Fabrication of Alternating Copolymers Based on Cyclopentadithiophene-Benzothiadiazole Dicarboxylic Imide with Reduced Optical Band Gap: Synthesis, Optical, Electrochemical, Thermal, and Structural Properties

A series of alternating copolymers containing cyclopentadithiophene (CPDT) flanked by thienyl moieties as electron-donor units and benzothiadiazole dicarboxylic imide (BTDI) as electron-acceptor units were designed and synthesized for solar cell applications. Different solubilizing side chains, including 2-ethylhexyl chains and n-octyl chains were attached to CPDT units, whereas 3,7-dimethyloctyl chains and n-octyl chains were anchored to the BTDI moieties. The impact of these substituents on the solubilities, molecular weights, optical and electrochemical properties, and thermal and structural properties of the resulting polymers was investigated. PCPDTDTBTDI-EH, DMO was synthesized via Suzuki polymerization, whereas PCPDTDTBTDI-8, DMO, and PCPDTDTBTDI-EH, 8 were prepared through direct arylation polymerization. PCPDTDTBTDI-8, DMO has the highest number average molecular weight (Mn = 17,400 g mol−1) among all polymers prepared. The PCPDTDTBTDI-8, DMO and PCPDTDTBTDI-8, 8 which have n-octyl substituents on their CPDT units have comparable optical band gaps (Eg ~ 1.3 eV), which are around 0.1 eV lower than PCPDTDTBTDI-EH, DMO analogues that have 2-ethylhexyl substituents on their CPDT units. The polymers have their HOMO levels between −5.10 and −5.22 eV with PCPDTDTBTDI-EH, DMO having the deepest highest occupied molecular orbital (HOMO) energy level. The lowest unoccupied molecular orbital (LUMO) levels of the polymers are between −3.4 and −3.5 eV. All polymers exhibit good thermal stability with decomposition temperatures surpassing 350 °C. Powder X-ray diffraction (XRD) studies have shown that all polymers have the amorphous nature in solid state.


. Materials
All of the starting materials and reagents obtained from Sigma-Aldrich (Gillingham, UK) and Alfa Aesar (Heysham, UK) were utilized without further purification. The majority of the reactions were carried out under argon atmosphere. Anhydrous solvents used for the reactions were obtained from Grubbs solvent purification system within the Sheffield University/Chemistry Department. All the monomers used for preparing the polymers in this article were synthesized according to the procedure part.

Measurements
Here, 1 H and 13 C nuclear magnetic resonance (NMR) spectra for the monomers were measured either with a Bruker Avance AV 3HD 400 (400 MHz, Bruker, Berlin, Germany) spectrometer in deuterated chloroform (CDCl 3 ), deuterated acetone (CD 3 COCD 3 ), or deuterated dimethyl sulphoxide (CD 3 SOCD 3 ) as the solvents at room temperature. The 1 H NMR spectra for the polymers were measured with Bruker AV 3HD 500 (500 MHz, Bruker, Berlin, Germany) in deuterated 1,1,2,2-tetrachloroethane (C 2 D 2 Cl 4 ) as the solvent at 100 • C. The chemical shifts were measured in parts per million (ppm). The coupling constants (J) are calculated in Hertz (Hz). The 1 H and 13 C NMR spectra were analysed using Bruker TopSpin 3.2 software. Elemental analysis (CHN) was performed by either the Perkin Elmer 2400 CHNS/O Series II Elemental Analyser (Horiba, Northampton, UK) or Vario MICRO Cube CHN/S Elemental Analyser (Eltra, Chester, UK) for CHN analysis. Anion analysis (Br, I, and S) was performed by the Schöniger oxygen flask combustion method. Mass spectra for the monomers were recorded on Agilent 7200 accurate mass Q-TOF GC-MS spectrometer (Agilent, Santa Clara, CA, USA). Helium was used as a carrier gas at the rate of 1.2 mL min −1 , the injection volume was 1.0 µL, and the concentration of measured sample was 5 mg mL −1 in CHCl 3 solvent. The temperature program is between 60 and 320 • C at 10 • C min −1 . Mass spectra for the monomers were obtained by the electron ionization method (EI). Gel permeation chromatography (GPC) measurements accomplished by Viscotek GPC Max (Malvern Panalytical, Malvern, UK), a waters 410 instrument with a differential refractive index detector, two Polymer Labs PLgel 5µ Mixed C (7.5 × 300 mm) columns and a guard (7.5 × 50 mm). Molecular weights for the polymers were determined by preparing polymer solutions (2.5 mg mL −1 ) using HPLC grade CHCl 3 . The columns were thermostated at 40 • C using CHCl 3 . UV-VIS absorption spectra were measured by SPECORD S600 UV/visible Spectrophotometer (Hach, Düsseldorf, Germany) at room temperature. The absorbance of the polymers was measured in CHCl 3 solution using quartz cuvettes (light path length = 10 mm) and blank quartz cuvettes including CHCl 3 was used as a reference. The polymers were coated on quartz substrates from CHCl 3 solutions (1 mg mL −1 ) and blank quartz substrate was used as a reference. Thermogravimetric analysis (TGA) measurements were recorded by Perkin Elmer (Pyris 1) thermogravimetric Analyser (Eltra, Chester, UK). Platinum pans was used as sample holder and the weight of the measured samples was about (3 mg). Cyclic voltammograms were measured using a Model 263A Potentiostat/Galvanostat-Princeton Applied Research (Artisan Technology Group, Champaign, Indian, United State). A standard three electrode system was used based on a Pt disk working electrode, a silver wire reference electrode (Ag/Ag + ) inserted in (0.01 M) AgNO 3 solution in acetonitrile and put it in the electrolyte solution and a Pt wire counter electrode was purged with argon atmosphere during all measurements at room temperature. Tetrabutylammonium perchlorate in acetonitrile (Bu 4 NClO 4 /CH 3 CN) (0.1 M) was used as the electrolyte. Polymer thin films were drop cast onto the Pt disk from polymer solutions in CH 3 Cl (1 mg mL −1 ) and dried under nitrogen prior to measurement. Ferrocene (Fc/Fc + ) was used as a reference redox system. Powder X-ray diffraction (XRD) for the polymers was measured by Bruker D8 ADVANCE X-ray powder diffractometer (Bruker, Berlin, Germany). Infrared absorption spectra were recorded on ATR Perkin Elmer Rx/FT-IR system (Perkin Elmer, Melville, NY, USA) and Nicolet Model 205 FT-IR spectrometer (Nicolet Instrument, Sainte-Julie, QC, Canada).

Monomers and Polymers Synthesis
Synthesis of 2,5-dibromothiophene (1): Thiophene (25.00 g, 297.12 mmol) in DMF (250 mL) was added to a flask and cooled to −15 • C. To this solution, NBS (110.00 g, 618.04 mmol) in DMF (300 mL) was added dropwise in the dark, and the reaction was stirred overnight at RT. The reaction contents were put into ice and DCM and subsequently extracted with DCM, and the organic phase was washed with deionized H 2 O to a neutral pH. The organic layer was collected and dried over MgSO 4 and the solvent was concentrated to afford the product, which was purified by vacuum distillation and gave 1 as a yellow oil (59.30 g, 245 mmol, 82% yield) [22]. 1 H NMR (CDCl 3 , δ): 6.87 (s, 2H). 13 (2): Concentrated H 2 SO 4 (150 mL) and fuming H 2 SO 4 (150 mL, 20% free SO 3 ) were combined in a flask. This flask was cooled to 0 • C and 1 (26.00 g, 107.46 mmol) was added dropwise. Concentrated nitric acid (125 mL) was added dropwise, and the reaction contents were kept under 20 • C. During addition of nitric acid, yellow precipitate formed quickly. The mixture was stirred for 3 h at 20-30 • C. Then, the mixture was poured into ice, and upon melting of the ice, a yellow precipitate was filtrated and washed thoroughly with deionized H 2 O. The product recrystallized from methanol to afford 2 as yellow crystals (32.50 g, 98 mmol, 91% yield) [23]. 13  Synthesis of 3 ,4 -dinitro-2,2 :5 ,2"-terthiophene (3): In a flask, 2 (9.90 g, 29.82 mmol), 2-(tributylstannyl)thiophene (27.82 g, 74.54 mmol) and PdCl 2 (PPh 3 ) 2 (0.45 g, 0.64 mmol) were added. The system degassed under argon and anhydrous toluene (100 mL) was added and heated at 115 • C for 24 h. The flask was cooled to RT and the volatiles were removed to obtain the product, which was purified by column chromatography with gradient (petroleum ether, 0-50% DCM) to obtain an orange solid and the product further purified by recrystallization from methanol to afford 3 as orange crystals (9.10 g, 27 mmol, 90% yield) [24]. 