3,4-Ethylenedithio thiophene donor for NIR-II fluorophores with improved quantum yields

Bright fluorophores are vital for bioimaging in the second near-infrared (NIR-II, 1500-1700 nm) window. Donor engineering has been demonstrated to be effective at significantly improving the brightness of donor-acceptor-donor (D-A-D) NIR-II fluorophores. Developing new donor units to construct NIR-II fluorophores with improved quantum yields (QYs) is still challenging. Herein, 3,4-ethylenedithio thiophene (EDST) is employed as the donor unit for the first time to construct a new NIR-II fluorophore, IR-nFES. Compared with the fluorophore IR-nFE with the 3,4-ethylenedioxy thiophene (EDOT) donor, sulphur atom replacement can significantly increase the conjugation backbone distortion, affording an enhanced fluorescence QY of 5.5% in toluene for IR-nFES. Water soluble nanofluorophores (NFs) of IR-nFES are fabricated by encapsulating with an amphiphilic copolymer, and IR-nFE NFs show a QY of 1.3%, 4.8 times higher than that of the IR-nFE counterpart. It is further demonstrated that IR-nFES NFs are significantly brighter than IR-nFE NFs and indocyanine green (ICG), and also exhibit good optical stability. Consequently, IR-nFES NFs realize superior in vivo imaging of mouse cerebral vessels with a high signal-to-background ratio (SBR) of 4.99. Our studies reveal that sulphur substitution can afford new NIR-II molecular fluorophores with enhanced brightness.