Incorporation of Electron-Rich Indacenodithiophene Units into the Backbone of 2,6-Azulene-Based Conjugated Polymers for Proton-Responsive Materials and p-Type Polymeric Semiconductors

The 2,6-azulene unit is one of the most promising moieties for constructing organic conjugated materials. Herein, we reported the design and synthesis of 2,6-azulene-based conjugated copolymers P(AzIDT-C6), P(AzIDT-PhC6), and P(AzIDTT-PhC6) containing electron-rich indacenodithiophene (IDT) units. Because of the electron-rich features of IDT moieties, all polymers can be easily protonated in both solution and solid state. The protonation states can reach saturation with as little as 0.5% volume ratio of trifluoroacetic acid in solution accompanied by a substantial red-shift (>250 nm) in the UV–vis–NIR absorption spectra, which is the most sensitive proton responsiveness among reported azulene-based polymers. The proton responsiveness of these polymers in thin films is shown to be dynamically reversible with the color changing between deep blue (neutral state) and colorless (protonated state) due to shifting of the absorption bands between the visible and near-infrared regions. Electron paramagnetic resonance experiments reveal that the protonated polymers can be further oxidized to form radical cations with strong EPR signals. A membrane containing 3 wt % P(AzIDTT-PhC6) in Nafion shows a 72.2% higher conductivity than Nafion at 125 °C and 0% relative humidity, implying the potential application of these polymers in polymer electrolyte membranes. Preliminary organic field-effect transistor studies suggest that all conjugated polymers investigated display typical p-channel transport behavior with hole mobilities up to 0.46 cm2 V–1 s–1, demonstrating that the 2,6-azulene-based conjugated copolymers can behave as p-type semiconducting materials. Our work indicates that the incorporation of electron-rich large π-conjugated units into 2,6-azulene-based conjugated polymers is an effective strategy to develop highly sensitive proton-responsive materials and high-performance hole-transporting semiconductors.