Ultrafast Electronic Relaxation in 6-Methyluracil and 5-Fluorouracil in Isolated and Aqueous Conditions: Substituent and Solvent Effects.

We report ultrafast extreme ultraviolet photoelectron spectroscopy of 6-methyluracil (6mUra) and 5-fluorouracil (5FUra) in the gas phase and 6mUra and 5-fluorouridine in an aqueous environment. In the gas phase, internal conversion (IC) occurs from ππ* to nπ* states in tens of femtoseconds, followed by intersystem crossing to the ππ* state in several picoseconds. In an aqueous solution, 6mUra undergoes IC almost exclusively to the ground state (S) in about 100 fs, which is essentially the same process as that for unsubstituted uracil, but much faster than that for thymine (5-methyluracil). The different dynamics for C5 and C6 methylation suggest that IC from ππ* to S is facilitated by out-of-plane (OOP) motion of the C5 substituent. The slow IC for C5-substituted molecules in an aqueous environment is ascribed to the solvent reorganization that is required for this OOP motion to occur. The slow rate for 5FUrd may arise in part from an increased barrier height due to C5 fluorination.