Membrane‐Permeant, Bioactivatable Coumarin Derivatives for In‐Cell Labelling

The delivery of small molecule fluorophores with minimal compartmentalization is currently one of the most critical technical problems in intracellular labelling. Here we introduce sulfonated and phosphonated coumarin dyes, demonstrate rapid cell entry via a prodrug approach, and show a lack of interaction with membranes, organelles, or other compartments. The dyes show no specific localization and are evenly distributed in the cells. Our fluorogenic, clickable phosphonate derivatives successfully tagged model targets in intact cells and the increase in brightness upon click reaction was around 60‐fold. A pro‐drug approach delivered sulfonated and phosphonated fluorescent coumarin dyes to live cells without labeling internal membranes. The new dyes exhibited homogeneous cell distribution. In addition, the use of azide groups followed by click chemistry provided fluorogenicity.