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Electron-deficient 1,2,7,8-tetraazaperylene derivative: Efficient synthesis and copolymerization
European Polymer Journal 107 (2018) 67–73
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Electron-deficient 1,2,7,8-tetraazaperylene derivative: Efficient synthesis and copolymerization ⁎
Peirong Qianga,1, Ruizhi Tanga,1, Shuai Bia, Feng Qiub, Feng Liuc, , Fan Zhanga,
T
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a
School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China c School of Materials Science and Engineering, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiao Tong University, Xi’an 710049, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electron-deficient 1,2,7,8-tetraazaperylene derivative D-A conjugated polymers Small band gap
Development of electron-deficient organic semiconducting monomers is the key role in the construction of donor (D)-acceptor (A) conjugated polymers. In this work, a new type of 1,2,7,8-tetraazaperylene derivative 3,9-bis(5bromo-4-hexylthienyl)-1,2,7,8-tetraazaperylene (Br-TPAL) with two terminal bromine groups, has been efficiently synthesized. Upon transition-metal catalyzed cross-coupling, Br-TPAL was facile to copolymerize with various monomers, to form three donor–acceptor conjugated polymers PTPAL-OBDT, PTPAL-TBDT PTPAL-DPP in high yields. The electronic structures of the resulting polymers enable being finely tuned through changing the building blocks. They exhibited very rich optoelectronic properties, for example, their absorption bands covered the nearly whole visible regions, or even extending to the NIR regions. These promising characters render them highly desirable for high-performance electronic devices.
1. Introduction Organic conjugated polymers have received considerable attention owing to their unique advantages over small-molecule organic semiconductors, such as facile solution processing and excellent film forming properties [1–4]. Efforts in the direction of developing sustainable synthetic protocols for semiconducting polymers are in progress all the time [5,6]. Significant progress has been made in recent years both in the design and synthesis of donor-accept (D-A) conjugated polymers and their fabrication of various organic electronic devices, such as, organic solar cells (OSCs) [7–11], organic field effect transistors (OFETs) [12–14], polymer light emitting diodes (PLEDs) [15,16] and resistance memory [17]. In the process of pursuing the high-performance devices, the development of new kinds of organic building blocks with promising characters, play a crucial role. In comparison with substantial electron-donating building blocks, electron-deficient acceptors are still relatively rare [18–20]. Up to now, numerous excellent D-A polymer semiconductors, are mostly relevant to several typical electron-deficient acceptors, such as, benzothiadiazole [21–23], diketopyrrolopyrrole [24,25], benzobisthiadiazole [26–28], naphthalene diimide [29,30], and isoindigo [31,32], mainly because they are facile to copolymerize with various electron-donating monomers, and
their molecular frontier orbitals could be well coupled with those of donor parts for finely tuning the semiconducting properties of these polymers. Undoubtedly, exploring new type of electron-deficient building blocks is very vital to enlarging the regime of advanced organic semiconducting materials. Incorporating strong electronegative nitrogen atom into aromatics has become a popular method to form electron-deficient building blocks via lowering the lowest unoccupied molecular orbital (LUMO) energy level [33–35]. Meanwhile, the introduction of nitrogen atoms into π-conjugated skeletons enable not only improving the molecular stability, but also providing multiple intra- and intermolecular interactions, such as, hydrogen bonding and dipole–dipole interaction, and thus exerting strong influence either on molecular packing structures in solid state or electronic structures, favorable for the enhancement of carrier mobility and photophysical activity [36]. In our previous work, we reported a series of rich nitrogen-embedded aromatic molecules, whose coral unit is 1,2,7,8-tetraazaperylene, structurally analogous to perylene [37]. These molecules exhibit n-type semiconducting properties and substantial photophysical properties, which are well tunable by changing the terminal groups around the central core. On the other hand, such kinds of molecules could be concisely transformed from various common starting materials in very high
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Corresponding authors. E-mail addresses: [email protected] (P. Qiang), [email protected] (R. Tang), [email protected] (S. Bi), [email protected] (F. Qiu), [email protected] (F. Liu), [email protected] (F. Zhang). 1 These two authors contributed equally to this work. https://doi.org/10.1016/j.eurpolymj.2018.07.047 Received 26 June 2018; Received in revised form 20 July 2018; Accepted 28 July 2018 Available online 31 July 2018 0014-3057/ © 2018 Elsevier Ltd. All rights reserved.
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Ultraviolet–visible (UV–vis) spectra were recorded on a HITACHI U4100 Spectrophotometer; The fluorescence spectroscopy (PL) emission spectra were obtained with a FluoroMax-4 spectrophotometer; Cyclic voltammetry (CV): CV was performed on a Chen hua 650D electrochemical analyzer a conventional three electrode cell was used with a platinum working electrode (surface area of 0.3 mm2) and a platinum wire as the counter electrode. The Pt working electrode was routinely polished with a polishing alumina suspension and rinsed with acetone before use. The measured potentials were recorded with respect to Ag/ AgCl reference electrode. All electrochemical measurements were carried out under an atmospheric pressure of nitrogen.
overall yields [37]. These advantages make it possible to construct versatile π-conjugated systems by the incorporation of 1,2,7,8-tetraazaperylene unit into molecular backbones. In this work, a new π-conjugated oligomer consisting of a 1,2,7,8tetraazaperylene core and two α-bromothiophene units at the both sides, was successfully synthesized (denoted as Br-TPAL). Its chemical structure and physical properties were fully characterized, revealing its electron-deficient characters. Furthermore, such borominated molecule as building block was facile to alternatively copolymerize with the other building blocks. Upon transition-metal catalyzed cross coupling, Br-TPAL was smoothly copolymerized with the (4,8-bis((2-octyldodecyl)oxy)benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane), (4,8-bis(5-(2-octyldodecyl)thiophen-2-yl) benzo[1,2-b:4,5b′]dithiophene-2,6-diyl)bis(trimethylstannane) and 2,5-bis(2-octyldodecyl)-3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione,and resulting in three conjugated polymers denoted as PTPAL-OBDT, PTPAL-TBDT PTPAL-DPP, respectively, in high yields. The chemical structures and physical properties of these resulting polymers were characterized by 1 H NMR, thermal gravimetry (TG), optical spectroscopy, cyclic voltammetry analyses and DSC analysis. They exhibited good thermal stabilities, and rich photophysical properties and electrochemical behaviors with respect to their different building blocks. All the three copolymers possess narrow band gaps, associated with their donor-acceptor structures. In particular, for PTPAL-DPP, a strong light harvesting capability in the near infrared regions was even observed, featuring a very narrow band gap value of around 1.45 eV. These intriguing properties render them with potential application in some electronic devices.
2.3. Synthesis and characterization 2.3.1. Synthesis of 1,5-bis(5-bromo-4-hexylthenoyl)anthracene 2 To a solution of 2-bromo-3-hexyl-5-iodothiophene (9.3 g, 25.0 mmol) in THF (10 mL), was slowly added to chloro(1-methylethyl)-magnesium (12.5 mL, 25.0 mmol, 2 M) to form the Grignard reagent. Afterwards, the resulting Grignard reagent was slowly added to a solution of 1,5-di-(2-pyridylthiol)anthracenoate 3 (4.5 g, 10.0 mmol) in dried THF (100 mL) at 0 °C and stirred overnight. Then, the reaction was quenched with 10% HCl (50 mL) and extracted with CH2Cl2. The combined organic fractions were washed with 1.0 M NaHCO3 and water, then dried over MgSO4 and filtered. The solvent was removed under reduced pressure, which was purified by column chromatography using CH2Cl2/petroleum as eluent, to afford compound 2 as yellow solid (5.2 g, yield: 72%). 1H NMR (400 MHz, CDCl3, ppm): δ 8.88 (s, 2H), 8.15 (d, J = 8.0 Hz, 2H), 7.76 (d, J = 8.0 Hz, 2H), 7.38 (m, J = 4.0 Hz, 2H), 7.24 (s, 2H), 2.52 (t, J = 3.8 Hz, 4H), 1.26–1.56 (m, 12H), 0.84 (d, J = 8.0 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm): 188.4, 144.2, 143.4, 136.7, 135.2, 133.0, 132.6, 128.5, 125.7, 124.4, 122.2, 40.0, 32.5, 28.8, 25.8, 23.2, 14.3. HRMS. (C36H36Br2O2S2, ESI): calculated for [M+H]+ 723.0523, found: 723.0606.
