Synthesis and study of donor–acceptor conjugated polymers based on the dithienopyrrolobenzothiadiazole unit via a metal free aldol condensation polymerization strategy and their SCLC hole mobilities†

The synthesis of donor–acceptor conjugated polymers presents a challenge due to the reliance on costly transition-metal catalysts and potentially hazardous reagents, which can have adverse environmental impacts. Aldol polycondensation offers a promising, metal-free alternative for the polymerization of such materials. In this study, we describe the synthesis of three new symmetrical reduced bis-indolinone derivatives. Polymers ADRI, ADTRI and ADCRI were prepared through aldol condensation reactions between diformyl-dithienopyrrolobenzothiadiazole (DTPBT) and bis-indolinones. Photophysical, electrochemical, and thermogravimetric analysis, atomic force measurement (AFM) analysis, gel permeation chromatography (GPC), powder X-ray diffraction (PXRD), density functional theory (DFT) calculations and space charge limited current (SCLC) hole mobility measurements were performed for these polymers. Photophysical and electrochemical studies revealed high visible light absorptivity, along with HOMO energy levels close to −5.9 eV and low-lying LUMO energy levels near −4.0 eV for all polymers. TGA showed that all polymers were quite stable up to ∼300 °C. The measured values of the SCLC hole mobilities of polymers ADRI, ADTRI and ADCRI were 3.90 × 10⁻² cm² V⁻¹ s⁻¹, 8.73 × 10⁻² cm² V⁻¹ s⁻¹ and 8.51 × 10⁻² cm² V⁻¹ s⁻¹, respectively.