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 4.2 ± 0.7 $$ 4.2\pm 0.7 $$ μs in acetonitrile, and the formation quantum yield of the excited triplet state was as large as 0.79 ± 0.01 $$ 0.79\pm 0.01 $$ . 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, Φ Δ $$ {\varPhi}_{\Delta} $$ , was successfully determined to be 0.61 ± 0.02 $$ 0.61\pm 0.02 $$ under the oxygen-saturated condition. From the oxygen concentration dependence of the Φ Δ $$ {\varPhi}_{\Delta} $$ value, the fraction of triplet ta5F4TUrd quenched by dissolved oxygen which gives rise to the 1 O 2 * formation, S Δ $$ {S}_{\Delta} $$ , was successfully obtained to be 0.78 ± 0.01 $$ 0.78\pm 0.01 $$ , 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.