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 cm ² V ^-1 s ^-1 with on-off ratio (I _on/I _off) of 2.5 × 10 ², 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-C ₆₁-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.