Near-infrared fluorescent HaloTag ligands for efficient organelle labelling in live cells

A series of HaloTag ligands that incorporate near-infrared (NIR)-emissive phospha-rhodamine (POR) dyes have been developed. The PORs that contain dealkylated amino groups exhibit a hypsochromic shift in their absorption and emission wavelengths. Subsequent refinement of the POR spirocyclization equilibrium significantly enhanced membrane permeability, thereby leading to a substantial reduction in the time necessary for labelling. Consequently, target organelles could even be labeled at low POR dye concentrations in the tens of nM range.