Impact of composition ratio of donor and acceptor moieties in conjugated polymer: optical and electrochemical properties†

Designing donor–acceptor polymers by incorporating additional donor or acceptor units in the polymer backbone has attracted significant interest for further tuning of physical and chemical properties for organic electronic applications. In this study, we design and synthesize random donor–acceptor–donor–acceptor (D–A–D–A′, P1) and acceptor–donor–acceptor–acceptor (A–D–A′–A, P2) polymers using direct heteroarylation polymerization. Polymer P1 has alternating donor and acceptor units, whereas polymer P2 has an additional acceptor unit. For comparison purposes with parent polymers, three donor–acceptor polymers (P3–5) were synthesized. All polymers P1–5 were synthesized from three precursor units, 5-fluoro-2,1,3-benzothiadiazole, 5-diethylhexyl-3,6-bis(thiophene-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione and 3-hexylthiophene, using different compositions and sequences. Structural characterization of polymers P1–5 was carried out by ¹H NMR, GPC and FTIR spectroscopy. The electrochemical, stability and optical properties of polymers P1–5 were investigated by cyclic voltammetry and UV-vis-NIR absorption spectroscopy. All polymers exhibited narrow optical band gaps. The absorption of these polymers was red-shifted (30–100 nm) in the solid state compared to in a solution. It was observed that copolymers have different optical and electrical properties from their parent donor–acceptor polymers. Therefore, it is an effective method for the synthesis of donor–acceptor polymers with additional donor or acceptor units to tune the properties of the polymers for flexible electronic applications.