Tautomerism of uracil and thymine in aqueous solution: spectroscopic evidence

Excitation spectra for triplet state formation and fluorescence emission from uracil and thymine in neutral aqueous solution at room temperature are anomalous when compared to the absorption spectra of uracil and thymine. The experimental data are critically examined with respect to three molecular models; Model 1, which is characterized by increased intersystem crossing from higher vibrational levels of S(1); Model 2, in which fluorescence is attributed to a (pi,pi(*)) state, whereas intersystem crossing occurs from an (npi(*)) state; and Model 3, in which fluorescence is attributed to a tautomer I, while triplet yields originate from tautomer II. Reasons are presented for discarding Models 1 and 2, and it is demonstrated that the observed excitation spectra complement each other with respect to the absorption spectrum, such that the quantum yields of triplet formation and fluorescence emission become independent of exciting wavelength. It is suggested that the fluorescing tautomer I has the N(3)-C(4) enol structure, while the triplet-forming tautomer II is most likely the predominant diketo form.