Propylenedioxy Thiophene Donor to Achieve NIR-II Molecular Fluorophores with Enhanced Brightness

Developing molecular fluorophores with high brightness is of considerable importance to achieve superior biological imaging quality in the second near-infrared (NIR-II) window. It has been demonstrated that the improved fluorescence quantum yield (QY) can be obtained for NIR-II molecular fluorophores with S-D-A-D-S (S, shielding unit; D, donor; A, acceptor) structures. However, their absorption coefficient is relatively low, limiting their brightness for imaging. Here, 3,4-propylenedioxy thiophene (PDOT) is introduced as a donor unit to construct NIR-II fluorophores with better protection of the conjugated backbone and decreased backbone distortion, eventually leading to simultaneously improved QY and absorption coefficient. Thus, the new fluorophore IR-FP8P shows fluorescence emission with a peak of 1040 nm, a QY of 0.6% (with reference to IR-26 with a QY of 0.05% in dichloroethane), and a peak absorption coefficient of 1.3 × 10⁴ M^-1·cm^-1 in aqueous solutions. The higher brightness at 808 nm excitation endows IR-FP8P with superior imaging quality in the NIR-II window. When conjugated with a specific hormone, the targeting probe FSH@FP8 enables fast and unambiguous ovary imaging in mice, revealing the potential of this bright fluorophore for visualizing complicated biological systems in the NIR-II window.