| Abstract
| - The excited-state dynamics of 5-fluorouracil in acetonitrile has been investigated by femtosecond fluorescenceupconversion spectroscopy in combination with quantum chemistry TD-DFT calculations ((PCM/TD-PBE0).Experimentally, it was found that when going from water to acetonitrile solution the fluorescence decay of5FU becomes much faster. The calculations show that this is related to the opening of an additional decaychannel in acetonitrile solution since the dark n/π* excited state becomes near degenerate with the brightπ/π* state, forming a conical intersection close to the Franck−Condon region. In both solvents, a S1−S0conical intersection, governed by the out-of-plane motion of the fluorine atom, is active, allowing an ultrafastinternal conversion to the ground state.
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