Near‐Infrared‐II Fluorophore with Inverted Dependence of Fluorescence Quantum Yield on Polarity as Potent Phototheranostics for Fluorescence‐Image‐Guided Phototherapy of Tumors

Abstract Organic phototheranostics simultaneously having fluorescence in the second near‐infrared (NIR‐II, 1000–1700 nm) window, and photothermal and photodynamic functions possess great prospects in tumor diagnosis and therapy. However, such phototheranostics generally suffer from low brightness and poor photodynamic performance due to severe solvatochromism. Herein, an organic NIR‐II fluorophore AS1, which possesses an inverted dependence of fluorescence quantum yield on polarity, is reported to serve as potent phototheranostics for tumor diagnosis and therapy. After encapsulation of AS1 into nanostructures, the obtained phototheranostics (AS1^R) exhibit high extinction coefficients (e.g., 68200 L mol⁻¹ cm⁻¹ at 808 nm), NIR‐II emission with high fluorescence quantum yield up to 4.7% beyond 1000 nm, photothermal conversion efficiency of ≈65%, and ¹O₂ quantum yield up to 4.1%. The characterization of photophysical properties demonstrates that AS1^R is superior to other types of organic phototheranostics in brightness, photothermal effect, and photodynamic performance at the same mass concentration. The excellent phototheranostic performance of AS1^R enables clear visualization and complete elimination of tumors using a single and low injection dose. This study demonstrates the merits and prospects of NIR‐II fluorophore with inverted polarity dependence of fluorescence quantum yield as high‐performance phototheranostic agents for fluorescence imaging and phototherapy of tumors.