The unusual fluorescence properties of 8-methoxy-4-methyl-2 H-benzo [g] chromen-2-one (1) are described. The fluorophore 1 is almost nonfluorescent in aprotic solvent (eg, fluorescence quantum yield Φf< 0.0003 in n-hexane), whereas it strongly fluoresces at long wavelengths (> 450 nm) in protic solvent (eg, Φf= 0.21 in methanol). The fluorophore 1 also shows good applicability in developing a new fluorogenic (fluorescent “off-on”) sensor.