Facile synthesis of ultralow-band-gap alkoxythiophene-flanked diketopyrrolopyrrole homopolymers via FeCl 3 -mediated oxidative polymerization

Abstract

Conjugated polymers with strong absorption in the near-infrared-II (NIR-II, 1000-1700 nm) region are highly attractive for optoelectronic and biomedical applications. However, their availability remains limited, and their synthesis often relies on costly transition-metal catalysts and multistep monomer functionalization. Here, we report an economical and operationally simple FeCl3-mediated oxidative polymerization strategy for the synthesis of two alkoxythiophene-flanked diketopyrrolopyrrole (DPP) homopolymers, PDPPC20OT-C8 and PDPPC12OT-C12. Both polymers exhibit intense NIR-II absorption, with maximum absorption wavelengths reaching ~1190 nm, and ultralow optical band gaps as low as 0.87 eV.Side-chain engineering enables good solubility, suppressed crosslinking, and more coplanar backbone conformation, leading to ambipolar charge transport in organic thin-film transistors. Optimized devices based on PDPPC12OT-C12 show balanced hole and electron mobilities of up to 1.74 × 10 -3 and 2.24 × 10 -3 cm 2 V -1 s -1 , respectively. Density functional theory calculations reveal enhanced backbone planarity arising from intramolecular S•••O interactions, consistent with the experimentally observed red-shifted absorption and reduced band gaps. This work establishes FeCl3-mediated oxidative polymerization as a viable route to high-performance ultralow-band-gap DPP homopolymers and highlights their potential for ambipolar electronics, NIR-II photodetection, and photothermal applications.