2. Materials and methods 2.1. Materials
2.3.2. Synthesis of 1,5-bis(5-bromo-4-hexylthienyl)anthraquinone 1 To a suspension of 2 (723.0 mg, 1.0 mmol) in 15 mL glacial acetic acid, was added a solution of CrO3 (300.0 mg, 3.0 mmol) in 15 mL glacial acetic acid and water (4:1). The reaction mixture was maintained at 75 °C for 1.5 h. Compound 1 was obtained (as light yellow solid), which was used directly in the next step without purification. 1H NMR (400 MHz, CDCl3, ppm): δ 8.33 (d, J = 8.0 Hz, 2H), 7.85 (t, J = 8.0 Hz, 2H), 7.69 (d, J = 8.0 Hz, 2H), 6.95 (s, 2H), 2.46 (t, J = 3.8 Hz, 4H), 1.23–1.47 (m, 12H), 0.81 (d, 6H, J = 8.0 Hz). HRMS. (C36H35Br2O4S2, ESI): calculated for [M+H]+ 753.0265, found: 753.0327.
All of the solvents used for chemical reactions in this work, were AR grade. Of these, diethyl ether and toluene, were purchased from Sinopharm Chemical Reagent Co., Ltd., THF, petroleum ether, o-dichlorobenzene, trimethylamine, dichloromethane, ethanol, hydrazine hydrate, Sodium carbonate anhydrous, magnesium sulfate and sodium chloride were purchased from Shanghai Titan Scientific Co., Ltd. (4,8bis((2-octyldodecyl)oxy)benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl)bis (trimethylstannane), (4,8-bis(5-(2-octyldodecyl)thiophen-2-yl) benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) and 2,5-bis (2-octyldodecyl)-3,6-bis (5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)-thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione were purchased from SunaTechInc. Co., Ltd. The solvents was carefully dried and collected before used, Tetrahydrofuran USES metal sodium to remove water and oxygen with diphenylketone as indicator, when the solvent turns blue, then collection and use. The reactions were carried out by using glovebox techniques and standard Schlenk techniques.
2.3.3. Synthesis of 3,9-bis(5-bromo-4-hexylthienyl)-1,2,7,8-tetraazaperylene Br-TAPL A suspension of hydrazine hydrate (30.0 mg, 0.60 mmol) and 1 (150.0 mg, 0.20 mmol) in ethanol (30 mL) was stirred at 78 °C under N2. After cooled to room temperature, the solvent was removed under reduced pressure. After washed with ethanol, the product Br-TAPL was obtained as yellow solid (131.0 mg, 95%). 1H NMR (400 MHz, CDCl3, ppm): δ 9.34 (d, 7.2 Hz, 2H), 8.51 (d, J = 8.0 Hz, 2H), 8.11 (t, J = 7.6 Hz, 2H), 7.50 (s, 2H), 2.63 (t, J = 8.0 Hz, 4H), 1.36–1.56 (m, 12H), 0.92 (t, 6H). HRMS (C36H35Br2N4S2, ESI): calculated for [M +H]+ 745.0592, Found: 745.0630.
2.2. Instrumentation Nuclear Magnetic Resonance and Fourier Transform Infrared Spectroscopy: 1H and 13C NMR spectra were recorded on Mercury Plus 400 (400 MHz for proton, 100 MHz for carbon) spectrometer with tetramethylsilane as the internal reference using CDCl3 as solvent in all cases; Molecular weights of the polymers were obtained by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as the eluent against polystyrene (PS) standards with a refractive index detector (Shimadzu RID-10A) and the measurements were performed at 40 °C with a flow rate of 1.0 mL min−1. Thermogravimetric Analysis (TGA): TGA was measured by a TAQ5000IR with a heating rate of 10 °C/min under flowing N2. Differential scanning calorimetry (DSC) analysis were scanned on a DSC-204F1 Calorimeter with a heating rate 10 °C min−1 and cooling rate 20 °C min−1; Absorption Measurements:
2.3.4. Synthesis of polymer PTAPL-ODBT 3,9-bis (5-bromo-4-hexylthienyl)-1,2,7,8-tetraazaperylene (148.0 mg, 0.2 mmol) and (4,8-bis((2-octyldodecyl)oxy)benzo-[1,2-b:4,5-b′] dithiophene-2,6-diyl)bis(trimethylstannane) (222.0 mg, 0.2 mmol) were employed in 15 mL chlorobenzene. After the mixture was degassed for 30 min, Pd2(dba)3 (1.8 mg) and P(o-tol)3 (3.7 mg) was added, then heated up to 110 °C and stirred for 72 h. The resulting mixture was poured into methanol for precipitation. The solid were further purified by 68
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confirming their chemical structures. By using Pd-catalyzed stille coupling reaction, Br-TAPL as the key building block was smoothly copolymerized with the other distannylated building blocks (4,8-bis((2-octyldodecyl)oxy)benzo[1,2b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane), (4,8-bis(5-(2-octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis (trimethylstannane) to afford two kinds of copolymers denoted as PTAPL-ODBT and PTAPL-TDBT, respectively. While, upon Pd-catalyzed Suzuki cross coupling reaction, compound Br-TAPL was efficiently reacted with boronic ester-substituted building block 2,5-bis(2octyldodecyl)-3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione to afford the copolymer PTAPL-DPP. All the resulting polymer samples were purified by Soxhlet extraction in different solvents with a sequence of methanol, acetone and hexane, for mainly removing off the low-molecular-weight fractions and catalyst residues. Finally, the resulting solid powders were extracted into chloroform. The chemical structures of these polymers were characterized by 1H NMR spectroscopy (detailed see the above experimental part). The 1H NMR profiles of PTAPL-ODBT and the key building block Br-TAPL was combined in Fig. 1 for comparison. As expected, the rigid polymer PTAPL-ODBT exhibits a set of relatively broad, Br-TAPL exhibits well-resolved peaks as compared with PTAPL-ODBT. The two peaks with respect to the protons (signed as H-1 and H-2) attached to (4,8-bis((2-octyldodecyl)oxy)benzo[1,2b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) and thiophene moieties, appeared at 6.94 and 7.76 ppm, respectively. While the phenyl protons of TAPL (H-3, H-4 and H-5), gave peaks at 8.62, 8.10 and 9.29 ppm, respectively. The two signals at 3.74 ppm and 3.01 ppm, were attributed to the protons of the methylene moieties attached to oxygen (H-6) and thiophene ring (H-7), respectively. Their molecular weights were measured by the Gel Permeation Chromatography (GPC), using the chloroform as the eluent. The number-averaged molecular weights (Mn) of the resulting polymers PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP were 58.67, 16.63 and 11.50 kDa, with the polydispersity index (PDIs) of 1.69, 1.78 and 1.82, respectively (Table 1).
