Synthesis of 2,7-Dihydrooxepine-spirodithiophene Derivatives via an Intramolecular Ring Closure Reaction.

Spiro architectures with π-conjugation have improved thermal stability and stronger photosensitivity, making them potentially useful for organic optoelectronic devices. Our recent work has demonstrated the synthetic chemistry of a novel thiophene oligomer combining 2,7-dihydrooxepine and dispiro structure and derived it into A-D-A-type compounds. The optical spectroscopy and electrochemical characteristics were investigated. The results show that the presence of the alkyl side chains enhances the nucleophilicity of aromatic anions but induces strong steric hindrance so that the selectivity toward a dispiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,4'-dithieno[3,2-c:2',3'-]oxepine-6',4″-cyclopenta[2,1-b:3,4-b']dithiophene] (DSOCT) core is preferred. The A-D-A-type DSOCT derivatives show an increased light absorption wavelength and a reduced optical band gap. The TD-DFT study exhibited consistent results with the experimental analysis. Regarding application to organic solar cells of both materials, -based solar cells exhibited better power conversion efficiency (PCEs) compared to -based devices. This improvement can be attributed to the higher current density and fill factor, which are facilitated by the more efficient charge excitation, separation, and transport resulting from the molecular "fluorination effect.″.