Facile synthesis of ultralow-band-gap alkoxythiophene-flanked diketopyrrolopyrrole homopolymers via FeCl3-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 cm² 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 thin film transistors, NIR-II photodetection, and photothermal applications.