Soxhlet extraction subsequently in methanol, acetone, hexane, and then dissolved in chloroform. The solvent was removed under reduced pressure to get the target polymer PTAPL-ODBT as dark purple solid (237.3 g, 85.0% yield). 1H NMR (400 MHz, CDCl3, ppm): δ 9.29 (br, 2H), 8.62 (br, 4H), 8.10 (br, 4H), 7.76 (br, 2H), 3.97 (br, 4H), 3.01 (br, 4H), 0.75–1.85 (br, 106H). Mn = 58.6 7 kDa, PDI = 1.69. 2.3.5. Synthesis of polymer PTAPL-TDBT Br-TAPL (148.0 mg, 0.2 mmol) and (4,8-bis(5-(2-octyldodecyl) thiophen-2-yl) benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) (242.0 mg, 0.2 mmol) were dissolved in 15 mL chlorobenzene. After the mixture was degassed for 30 min, Pd2(dba)3 (1.8 mg) and P(o-tol)3 (3.7 mg) was added, then the mixture was heated to 110 °C and stirred for 72 h. The reactant was poured into methanol to get solid. The solid were further purified by Soxhlet extraction with different solvents in a sequence of methanol, acetone, hexane, and finally was extracted into chloroform. The solvent was removed under reduced pressure to offer PTAPL-TDBT as dark purple solid (272.9 g, 89.0%). 1H NMR (400 MHz, CDCl3, ppm): δ 9.41 (br, 2H), 8.62 (br, 2H), 8.12(br, 2H), 7.83 (br, 2H), 7.71 (br, 2H), 7.34 (br, 2H), 6.90 (br, 2H), 2.87 (br, 4H), 0.87–2.02 (br, 110H). Mn = 16.63 kDa, PDI = 1.78. 2.3.6. Synthesis of polymer PTAPL-DPP Compound Br-TAPL (148.0 mg, 0.2 mmol) and 2,5-bis(2-octyldodecyl)-3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (222.0 mg, 0.2 mmol) were dissolved in 15 mL toluene. After degassed for 30 min, Pd(PPh3)4 (9.2 mg) and P(o-tol)3 (6.1 mg) was added, then the mixture was heated up to 90 °C and stirred for 72 h. Then, the resulting mixture was poured into methanol to form heavy precipitation. After filtration, the resulting residue was collected, and further purified by Soxhlet extraction by methanol, acetone, hexane in sequence, then dissolved in chloroform. The solvent was removed under reduced pressure to get PTAPL-DPP (236.2 g, 80.0% yield) as dark purple solid. NMR (400 MHz, CDCl3, ppm): δ 9.34 (br, 2H), 9.02 (br, 2H), 8.84 (br, 2H), 8.55 (br, 2H), 8.16 (br, 2H), 7.75 (br, 2H), 4.05 (br, 2H), 2.99 (br, 2H), 0.83–1.99 (br, 100H). Mn: 11.50 kDa, (PDI: 1.82).
3.2. Thermal properties of polymers 3. Results The TGA curves of the polymers heated at a rate of 10 °C/min were measured in nitrogen (Fig. 2). The thermal degradation data of these polymers, including 5%, 10% and 50% mass-loss temperatures (T5–T50) were listed in Table 1. The decomposition temperatures (Td, with 5% weight loss) of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP were 334.1 °C, 391.8 °C and 377.6 °C, respectively. Notably, the Td of PTAPL-ODBT was remarkably lower than PTAPL-TDBT, but the molecular weight of the former is higher than the latter one as shown in the GPC results. Such phenomenon seems to be attributed to their different binding modes between the alkyl side chains and the aromatic moieties in the main backbones, namely, the thermal stability of CeC bond for PTAPL-TDBT was better than that of CeO bond for PTAPLODBT. While, the Td of PTAPL-DPP falling in between PTAPL-TDBT and PTAPL-DPP, likely indicated that the thermal stability of CeN bond is stronger than that of CeO, but weaker than that of CeC.
3.1. Synthesis of building block and conjugated polymers The synthetic routes to the target compounds, were depicted in Scheme 1. Compound 3 1,5-di-(2-pyridylthiol)anthracenoate was synthesized according to a modified protocol as our previously reported [37]. The treatment of anthracene-1,5-dicarbonyl dichloride with 2mercaptopyridine, using triethylamine (TEA) as a catalyst, afforded compound 3 as a pale-yellow solid in a yield of 68%. By this method, the two mercaptopyridine were effectively attached on compound 3 as good leaving groups for further substitution reaction. In order to directly construct a molecular backbone with two active bromo-subsitutents, (5-bromo-4-hexylthiophen-2-yl)magnesium chloride was produced by the treatment of 2-bromo-3-hexyl-5-iodothiophene with Grignard reagent, which was directly used in the next step. The key precursor compound 3, was synthesized according to our previous report [37]. The reaction of 3 with (5-bromo-4-hexylthiophen2-yl) magnesium chloride in THF at 0 °C and stirring overnight, smoothly led to the formation of 1,5-bis(5-bromo-4-hexylthiophen-2-ly)anthracene 2, which was purified via column chromatography (eluent: CH2Cl2/ petroleum) as a yellow solid in a yield of 72%. Upon oxidation with CrO3, 1,5-bis(5-bromo-4-hexylthiophen-2-yl)anthraquinone 1 was obtained in a nearly quantitative yield. After the treatment with hydrazine hydrate, this intermediate was readily converted to the target molecule 3,9-bis(5-bromo-4-hexylthiophen-2-yl)-1,2,7,8-tetraazaperylene BrTAPL in a very high yield (97%), respectively. All new compounds have been fully characterized by NMR spectra, HRMS analyses for essentially
3.3. Optical properties of the as-prepared polymers The UV–vis absorption spectra of these polymers were measured in CH2Cl2. The polymers PTAPL-ODBT and PTAPL-TDBT showed similar absorption profiles, attributable to their similar π-conjugated main backbones. The absorption bands cover the most of the ultraviolet and visible regions. The one shoulder peak at λabs = 347 nm, and the two main absorption at λabs = 415 and 551 nm, were observed for PTAPLODBT. Correspondingly, the three red-shifted main peaks at λabs = 350, 453 and 572 nm were found for PTAPL-TDBT. Such phenomenon is reasonably arising from the difference in the pendant side chains of the 69
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Scheme 1. Synthetic routes and conditions: (a) CrO3,HAc; (b) NH2-NH2, C2H5OH; (c) Pd2(dba)3, P(o-tol)3, chlorobenzene, 110 °C; (d) Pd2(dba)3, P(o-tol)3, toluene, 90 °C.
Table 1 Molecular weight and thermal stability of PTPAL-OBDT, PTPAL-TBDT and PTAPL-DPP. Sample
Mn/kDa
Mw/kDa
PDI
T5 (°C)
T10 (°C)
T50 (°C)
PTAPL-ODBT PTAPL-TDBT PTAPL-DPP
58.67 16.63 11.50
99.46 29.59 20.93
1.69 1.78 1.82
334.1 391.8 377.6
344.9 409.1 395.7
386.7 463.4 457.3
PTAPL-ODBT. The absorption peaks in the high energy regions of 300–450 nm were originated from the π−π* transitions of the aromatic skeletons of the two polymers. And the absorption at λabs = 470–700 nm, can be attributed to the intramolecular charge transfer (ICT) from the electron-donating benzothiophene-cored segment to the electron acceptor 1,2,7,8-tetraazaperylene. The lowest energy absorption peak (λmax = 572 nm) of PTAPL-TDBT, was significantly red-shifted over 20 nm in comparison with that of PTAPLODBT, manifesting a much facile electron transfer between donor and acceptor in the former case, because of its much stronger electron donating character associated with the thienyl groups on the pendent side chains. These results clearly demonstrated that the optical behaviors of such kinds of conjugated polymers can be significantly tuned via the
Fig. 1. 1H NMR spectra of Br-TAPL and PTAPL-ODBT in CDCl3.
two polymers. Obviously, the incorporation of a thiophene unit in the perpendicular direction to the main conjugated backbone for PTAPLTDBT, could result in an extend π conjugation by compared with 70
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Table 2 Photophysical data of PTPAL-OBDT, PTPAL-TBDT and PTAPL-DPP in CH2Cl2, respectively.