1  Synthesis of 3 ,4 -diamino-2,2 :5,2"-terthiophene (4): EtOH (31 mL) and HCl (62 mL, 35%) were added to 3 (3.00 g, 8.86 mmol) in a flask. To this mixture, anhydrous tin (II) chloride (31.00 g, 163.50 mmol) in ethanol (62 mL) was added and stirred at 30 • C for 24 h. The mixture was cooled to RT and put into cold NaOH. To this mixture, toluene was added and then stirred vigorously and filtered through celite. The product was extracted with toluene and the organic phases were washed with NaCl and subsequently dried over MgSO 4 . The solvent was concentrated to obtain the 4 as a brown solid (2.40 g, 9 mmol, 97% yield) [25]. 1 (30 mL) in a flask and degassed under argon. To this mixture, N-thionylaniline (1.60 g, 11.49 mmol) was added dropwise and chlorotrimethylsilane (4.50 g, 41.42 mmol) was then added dropwise, resulting in a dark blue colour. The reaction contents were stirred for 3 h at RT and then put into DCM. The solution was washed with HCl and with deionized water and then extracted with DCM. The organic phase was dried over anhydrous MgSO 4 and subsequently filtered.
The solvent was evaporated to afford the product, which purified via chromatography with DCM to afford 5 as blue crystals (1.72 g, 6 mmol, 93% yield) [26]. 1  Synthesis of 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dimethyl ester (6): 5 (1.86 g, 6.06 mmol) and dimethyl acetylenedicarboxylate (1.73 g, 12.17 mmol) were combined in a flask. The system was evacuated and refilled with argon for three cycles before anhydrous xylene (40 mL) was added. The reaction contents were refluxed for 24 h. The flask was cooled to RT and the solvent was removed to afford the product, which was purified by column chromatography with gradient (petroleum ether, 0-50% DCM) to afford 6 as yellow crystals (2.37 g, 6 mmol, 94% yield) [27]. 1   Synthesis of 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic acid (7): Sodium hydroxide (4.00 g, 100.00 mmol) dissolved in deionized water (30 mL) was added to a flask. To this solution, ethanol (200 mL) and 6 (2.27 g, 5.45 mmol) were added and the reaction contents were refluxed for 24 h. The flask was cooled to RT and deionized H 2 O was added. This mixture was cooled to 0 • C and neutralized by HCl to precipitate the product. The precipitate was filtered and subsequently washed with deionized H 2 O. The precipitate was dried under high vacuum to afford 7 as yellow solid (1.80 g, 5 mmol, 85% yield) [28]. 1  Synthesis of 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic anhydride (8): 7 (1.15 g, 2.96 mmol) and anhydrous acetic anhydride (10.00 g, 97.95 mmol) were combined in a flask. The system was evacuated and refilled with argon for three cycles before anhydrous xylene (30 mL) was added. The mixture was heated at 130 • C for 6 h. The mixture was cooled to RT, and the solvent was evaporated to obtain 8 as red solid (1.06 g, 3 mmol, 97% yield) [29]. 1  Synthesis of 3,7-dimethyloctyl bromide (9): Triphenylphosphine (21.10 g, 80.44 mmol) was added to a mixture of 3,7-dimethyloctyl alcohol (12.61 g, 79.69 mmol) and dichloromethane (250 mL) and stirred in a flask. To this mixture, NBS (14.26 g, 80.14 mmol) was added portionwise and stirred at RT for 90 min. The mixture was washed with NaHCO 3 solution, dried over MgSO 4 , and filtered, and the solvent was evaporated. The substance was stirred in petroleum ether for 1 h at RT and then filtered, and the filtrate was evaporated. The product was purified by chromatography with petroleum ether to yield 9 as colourless oil (23.00 g, 59 mmol, 73% yield) [30]. 1  Synthesis of N-(3,7-dimethyloctyl)phthalimide (10): 9 (4.07 g, 18.40 mmol) and anhydrous DMF (20 mL) were added into a flask. To this mixture, potassium phthalimide (3.75 g, 20.27 mmol) was added, and the reaction contents were heated to 90 • C for 17 h. The mixture was cooled to RT and put in deionized H 2 O, and the product was subsequently extracted with DCM. The organic extracts were combined, and then washed with KOH and deionized water. The organic phase was dried over MgSO 4 , and the solvent was evaporated to obtain the product, which was purified via chromatography with DCM to yield 10 as colourless oil (5.29 g, 18 mmol, 91% yield) [31]. 