Abstract
The pH sensing properties of five (four new) small-molecule fluorescent “turn-on” pH probes based on 3-amino- and 3-dimethylamino-coumarin skeleton were investigated. Protonation of their amino- or dimethylamino- group leads to 12–1400-fold increase in their fluorescence intensity in highly acidic conditions. Whereas neutral molecules exhibit only poor fluorescence, fluorescence de-excitation pathway in the protonated forms can compete with effective intersystem crossing. Performed quantum chemical calculations based on frontier orbital analysis support the photoinduced electron transfer (PET) quenching mechanism of fluorescence in the neutral forms. Ability of the studied 3-amino-/3-dimethylamino-coumarins to stain vacuolar lumen with fast stain response and to monitor the pH of commercially available apple juice samples was demonstrated.
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