Ab initio investigation of the methylation and hydration effects on the electronic spectra of uracil and thymine

In this work we investigated the lowest-lying electronic excitations for a series of methyl-substituted uracil derivatives, i.e., uracil, 1-methyluracil, 3-methyluracil, thymine, 1-methylthymine, 1,3-dimethyluracil, 3-methylthymine, 1,3-dimethylthymine, and their microhydrated complexes by means of coupled cluster singles and approximate doubles (CC2) and density functional theory (DFT) methods. The bulk water environment was mimicked by a combination of microhydration and the conductor-like screening model (COSMO). We find that the shift of the electronic excitation energies due to methylation and hydration depend on the character of the wave function and on the position of the methyl substituent. The lowest-lying singlet and triplet n -> pi* states are insensitive to methylation but are strongly blue-shifted by microhydration and bulk water solvation. The largest red-shift of the first (1)(pi -> pi*) excitation occurs upon methylation at N-1 followed by substitution at C-5 whereas no effect is obtained for a methylation at N-3. For this state, the effects of methylation and hydrogen bonding partially cancel. Upon microhydration with six water molecules, the order of the (1)(n -> pi*) and (1)(pi -> pi*) states is reversed in the vertical spectrum. Electrostatic solute-solvent interaction in bulk water leads to a further increase of their energy separation. The n -> pi* states are important intermediates for the triplet formation. Shifting them energetically above the primarily excited (1)(pi -> pi*) state will considerably decrease the triplet quantum yield and thus increase the photostability of the compounds, in agreement with experimental observations.