Low-band gap copolymers containing thienothiadiazole units: Synthesis, optical, and electrochemical properties

Novel low-band gap alternating copolymers consisting of 9,9-bis(2-ethylhexyl)fluorene and 4,6-di(2-thienyl)thieno[3,4-c][1,2,5]thiadiazole and its 3,3″-dialkyl derivatives were synthesized by Suzuki copolymerization reaction, and their photophysical and electrochemical properties were studied. The copolymers possess small optical band gap 1.3-1.4 eV. The absorption covers the whole visible spectral region. The long-wavelength absorption maxima in thin films located at approximately 750-785 nm are significantly red shifted compared with those in solution, indicating strong intermolecular interactions. The introduction of alkyl chains to the thiophene units increases the molecular weights of soluble fractions and solubility of the final copolymers, leading to the improved processability of thin films. Polymer solutions exhibited solvatochromism and thermochromism, which is strongly supported by the involvement of the alkyl chains. The copolymers exhibited ambipolar redox properties and reversible electrochromic behavior. The electronic properties are influenced only slightly by alkyl substituents.