Phenolic dyes with high pKas – such as 7-hydroxycoumarin (often called umbelliferone) and the more common 7-hydroxy-4-methylcoumarin – are not fully deprotonated and therefore not fully fluorescent unless the pH of the reaction mixture is raised to above pH ~10. Thus, substrates derived from these fluorophores are seldom used for continuous measurement of enzymatic activity in solution or live cells. The similar 3-cyano-7-hydroxycoumarin and 6,8-difluoro-7-hydroxy-4-methylcoumarin have lower pKas, making them suitable for a broader range of applications. Moreover, the -trifluoromethylumbelliferone product has longer-wavelength spectra and stronger absorbance than -methylumbelliferone. Ether, ester and phosphate substrates derived from these phenolic dyes may be fluorescent but invariably exhibit shorter-wavelength absorption and emission spectra.