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- b ;3,4- b ']dithiophene] ( SCT ) 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 SCT and its derivatives, where a structurally symmetric 3,3'-dibromo-5,5'-bis(trimethylsilyl)-2,2'-bithiophene ( 2 ) serves as the precursor for both the synthesis of 4 H -cyclopenta[2,1- b :3,4- b ']dithiophen-4-one ( 4 ) and 4-(5,5'-bis(trimethylsilyl)-2,2'-bithiophen-3-yl)-2,6-bis(trimethylsilyl)-4-hydroxy-cyclopenta[2,1- b ;3,4- b ']dithiophene ( 5 ). The later one is the key intermediate for the final brominated SCT building block. Such a "two birds with one stone" strategy simplifies the synthetic approach to the SCT core. Functionalization on the SCT core with different terminal electron-deficient groups, including 1 H -indene-1,3(2 H )-dione ( ID ), 2-(3-oxo-2,3-dihydro-1 H -inden-1-ylidene)malononitrile ( IC ), and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1 H -inden-1-ylidene)malononitrile ( FIC ), was carried out, yielding three spiro-conjugated A-D-A type molecules, SCT-(TID) 4 , SCT-(TIC) 4 , SCT-(TFIC) 4 , respectively. The optical spectroscopy and electrochemical properties of these three compounds were investigated and compared to the corresponding linear oligomers. Results revealed that the IC and TFIC 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 SCT-(TFIC) 4 based cell, demonstrating the application perspective of 3D molecules.