Quinoid Conjugated Polymer Nanoparticles with NIR‐II Absorption Peak Toward Efficient Photothermal Therapy**

Recently, extensive efforts have been devoted to the development of the second near-infrared bio-window (NIR-II, 1000-1700 nm) theranostic agents owing to the excellent tissue-penetration capability of NIR-II light. The exploration of organic NIR-II photothermal therapy materials, especially those with absorption peak over 1000 nm, is an appealing yet significantly challenging task. Herein, we have designed conjugated polymer nanoparticles (PIS NPs) with NIR-II absorption peak at 1026 nm through a combined strategy of introducing quinoid donor-acceptor (D-A) structures, constructing intramolecular "conformational locks" and extending the conjugation area to narrow the band gap. Irradiated at 1064 nm, PIS NPs showed remarkable photothermal conversion performance for efficient photothermal ablation of tumor cells in vitro and in vivo. This study provides useful insights into the rational design of organic NIR-II photothermal agents based on multiple strategies.