The effect of methylation on the excited state dynamics of aminouracils

The excited state properties of two amino substituted uracils, 5-aminouracil (5AU) and 5-dimethylaminouracil (5DMAU) have been studied by steady-state and time-resolved fluorescence spectroscopy. In addition, the electronic transitions corresponding to photon absorption and emission were calculated using the TD-DFT/PCM method (with PBEO and CAM-B3LYP functionals), explicitly including water molecules of the first solvent shell. The fluorescence decays, recorded by fluorescence upconversion, reveal that the emitting states of both molecules are substantially longer-lived than that of uracil. However, while the 5AU fluorescence decays on a picosecond timescale, the 5DMAU fluorescence contains a weak, long-lived component, continuing beyond 100 ps. Calculations show that the excited state deactivation of 5AU is governed by the existence of a minimum on the pi pi* excited state potential energy surface. For 5DMAU, on the other hand, a very long-lived but weakly fluorescent intramolecular charge transfer (ICT) state, involving a significant charge transfer between the dimethylamino group and the pyrimidine ring, is found. This stable ICT state is characterized by a rotation of the dimethylamino group towards a quasi-perpendicular geometry. (c) 2012 Elsevier B.V. All rights reserved.