Coumarins remain one of the most important groups of fluorescent bio-probes, thanks to their high quantum yields, moderate photostability, efficient cell permeation and low (cyto)toxicity. Herein, we introduce new 3-aminocoumarins as turn-on pH probes under strongly acidic conditions and for indicators capable of significantly improving yeast vacuolar lumen staining compared to the commercial CMAC derivatives. We present the details of the on-off switching mechanism revealed by the TD-DFT and ab initio calculations complemented by a Franck-Condon analysis of the probes' emission profiles.