Thermal Conversion of Precursor Polymer to Low Bandgap Conjugated Polymer Containing Isothianaphthene Dimer Subunits

Abstract

Thermal conversion strategy has been utilized in the synthesis of a novel low bandgap polymer containing isothianaphthene (ITN) dimer structure and benzodithiophene (BDT) unit in the backbone (PBIBDT). First, a highly soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]octadiene-fused thiophene dimer and BDT (PPBIBDT) was synthesized by a palladium(0)-catalyzed Stille coupling reaction. Then, heating of the yellow PPBIBDT film spin-coated on a glass plate yielded a dark blue film of PBIBDT that was insoluble in any organic solvents. Thermogravimetric analysis of PPBIBDT showed 14% weight loss with an onset at 230 °C, corroborating the occurrence of the thermally induced retro-Diels–Alder reaction. The PBIBDT film showed red-shifted, broad absorption in the visible and near-infrared regions with a maximum at 706 nm compared to the precursor polymer PPBIBDT with an absorption peak at 445 nm. The introduction of an ITN dimer unit in the backbone lowered the bandgap owing to th...