist and can hardly be broken even in hot C2D2Cl4 solution at 373 K. Investigation of thermal stability revealed that only 5% weight loss was observed for both polymers after heating up to 390 °C for PBTTQ-TT and 368 °C for PBTTQCN-TT (Figure S3). Single-Crystal Structure Analysis. To confirm the molecular structure of the newly developed TQ acceptor E


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Figure 4. Absorption spectra of the model molecules TQ-TIPS (4a) and TQCN-TIPS (4b), comonomers BTTQ-2Br (4c) and BTTQCN-2Br (4d), and polymers PBTTQ-TT (4e) and PBTTQCN-TT (4f).

Table 2. Optical Properties, Redox Potentials, Energy Levels, and Band Gaps of the Newly Developed TQ-Based Molecules sample TQ-TIPS TQCN-TIPS BTTQ-2Br BTTQCN-2Br PBTTQ-TT PBTTQCN-TT

λsol max [nm] 533, 572 570, 616 720 575, 795 1090 1215

λfilm max [nm] 538, 580 578, 630 716, 803 645, 803 1108 1270

λfilm onset [nm] 610 685 900 1050 1648 1880

a Eopt [eV] g

Eonset [V] ox

EHOMO [eV]

Eonset red [V]

ELUMOd [eV]

1.05 1.11 0.64 0.78

−6.12 −6.04b −5.47c −5.53c −5.06c −5.20c

−0.33 −0.19 −0.38 −0.22 −0.52 −0.32

−4.09 −4.23 −4.04 −4.20 −3.90 −4.10

b

2.03 1.81 1.38 1.18 0.75 0.66

e Ecv g [eV]

f Ecal g [eV]

1.43 1.33 1.16 1.10

2.29 2.09 1.71 1.61 1.13 0.98

a c film b opt onset Calculated optical band gap using Eopt + 4.42) g = 1240/λonset. Determined using EHOMO = (ELUMO − Eg ) eV. Calculated using EHOMO = −(Eox e f cv cal + 4.42) eV. Calculated using E = (E − E ) eV. DFT-calculated band gaps using E eV. dCalculated using ELUMO = −(Eonset red g LUMO HOMO g = (ELUMO − EHOMO) eV.

obtained from chloroform (ca. 10−5 M) and thin film are collected in Table 2. In chloroform, both model molecules show multiple absorption peaks (Figure 4a,b), similar to the reported TQ-based heteroacenes.58,63 It is found that incorporation of cyano groups endows TQCN-TIPS with extended absorption onset (650 nm) when compared with TQ-TIPS (600 nm) because of enhanced D−A interaction and overwhelmingly electron-deficient capacity of cyano groups. For both thiophene-containing comonomers, typical threeband absorption curves are observed (Figure 4c,d); moreover, the charge-transfer peaks (λmax) of BTTQ-2Br and BTTQCN2Br are 720 and 795 nm, respectively. The molar extinction coefficients, determined from the solution absorption peaks at ca. 350 nm (Figure S6a,b), are ca. 4.8 × 104 M−1 cm−1 for TQTIPS, 3.5 × 104 M−1 cm−1 for TQCN-TIPS, 3.8 × 104 M−1 cm−1 for BTTQ-2Br, and 4.7 × 104 M−1 cm−1 for BTTQCN2Br. Compared with solution absorption, the maximum absorption onsets of the films are further red-shifted by ca. 30 nm for BTTQ-2Br and ca. 80 nm for BTTQCN-2Br, indicative of strong solid-state aggregation or molecular organization.8,64 In comparison with comonomers, both polymers achieve much broader light-capturing bands covered from ultraviolet to NIR bands (as far as ca. 1880 nm, Figure 4c,d). Particularly, a typical charge-transfer band at ca. 600−2100 nm a and π−π* transition band at ca. 300−600 nm can be found for both polymers. Because of stronger D−A interactions, the λmax value of PBTTQCN-TT (1270 nm) film exhibits 162 nm red shift

than that of PBTTQ-TT (1108 nm). As can be seen from Figure S6c, the absorption coefficients at the λmax peaks are determined to be ca. 12.0 M−1 cm−1 for PBTTQ-TT and 11.1 M−1 cm−1 for PBTTQCN-TT. The optical band gaps for TQTIPS, TQCN-TIPS, BTTQ-2Br, BTTQCN-2Br, PBTTQ-TT, and PBTTQCN-TT are estimated to be 2.03, 1.81, 1.38, 1.18, 0.75, and 0.66 eV, respectively. Such low Eopt g values for these materials, especially for PBTTQCN-TT, are among the lowest ones (≲0.7 eV) for the solution-processable polymers reported to date and even compared to the reported benzobisthiadiazole-based polymers such as PBBTTT (0.56 eV)7 and PBBTQT (0.70 eV).65 Notably, this low Eopt ≈ 0.66 eV g suggests that the π-electrons of PBTTQCN-TT can be extensively delocalized in the polymer backbones, clearly associating with strong intramolecular charge transfer between the 2,5-bis(3-(2-decyltetradecyl)thiophen-2-yl)thieno[3,2-b]thiophene donor and the TQ acceptor.7−9 Electrochemical Properties and Theoretical Simulation. The electrochemical properties for these TQ-based derivatives were investigated by using a three-electrode cyclic voltammetry (CV) system. To calculate EHOMO and ELUMO levels, we calibrated redox potentials using ferrocene/ ferrocenium (Fc/Fc+) as the internal standard (0.38 V vs Ag/AgCl). During negative and positive scans, two couples of reversible reduction peaks are observed for both model molecules TQ-TIPS and TQCN-TIPS, while no obvious oxidation peaks are detected (Figure 5a). The ELUMO levels of TQ-TIPS and TQCN-TIPS estimated from the reduction F