1  Synthesis of 3,7-dimethyl-1-octanamine (11): 10 (6.03 g, 20.98 mmol), hydrazine hydrate (4.0 mL, 65.0 mmol, 51%), and methanol (100 mL) were combined in a flask. The reaction contents were refluxed until the starting material disappeared. Upon completion, excess HCl was added and the mixture was refluxed for 1 h and then cooled to RT. The precipitate was filtered and washed with water. The methanol was concentrated and the residue was diluted with dichloromethane. The organic layer was washed with KOH, and the product was extracted with dichloromethane. The organic phase was washed with NaCl and dried over MgSO 4 , and the solvent was concentrated to yield 11 as a brown oil (2.85 g, 18 mmol, 86% yield) [32]. 1  Synthesis of 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-N-(3,7-dimethyloctyl)dicarboxylic imide (M1): 8 (1.00 g, 2.69 mmol), acetic acid (50 mL, 100%), and 11 (0.88 g, 5.59 mmol) were combined in a flask. The system was evacuated, refilled with argon for three cycles, and heated overnight at 110 • C. The mixture was cooled to RT, and then, acetic anhydride (20 mL) was added and heated at 110 • C for 6 h. The mixture was cooled to RT, and the solvent was concentrated to yield the product, which was purified by chromatography with 60:10, petroleum ether: ethyl acetate to afford M1 as an orange solid (1.15 g, 2.3 mmol, 84% yield) [29]. 1  Synthesis of 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-N-octyl-dicarboxylic imide (M2): M2 was prepared followed by the same procedure for synthesis of M1 except Noctylamine (1.20 g, 9.28 mmol) was used. M2 was obtained as an orange solid (1.20 g, 2.5 mmol, 93% yield) [29]. 1   Synthesis of 4,7-di(5-bromo-thien-2-yl)-2,1,3-benzothiadiazole-5,6-N-(3,7-dimethyloctyl) dicarboxylic imide (M3): M1 (1.00 g, 1.96 mmol) and THF (100 mL) were combined in a flask. To this mixture, NBS (1.74 g, 9.77 mmol) was added and stirred overnight at RT in the dark. The solvent was evaporated to obtain the product as red solid, and it was subsequently washed with cold CH 3 OH, filtered, and dried. The product was purified via chromatography with DCM to yield M3 as red solid (1.28 g, 2 mmol, 98% yield) [28]. 1 13  Synthesis of bis(3-thienyl)methanol (12): In a flask, 3-bromothiophene (19.56 g, 119.97 mmol) was added and then degassed under argon before anhydrous Et 2 O (150 mL) was added. The flask was cooled to −78 • C and n-BuLi (48.00 mL, 120.00 mmol) was added dropwise and stirred for 4 h. To this mixture, thiophene-3-carboxaldehyde (13.44 g, 119.83 mmol) was added dropwise. The reaction contents were stirred at −78 • C for 3 h and the temperature was raised to RT and stirred overnight. NH 4 Cl solution was added and the product extracted with CHCl 3 . The organic layer was separated and washed with NaCl solution. The combined organic layers were dried over MgSO 4 and filtered, and the solvent was evaporated to yield the product. It was purified via column chromatography using petroleum ether: ethyl acetate (80:20) to obtain 12 as yellow oil (20.00 g, 102 mmol, 85% yield) [33]. 1  Synthesis of bis(2-iodo-3-thienyl)methanol (13): In a flask, 12 (10.00 g, 50.94 mmol) was added and then degassed under argon before anhydrous Et 2 O (50 mL) was added. The flask was cooled to −78 • C and n-BuLi (62.5 mL, 156.25 mmol) was added dropwise at this temperature during 2 h and stirred for 2 h. The reaction contents were stirred at RT for 2 h. The mixture was cooled to −78 • C and subsequently iodine (42.70 g, 168.24 mmol) dissolved in anhydrous Et 2 O (250 mL) was added dropwise. The reaction was stirred overnight at RT. Sodium thiosulphate solution was added, and the product was extracted with Et 2 O. The organic layers were separated and dried over MgSO 4 and then filtered. The solvent was concentrated to afford the product. It was purified via chromatography using petroleum ether: DCM (70:30) to afford 13 as cream-coloured crystals (18.00 g, 40 mmol, 79% yield) [34]. 