PTAPL-ODBT PTAPL-TDBT PTAPL-DPP
λabs (nm)
λemission (nm)
Τ (ns)
347,415,551 350,453,572 420,662,720
410,437,459 410,438,459 410,438,460
1.91 2.02 1.97
in side chain or main backbone. In particular, the 1,2,7,8-tetraazaperylene moiety could be well compatible with those typical building blocks to architect new conjugated polymers with promising optical properties. In addition, all polymers show very weak fluorescence emissions in CH2Cl2 (Fig. 3b), which could be attributed to the fluorescence quenching effect of 1,2,7,8-tetraazaperylene derivative [37]. Typically, a nitrogen embedded acene always exhibits strong trapping effect [38] or intersystem crossing effect associated with electronegative nitrogen atom with a lone pair of electrons in aromatics [39]. The fluorescence lifetime values of PTPAL-OBDT, PTPAL-TBDT and PTAPL-DPP are 1.91 ns, 2.02 ns and 1.97 ns, respectively (Table 2).
Fig. 2. TGA curves of the resulting polymers measured with a heating rate of 10 °C/min under flowing N2.
variation of the pendant side chains attached on the conjugated main backbone. Moreover, in the case of PTAPL-DPP, 2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4dione as the key building block, took place of (4,8-bis((2-octyldodecyl) oxy)benzo [1,2-b:4,5-b′]dithiophene for PTAPL-ODBT or (4,8-bis(5-(2octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene for PTAPLTDBT. Interestingly, PTAPL-DPP showed one sharp absorption peak at 480 nm, and a relatively broad peak centered at 662 nm with a shoulder peak at 720 nm. These absorption bands are from 400 to 900 nm, covering the nearly whole visible regions and part of the NIR regions, exhibiting excellent light harvesting capability. The absorption max at 662 nm, was remarkably red-shifted by about 90 nm, as compared with that of PTAPL-TDBT. Such result essentially revealed the presence of donor-acceptor structure of PTAPL-DPP. Different from the other two polymers, this donor-acceptor structure in the π-conjugated main backbone, should be consisting of the two kinds of acceptors 1,2,7,8tetraazaperylene (A1) and diketodipyrrole (A2), and one kind of bithiophene (D), to form D-A1-D-A2 fashion (Scheme 1). The building blocks in the main backbone of PTAPL-DPP, could construct a π-extended conjugated system through rationally matching these functional units with their frontier molecular orbital energy levels. Accordingly, the optical band gaps of the three conjugated polymers were evaluated by the onsets of their absorption edges in such a sequence: 1.47 eV for PTAPL-DPP < 1.76 eV for PTAPL-TDBT < 1.88 eV for PTAPL-ODBT, indicative of a declined tendency along with the decreased π-conjugation. Meanwhile, the relatively narrow bands for all of these three polymers observed here, could be contributable to their D-A structural characters. These phenomena manifested that the finely tunable electronic structures of conjugated polymers via the modification of either
3.4. Electrochemical behavior In order to make insight into the electronic structures of these asprepared polymers, the electrochemical behaviors of the as-prepared polymers were investigated through cyclic voltammetries (CVs). The CV profiles of PTAPL-ODBT and PTAPL-TDBT showed one irreversible oxidation processes and one reduction processes under the measurement conditions, with the initial oxidation potentials at 0.70 and 0.95 V, and initial reduction potentials at −1.12 and −1.06 V. respectively (Fig. 4). PTAPL-TDBT was easily oxidized in compared with PTAPL-ODBT, likely due to the much electron-rich character of the former, relevant to its thiophenyl side groups. Such result is well in line with their optical properties. Notably, PTAPL-DPP exhibited the two consecutive reversible reduction peaks at −1.41 and −1.79 V, and one quasi-reversible oxidation peak at 0.75 V. The HOMO energy levels of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP, evaluated from the onsets of the first oxidation potential, were −5.50, −5.75 and −5.27 eV respectively. Correspondingly, evaluated from the onsets of the first reduction potential, the LUMO energy levels of the polymers were calculated as −3.65, −3.74 and −3.39 eV (Table 3). On the other hand, its electrochemical band gap was calculated as 1.82, 2.01 and1.88 eV respectively, which is similar to the optical band gap 1.88, 1.76 and1.43 eV. Obviously, the electrochemical behavior of these conjugated polymers clearly revealed the influence of the building blocks on their electronic structures, demonstrating their rich semiconducting properties.
Fig. 3. Normalized UV–vis absorption spectra (a) and normalized emission spectra (b) of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP in CH2Cl2, respectively. 71
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Fig. 4. Cyclic voltammograms of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP in CH2Cl2 (0.1 mol/L n-Bu4NPF6) at a scan rate of 100 mV/s. Potentials were given against ferrocene/ferrocenium ion couple (Fc/Fc+). Table 3 Summary electrochemical properties of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP.
PTAPL-ODBT PTAPL-TDBT PTAPL-DPP
E1 (V)
E2 (V)
E3 (V)
a
0.70 0.95 0.47
−1.12 −1.06 −1.41
– – −1.79
−5.50 −5.75 −5.27
HOMO (eV)
b
LUMO (eV)
−3.68 −3.74 −3.39
Eg1 (eV)
Eg2 (eV)
1.82 2.01 1.88
1.88 1.76 1.43
HOMO = −E1 − 4.80 eV, E1: the first oxidation potential, evaluated from the onset. LUMO = −E2 − 4.80 eV, E2: the first reduction potential, evaluated from onset of the first reduction potential for PTAPL-ODBT and PTAPL-TDBT, evaluated from half-wave potential for PTAPL-DPP; E3: the second reduction potential, evaluated from half potential; Eg1: electrochemical energy gap, Eg1 = LUMO-HOMO; Eg2: optical band gap, estimated from UV–vis absorption edge. a
b
Conflicts of Interest
4. Conclusions
There are no conflicts to declare.
We have successfully developed a new electron-deficient 1,2,7,8tetraazaperylene derivative with two terminal boromo groups (BrTPAL). Upon transition-metal catalyzed cross coupling, such compound was readily copolymerized with some traditional functional monomers to form three kinds of donor–acceptor (D–A) conjugated polymers. The resulting copolymers showed tunable electronic structures via the modification of the main chains and side chains. Some excellent optical properties, such as, strong light harvesting capability in the NIR regions, were observed. Altering the building block boned with TPAL can tune the TGA, Vis-UV and CV properties. In particular, the intriguing characters of polymer PTAPL-DPP render them desirable for developing promising semiconducting materials potentially useful for highperformance organic electronic devices. It is also can be foreseen that the developed electron-deficient 1,2,7,8-tetraazaperylene derivative (Br-TPAL) will be significantly contributable to expanding the regime of D-A conjugated polymers. The corresponding applications of these polymers are in progress.
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Author contributions Fan Zhang and Feng Liu conceived and designed the experiments; Peirong Qiang and Ruizhi Tang performed the experiments of synthesis of Monomer and polymers; Shuai Bi characterized the electrochemical behavior; Peirong Qiang, Feng Qiu, Ruizhi Tang, and Fan Zhang analyzed the data; Peirong Qiangand Fan Zhang wrote the paper.
