New NIR-emitting probes for in vivo visualization of cellular organelles such as lysosomes are critical for basic medical research and clinical applications. Cyanine dyes 1 and 2 (λem~700 nm) with a large Stokes’ shift (△λ≈234 nm) are synthesized by coupling 2-(2’-hydroxyphenyl) benzothiazole (HBT) and benzoxazole (HBO) with a benzothiazolium-derived unit. In the molecular design, excited-state intramolecular proton transfer (ESIPT) is used to generate the excited keto tautomer 1b, which enabled the donor-acceptor interaction over an extended π-conjugation frame. We demonstrate that the resulting 1b includes a cyanine chromophore that is only generated in the excited state. The study thus illustrates a general strategy to design an NIR-emitting chromophore with a large Stokes’ shift by using ESIPT. The phenolic proton, which controls ESIPT, also acts as a switch to respond to acidic environment (pH=4-6). Staining the normal human lung fibroblast cells reveals that 1 is selectively binding to lysosome organelles, giving strong fluorescence in acidic medium. The NIR probe could offer significant potential for in vivo study of cellular imaging, as it does not exhibit “an alkalinizing effect” (an adverse effect which the current probes suffer).

Lysosomes targeting probes with large stokes’ shifts via cyanine coupling with excited state intramolecular proton transfer (ESIPT)

254th National Meeting (2017)

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