Synthesis of 2,7-Dihydrooxepine-spirodithiophene\nDerivatives\nvia an Intramolecular Ring Closure Reaction

Spiro architectures with π-conjugation have improved\nthermal\nstability and stronger photosensitivity, making them potentially useful\nfor organic optoelectronic devices. Our recent work has demonstrated\nthe synthetic chemistry of a novel thiophene oligomer combining 2,7-dihydrooxepine\nand dispiro structure and derived it into A–D–A-type\ncompounds. The optical spectroscopy and electrochemical characteristics\nwere investigated. The results show that the presence of the alkyl\nside chains enhances the nucleophilicity of aromatic anions but induces\nstrong 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′-<i>e</i>]oxepine-6′,4″-cyclopenta[2,1-b:3,4-b’]dithiophene]\n(DSOCT) core is preferred. The A–D–A-type DSOCT derivatives\nshow an increased light absorption wavelength and a reduced optical\nband gap. The TD-DFT study exhibited consistent results with the experimental\nanalysis. Regarding application to organic solar cells of both materials, <b>PM6:DSOCT-(TFIC)</b><sub><b>6</b></sub>-based solar cells exhibited better power conversion efficiency (PCEs)\ncompared to <b>PM6:DSOCT-(TIC)</b><sub><b>6</b></sub>-based devices. This improvement can be attributed to\nthe higher current density and fill factor, which are facilitated\nby the more efficient charge excitation, separation, and transport\nresulting from the molecular “fluorination effect.″.