Excited state properties of 5-fluoro-4-thiouridine derivative^†

The excited state properties of thionated 5-fluorouridine (2',3',5'-tri-O-acetyl-5-fluoro-4-thiouridine; ta5F4TUrd), synthesized with Lawesson's reagent, have been intensively investigated with nanosecond transient absorption spectroscopy, time-resolved thermal lensing, near-infrared emission, and quantum chemical calculation. The intrinsic triplet lifetime of ta5F4TUrd was determined to be $$ μs in acetonitrile, and the formation quantum yield of the excited triplet state was as large as $$ . The quenching rate constants of the triplet ta5F4TUrd by the dissolved oxygen molecule and by the self-quenching process were found to be nearly equal to the diffusion-controlled rate of acetonitrile. The quantum yield of the singlet molecular oxygen produced through energy transfer between the triplet ta5F4TUrd and the dissolved oxygen, $$ , was successfully determined to be $$ under the oxygen-saturated condition. From the oxygen concentration dependence of the $$ value, the fraction of triplet ta5F4TUrd quenched by dissolved oxygen which gives rise to the ¹ O₂ * formation, $$ , was successfully obtained to be $$ , which was the largest among the thionucleobases and the thionucleosides reported so far. This could be due to the lower energy and/or the ππ* character of the triplet state.