Multidimensional Molecular Engineering of Benzobisthiadiazole‐Based NIR‐II AIEgens for Pareto‐Balanced Imaging and Photothermal Performance

The second near‐infrared (NIR‐II, 1000–1700 nm) window enables deep‐tissue optical interrogation with high spatial resolution for biomedical applications, but molecular design is hampered by trade‐offs among five parameters: absorption and emission wavelengths (λ abs , λ em ), fluorescence quantum yield (QY), molar extinction coefficient (ε), and photothermal conversion efficiency (η). Here, we report a molecular engineering strategy that balances radiative and nonradiative decay within an aggregation‐induced emission (AIE) framework. We construct a benzobisthiadiazole (BBT)‐based D–A–D–A–D–A–D chromophore, MTBTMT‐BBT, comprising a rigid planar π‐core and twisted peripheral donors that yield an AIE–ACQ–AIE framework. In F127‐encapsulated nanoparticles (MTBTMT‐BBT NPs), this architecture affords increased NIR‐II fluorescence quantum yield and sustained long‐wavelength absorption/emission with high photostability, and retains strong absorption capacity (high molar extinction coefficient) together with efficient photothermal energy conversion under 808 nm irradiation. Spectroscopic analysis and calculations link the design to controlled aggregate packing that suppresses nonradiative decay yet preserves sufficient heat generation. In vivo, MTBTMT‐BBT NPs enable high‐contrast NIR‐II vascular and tumor imaging and, upon laser irradiation, achieve complete 4T1 tumor eradication without recurrence or systemic toxicity, establishing a general design principle for Pareto‐balanced NIR‐II chromophores and multifunctional molecular theranostics.