1  and Cu powder (1.49 g, 23.44 mmol) was combined in a flask. The mixture was evacuated and refilled with argon for three cycles before anhydrous DMF (25 mL) was added and heated at 125 • C for 3 h. The reaction contents were cooled to RT, dissolved in toluene, and then filtered. NaHCO 3 solution was added to the filtrate and the mixture was extracted with toluene, and the organic phases were combined and washed with deionized H 2 O several times until became neutral. The organic phase was dried over anhydrous MgSO 4 and filtered, and the solvent was evaporated to afford the product, which recrystallized from isopropanol to yield 15 as purple crystals (1.47 g, 8 mmol, 98% yield) [35]. 1  84 mmol), potassium hydroxide (2.47 g, 44.02 mmol) and hydrazine hydrate (15 mL, 64%) were combined in a flask. The system was evacuated and refilled with argon for three cycles before triethylene glycol (247 mL) was added and heated at 180 • C for 17 h. The flask was cooled to RT and deionized H 2 O was added, and the product was extracted with diethyl ether. The organic layer was washed with NH 4 Cl solution. The organic layer was dried over MgSO 4 and filtered, and the solvent was concentrated to obtain the product. It was purified via chromatography with petroleum ether to afford 16 as white crystals (1.83 g, 10 mmol, 80% yield) [36]. 1  , and NaI (0.02 g, 0.13 mmol) were combined in a flask. The system was purged with three vacuum/argon cycles before anhydrous DMSO (8.5 mL) was added and cooled to 0 • C. To this mixture, potassium hydroxide (0.31 g, 5.61 mmol) was added and stirred for 17 h at RT. Deionized H 2 O was added to the mixture and extracted with n-hexane. The organic phase was separated, dried over anhydrous MgSO 4 , and filtered. The solvent was evaporated to obtain the product. It was purified using chromatography with petroleum ether to afford 17 as yellow oil (0.50 g, 1 mmol, 89% yield) [37]. 1  The organic layer was separated, dried over MgSO 4 , and filtered. The solvent was concentrated to obtain the product. It was purified using chromatography with petroleum ether to afford M4 as yellow oil (1.30 g, 2 mmol, 88% yield) [38]. 1  were added to a flask and degassed under argon. Anhydrous THF (10 mL) followed by sodium hydrogen carbonate solution (2.5 mL, 5% wt, degassed) was added and the system was degassed again. To this mixture, Pd(OAc) 2 (3.7 mg, 0.0168 mmol) and P(o-tol) 3 (10.2 mg, 0.0336 mmol) were added, degassed, and heated at 90 • C for 30 h. The flask was cooled to RT, the polymer was dissolved in CHCl 3 (200 mL), an NH 4 OH solution (50 mL, 35% in H 2 O) was added, and the mixture was stirred overnight. The organic phase was separated and washed with deionized H 2 O. The organic phase was concentrated to around (50 mL) and put into methanol (300 mL) and stirred overnight. The mixture was filtered, and the polymer was cleaned using Soxhlet extraction with methanol (300 mL), acetone (300 mL), hexane (300 mL), and then toluene (300 mL). The toluene fraction was concentrated (to around 50 mL) and then put into methanol (300 mL). The mixture was stirred overnight, and the pure polymer was recovered by filtration to afford PCPDTDTBTDI-EH, DMO as dark green powders (16 mg, 0.02 mmol, 8% yield) [40]. GPC: toluene fraction, M n = 5200 g mol −1 , M w = 10,100 g mol −1 , PDI = 1.9 and Dp = 6. 1

Polymers Synthesis
In this work, the preparations of four low band gap copolymers either through Suzuki or direct arylation polymerizations are discussed. PCPDTDTBTDI-EH, DMO was synthesized via Suzuki polymerization between diboronic ester of CPDT monomer (M6) and dibrominated BTDI monomer (M3) (Scheme 3). A direct arylation polymerization as a new synthetic method was used to prepare the other three copolymers based on CPDT units. One of the advantages of the direct arylation polymerization is that it requires fewer synthetic steps compared to Suzuki and Stille polymerizations [41]. It eliminates the need to prepare boronic ester and stannylated monomeric compounds, which are sometimes challenging to purify from their by-products as in the case for M6. It also avoids the use of toxic compounds especially tin compounds [41]. PCPDTDTBTDI-8, DMO, PCPDTDTBTDI-8, 8 and