Highly sensitive water-soluble fluorescent pH sensors have been synthesized employing the 7-amino-1-methylquinolinium (7AMQ) chromophore. These compounds, which contained oligomethylene spacers and substituted amine receptor units, were prepared in high yields and purity by a single reaction from the readily available 7-fluoro-1-methylquinolinium iodide. Density functional theory (DFT) and semiempirical INDO/s calculations have been performed to describe the ground state and the locally excited state of the chromophore. The photophysics and the sensor characteristics have been investigated. Dissociation constants pK(A)* ranging from 5.8 to 9.9 have been obtained. An additional quenching process at pH 11, due to excited state deprotonation at N7, has been observed. Fluorescence quantum yields in the protonated "on-state" between 0.75 and 0.85 and fluorescence enhancements (FE) between 2 and 55 were determined. These values are significantly higher than those of molecules based on other CT-chromophores that contain identical spacer-receptor units. The high fluorescence enhancements may be explained by the low rate of fluorescence ( approximately 6 x 10(7) s(-1)) and the high excited state reduction potential ( approximately 1.6 eV) of the 7AMQ chromophore.