Investigation of the conditions for preparing organic-semiconductor acceptor precursor: 2-(3-fluoro-5-oxo-5,6-dihydro-7H-cyclopenta[b]thiophene-2,3-d]thiophene-7-methyl)malononitrile and donor precursor: 2,5-bis(2-ethylhexyl)-3,6-bis(thiophen-2-yl)-2,5-dihydropyrrolopyrrole-1,4-dione

The study aims to introduce fluorine substituent at the molecular terminal of the organic semiconductor acceptor 2-(5-oxo-5,6-dihydro-7H-cyclopenta[b]thieno[2,3-d]thiophen-7-ylidene)malononitrile (TTC) to enhance its optoelectronic performance.Acceptor – 2-(3-fluoro-5-oxo-5,6-dihydro-7H-cyclopenta[b]thiophene-2,3-d]thiophene-7-methyl)malononitrile (FTTC) was prepared via two approaches, i.e. later-fluoro-introduction pathway and initial-fluoro-introduction pathway. The former one started from 3,4-dibromothiophene via 1) formylation to give 3,4-dibromothiophene-2-carbaldehyde in 40% yield, 2) intramolecular cyclization to provide ethyl 6-bromothieno[3,2-b]thiophene-2-carboxylate in 89% yield, 3) deprotection to give 6-bromothieno[3,2-b]thiophene-2-carboxylic acid in 98% yield and 4) chlorination to generate the intermediate 6-bromothieno[3,2-b]thiophene-2-carbonyl chloride as well as 5) Friedel–Crafts acylation to provide 3-bromo-5H-cyclopenta[b]thiophene-2,3-d]thiophene-5,7(6H)-dione in 13% yield. A total yield of 6% yield through five steps was obtained. The subsequent aryl trimethylstannylation did not give the desired product probably due to the difficult-to-purify starting material.The other pathway of adopting initial-fluoro-introduction pathway started from ethyl 6-bromothieno[3,2-b]thiophene-2-carboxylate through aryl tributylstannylation to give stannylated intermediate ethyl 6-(tributylstannyl)thieno[3,2-b]thiophene-2-carboxylate in 27% yield followed by the fluorination to give ethyl 6-fluorothieno[3,2-b]thiophene-2-carboxylate in 15% yield. The donor moieties started from 2-thiophenecarbonitrile via intramolecular cyclization to afford 3,6-bis(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DTPP) in 76% yield. Through alkylation, the product 2,5-bis(2-ethylhexyl)-3,6-bis(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (ethylhexyl DTPP) was obtained in 42% yield. The subsequent deprotonation with lithium diisopropylamide followed by nucleophilic addition with DMF did not produce the desired product 5,5'-(2,5-bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)bis(thiophene-2-carbaldehyde) (ethylhexyl DTPP CHO).