Spectral and photophysical properties of the lowest excited triplet state of 4-thiouridine and its 5-halogeno derivatives

The properties of the lowest excited triplet state (T1) of 2′,3′,5′-tri-O-acetyl-4-thiouridine (TU) and its 5-halogeno substituted derivatives (XTU, X = F, Cl, Br and I) in CCl4 solutions are presented. Steady-state and pulsed techniques were used to characterize the T1 state of TU and XTU in terms of the phosphorescence quantum yields (φp0), intrinsic lifetimes (τT10) and the self-quenching rate constants (ksq). The quantum yields of photochemical decay (ϕpch) of TU and XTU were also determined. From the values of quantum yields and under assumption that the formation of the T1 state can be approximated by ϕT1=1, the rate constants of radiative (kp) and non-radiative (knr) decay processes from the T1 state were calculated. The results of the studies show that the T1 states of TU and XTU have a ππ∗ nature. Comparison of the absorption and phosphorescence spectral properties of TU and XTU molecules with those of 1,3-dimethyl-4-thiouracil (DMTU) confirms that TU and XTU exist in CCl4 solution as (C-2)-keto-(C-4)-thione tautomers, exclusively, and that the phosphorescence arises from the excited T1 state of these single tautomeric forms. The effect of the halogen substituent on the T1 state properties of XTU molecules is discussed.