Luminescent and DFT Study of Keto-Enol Tautomers of 5-Fluorouracil and Its Derivatives in Aqueous Solutions.

Fluorescence (FL) spectra were recorded and identified for the following 5-fluorouracil (FU) tautomers: 5-fluoropyrimidine-2,4(1H,3H)-dione (A), 5-fluoro-2-hydroxypyrimidine-4(3H)-one (B), and 5-fluoro-4-hydroxypyrimidine-2(1H)-one (D), as well as the corresponding tautomers A and D of 1-(tetrahydrofuranyl-2)-5-fluorouracil and 1-methyl-5-fluorouracil, tautomers A and B of 3-methyl-5-fluorouracil, and the diketo tautomer of 1,3-dimethyl-5-fluorouracil. It was shown that the FL of rare tautomers of FU derivatives occurs by the excitation of uracil homoassociates followed by intramolecular proton transfer (IPT), formation of a pair of rare tautomers, and radiative deactivation of one of them. The FL quantum yields φi were estimated. In the case of rare tautomers, the φi value includes the probability of IPT in the homoassociate, and density functional theory calculations in the TPSSTPSS/6-311+G(d,p) approximation were used to determine the relative energies of tautomers, associated FU forms, and their spectral properties that agree with the observed luminescence pattern.