AbstractsChemistry

Fluorescence of adenine, guanine, thymine, cytosine, and uracil has been detected at room temperature in neutral aqueous solution using a digital signal accumulation technique and right angle fluorescence detection from solutions with absorbances between 0. 4 and 0. 7. The quantum yields are respectively 2.6 x 10⁻⁴, 3.0 x 10⁻⁴, 1 x 10⁻⁴ 0.8 x 10⁻⁴, and 0.5 x 10⁻⁴ when excited at their low energy absorption maxima. Singlet lifetimes calculated from these yields may mean a recalculation of excited state processes occurring in DNA which were previously based on low temperature data. Corrected relative emission spectra are presented and compared with reported low temperature data. The corrected relative excitation spectra all show significant differences from the absorption spectrum when both are determined under identical conditions of concentration and spectral bandwidth on the same instrument. This behavior is discussed in terms J. of a) n-π and π-π states, b) emission from a minor tautomer and c) kinetics of competing deactivation processes. From a development of c), rate constants for processes occurring from the 0' level are calculated for intersystem crossing, internal conversion, and (singlet) photochemical reaction in thymine and uracil. From solvent induced shifts of absorption and fluorescence spectra, the excited state dipole moment of thymine μe = 8. 0 ± 2. 20) and the π-π* nature of the transitions responsible for fluorescence are deduced. Consideration of the singlet lifetime of thymine in relation to the rates of excited state process implies that fluorescence may occur from a Franck-Condon excited state and this is discussed in terms of possible solute-solvent interactions. A study of the pH effects on the spectral shape, magnitude, and polarization of thymine emission concludes that the emitting states of J. ionized thymine are of a π-π* nature , that fluorescence may arise within the 1, 2,4, 5, and 6 π- conjugated system in the thymine molecule, and that excited state ionizations do not occur in the fluorescent forms of thymine. These deductions are discussed in relation to DNA excited state processes.