ALTERNATING DONOR-ACCEPTOR COPOLYMERS CONTAINING THERMALLY REMOVABLE SOLUBILIZING GROUPS FOR USE IN PHOTOVOLTAIC DEVICES

3-5 The following research combines the benefits of a thermally removable solubilizing group onto alternating donor-acceptor copolymers with varying backbone structure. Experimental Materials. All reagents were purchased from Aldrich and used without further purification unless otherwise noted. Tetrahydrofuran (THF) was refluxed over sodium and benzophenone and then distilled. N,NDimethylformamide (DMF) was dried over 4A molecular sieves, filtered and distilled. Toluene was dried by distillation after stirring over calcium hydride. 1,4-Dibromo-2,5-dimethoxybenzene (99%) and copper(I) bromide (98%) were purchased from Fisher and used without further purification. Instrumentation. 1 H and 13 C NMR spectra were acquired in deuterated solvents on a Bruker 400 AVANCE spectrometer. Molecular weights, relative to narrow polystyrene standards, were measured using a Waters GPC system using RI detection. The measurements were taken at 35 °C with THF as the mobile phase on three columns (Waters Styragel HR2, HR4, HR5). Thermal transitions were measured with a Seiko 220C DSC on the second heat with a heating rate of 10 °C/min. Thermogravimetric analysis was carried out using a Perkin Elmer TGA with a heating rate of 10 °C/min in a N2 atmosphere. Synthesis of 2,5-Dibromothiophene-3-carboxylic Acid. To a mixture of thiophene-3-carboxylic acid (5.0 g, 39.0 mmol) and 60 mL acetic acid 10 mL of Br2 was added dropwise and stirred at 60 °C for 8 h. The mixture was allowed to cool and precipitated into 300 mL of cold water and Na2SO3 was added to decolorize. After filtering a yellow precipitate was collected and recrystallized from water and ethanol (2:1) to afford off-white crystals (6.92 g, 62% recovery). Synthesis of 2-Methyl-2-hexyl-2,5-dibromothiophene-3-carboxylate. To a mixture of 2,5-dibromothiophene-3-carboxylic acid (3.0 g, 10.5 mmol) and 2-methyl-2-hexanol (1.6 g, 13.7 mmol) in pyridine (15 mL) was added 2chloro-3,5-dinitropyridine (2.1 g, 10.5 mmol). The mixture was stirred at 115 °C for 30 min. The reaction mixture was allowed to cool to room temperature and poured into 6% aqueous NaHCO3 (300 mL) and extracted with diethyl ether. The combined organic phases were washed with water then dried over anhydrous Na2SO4. The solvent was removed and the residue was purified by column chromatography (SiO2) using ethyl acetate/hexane (1:7) as eluent. The desired product was obtained as a light yellow oil (1.1 g, 27% recovery). Synthesis of 2-Methyl-2-hexyl-1,4-dibromobenzene-2-carboxylate. The same procedure as the synthesis of 2,methyl-2-hexyl-2,5dibromothiophene-3-carboxylate was employed using 2,5-dibromobenzoic acid (3.0 g, 10.0 mmol) resulting in a light yellow oil (1.9 g, 50% recovery). Synthesis of 2,5-Dibromobenzoylchloride. To a mixture of 2,5dibromobenzoic acid (4.3 g, 15.3 mmol) and 1,2-dichloroethane (50 mL) was added dropwise thionyl chloride (20 mL). The solution was allowed to reflux for 4 h then allowed to cool to room temperature. The excess solvent was distilled off resulting in a yellow solid (4.5 g, 100% recovery). Synthesis of 2,5-Dibromobenzophenone. To a solution of 2,5dibromobenzoylchloride (4.3 g, 14.4 mmol) in benzene (10 mL) in NO2CH3 (10 mL) in an ice bath was added AlCl3 (1.9 g, 14.4 mmol). The ice bath was removed and the solution was allowed to stir for 18 h at room temperature. The mixture was precipitated into cold acidic water and a light yellow powder was collected by filtration. The resulting solid was recrystallized from ethanol to give white crystals (2.6 g, 53% recovery). Synthesis of 3-Methoxythiophene. To a solution of 3-bromothiophene (8.15 g, 50 mmol) and CuBr (0.7 g, 5 mmol) in DMF (30 mL) was added NaOCH3 (4.0 g, 75 mmol) in methanol (30 mL). The solution was refluxed for 3 h, allowed to cool to room temperature, and poured into 200 mL water with 5 g of NaCN. The solution was extracted with diethyl ether and dried over MgSO4. The solvent was removed and the reddish-brown residue was purified by column chromatography using ethyl acetate and hexane (1:9) as eluent to afford a light yellow oil (1.2 g, 21% recovery). Synthesis of 2,5-Bis-trimethylstannanyl-3-methoxythiophene. To a solution of 3-methoxythiophene (2.0 g, 17.5 mmol) in THF (50 mL) at -78 °C was added butyllithium (25.5 mL of 2.0M in cyclohexane) and the mixture was allowed to stir for 1 h. SnCl(CH3)3 was added (51 mL of 1.0M in THF) and the solution was allowed to warm to room temperature and stir for 16 h. The reaction mixture was poured into water, extracted with diethyl ether, dried over MgSO4, and the solvent was removed. The crude product was recrystallized from ethanol to afford the product as white crystals. Synthesis of 1,4-Bis-trimethylstannanyl-2,5-dimethoxybenzene. The same procedure as the synthesis of 2,5-bis-trimethylstannanyl-3methoxythiophene was employed using 1,4-dibromo-2,5-dimethoxybenzene (5.0 g, 16.9 mmol) to afford the desired product as white crystals (2.7 g, 35% recovery). Synthesis of Alternating Copolymers. The following is a general procedure for the polymerization of the above monomers via Stille coupling. A solution of 1,4-bix-trimethylstannanyl-2,5-dimethoxybenzene (0.682 g, 1.47 mmol), 2,5-dibromobenzophenone (0.500 g, 1.47 mmol), Pd(PPh3)4 (0.085 g, 0.07 mmol), and Cu(I)Br (0.463 g, 3.23 mmol) in toluene (30 mL) and DMF (10 mL) was stirred at 120 °C for 24 h. The solution was allowed to cool to room temperature and precipitated in 300 mL of methanol and filtered. The polymer was washed by Soxhlet extraction with hexane for 10 h and subsequently by THF for 10 h. The extracted fraction was concentrated down and precipitated into methanol, filtered, and dried in a vacuum oven overnight (0.1515 g, 33% recovery).