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Figure 5. (a) CV curves for the TQ-based model molecules and comonomers measured in dichloromethane. (b) CV curves for the polymer films measured in acetonitrile. (c) Molecular orbitals of the TQ-based model molecules, comonomers, and model dimers, together with the alignment of the HOMO and LUMO values obtained from CV and DFT data.

Figure 6. Typical J−V curves of the OFET devices fabricated from PBTTQ-TT (a,b) and PBTTQCN-TT (c,d).

(Ered onset) potentials are −4.09 and −4.23 eV, respectively. Interestingly, the observed ELUMO levels are lower than those of the reported TIPS-containing TQ analogues (Figure S7), such as TIPS-APhTQ (−3.82 eV),58 TIPS-PhTQ (−3.95 eV),58 TIPS-BDTTQ (−3.99 eV),58 and TIPS-PhNTQ (−3.99 eV).63 These results prove that indanone and malononitrile are more effective groups for improving electron affinity of the TQ-based derivatives. Unlike two model molecules, both TQbased comonomers (BTTQ-2Br and BTTQCN-2Br) exhibit reversible reduction and oxidation processes because of attaching two electron-rich thiophene units. Compared with BTTQ-2Br, the highly electron-withdrawing cyano groups also endow BTTQCN-2Br with reduced EHOMO (−5.53 eV) and ELUMO (−4.20 eV) values. The same tendency is also observed

for both TQ-based copolymers. As shown in Figure 5b, the cyano-containing PBTTQCN-TT displays low-lying EHOMO (−5.20 eV) and ELUMO (−4.10 eV), slightly lower by ca. 0.14−0.20 eV than those of PBTTQ-TT (Table 2). Moreover, note that both EHOMO and ELUMO values for PBTTQCN-TT are remarkably lower than those of the classical BBTcontaining polymers, such as PBBTTT (EHOMO = −4.36 eV and ELUMO = −3.80 eV)7 and PBBTQT (EHOMO = −4.60 eV and ELUMO = −3.80 eV).65 These findings indicate that incorporation of malononitrile substitutes into the parent TQ core is a very facile strategy to obtain stronger acceptor units for low band gap polymers, thus achieving deeper HOMO and LUMO levels. Such deep-lying ELUMO and suitable EHOMO values would facilitate both electron and hole injections from G


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for PBTTQCN-TT. The results suggest that PBTTQCN-TT has a relatively good film organization, mainly adopting a faceon model packing.68 Compared with PBTTQ-TT film, the enhanced ordering for PBTTQCN-TT film might provide an effective transport channel for the higher carrier mobilities. Furthermore, the difference of film organization can also be revealed from both in-plane and out-of-plane pattern 1DGIXRD (Figures S9c,d). The calculated lamellar stacking distances are 25.2 Å for PBTTQ-TT and 26.0 Å for PBTTQCN-TT, and the π−π packing distance is 3.58 Å for PBTTQCN-TT, as determined from the out-of-plane pattern GIXRD (Figure S9d). Generally, such short π−π stacking distance might give a partial evidence to explain higher ambipolar mobilities of PBTTQCN-TT than those of PBTTQ-TT.

