Efficient Red/Near-Infrared Fluorophores Based on Benzo[1,2-b:4,5-b′]dithiophene 1,1,5,5-Tetraoxide for Targeted Photodynamic Therapy and In Vivo Two-Photon Fluorescence Bioimaging

Red/near-infrared dyes are highly demanded for biological applications but most of them are far from satisfactory. In this work, a series of red/near-infrared fluorophores based on electron-withdrawing benzo[1,2-b:4,5-b′]dithiophene 1,1,5,5-tetraoxide (BDTO) are synthesized and characterized. They possess both aggregation-induced emission, and hybridized local and charge-transfer characteristics. Crystallographic, spectroscopic, electrochemical and computational results reveal that the oxidation of benzo[1,2-b:4,5-b′]dithiophene to BDTO can endow the fluorophores with greatly red-shifted emission, enhanced emission efficiency, reduced energy levels, enlarged two-photon absorption cross section, and increased reactive oxygen species generation efficiency. The nanoparticles fabricated with a near-infrared fluorophore TPA-BDTO show high photostability and biocompatibility with good performance in targeted photodynamic ablation of cancer cells and two-photon fluorescence imaging of intravital mouse brain vasculature.