Acknowledgments This work was financially supported by Shanghai Committee of Science and Technology (16JC1400703, 15JC1490500), National Natural Science Foundation of China (21574080, 21774072), Open Project Programs of the State Key Laboratories (SKLPEE-KF201702, Fuzhou University; 20161803, Xi’an Jiaotong University); State Key Laboratory of Supramolecular Structure and Materials (sklssm201732, Jilin University). 72
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Electron-deficient 1,2,7,8-tetraazaperylene derivative: Efficient synthesis and copolymerization ⁎
Peirong Qianga,1, Ruizhi Tanga,1, Shuai Bia, Feng Qiub, Feng Liuc, , Fan Zhanga,
T
⁎
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, PR China c School of Materials Science and Engineering, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiao Tong University, Xi’an 710049, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electron-deficient 1,2,7,8-tetraazaperylene derivative D-A conjugated polymers Small band gap
Development of electron-deficient organic semiconducting monomers is the key role in the construction of donor (D)-acceptor (A) conjugated polymers. In this work, a new type of 1,2,7,8-tetraazaperylene derivative 3,9-bis(5bromo-4-hexylthienyl)-1,2,7,8-tetraazaperylene (Br-TPAL) with two terminal bromine groups, has been efficiently synthesized. Upon transition-metal catalyzed cross-coupling, Br-TPAL was facile to copolymerize with various monomers, to form three donor–acceptor conjugated polymers PTPAL-OBDT, PTPAL-TBDT PTPAL-DPP in high yields. The electronic structures of the resulting polymers enable being finely tuned through changing the building blocks. They exhibited very rich optoelectronic properties, for example, their absorption bands covered the nearly whole visible regions, or even extending to the NIR regions. These promising characters render them highly desirable for high-performance electronic devices.
1. Introduction Organic conjugated polymers have received considerable attention owing to their unique advantages over small-molecule organic semiconductors, such as facile solution processing and excellent film forming properties [1–4]. Efforts in the direction of developing sustainable synthetic protocols for semiconducting polymers are in progress all the time [5,6]. Significant progress has been made in recent years both in the design and synthesis of donor-accept (D-A) conjugated polymers and their fabrication of various organic electronic devices, such as, organic solar cells (OSCs) [7–11], organic field effect transistors (OFETs) [12–14], polymer light emitting diodes (PLEDs) [15,16] and resistance memory [17]. In the process of pursuing the high-performance devices, the development of new kinds of organic building blocks with promising characters, play a crucial role. In comparison with substantial electron-donating building blocks, electron-deficient acceptors are still relatively rare [18–20]. Up to now, numerous excellent D-A polymer semiconductors, are mostly relevant to several typical electron-deficient acceptors, such as, benzothiadiazole [21–23], diketopyrrolopyrrole [24,25], benzobisthiadiazole [26–28], naphthalene diimide [29,30], and isoindigo [31,32], mainly because they are facile to copolymerize with various electron-donating monomers, and
their molecular frontier orbitals could be well coupled with those of donor parts for finely tuning the semiconducting properties of these polymers. Undoubtedly, exploring new type of electron-deficient building blocks is very vital to enlarging the regime of advanced organic semiconducting materials. Incorporating strong electronegative nitrogen atom into aromatics has become a popular method to form electron-deficient building blocks via lowering the lowest unoccupied molecular orbital (LUMO) energy level [33–35]. Meanwhile, the introduction of nitrogen atoms into π-conjugated skeletons enable not only improving the molecular stability, but also providing multiple intra- and intermolecular interactions, such as, hydrogen bonding and dipole–dipole interaction, and thus exerting strong influence either on molecular packing structures in solid state or electronic structures, favorable for the enhancement of carrier mobility and photophysical activity [36]. In our previous work, we reported a series of rich nitrogen-embedded aromatic molecules, whose coral unit is 1,2,7,8-tetraazaperylene, structurally analogous to perylene [37]. These molecules exhibit n-type semiconducting properties and substantial photophysical properties, which are well tunable by changing the terminal groups around the central core. On the other hand, such kinds of molecules could be concisely transformed from various common starting materials in very high
⁎
Corresponding authors. E-mail addresses: [email protected] (P. Qiang), [email protected] (R. Tang), [email protected] (S. Bi), [email protected] (F. Qiu), [email protected] (F. Liu), [email protected] (F. Zhang). 1 These two authors contributed equally to this work. https://doi.org/10.1016/j.eurpolymj.2018.07.047 Received 26 June 2018; Received in revised form 20 July 2018; Accepted 28 July 2018 Available online 31 July 2018 0014-3057/ © 2018 Elsevier Ltd. All rights reserved.
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Ultraviolet–visible (UV–vis) spectra were recorded on a HITACHI U4100 Spectrophotometer; The fluorescence spectroscopy (PL) emission spectra were obtained with a FluoroMax-4 spectrophotometer; Cyclic voltammetry (CV): CV was performed on a Chen hua 650D electrochemical analyzer a conventional three electrode cell was used with a platinum working electrode (surface area of 0.3 mm2) and a platinum wire as the counter electrode. The Pt working electrode was routinely polished with a polishing alumina suspension and rinsed with acetone before use. The measured potentials were recorded with respect to Ag/ AgCl reference electrode. All electrochemical measurements were carried out under an atmospheric pressure of nitrogen.
overall yields [37]. These advantages make it possible to construct versatile π-conjugated systems by the incorporation of 1,2,7,8-tetraazaperylene unit into molecular backbones. In this work, a new π-conjugated oligomer consisting of a 1,2,7,8tetraazaperylene core and two α-bromothiophene units at the both sides, was successfully synthesized (denoted as Br-TPAL). Its chemical structure and physical properties were fully characterized, revealing its electron-deficient characters. Furthermore, such borominated molecule as building block was facile to alternatively copolymerize with the other building blocks. Upon transition-metal catalyzed cross coupling, Br-TPAL was smoothly copolymerized with the (4,8-bis((2-octyldodecyl)oxy)benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane), (4,8-bis(5-(2-octyldodecyl)thiophen-2-yl) benzo[1,2-b:4,5b′]dithiophene-2,6-diyl)bis(trimethylstannane) and 2,5-bis(2-octyldodecyl)-3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione,and resulting in three conjugated polymers denoted as PTPAL-OBDT, PTPAL-TBDT PTPAL-DPP, respectively, in high yields. The chemical structures and physical properties of these resulting polymers were characterized by 1 H NMR, thermal gravimetry (TG), optical spectroscopy, cyclic voltammetry analyses and DSC analysis. They exhibited good thermal stabilities, and rich photophysical properties and electrochemical behaviors with respect to their different building blocks. All the three copolymers possess narrow band gaps, associated with their donor-acceptor structures. In particular, for PTPAL-DPP, a strong light harvesting capability in the near infrared regions was even observed, featuring a very narrow band gap value of around 1.45 eV. These intriguing properties render them with potential application in some electronic devices.
2.3. Synthesis and characterization 2.3.1. Synthesis of 1,5-bis(5-bromo-4-hexylthenoyl)anthracene 2 To a solution of 2-bromo-3-hexyl-5-iodothiophene (9.3 g, 25.0 mmol) in THF (10 mL), was slowly added to chloro(1-methylethyl)-magnesium (12.5 mL, 25.0 mmol, 2 M) to form the Grignard reagent. Afterwards, the resulting Grignard reagent was slowly added to a solution of 1,5-di-(2-pyridylthiol)anthracenoate 3 (4.5 g, 10.0 mmol) in dried THF (100 mL) at 0 °C and stirred overnight. Then, the reaction was quenched with 10% HCl (50 mL) and extracted with CH2Cl2. The combined organic fractions were washed with 1.0 M NaHCO3 and water, then dried over MgSO4 and filtered. The solvent was removed under reduced pressure, which was purified by column chromatography using CH2Cl2/petroleum as eluent, to afford compound 2 as yellow solid (5.2 g, yield: 72%). 1H NMR (400 MHz, CDCl3, ppm): δ 8.88 (s, 2H), 8.15 (d, J = 8.0 Hz, 2H), 7.76 (d, J = 8.0 Hz, 2H), 7.38 (m, J = 4.0 Hz, 2H), 7.24 (s, 2H), 2.52 (t, J = 3.8 Hz, 4H), 1.26–1.56 (m, 12H), 0.84 (d, J = 8.0 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm): 188.4, 144.2, 143.4, 136.7, 135.2, 133.0, 132.6, 128.5, 125.7, 124.4, 122.2, 40.0, 32.5, 28.8, 25.8, 23.2, 14.3. HRMS. (C36H36Br2O2S2, ESI): calculated for [M+H]+ 723.0523, found: 723.0606.
