This paper describes the synthesis and characterization of a series of thiophene- and N-methylpyrrole-based low band gap polymers incorporating o-quinoid acceptors such as thienopyrazine, quinoxaline, benzothiadiazole, pyrazinoquinoxaline, thiadiazoloquinoxaline and benzo(bis)thiadiazole. Structure-property relationships based on donor and acceptor strengths and geometry were measured and the smallest band gap polymers were based on the benzo(bis)thiadiazole acceptor. Monomer and polymer HOMO/LUMO levels were determined by cyclic voltammetry and confirmed acceptor strengths and geometry effects depending on upon the donors and acceptors used.
The synthetic schemes and experimental section provides reliable procedures for the synthesis of these compounds. With slight optimizations, product yields can be improved and purifications can be simplified resulting in less waste and improved efficiency.
The following observations and modifications were made for the synthesis of the following compounds (11, 12 and 4c):
To a solution of 4,7-dibromo-5,6-dinitro-2,1,3-benzothiadiazole (3.80 g, 9.9 mmol) and tributyl(thien-2-yl)stannane (8.51 g, 22.8 mmol) in THF (30 mL), PdCl2(PPh3)2 (143 mg, 2 mol %) was added. The mixture was refluxed for 3 h. After cooling, an orange solid appeared. The solid was filtered off, washed with MeCN, and collected. Recrystallization from THF gave the title compound 11 (1.82 g, 47%) as an orange solid.
If the reaction time of the Stille coupling is increased from 3 h to overnight (~ 12 h), the yield can be increased to ~ 70-80 % compared to the 47 % originally reported for a 3 h reaction time. Alternate catalysts such as Pd2dba3 and tri-o-tolylphoshine can also be used. This reaction is quite robust and can be performed with regular THF out of the bottle. It is not necessary for the THF to be distilled or strictly anhydrous.
A mixture of dinitro compound 11 (780 mg, 2.0 mmol) and iron dust (1.33 g, 24.0 mmol) in acetic acid (40 mL) was stirred at 30 °C for 4 h. The reaction mixture was poured into cold 5% NaOH (50 mL), and then a yellow solid appeared. The solution was extracted with Et2O. The organic layer was washed with brine and dried over Na2SO4. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel (eluent CH2Cl2). Recrystallization from CHCl3 gave the title compound 12 (383 mg, 58%) as yellow plates.
After pouring the reaction into the cold NaOH, it is possible to use a magnet to collect the iron in the bottom of the flask and then rinse out the flask and collect the solid by filtration and rinse with some excess NaOH and water and then use as is. This can prevent materials loss through extraction and column purifications. This is especially advantageous for less soluble derivatives.
Original Procedure: A mixture of 0.1-0.2 mmol of diamine 12 and 1.4 equiv of 2,3-dihydroxy-1,4-dioxane (for compound 4a) or 1.6-2.0 equiv of 1,2-diketones (diacetyl and 7,8-tetradecanedione for compound 4b and 4c, respectively) was stirred in acetic acid at room temperature for 10 min. After removal of the solvent under reduced pressure, the residue was purified by column chromatography and recrystallization.
Depending on the diketone used, reaction times may need to be increased, especially for diphenyl diketones. Reaction can be monitored by TLC.