Development of a benzothiazole-functionalized red-emission pyronin dye and its dihydro derivative for imaging lysosomal viscosity and tracking endogenous peroxynitrite.

Although strategies based on pi-conjugation expansion and one-atom replacement have routinely been used to extend the emission wavelengths of rhodamine dyes from the visible region to the biologically more favorable red to near-infrared (NIR) region, the strategy of introducing electron-withdrawing groups to the meso-position of pyronin dyes for the same goal is still in its infant phase. In this work, we present a benzothiazole-functionalized pyronin dye BTP as a red-emission fluorescent dye platform for bioimaging applications. Due to the electron-withdrawing nature of the meso-substituted benzothiazole unit, BTP exhibited a large red-shift in absorption and emission wavelengths compared to classic rhodamines. Interestingly, BTP could not only behave like a molecular rotor to fluorescently respond to viscosity changes, but also specifically target lysosomes and light up them assisted by a lysosomal viscous microenvironment. Furthermore, based on the BTP platform, we developed its "dihydro" derivative, i.e., HBTP, and evaluated its sensing performance to reactive oxygen species (ROS). The obtained results showed that HBTP is a highly selective fluorescent probe for sensing endogenous peroxynitrite (ONOO-) with quite rapid fluorescence off-on response and high sensitivity. It is greatly expected that the present study could stimulate research interest in exploiting various rhodamine-inspired fluorescent dyes or probes for bioimaging applications.