2. Materials and methods 2.1. Materials
2.3.2. Synthesis of 1,5-bis(5-bromo-4-hexylthienyl)anthraquinone 1 To a suspension of 2 (723.0 mg, 1.0 mmol) in 15 mL glacial acetic acid, was added a solution of CrO3 (300.0 mg, 3.0 mmol) in 15 mL glacial acetic acid and water (4:1). The reaction mixture was maintained at 75 °C for 1.5 h. Compound 1 was obtained (as light yellow solid), which was used directly in the next step without purification. 1H NMR (400 MHz, CDCl3, ppm): δ 8.33 (d, J = 8.0 Hz, 2H), 7.85 (t, J = 8.0 Hz, 2H), 7.69 (d, J = 8.0 Hz, 2H), 6.95 (s, 2H), 2.46 (t, J = 3.8 Hz, 4H), 1.23–1.47 (m, 12H), 0.81 (d, 6H, J = 8.0 Hz). HRMS. (C36H35Br2O4S2, ESI): calculated for [M+H]+ 753.0265, found: 753.0327.
All of the solvents used for chemical reactions in this work, were AR grade. Of these, diethyl ether and toluene, were purchased from Sinopharm Chemical Reagent Co., Ltd., THF, petroleum ether, o-dichlorobenzene, trimethylamine, dichloromethane, ethanol, hydrazine hydrate, Sodium carbonate anhydrous, magnesium sulfate and sodium chloride were purchased from Shanghai Titan Scientific Co., Ltd. (4,8bis((2-octyldodecyl)oxy)benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl)bis (trimethylstannane), (4,8-bis(5-(2-octyldodecyl)thiophen-2-yl) benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) and 2,5-bis (2-octyldodecyl)-3,6-bis (5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)-thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione were purchased from SunaTechInc. Co., Ltd. The solvents was carefully dried and collected before used, Tetrahydrofuran USES metal sodium to remove water and oxygen with diphenylketone as indicator, when the solvent turns blue, then collection and use. The reactions were carried out by using glovebox techniques and standard Schlenk techniques.
2.3.3. Synthesis of 3,9-bis(5-bromo-4-hexylthienyl)-1,2,7,8-tetraazaperylene Br-TAPL A suspension of hydrazine hydrate (30.0 mg, 0.60 mmol) and 1 (150.0 mg, 0.20 mmol) in ethanol (30 mL) was stirred at 78 °C under N2. After cooled to room temperature, the solvent was removed under reduced pressure. After washed with ethanol, the product Br-TAPL was obtained as yellow solid (131.0 mg, 95%). 1H NMR (400 MHz, CDCl3, ppm): δ 9.34 (d, 7.2 Hz, 2H), 8.51 (d, J = 8.0 Hz, 2H), 8.11 (t, J = 7.6 Hz, 2H), 7.50 (s, 2H), 2.63 (t, J = 8.0 Hz, 4H), 1.36–1.56 (m, 12H), 0.92 (t, 6H). HRMS (C36H35Br2N4S2, ESI): calculated for [M +H]+ 745.0592, Found: 745.0630.
2.2. Instrumentation Nuclear Magnetic Resonance and Fourier Transform Infrared Spectroscopy: 1H and 13C NMR spectra were recorded on Mercury Plus 400 (400 MHz for proton, 100 MHz for carbon) spectrometer with tetramethylsilane as the internal reference using CDCl3 as solvent in all cases; Molecular weights of the polymers were obtained by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as the eluent against polystyrene (PS) standards with a refractive index detector (Shimadzu RID-10A) and the measurements were performed at 40 °C with a flow rate of 1.0 mL min−1. Thermogravimetric Analysis (TGA): TGA was measured by a TAQ5000IR with a heating rate of 10 °C/min under flowing N2. Differential scanning calorimetry (DSC) analysis were scanned on a DSC-204F1 Calorimeter with a heating rate 10 °C min−1 and cooling rate 20 °C min−1; Absorption Measurements:
2.3.4. Synthesis of polymer PTAPL-ODBT 3,9-bis (5-bromo-4-hexylthienyl)-1,2,7,8-tetraazaperylene (148.0 mg, 0.2 mmol) and (4,8-bis((2-octyldodecyl)oxy)benzo-[1,2-b:4,5-b′] dithiophene-2,6-diyl)bis(trimethylstannane) (222.0 mg, 0.2 mmol) were employed in 15 mL chlorobenzene. After the mixture was degassed for 30 min, Pd2(dba)3 (1.8 mg) and P(o-tol)3 (3.7 mg) was added, then heated up to 110 °C and stirred for 72 h. The resulting mixture was poured into methanol for precipitation. The solid were further purified by 68
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confirming their chemical structures. By using Pd-catalyzed stille coupling reaction, Br-TAPL as the key building block was smoothly copolymerized with the other distannylated building blocks (4,8-bis((2-octyldodecyl)oxy)benzo[1,2b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane), (4,8-bis(5-(2-octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis (trimethylstannane) to afford two kinds of copolymers denoted as PTAPL-ODBT and PTAPL-TDBT, respectively. While, upon Pd-catalyzed Suzuki cross coupling reaction, compound Br-TAPL was efficiently reacted with boronic ester-substituted building block 2,5-bis(2octyldodecyl)-3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione to afford the copolymer PTAPL-DPP. All the resulting polymer samples were purified by Soxhlet extraction in different solvents with a sequence of methanol, acetone and hexane, for mainly removing off the low-molecular-weight fractions and catalyst residues. Finally, the resulting solid powders were extracted into chloroform. The chemical structures of these polymers were characterized by 1H NMR spectroscopy (detailed see the above experimental part). The 1H NMR profiles of PTAPL-ODBT and the key building block Br-TAPL was combined in Fig. 1 for comparison. As expected, the rigid polymer PTAPL-ODBT exhibits a set of relatively broad, Br-TAPL exhibits well-resolved peaks as compared with PTAPL-ODBT. The two peaks with respect to the protons (signed as H-1 and H-2) attached to (4,8-bis((2-octyldodecyl)oxy)benzo[1,2b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) and thiophene moieties, appeared at 6.94 and 7.76 ppm, respectively. While the phenyl protons of TAPL (H-3, H-4 and H-5), gave peaks at 8.62, 8.10 and 9.29 ppm, respectively. The two signals at 3.74 ppm and 3.01 ppm, were attributed to the protons of the methylene moieties attached to oxygen (H-6) and thiophene ring (H-7), respectively. Their molecular weights were measured by the Gel Permeation Chromatography (GPC), using the chloroform as the eluent. The number-averaged molecular weights (Mn) of the resulting polymers PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP were 58.67, 16.63 and 11.50 kDa, with the polydispersity index (PDIs) of 1.69, 1.78 and 1.82, respectively (Table 1).