Au source/drain electrodes to semiconductor layers, suggesting that both polymers would be operated as ambipolar carrier transport in organic field-effect transistor (OFET) devices. In order to combine the experimental results with theoretical model data, DFT calculations were conducted at the B3LYP/ 6-31G (d) level.66 Both polymers were simplified as the two model dimers bearing methyl substitutes to reduce DFT operation time. Figure 5c shows the electron density distributions and comparative alignments of the HOMO and LUMO levels obtained from CV and DFT data. The optimized molecular orbitals reveals that the π-electrons are delocalized evenly over the whole HOMO and LUMO orbitals (Figure 5c). The result implies that all the TQ-based small molecules and copolymers have a good π-conjugation and backbone coplanarity, which is helpful for charge carrier transport, intramolecular charge transfer, as well as reducing band gaps. Note that the calculated HOMO/LUMO values are much (ca. 0.15−0.62 eV) higher than the actual values, while the change trend is similar with the experimental results. For example, the replacement of the carbonyl group by the malononitrile (TQTIPS → TQCN-TIPS, BTTQ-2Br → BTTQCN-2Br, and PBTTQ-TT → PBTTQCN-TT) leads to reduced HOMO and LUMO values simultaneously. Accordingly, a similar trend can also be found by comparing the calculated band gaps and the experimental ones. Charge Transport Property and Film Organization. To characterize the charge carrier mobilites of both polymers, OFET devices were fabricated on the surface of Corning glass substrates with a classical top-gate/bottom-contact structure. As we excepted, both polymers exhibit typical ambipolar transport behaviors. For the optimized PBTTQ-TT-based OFET devices, the extracted hole and electron mobilities are equal to 4.7 × 10−4 and 1.4 × 10−3 cm2 V−1 s−1 (Figure 6a,b). Interestingly, the cyano groups containing PBTTQCN-TT obtained the improved ambipolar mobilities relative to PBTTQ-TT, affording the highest mobilities of 1.3 × 10−3 cm2 V−1 s−1 for holes and 2.0 × 10−3 cm2 V−1 s−1 for electrons (Figure 6c,d). Note that the mobilities of both PBTTQ-TT and PBTTQCN-TT are slightly lower than those of most reported symmetric TQ-based polymers.8,9,46,48 The main reason is that the asymmetric structure of both TTQ and TQCN acceptors results in a region-irregular structure of the polymer backbones, which is disadvantageous for charge transport properties in the solid-state film.67 In order to understand the difference of mobilities, both atomic force microscopy (AFM) and grazing incidence diffraction (GIXRD) techniques were utilized to characterize film surface morphology and organization, respectively. As shown in Figure S8, both PBTTQ-TT and PBTTQCN-TT films display a similar microstructure and a uniform morphology, together with a very small root-mean-square (rms) surface roughness of 0.95 and 0.81 nm, respectively. Such small rms and smooth surface morphology would facilitate to form a good boundary contact between the active layer and the adjacent dielectric layer.64 The 2D-GIXRD pattern for PBTTQ-TT, as can be seen from Figure S9a, shows only one isotropic lamellar reflection (100) and a strong amorphous halo of alkyl side chain, while no clear (010) reflection peak can be observed, indicating a randomly arranged lamellar packing toward the substrate.47 This disordered assembly might be one factor responsible for the relative low ambipolar mobilities.47 From the qz direction in Figure S9b, we can observe both the lamellar reflection (100) and the π−π stacking reflection (010)

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CONCLUSIONS

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ASSOCIATED CONTENT

We have demonstrated that the condensation coupling of indanone derivatives is an effective method for developing two types of highly electron-deficient indanone-fused TQ acceptor building blocks (BTTQ-2Br and BTTQCN-2Br). Compared with the reported TQ acceptor units, the attachment of the auxiliary carbonyl and cyano groups into the parent TQ core endows BTTQ-2Br and BTTQCN-2Br with enhanced electron affinities and reduced LUMO values (as low as ca. −4.20 eV). By using both BTTQ-2Br and BTTQCN-2Br as the electron acceptor units, we have prepared two novel D−A conjugated polymers with low optical band gaps of 0.75 eV for PBTTQ-TT and 0.66 eV for PBTTQCN-TT. It is found that the optical properties, band gaps, solid-sate structures, as well as charge carrier mobilities can be manipulated through adjusting substitute groups, including carbonyl and cyano groups. Comparative studies reveal that the cyano-containing PBTTQCN-TT achieves an extended NIR absorption, downshifted HOMO and LUMO levels, and relatively good film organizations than those of the carbonyl-containing PBTTQTT. Preliminary OFET investigations indicate that typical ambipolar transport behaviors have been demonstrated for both polymers. Compared with PBTTQ-TT, relatively good film organization endows PBTTQCN-TT with higher hole (1.3 × 10−3 cm2 V−1 s−1) and electron mobilities (2.0 × 10−3 cm2 V−1 s−1). The observed results suggest that the annulation of another electron-withdrawing units into the parent TQ core is an effective approach for the creation of novel electron acceptor units with high electron affinity. This method can be extended to construct novel low band gap π-conjugated systems for various applications, especially in flexible electronics, NIR-II photothermal conversion, and NIR-II biological imaging.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00834. Information regarding characterization, device fabrication procedures, and material characterization data, such as GPC, FI-IR, TGC, crystal data, absorption coefficient, AFM, GIXRD, 1H NMR, and 13C NMR (PDF) H


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.C.). *E-mail: [email protected] (Y.G.). *E-mail: [email protected] (Y.L.). ORCID

Huajie Chen: 0000-0003-0366-8826 Yunlong Guo: 0000-0003-1602-769X Yunqi Liu: 0000-0001-5521-2316 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully thank the scientists from the Beijing Synchrotron Radiation Facility for their assistance during the GIXRD experiments and acknowledge the assistance of Prof. Zebing Zeng from Hunan University during the absorption spectra experiments. This research was supported by the National Natural Science Foundation of China (21875202 and 51873216), the Hunan Provincial Natural Science Foundation of China (2018JJ1024), and the Science and Technology Planning Project of Hunan Province (2017RS3048).

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