PCPDTDTBTDI-EH, 8 were synthesized successfully through direct arylation polymerization using Pd 2 (dba) 3 .CHCl 3 /P(o-MeOPh) 3 catalyst, caesium carbonate base, and pivalic acid in anhydrous toluene: DMF as a cosolvent. M4 was copolymerized with both M1 and M2 to form PCPDTDTBTDI-8, DMO and PCPDTDTBTDI-8, 8, respectively. PCPDTDTBTDI-EH, 8 was obtained by copolymerizing M5 with M2. All polymerizations were left running between 17 and 96 h with large amounts of dark green precipitates forming as the reactions proceeded. The polymers were then dissolved in chloroform and an ammonia solution was added, and then, the mixture was stirred overnight to remove the Pd metal catalyst residues by forming Pd(NH 3 ) 4 (OH) 2 soluble complexes. The polymers were obtained by precipitation from methanol followed by filtration. The polymers were purified via Soxhlet extraction with methanol, acetone, hexane, toluene, and finally chloroform. The small molecules, oligomers, and impurities were removed in the methanol, acetone, and hexane fractions. The toluene and chloroform fractions of the polymers were subsequently collected and concentrated in vacuo and reprecipitated in methanol followed by filtration to yield the purified polymers. The structures of the PCPDTDTBTDI-EH, DMO, PCPDTDTBTDI-8, DMO, PCPDTDTBTDI-8, 8, and PCPDTDTBTDI-EH, 8 were confirmed by the 1 H NMR spectroscopy, FT-IR spectroscopy, and elemental analysis. The 1 H NMR spectra of the polymers are available in the Supplementary Materials Information.

Molecular Weights and Yields
Molecular weights of the polymers were measured by GPC in chloroform solution at 40 • C relative to polystyrene standards (Table 1). Although, the polymerization of PCPDTDTBTDI-EH, DMO was performed for 48 h, it was synthesized in very low yield (<10%) and with a low number average molecular weight (M n~5 000 g mol −1 ). There are several factors that would lead to the low M n value and low yield of this polymer. One of the main reasons is probably due to severe steric hindrance between two branched alkyl chains (2-ethylhexyl and 3,7-dimethyloctyl) on CPDT and BTDI repeat units, respectively. The second reason is that the bis-boronic ester monomer (M6) contains some impurities such as unreacted staring material and monoboronic ester, since M6 was used for polymerization without further purification. Since the monomer was unstable, it was difficult to purify by column chromatography. Finally, the polymer contains too much solubilizing side chains. It is well known that both steric hindrance and impurities disrupt the effective conjugation length (ECL) and lead to low molecular weight polymers. PCPDTDTBTDI-8, DMO and PCPDTDTBTDI-8, 8 were synthesized twice under the same experimental conditions but with different reaction times. Substituting 3,7-dimethyloctyl chains in PCPDTDTBTDI-8, DMO for n-octyl chains in PCPDTDTBTDI-8, 8 on the BTDI units led to lower M n values for the toluene fractions of the polymers in the first polymerization. However, in the second polymerization, by extending the reaction time, the M n value of the former polymer increased slightly, while M n value of the latter polymer increased significantly for the toluene fractions. In addition, PCPDTDTBTDI-8, DMO afforded another fraction in chloroform of a higher M n value. The higher molecular weight polymers could be obtained by prolonging the polymerization times as well as by substituting n-octyl chains to 3,7-dimethyloctyl chains on the BTDI building blocks. Furthermore, replacing n-octyl chains in PCPDTDTBTDI-8, 8 by 2-ethylhexyl chains in PCPDTDTBTDI-EH, 8 on the CPDT moieties results in a polymer with the higher M n value for toluene fractions. Moreover, PCPDTDTBTDI-EH, 8 has the highest M n value for toluene fractions among all polymers prepared. This could be attributed to the effect of the branched chains in both PCPDTDTBTDI-8, DMO and PCPDTDTBTDI-EH, 8, which provide greater solubilities and higher molecular weight fractions. The yields of the direct arylation polymerization are high between 72% and 95%, and PCPDTDTBTDI-8, DMO has the highest yield.

