Conjugated Ladder Poly(thienobenzothiazine): Synthesis, Electronic Structure, Optical Properties, and Electrical Conductivity of a Narrow Bandgap p-Type Semiconducting Polymer

We report herein the synthesis, electronic structure, experimental and density functional theory (DFT) calculated optical properties, and electrical conductivity of a new π-conjugated ladder polymer, ladder poly(thienobenzothiazine) (LTBT), and its model compound, thienobis(benzothiazine) (TBBT). The synthesis of LTBT in a mixed polyphosphoric acid/sulfolane solvent system gave an intrinsic viscosity of 2.0 dL/g at 30 °C in methanesulfonic acid, and the polymer could form large-area freestanding films. Single-crystal X-ray structure of TBBT showed that it crystallizes in an orthorhombic structure with a coplanar thienobenzothiazine ladder backbone, which is in agreement with the DFT calculated structure. The electrochemically derived HOMO and LUMO energy levels of −5.17 eV and −3.75 eV of LTBT thin films revealed the p-type semiconducting nature of the conjugated ladder polymer. We thus p-doped LTBT thin films with FeCl3, leading to an average electrical conductivity of 0.3 S/cm. These results demonstrate the potential of the new ladder polymer LTBT in organic electronics while providing valuable insights into the structure–property relationships of semiconducting ladder polymers.