Soxhlet extraction subsequently in methanol, acetone, hexane, and then dissolved in chloroform. The solvent was removed under reduced pressure to get the target polymer PTAPL-ODBT as dark purple solid (237.3 g, 85.0% yield). 1H NMR (400 MHz, CDCl3, ppm): δ 9.29 (br, 2H), 8.62 (br, 4H), 8.10 (br, 4H), 7.76 (br, 2H), 3.97 (br, 4H), 3.01 (br, 4H), 0.75–1.85 (br, 106H). Mn = 58.6 7 kDa, PDI = 1.69. 2.3.5. Synthesis of polymer PTAPL-TDBT Br-TAPL (148.0 mg, 0.2 mmol) and (4,8-bis(5-(2-octyldodecyl) thiophen-2-yl) benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane) (242.0 mg, 0.2 mmol) were dissolved in 15 mL chlorobenzene. After the mixture was degassed for 30 min, Pd2(dba)3 (1.8 mg) and P(o-tol)3 (3.7 mg) was added, then the mixture was heated to 110 °C and stirred for 72 h. The reactant was poured into methanol to get solid. The solid were further purified by Soxhlet extraction with different solvents in a sequence of methanol, acetone, hexane, and finally was extracted into chloroform. The solvent was removed under reduced pressure to offer PTAPL-TDBT as dark purple solid (272.9 g, 89.0%). 1H NMR (400 MHz, CDCl3, ppm): δ 9.41 (br, 2H), 8.62 (br, 2H), 8.12(br, 2H), 7.83 (br, 2H), 7.71 (br, 2H), 7.34 (br, 2H), 6.90 (br, 2H), 2.87 (br, 4H), 0.87–2.02 (br, 110H). Mn = 16.63 kDa, PDI = 1.78. 2.3.6. Synthesis of polymer PTAPL-DPP Compound Br-TAPL (148.0 mg, 0.2 mmol) and 2,5-bis(2-octyldodecyl)-3,6-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (222.0 mg, 0.2 mmol) were dissolved in 15 mL toluene. After degassed for 30 min, Pd(PPh3)4 (9.2 mg) and P(o-tol)3 (6.1 mg) was added, then the mixture was heated up to 90 °C and stirred for 72 h. Then, the resulting mixture was poured into methanol to form heavy precipitation. After filtration, the resulting residue was collected, and further purified by Soxhlet extraction by methanol, acetone, hexane in sequence, then dissolved in chloroform. The solvent was removed under reduced pressure to get PTAPL-DPP (236.2 g, 80.0% yield) as dark purple solid. NMR (400 MHz, CDCl3, ppm): δ 9.34 (br, 2H), 9.02 (br, 2H), 8.84 (br, 2H), 8.55 (br, 2H), 8.16 (br, 2H), 7.75 (br, 2H), 4.05 (br, 2H), 2.99 (br, 2H), 0.83–1.99 (br, 100H). Mn: 11.50 kDa, (PDI: 1.82).
3.2. Thermal properties of polymers 3. Results The TGA curves of the polymers heated at a rate of 10 °C/min were measured in nitrogen (Fig. 2). The thermal degradation data of these polymers, including 5%, 10% and 50% mass-loss temperatures (T5–T50) were listed in Table 1. The decomposition temperatures (Td, with 5% weight loss) of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP were 334.1 °C, 391.8 °C and 377.6 °C, respectively. Notably, the Td of PTAPL-ODBT was remarkably lower than PTAPL-TDBT, but the molecular weight of the former is higher than the latter one as shown in the GPC results. Such phenomenon seems to be attributed to their different binding modes between the alkyl side chains and the aromatic moieties in the main backbones, namely, the thermal stability of CeC bond for PTAPL-TDBT was better than that of CeO bond for PTAPLODBT. While, the Td of PTAPL-DPP falling in between PTAPL-TDBT and PTAPL-DPP, likely indicated that the thermal stability of CeN bond is stronger than that of CeO, but weaker than that of CeC.
3.1. Synthesis of building block and conjugated polymers The synthetic routes to the target compounds, were depicted in Scheme 1. Compound 3 1,5-di-(2-pyridylthiol)anthracenoate was synthesized according to a modified protocol as our previously reported [37]. The treatment of anthracene-1,5-dicarbonyl dichloride with 2mercaptopyridine, using triethylamine (TEA) as a catalyst, afforded compound 3 as a pale-yellow solid in a yield of 68%. By this method, the two mercaptopyridine were effectively attached on compound 3 as good leaving groups for further substitution reaction. In order to directly construct a molecular backbone with two active bromo-subsitutents, (5-bromo-4-hexylthiophen-2-yl)magnesium chloride was produced by the treatment of 2-bromo-3-hexyl-5-iodothiophene with Grignard reagent, which was directly used in the next step. The key precursor compound 3, was synthesized according to our previous report [37]. The reaction of 3 with (5-bromo-4-hexylthiophen2-yl) magnesium chloride in THF at 0 °C and stirring overnight, smoothly led to the formation of 1,5-bis(5-bromo-4-hexylthiophen-2-ly)anthracene 2, which was purified via column chromatography (eluent: CH2Cl2/ petroleum) as a yellow solid in a yield of 72%. Upon oxidation with CrO3, 1,5-bis(5-bromo-4-hexylthiophen-2-yl)anthraquinone 1 was obtained in a nearly quantitative yield. After the treatment with hydrazine hydrate, this intermediate was readily converted to the target molecule 3,9-bis(5-bromo-4-hexylthiophen-2-yl)-1,2,7,8-tetraazaperylene BrTAPL in a very high yield (97%), respectively. All new compounds have been fully characterized by NMR spectra, HRMS analyses for essentially
3.3. Optical properties of the as-prepared polymers The UV–vis absorption spectra of these polymers were measured in CH2Cl2. The polymers PTAPL-ODBT and PTAPL-TDBT showed similar absorption profiles, attributable to their similar π-conjugated main backbones. The absorption bands cover the most of the ultraviolet and visible regions. The one shoulder peak at λabs = 347 nm, and the two main absorption at λabs = 415 and 551 nm, were observed for PTAPLODBT. Correspondingly, the three red-shifted main peaks at λabs = 350, 453 and 572 nm were found for PTAPL-TDBT. Such phenomenon is reasonably arising from the difference in the pendant side chains of the 69
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Scheme 1. Synthetic routes and conditions: (a) CrO3,HAc; (b) NH2-NH2, C2H5OH; (c) Pd2(dba)3, P(o-tol)3, chlorobenzene, 110 °C; (d) Pd2(dba)3, P(o-tol)3, toluene, 90 °C.
Table 1 Molecular weight and thermal stability of PTPAL-OBDT, PTPAL-TBDT and PTAPL-DPP. Sample
Mn/kDa
Mw/kDa
PDI
T5 (°C)
T10 (°C)
T50 (°C)
PTAPL-ODBT PTAPL-TDBT PTAPL-DPP
58.67 16.63 11.50
99.46 29.59 20.93
1.69 1.78 1.82
334.1 391.8 377.6
344.9 409.1 395.7
386.7 463.4 457.3
PTAPL-ODBT. The absorption peaks in the high energy regions of 300–450 nm were originated from the π−π* transitions of the aromatic skeletons of the two polymers. And the absorption at λabs = 470–700 nm, can be attributed to the intramolecular charge transfer (ICT) from the electron-donating benzothiophene-cored segment to the electron acceptor 1,2,7,8-tetraazaperylene. The lowest energy absorption peak (λmax = 572 nm) of PTAPL-TDBT, was significantly red-shifted over 20 nm in comparison with that of PTAPLODBT, manifesting a much facile electron transfer between donor and acceptor in the former case, because of its much stronger electron donating character associated with the thienyl groups on the pendent side chains. These results clearly demonstrated that the optical behaviors of such kinds of conjugated polymers can be significantly tuned via the
Fig. 1. 1H NMR spectra of Br-TAPL and PTAPL-ODBT in CDCl3.
two polymers. Obviously, the incorporation of a thiophene unit in the perpendicular direction to the main conjugated backbone for PTAPLTDBT, could result in an extend π conjugation by compared with 70
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Table 2 Photophysical data of PTPAL-OBDT, PTPAL-TBDT and PTAPL-DPP in CH2Cl2, respectively.