Optical Properties
UV-visible spectrophotometer is an efficient technique used to study the UV-VIS absorption spectra of various types of conductive polymers, polymer electrolytes, and composites [46][47][48][49][50]. The UV-VIS absorption spectra of all polymers were investigated in chloroform solutions and in thin films ( Figure 1 and Table 2). In solutions, PCPDTDTBTDI-8, DMO, PCPDTDTBTDI-8, 8, and PCPDTDTBTDI-EH, 8 display comparable absorption maxima, which are red-shifted around 40 nm relative to those of PCPDTDTBTDI-EH, DMO analogue probably as a result of the low molecular weight of the latter polymer. In thin films, the absorption spectra of the polymers show quite strong bathochromic shift absorption maxima between 29 and 86 nm relative to their absorption in solutions. This could be explained by stronger intermolecular π-π interaction and more planar structures in the solid state. The E g of the polymers is estimated from the absorption onsets in thin films. PCPDTDTBTDI-8, DMO and PCPDTDTBTDI-8, 8 have comparable E g (ca. 1.3 eV), which is around 0.1 eV lower than those of PCPDTDTBTDI-EH, DMO and PCPDTDTBTDI-EH, 8 analogues. The results indicate that substituting 2-ethylhexyl chains by n-octyl chains on CPDT units would lead to lower E g of the polymers, while changing 3,7-dimethyloctyl chains by n-octyl chains on BTDI moieties has a minimal effect on the E g of the polymers. These polymers are good candidates as donor materials fabricated with fullerene derivatives as top BHJ cell in tandem solar cells due to their low optical band gaps [51].
PCPDTDTBTDI-EH, DMO and PCPDTDTBTDI-EH, 8 have comparable absorption coefficients (ε) and their coefficients are slightly higher than PCPDTDTBTDI-8, 8. PCPDT DTBTDI-8, DMO has the highest absorption coefficient among all polymers prepared. This could be attributed to the PCPDTDTBTDI-8, DMO having the highest absorption maxima of about 759 nm in solid state, and it is red-shifted by more than 80 nm compared to solution among all polymers (Table 2).

Electrochemical Properties
Cyclic voltammetry was used to study the electrochemical properties of the polymers. The electrochemical energy storage devices were investigated using cyclic voltammetry [54][55][56][57][58]. The LUMO and HOMO levels of the polymers are calculated from the onsets of reduction and oxidation potentials, respectively ( Figure 2 and Table 3   The HOMO levels of the polymers are higher around 0.1-0.2 eV, while the LUMO energy levels are almost identical relative to its PCPDTBT analogue [5]. The HOMO levels of the polymers are shallower than the HOMO level of PCPDTTPD (−5.43 eV), while their LUMO energy levels are lower than the LUMO energy level of PCPDTTPD (−3.25 eV) [18].

Thermal Properties
Thermal properties of the polymers were studied by TGA (Figure 3 and Table 3). All polymers show high thermal stability with decomposition temperatures up to 370 • C. It is interesting to note that the thermal stability of the polymers with linear n-octyl chains on CPDT repeat units is higher than those polymers with branched 2-ethylhexyl chains. In addition, the thermal stability of the PCPDTDTBTDI-EH DMO and PCPDTDTBTDI-EH, 8 is not affected by the nature of substituents on BTDI units, while changing the 3,7-dimethyloctyl chains in PCPDTDTBTDI-8, DMO to n-octyl chains in PCPDTDTBTDI-8, 8 on the acceptor moieties has a negative impact on the thermal stability of the polymers.
