Design and Synthesis of Conjugated Polymers and Small Molecules Based on Thiophene-Substituted Isoindigo, 5-acetyl-4Hcyclopenta[c]thiophene-4,6(5H)-dione, and Diketopyrrolopyrrole Electron-Deficient Units

Pi-Conjugated polymers and small molecules are useful for their semiconductor properties in organic electronic devices such as organic photovoltaics, light emitting diodes, and thin film transistors. They also find application in chemical detection for their high sensitivity to fluorescence quenching species. Described herein are the syntheses of two new electron deficient monomer units for polymers and small molecules. 5-acetyl-1,3-dibromo-4H- cyclopenta[c]thiophene-4,6(5H)-dione was synthesized for the first time. It can be conveniently polymerized by Stille coupling to form polymers initially insoluble in common solvents, which can be rendered soluble by deprotonation with various organic amines. Solutions in methanol are highly fluorescent and show strong quenching when exposed to Ni2+ and Cu2+. Additionally, films can be cast from chloroform or dichlorobenzene solutions with triethylamine. New polymers and small molecules based on the recently reported thiophene substituted isoindigo were also synthesized and employed in photovoltaic devices reaching a maximum of 3.75 % power conversion efficiency, the highest yet reported for this unit. Also discussed is a series of low band gap small molecule semiconductors incorporating two diketopyrrolopyrrole units and their photovoltaic applications. The best power conversion efficiency for a molecule in this series was 2.22 %.