Incorporation of Robust NIR‐II Fluorescence Brightness and Photothermal Performance in a Single Large π‐Conjugated Molecule for Phototheranostics

Second near‐infrared (NIR‐II, 1000–1700 nm) window fluorescence imaging‐guided photothermal therapy probes are promising for precise cancer phototheranostics. However, most of the currently reported probes do not demonstrate high NIR‐II fluorescent brightness (molar absorption coefficient (ε) × quantum yield (QY)) and photothermal performance (ε × photothermal conversion efficiency (PCE)) in a single molecule. Herein, a versatile strategy to solve this challenge is reported by fabricating a large π‐conjugated molecule (BNDI‐Me) with a rigid molecular skeleton and flexible side groups. The proposed BNDI‐Me nanoprobe boosts the ε and simultaneously optimizes its QY and PCE. Therefore, high NIR‐II fluorescent brightness (ε × QY = 2296 m (−1) cm(−1)) and strong photothermal performance (ε × PCE = 82 000) are successfully incorporated in a single small molecule, and, to the best of knowledge, either of these two parameters is better than the best currently available fluorescent or photothermal probes. Thus, superior NIR‐II imaging effect in vivo and high photothermal tumor inhibition rate (81.2%) at low systemic injection doses are obtained. The work provides further insights into the relationship of photophysical mechanisms and structures, and presents promising molecular design guidelines for the integration of more efficient multiple theranostic functions in a single molecule.