We selected Haugland's precipitating fluorophore PQ-OH (Fig. ) as our proof-of-concept fluorogenic phenolic cargo. Acylating its phenol completely mechanistically quenches its highquantum-yield, environment-independent, large-Stokes-shift fluorescence (ex/em 360/520 nm), since this depends on excited state intramolecular proton transfer (ESIPT) of the phenolic hydrogen. Therefore PQ-releasing probes can operate as true turn-on systems, with zero self-background and excellent signal-to-noise ratios, typically above 300 even without background signal subtraction. There are many further advantages and few drawbacks of PQ-OH for cellfree and cellular proof-of-concept studies (see Supporting Note 4).