PTAPL-ODBT PTAPL-TDBT PTAPL-DPP
λabs (nm)
λemission (nm)
Τ (ns)
347,415,551 350,453,572 420,662,720
410,437,459 410,438,459 410,438,460
1.91 2.02 1.97
in side chain or main backbone. In particular, the 1,2,7,8-tetraazaperylene moiety could be well compatible with those typical building blocks to architect new conjugated polymers with promising optical properties. In addition, all polymers show very weak fluorescence emissions in CH2Cl2 (Fig. 3b), which could be attributed to the fluorescence quenching effect of 1,2,7,8-tetraazaperylene derivative [37]. Typically, a nitrogen embedded acene always exhibits strong trapping effect [38] or intersystem crossing effect associated with electronegative nitrogen atom with a lone pair of electrons in aromatics [39]. The fluorescence lifetime values of PTPAL-OBDT, PTPAL-TBDT and PTAPL-DPP are 1.91 ns, 2.02 ns and 1.97 ns, respectively (Table 2).
Fig. 2. TGA curves of the resulting polymers measured with a heating rate of 10 °C/min under flowing N2.
variation of the pendant side chains attached on the conjugated main backbone. Moreover, in the case of PTAPL-DPP, 2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4dione as the key building block, took place of (4,8-bis((2-octyldodecyl) oxy)benzo [1,2-b:4,5-b′]dithiophene for PTAPL-ODBT or (4,8-bis(5-(2octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene for PTAPLTDBT. Interestingly, PTAPL-DPP showed one sharp absorption peak at 480 nm, and a relatively broad peak centered at 662 nm with a shoulder peak at 720 nm. These absorption bands are from 400 to 900 nm, covering the nearly whole visible regions and part of the NIR regions, exhibiting excellent light harvesting capability. The absorption max at 662 nm, was remarkably red-shifted by about 90 nm, as compared with that of PTAPL-TDBT. Such result essentially revealed the presence of donor-acceptor structure of PTAPL-DPP. Different from the other two polymers, this donor-acceptor structure in the π-conjugated main backbone, should be consisting of the two kinds of acceptors 1,2,7,8tetraazaperylene (A1) and diketodipyrrole (A2), and one kind of bithiophene (D), to form D-A1-D-A2 fashion (Scheme 1). The building blocks in the main backbone of PTAPL-DPP, could construct a π-extended conjugated system through rationally matching these functional units with their frontier molecular orbital energy levels. Accordingly, the optical band gaps of the three conjugated polymers were evaluated by the onsets of their absorption edges in such a sequence: 1.47 eV for PTAPL-DPP < 1.76 eV for PTAPL-TDBT < 1.88 eV for PTAPL-ODBT, indicative of a declined tendency along with the decreased π-conjugation. Meanwhile, the relatively narrow bands for all of these three polymers observed here, could be contributable to their D-A structural characters. These phenomena manifested that the finely tunable electronic structures of conjugated polymers via the modification of either
3.4. Electrochemical behavior In order to make insight into the electronic structures of these asprepared polymers, the electrochemical behaviors of the as-prepared polymers were investigated through cyclic voltammetries (CVs). The CV profiles of PTAPL-ODBT and PTAPL-TDBT showed one irreversible oxidation processes and one reduction processes under the measurement conditions, with the initial oxidation potentials at 0.70 and 0.95 V, and initial reduction potentials at −1.12 and −1.06 V. respectively (Fig. 4). PTAPL-TDBT was easily oxidized in compared with PTAPL-ODBT, likely due to the much electron-rich character of the former, relevant to its thiophenyl side groups. Such result is well in line with their optical properties. Notably, PTAPL-DPP exhibited the two consecutive reversible reduction peaks at −1.41 and −1.79 V, and one quasi-reversible oxidation peak at 0.75 V. The HOMO energy levels of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP, evaluated from the onsets of the first oxidation potential, were −5.50, −5.75 and −5.27 eV respectively. Correspondingly, evaluated from the onsets of the first reduction potential, the LUMO energy levels of the polymers were calculated as −3.65, −3.74 and −3.39 eV (Table 3). On the other hand, its electrochemical band gap was calculated as 1.82, 2.01 and1.88 eV respectively, which is similar to the optical band gap 1.88, 1.76 and1.43 eV. Obviously, the electrochemical behavior of these conjugated polymers clearly revealed the influence of the building blocks on their electronic structures, demonstrating their rich semiconducting properties.
Fig. 3. Normalized UV–vis absorption spectra (a) and normalized emission spectra (b) of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP in CH2Cl2, respectively. 71
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Fig. 4. Cyclic voltammograms of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP in CH2Cl2 (0.1 mol/L n-Bu4NPF6) at a scan rate of 100 mV/s. Potentials were given against ferrocene/ferrocenium ion couple (Fc/Fc+). Table 3 Summary electrochemical properties of PTAPL-ODBT, PTAPL-TDBT and PTAPL-DPP.
PTAPL-ODBT PTAPL-TDBT PTAPL-DPP
E1 (V)
E2 (V)
E3 (V)
a
0.70 0.95 0.47
−1.12 −1.06 −1.41
– – −1.79
−5.50 −5.75 −5.27
HOMO (eV)
b
LUMO (eV)
−3.68 −3.74 −3.39
Eg1 (eV)
Eg2 (eV)
1.82 2.01 1.88
1.88 1.76 1.43
HOMO = −E1 − 4.80 eV, E1: the first oxidation potential, evaluated from the onset. LUMO = −E2 − 4.80 eV, E2: the first reduction potential, evaluated from onset of the first reduction potential for PTAPL-ODBT and PTAPL-TDBT, evaluated from half-wave potential for PTAPL-DPP; E3: the second reduction potential, evaluated from half potential; Eg1: electrochemical energy gap, Eg1 = LUMO-HOMO; Eg2: optical band gap, estimated from UV–vis absorption edge. a
b
Conflicts of Interest
4. Conclusions
There are no conflicts to declare.
We have successfully developed a new electron-deficient 1,2,7,8tetraazaperylene derivative with two terminal boromo groups (BrTPAL). Upon transition-metal catalyzed cross coupling, such compound was readily copolymerized with some traditional functional monomers to form three kinds of donor–acceptor (D–A) conjugated polymers. The resulting copolymers showed tunable electronic structures via the modification of the main chains and side chains. Some excellent optical properties, such as, strong light harvesting capability in the NIR regions, were observed. Altering the building block boned with TPAL can tune the TGA, Vis-UV and CV properties. In particular, the intriguing characters of polymer PTAPL-DPP render them desirable for developing promising semiconducting materials potentially useful for highperformance organic electronic devices. It is also can be foreseen that the developed electron-deficient 1,2,7,8-tetraazaperylene derivative (Br-TPAL) will be significantly contributable to expanding the regime of D-A conjugated polymers. The corresponding applications of these polymers are in progress.
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Author contributions Fan Zhang and Feng Liu conceived and designed the experiments; Peirong Qiang and Ruizhi Tang performed the experiments of synthesis of Monomer and polymers; Shuai Bi characterized the electrochemical behavior; Peirong Qiang, Feng Qiu, Ruizhi Tang, and Fan Zhang analyzed the data; Peirong Qiangand Fan Zhang wrote the paper.
Acknowledgments This work was financially supported by Shanghai Committee of Science and Technology (16JC1400703, 15JC1490500), National Natural Science Foundation of China (21574080, 21774072), Open Project Programs of the State Key Laboratories (SKLPEE-KF201702, Fuzhou University; 20161803, Xi’an Jiaotong University); State Key Laboratory of Supramolecular Structure and Materials (sklssm201732, Jilin University). 72
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