Extended isoindigos as building blocks for developing D–A-type conjugated polymers

Abstract

By using 5-bromo-N-tetradecylindolin-2-one and different dialdehydes in a double Knoevenagel condensation procedure, three new π-extended isoindigo-based monomers (3, 5 and 7) were synthesized. π-Extended isoindigo-based monomers were coupled with 2,2-bithiophene to create donor–acceptor conjugated polymers P-1, P-2 and P-3 by using a Stille coupling procedure. The polymers were characterized by ¹H NMR, thermogravimetric analysis (TGA) and gel permeation chromatography (GPC). Photophysical and electrochemical studies were carried out for the monomers and polymers. Polymers P-1, P-2 and P-3 show band gap values of 2.55 eV, 2.16 eV and 2.15 eV, respectively. The polymers have good thermal stability with a decomposition temperature (T_d) above 200 °C. The structural aspects of polymers P-1, P-2 and P-3 were studied by DFT calculations on model polymers (by replacing N-alkyl by N-ethyl groups) comprising the repeat unit of each parent copolymer, which indicates that the polymers are highly planar with the dihedral angle less than 30°. The powder X-ray diffraction (PXRD) analysis showed that polymer P-3 exhibited greater crystallinity compared to polymer P-1 and it surpasses polymer P-2 in this aspect. The measured space-charge-limited current (SCLC) hole mobilities were found to be 7.11 × 10⁻² cm² V⁻¹ s⁻¹, 6.43 × 10⁻² cm² V⁻¹ s⁻¹ and 8.19 × 10⁻² cm² V⁻¹ s⁻¹ for polymers P-1, P-2 and P-3, respectively.