A Highly Efficient Catalyst for the Synthesis of Alternating Copolymers with Thieno[3,4-c]pyrrole-4,6-dione Units via Direct Arylation Polymerization

pi-Conjugated polymers with a donor acceptor (DA) combination of repeating units possess a narrow HOMO-LUMO gap, thus resulting in a high device performance in solar cells. This paper reports an improved catalytic system for the synthesis of DA polymers containing 5-(2-hexyldecyl)-5H-thieno[3,4-c]pyrrole-4,6-dione-1,3-diyl (TPD) group as the acceptor unit via palladium-catalyzed direct arylation polymerization. Although a related study has been reported (Angew. Chem. Int. Ed. 2012, 51, 2068), we attempted to reduce the catalyst loading because the palladium residue in if-conjugated polymers has been known to produce a detrimental effect on device performance. As a result, the amount of palladium could be reduced to 1/8 by using PdCl2(MeCN)(2) and P(C6H4-o-OMe)(3) (L1) as catalyst precursors. The polymerization smoothly proceeds at 100 degrees C in THF in the presence of pivalic acid and Cs2CO3 to afford TPD-based DA polymers 3a-3d containing the following donor units in almost quantitative yields: 4,4'-dioctyl-2,2'-bithiophene-5,5'-diyl (3a, M-n = 36800, M-w/M-n = 2.20), 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl (3b, M-n = 31100, M-w/M-n = 2.44), 3,4-(2,2'-dioctylpropylenedioxy)thiophene-2,5-diyl (3c, M-n = 68200, M-w/M-n = 3.04), and 2,5-bis(2-ethylhexyloxy)-1,4-phenylene (3d, M-n = 65500, M-w/M-n = 2.21). A detailed analysis of the structure of 3a is reported.