"Donor-two-acceptor" dye design: a distinct gateway to NIR fluorescence.

The detection of chemical or biological analytes upon molecular reactions relies increasingly on fluorescence methods, and there is a demand for more sensitive, more specific, and more versatile fluorescent molecules. We have designed long wavelength fluorogenic probes with a turn-ON mechanism based on a donor-two-acceptor π-electron system that can undergo an internal charge transfer to form new fluorochromes with longer π-electron systems. Several latent donors and multiple acceptor molecules were incorporated into the probe modular structure to generate versatile dye compounds. This new library of dyes had fluorescence emission in the near-infrared (NIR) region. Computational studies reproduced the observed experimental trends well and suggest factors responsible for high fluorescence of the donor-two-acceptor active form and the low fluorescence observed from the latent form. Confocal images of HeLa cells indicate a lysosomal penetration pathway of a selected dye. The ability of these dyes to emit NIR fluorescence through a turn-ON activation mechanism makes them promising candidate probes for in vivo imaging applications.