Enhanced Diradical Feature of Quinoid‐Donor–Acceptor Semiconducting Polymers by Strong Pro‐Quinoidal Acceptor Engineering for Efficient NIR‐II Photothermal Theranostics

The development of semiconducting conjugated polymer dyes excitable in the second near‐infrared window (NIR‐II, 1000–1700 nm) is essential for achieving deep‐tissue penetration and maximizing permissible exposure in cancer photothermal therapy (PTT). Herein, the first design of a quinoid–donor–acceptor (Q–D–A) dye, PAQM‐EH‐BBT, for photoacoustic imaging (PAI)‐guided NIR‐II PTT is reported. With enhanced acceptor strength, backbone planarity, and diradical character, PAQM‐EH‐BBT exhibits an ultrabroad absorption spectrum, peaking at 1138 nm, with the tail extending to 1600 nm and shorter nonradiative (NR) relaxation. After encapsulation with hydrophilic DSPE‐mPEG 2000 , the resulting PAQM‐EH‐BBT nanoparticles (NPs) demonstrate an excellent mass extinction coefficient of 22.98 L g −1 cm −1 , remarkable biocompatibility, photostability, and achieve the record‐setting photothermal conversion efficiency (PCE) of 73.0%, surpassing all reported quinoidal‐aromatic polymers. Notably, PAQM‐EH‐BBT NPs exhibit a pronounced PTT effect against cancer cells in vitro under 1064 nm laser irradiation. Furthermore, highly effective NIR‐II PTT is mediated for tumor eradication in vivo under the guidance of NIR‐II PAI, without eliciting any significant side effects. This work pioneers the use of a Q–D–A type semiconducting polymeric phototherapeutic agent operating in the NIR‐II window and offers a new paradigm for the design of an anticancer treatment agent.