A Cost-Effective Semiconducting Polymer with an Ether Chain-Substituted Bithiophene as the Donor Unit Enabling Effective NIR-II Photoacoustic Imaging

Due to their tunability in the chemical structure and light absorption, semiconducting polymers (SPs) have proven their promising potential for photoacoustic (PA) imaging within the second near-infrared (NIR-II) window. However, the syntheses of these polymers are generally lengthy and costly, limiting their potential applications. Herein, inspired by the commonly cost-effective thiophene-based compounds, we report a semiconducting polymer PTT-ATQ based on the non-fused ether chain-substituted donor unit 3,3′-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-2,2′-bithiophene (TT). With the side chain substituted by the biocompatible ether chains, the TT unit possesses an even stronger electron-donating ability and thus, a strong intramolecular charge-transfer (ICT) effect in the polymer backbone; hence, an absorption band (peaked at ca. 1370 nm) within the NIR-II window for PTT-ATQ is realized. Meanwhile, the long and bulky side chains of the donor and acceptor units lead to a non-coplanarity in the polymer backbone, resulting in a twisted ICT effect that can induce high PA signals. Further results show that the PTT-ATQ has good biocompatibility and high in vivo PA imaging performance within the NIR-II window. This work highlights the effective strategy to realize high-performance NIR-II in vivo PA imaging SP contrast agents by using the cost-effective thiophene-based building blocks.