Room temperature fluorescence of 5-chlorouracil tautomers

Fluorescence and absorption spectra of 10^-4 M 5-chlorouracil in aqueous solutions at different pH values are reported at room temperature. The shape of the fluorescence spectrum is found to change when the excitation wavelength is varied. In addition to the predominant diketo tautomer (I), the keto-enol tautomer, namely 5-chloro-2-hydroxy-4(3H)-pyrimidinone (II), is believed to contribute to the absorption spectrum of 5-chlorouracil in the spectral range 240-330 nm. Using picosecond laser pulses, lifetimes and time-resolved spectra have also been recorded. Both experiments support the explanation of obtained results through acid-base equilibrium and the tautomeric contribution in the ground state.