Single Boron Coordination on a Strong [1,2,5]Thiadiazolo[3,4-g]Quinoxaline Acceptor Enables Intense NIR-II Absorption for Photoacoustic Imaging-Guided Photothermal Therapy

Abstract

Strong near-infrared-II (NIR-II, 1000–1700 nm) absorbers with high photothermal efficiency are highly desirable for deep-tissue photoacoustic imaging and photothermal therapy. However, donor–acceptor (D–A) molecular design strategies that simultaneously achieve structural robustness and intense NIR-II absorption remain scarce. Herein, we demonstrate that single-boron coordination can be implemented on the intrinsically strong acceptor [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ), an unachieved regime by previously reported boron-engineered strategies, which have typically been based on weaker electron-deficient acceptors. Single intramolecular N→B–N coordination converts TQ into an even stronger electron-deficient motif than the widely used benzobis(thiadiazole) (BBT) acceptor. Through donor engineering and planarity modulation, the resulting borylated TQ derivative, BLD3, exhibits intense NIR‑II absorption with a high molar extinction coefficient (ε > 3 × 10⁴ M⁻¹ cm⁻¹) around 1000 nm in toluene. Featuring an ultrafast nonradiative decay of 2.1 ps, BLD3 nanoparticles (NPs) show a high photothermal conversion efficiency of 68% under 1060 nm laser irradiation, as well as a robust photoacoustic (PA) output under 1064 nm excitation. Combining excellent photo/thermal stability, favorable tumor accumulation, and good biocompatibility, BLD3 NPs enable effective NIR-II PA imaging-guided photothermal therapy with pronounced tumor ablation. This work expands the library of strong acceptors for boron‑coordination chemistry, providing a general platform for constructing powerful electron‑deficient building blocks and high‑performance NIR‑II phototheranostic materials.