Synthesis and characterization of 1,3,4,6-tetraarylpyrrolo[3,2-b]-pyrrole-2,5-dione (isoDPP)-based donor–acceptor polymers with low band gap

Abstract

The synthesis of new π-conjugated donor–acceptor (D–A) polymers containing tetraaryl-diketopyrrolo[3,2-b]-pyrrole (isoDPP) as the building block is described. Polymers were prepared upon palladium-catalyzed Stille coupling of 3,6-bis(5-bromothien-2-yl)-1,4-diphenyl-pyrrolo[3,2-b]pyrrole-2,5-dione (M1) with 4,4′-bis(2-ethylhexyl)-5,5′-bis(trimethyltin)dithieno[3,2-b:2′,3′-d]silole (M2) or (4,8-bis-(2-ethylhexyloxy)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl)bis(trimethylstannane) (M3). The polymers exhibit low band gaps of 1.53 and 1.67 eV, broad absorption bands with maxima of 523 nm and 620 nm, and high extinction coefficients of 6.5 × 10⁴ L mol⁻¹ cm⁻¹, and 2.6 × 10⁴ L mol⁻¹ cm⁻¹, respectively. Low fluorescence quantum yields between 0.13% and 0.82% were detected. Quantum chemical calculations indicated a nearly planar backbone with highly delocalized HOMO orbitals and localization of the LUMO electrons at the thienyl-isoDPP units. The X-ray structure analysis of M1 indicated a dihedral angle of 14.1° between the thienyl groups and the isoDPP core, and a 76.6° dihedral angle of the lactam phenyl group and the isoDPP core, the latter preventing any π-stacking of the monomers. The polymers show high glass transitions (T_g) and excellent thermal stability. Photoirradiation studies indicate that the polymers are extremely stable to UV and visible light in solution. Spectroelectrochemical studies indicated a reversible electrochromism with isosbestic point near 710 nm.