Unimolecular Micellization Strategy for Achieving NIR-II Excited Fluorophores with Enhanced Brightness in Aqueous

Fluorescence imaging in the second near-infrared window (NIR-II) is advantageous for the in situ detection of living organisms owing to its enhanced imaging depth and high spatial-temporal resolution. The development of high-performance fluorescence probes is crucial for enhancing imaging effectiveness. NIR-II (e.g. 1064 nm) excited molecular fluorophores exhibit favorable imaging performance and biocompatibility, but are susceptible to non-radiative decay, leading to substantial fluorescence quenching in aqueous compared to the NIR-I (e.g. 808 nm) excited counterparts. Previous studies have revealed that reducing the interaction between the excited-state center of the fluorophore and water molecules is a key point for improving fluorescence quantum yield (QY) in aqueous. Herein, an innovative unimolecular micellization strategy is proposed to achieve high-brightness organic fluorescence probes with NIR-II excitation (1064 nm) and emission (1000-1700 nm) in aqueous. We designed a series of NIR-II excited star-shaped amphiphilic molecules that demonstrate the ability to self-assemble into stable unimolecular micelles (UIMs) in aqueous. Upon unimolecular micellization, the intramolecular long alkyl chains can form a compact hydrophobic layer, effectively confining intermolecular interactions and shielding the excited-state center of the fluorophore from water-induced quenching, thereby maintaining high QY in aqueous. The IR-FCT8CP UIMs exhibit absorption and emission maximum wavelengths at 979 and 1181 nm, respectively, with a high QY of 0.0501% and a molar absorption coefficient of 1.67 × 104 M−1·cm−1 in aqueous, resulting in a brightness enhancement exceeding 28-fold compared to IR-FCDP UIMs. The exceptional fluorescence properties of IR-FCT8CP UIMs enable dynamic imaging of vessels using a 1500 nm long-pass filter, revealing a distinct vessel network with an optimal signal-to-background ratio. The concept of "unimolecular micellization for spatial confinement enhancement" offers innovative insights for developing NIR-II excited molecular fluorophores to maintain high brightness and stability in physiological environments for highly efficient bioimaging.