Simplified Synthetic Approach to Tetrabrominated Spiro-Cyclopentadithiophene and the Following Derivation to A-D-A Type Acceptor Molecules for Use in Polymer Solar Cells.

4,4'-Spiro-bis[cyclopenta[2,1-;3,4-']dithiophene] () is a versatile building block for constructing three-dimensional (3D) π-conjugated molecules for use in organic electronics. In this paper, we report a more convenient synthetic route to and its derivatives, where a structurally symmetric 3,3'-dibromo-5,5'-bis(trimethylsilyl)-2,2'-bithiophene () serves as the precursor for both the synthesis of 4-cyclopenta[2,1-:3,4-']dithiophen-4-one () and 4-(5,5'-bis(trimethylsilyl)-2,2'-bithiophen-3-yl)-2,6-bis(trimethylsilyl)-4-hydroxy-cyclopenta[2,1-;3,4-']dithiophene (). The later one is the key intermediate for the final brominated building block. Such a "two birds with one stone" strategy simplifies the synthetic approach to the core. Functionalization on the core with different terminal electron-deficient groups, including 1-indene-1,3(2)-dione (), 2-(3-oxo-2,3-dihydro-1-inden-1-ylidene)malononitrile (), and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1-inden-1-ylidene)malononitrile (), was carried out, yielding three spiro-conjugated A-D-A type molecules, , , , respectively. The optical spectroscopy and electrochemical properties of these three compounds were investigated and compared to the corresponding linear oligomers. Results revealed that the and terminated compounds showed low-lying HOMO/LUMO energy levels with reduced optical bandgap, making them more suitable for use in polymer solar cells. A power conversion efficiency of 3.73% was achieved for the based cell, demonstrating the application perspective of 3D molecules.