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

By using 5-bromo-N-tetradecylindolin-2-one and different dialdehydes in a double Knoevenagel condensation procedure, three new pi-extended isoindigo-based monomers (3, 5 and 7) were synthesized. pi-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 (Td) above 200 degrees 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 degrees. 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 x 10-2 cm2 V-1 s-1, 6.43 x 10-2 cm2 V-1 s-1 and 8.19 x 10-2 cm2 V-1 s-1 for polymers P-1, P-2 and P-3, respectively. pi-Extended isoindigo-based conjugated polymers were developed, which exhibited excellent Space-Charge-Limited Current (SCLC) hole mobilities.