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

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 the stabilized quinoid resonance structure. The field-effect hole mobility of PBIBDT was determined to be 1.1 × 10^–4 cm² V^–1 s^–1 with an on–off ratio of 2.5 × 10², while the PPBIBDT-based device revealed no p- and n-type responses. Organic photovoltaic devices were fabricated based on the planar heterojunction structure of PBIBDT and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and showed a power conversion efficiency of 0.07% under standard AM1.5 sunlight (100 mW cm^–2). These results obtained here will provide fundamental information on the design of thermally induced low bandgap polymers for device applications

Donor–Acceptor Alternating Copolymer Based on Thermally Converted Isothianaphthene Dimer and Thiazolothiazole Subunits

A novel donor–acceptor-conjugated polymer PBITT consisting of isothianaphthene (ITN) dimer donor unit and thiazolothiazole acceptor unit was synthesized by thermal conversion method. First, a soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]­octadiene (BCOD)-fused thiophene dimer and benzodithiophene (PPBITT) was synthesized by palladium(0)-catalyzed Stille coupling reaction. The BCOD moiety underwent the retro-Diels–Alder reaction by the thermal treatment of a red PPBITT film to afford a dark blue film of PBITT that was insoluble in any organic solvents. The optical bandgap of PBITT (1.3 eV) became significantly narrow compared with that of PPBITT (2.1 eV) due to the stabilized quinoid resonance structure of the PBITT main chain. The field-effect hole mobility (μ_h) of PBITT was determined to be 2.2 × 10^–4 cm² V^–1 s^–1 with on–off ratio (<i>I</i>_on/<i>I</i>_off) of 2.5 × 10², whereas the corresponding PPBITT-based device did not show any p- and n-type response. Organic photovoltaic (OPV) devices were fabricated based on the bulk heterojunction film of the polymers and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). The device with the PBITT:PCBM film exhibited higher short-circuit current and lower open-circuit voltage than those of the PPBITT:PCBM-based device, resulting in the comparable power conversion efficiency (∼0.3%). These results obtained here will provide fundamental information on the design of thermally induced donor–acceptor alternating polymers for organic electronics