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

Developing molecular fluorophores with high brightness is of high importance to achieve superior biological imaging quality in the second near-infrared (NIR-II) window. It has been demonstrated that 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 is introduced as the donor unit to construct NIR-II fluorophores with better protection of 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 peak of 1040 nm and a QY of 0.6% (Reference to IR-26 with 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 affords IR-FP8P with superio...