It was tentatively hypothesized that the polymers with n-octyl chains are more planar than those with 2-ethylhexyl chains; therefore, they might need higher temperature to decompose. Moreover, the differences in thermal stability of the polymers are probably due to different molecular weights of the polymers.

Powder X-ray Diffraction of the Polymers
X-ray diffraction (XRD) was used to study the amorphous nature of polymer electrolytes and composites [59][60][61][62][63]. The structural properties of the PCPDTDTBTDI-8, DMO, PCPDTDTBTDI-8, 8, and PCPDTDTBTDI-EH, 8 were investigated by powder X-ray diffraction (XRD) in solid state ( Figure 4). However, PCPDTDTBTDI-EH, DMO was not studied by powder XRD because the amount obtained from the Suzuki polymerization was not enough to undertake measurements. The XRD results of PCPDTDTBTDI-8, DMO, PCPDTDTBTDI-8, 8, and PCPDTDTBTDI-EH, 8 show diffraction peaks at 24.7, 24.5, and 24.4 • corresponding to the π-π stacking distance of 3.60, 3.62, and 3.64 Å, respectively. The results show that all polymers have an amorphous nature. Similarly, PCPDTBT does not show crystallinity by XRD study as reported in previous literature report [64].

Conclusions
Four novel low band gap alternating copolymers including cyclopentadithiophene (CPDT) flanked by thienyl units as electron donor moieties and benzothiadiazole dicarboxylic imide (BTDI) as electron acceptor units were synthesized via two different palladium catalysed cross coupling polymerizations. PCPDTDTBTDI-EH, DMO was prepared by copolymerizing the diboronic ester of CPDT (M6) with dibrominated BTDI (M3) via Suzuki polymerization. The yield of the polymerization was too low (<10%), and the polymer had a low M n value around 5000 g mol −1 . To circumvent these issues, direct arylation polymerization as a new alternative preparation method was utilized to prepare the PCPDTDTBTDI-8, DMO, PCPDTDTBTDI-8, 8, and PCPDTDTBTDI-EH, 8. All polymers were synthesized in good yields and they had excellent solubility in common organic solvents. Two distinct side chains (n-octyl vs. 2-ethylhexyl) were attached to the CPDT units as well as two different side chains (n-octyl vs. 3,7-dimethyloctyl) were anchored to the BTDI units to investigate the effect of these substituents on the solubilities, molecular weights, optical and electrochemical properties, thermal and structural properties of the resulting polymers. Changing 3,7-dimethyloctyl chains on the BTDI units in PCPDTDTBTDI-8, DMO for n-octyl chains in PCPDTDTBTDI-8, 8 as well as prolonging the polymerization times had a significant effect on the solubility and also on the M n values of the resulting polymers. PCPDTDTBTDI-8, DMO provided a toluene fraction, which had a M n value of 10,000 g mol −1 . In addition to the toluene fraction, another fraction from chloroform was obtained, which had a higher M n value of 17,400 g mol −1 . However, PCPDTDTBTDI-8, 8 was extracted in the toluene fraction, which had the M n value of 9100 g mol −1 . Moreover, substituting n-octyl chains in PCPDTDTBTDI-8, 8 for 2-ethylhexyl chains in PCPDTDTBTDI-EH, 8 on the CPDT units yielded the polymer with higher M n value of 15,900 g mol −1 for the toluene fraction. The polymers with one branched chain on either CPDT or BTDI units can provide greater solubilities and higher molecular weight fractions. In solutions, the polymers, which were synthesized by direct arylation polymerization show comparable absorption maxima and display bathochromic shift around 40 nm relative to PCPDTDTBTDI-EH, DMO. In thin-films, the absorption spectra of the polymers show red-shifted absorption maxima by 29-86 nm relative to their absorption in solutions. The optical band gaps of the polymers with n-octyl chains on the CPDT units are about 1.3 eV, which are about 0.1 eV lower than those analogues with 2-ethylhexyl chains. However, substituting 3,7-dimethyloctyl chains by n-octyl chains on BTDI units has little influence on the E g of the polymers. The low band gap of these polymers is beneficial to achieve high J sc values in BHJ solar cells. These polymers could also be used along with higher band gap conjugated polymers as top cells in tandem solar cells. The HOMO energy levels of the polymers are between −5.10 and −5.