Quinoidal π-Bridges for a Fused-Ring Acceptor with Enhanced Near-Infrared-II Photothermal Therapy and Fluorescent Emission beyond 1500 nm

The second near-infrared window (NIR-II, 1000–3000 nm) presents an attractive platform for phototheranostics due to improved tissue penetration. However, developing efficient phototheranostic agents for this range presents a significant challenge. Herein, we report an NIR-II organic theranostic agent, CNTIC-4F, based on a fused-ring acceptor structure, which features efficient absorption extending beyond 1200 nm and fluorescence emission reaching up to 1800 nm. The incorporation of a proaromatic π-bridge, thieno[3,4- b ]pyrazine (TP), enhances the quinoidal character of CNTIC-4F, reducing the optical bandgap and enhancing intermolecular interactions. In vivo fluorescence imaging of mouse vasculature with CNTIC-4F nanoparticles shows high-contrast imaging beyond 1500 nm under 1064 nm excitation. Furthermore, CNTIC-4F nanoparticles also exhibit a high photothermal conversion efficiency of 82% under 1064 nm excitation, enabling effective NIR-II imaging-guided photothermal therapy of 4T1 tumors in mice. This study illustrates the potential of quinoidal structures in developing advanced organic agents for high-contrast and deep-penetrating NIR-II phototheranostics.